\input texinfo.tex @c -*- texinfo -*- @c %**start of header @setfilename bashref.info @settitle Bash Reference Manual @c %**end of header @setchapternewpage odd @include version.texi @copying This text is a brief description of the features that are present in the Bash shell (version @value{VERSION}, @value{UPDATED}). This is Edition @value{EDITION}, last updated @value{UPDATED}, of @cite{The GNU Bash Reference Manual}, for @code{Bash}, Version @value{VERSION}. Copyright @copyright{} 1988--2011 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. @quotation Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover texts being ``A GNU Manual'', and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled ``GNU Free Documentation License''. (a) The FSF's Back-Cover Text is: You are free to copy and modify this GNU manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom.'' @end quotation @end copying @defcodeindex bt @defcodeindex rw @set BashFeatures @dircategory Basics @direntry * Bash: (bash). The GNU Bourne-Again SHell. @end direntry @finalout @titlepage @title Bash Reference Manual @subtitle Reference Documentation for Bash @subtitle Edition @value{EDITION}, for @code{Bash} Version @value{VERSION}. @subtitle @value{UPDATED-MONTH} @author Chet Ramey, Case Western Reserve University @author Brian Fox, Free Software Foundation @page @vskip 0pt plus 1filll @insertcopying @sp 1 Published by the Free Software Foundation @* 59 Temple Place, Suite 330, @* Boston, MA 02111-1307 @* USA @* @end titlepage @contents @ifnottex @node Top, Introduction, (dir), (dir) @top Bash Features This text is a brief description of the features that are present in the Bash shell (version @value{VERSION}, @value{UPDATED}). This is Edition @value{EDITION}, last updated @value{UPDATED}, of @cite{The GNU Bash Reference Manual}, for @code{Bash}, Version @value{VERSION}. Bash contains features that appear in other popular shells, and some features that only appear in Bash. Some of the shells that Bash has borrowed concepts from are the Bourne Shell (@file{sh}), the Korn Shell (@file{ksh}), and the C-shell (@file{csh} and its successor, @file{tcsh}). The following menu breaks the features up into categories based upon which one of these other shells inspired the feature. This manual is meant as a brief introduction to features found in Bash. The Bash manual page should be used as the definitive reference on shell behavior. @menu * Introduction:: An introduction to the shell. * Definitions:: Some definitions used in the rest of this manual. * Basic Shell Features:: The shell "building blocks". * Shell Builtin Commands:: Commands that are a part of the shell. * Shell Variables:: Variables used or set by Bash. * Bash Features:: Features found only in Bash. * Job Control:: What job control is and how Bash allows you to use it. * Command Line Editing:: Chapter describing the command line editing features. * Using History Interactively:: Command History Expansion * Installing Bash:: How to build and install Bash on your system. * Reporting Bugs:: How to report bugs in Bash. * Major Differences From The Bourne Shell:: A terse list of the differences between Bash and historical versions of /bin/sh. * GNU Free Documentation License:: Copying and sharing this documentation. * Indexes:: Various indexes for this manual. @end menu @end ifnottex @node Introduction @chapter Introduction @menu * What is Bash?:: A short description of Bash. * What is a shell?:: A brief introduction to shells. @end menu @node What is Bash? @section What is Bash? Bash is the shell, or command language interpreter, for the @sc{gnu} operating system. The name is an acronym for the @samp{Bourne-Again SHell}, a pun on Stephen Bourne, the author of the direct ancestor of the current Unix shell @code{sh}, which appeared in the Seventh Edition Bell Labs Research version of Unix. Bash is largely compatible with @code{sh} and incorporates useful features from the Korn shell @code{ksh} and the C shell @code{csh}. It is intended to be a conformant implementation of the @sc{ieee} @sc{posix} Shell and Tools portion of the @sc{ieee} @sc{posix} specification (@sc{ieee} Standard 1003.1). It offers functional improvements over @code{sh} for both interactive and programming use. While the @sc{gnu} operating system provides other shells, including a version of @code{csh}, Bash is the default shell. Like other @sc{gnu} software, Bash is quite portable. It currently runs on nearly every version of Unix and a few other operating systems @minus{} independently-supported ports exist for @sc{ms-dos}, @sc{os/2}, and Windows platforms. @node What is a shell? @section What is a shell? At its base, a shell is simply a macro processor that executes commands. The term macro processor means functionality where text and symbols are expanded to create larger expressions. A Unix shell is both a command interpreter and a programming language. As a command interpreter, the shell provides the user interface to the rich set of @sc{gnu} utilities. The programming language features allow these utilities to be combined. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories such as @file{/bin}, allowing users or groups to establish custom environments to automate their common tasks. Shells may be used interactively or non-interactively. In interactive mode, they accept input typed from the keyboard. When executing non-interactively, shells execute commands read from a file. A shell allows execution of @sc{gnu} commands, both synchronously and asynchronously. The shell waits for synchronous commands to complete before accepting more input; asynchronous commands continue to execute in parallel with the shell while it reads and executes additional commands. The @dfn{redirection} constructs permit fine-grained control of the input and output of those commands. Moreover, the shell allows control over the contents of commands' environments. Shells also provide a small set of built-in commands (@dfn{builtins}) implementing functionality impossible or inconvenient to obtain via separate utilities. For example, @code{cd}, @code{break}, @code{continue}, and @code{exec}) cannot be implemented outside of the shell because they directly manipulate the shell itself. The @code{history}, @code{getopts}, @code{kill}, or @code{pwd} builtins, among others, could be implemented in separate utilities, but they are more convenient to use as builtin commands. All of the shell builtins are described in subsequent sections. While executing commands is essential, most of the power (and complexity) of shells is due to their embedded programming languages. Like any high-level language, the shell provides variables, flow control constructs, quoting, and functions. Shells offer features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, command history and aliases. Each of these features is described in this manual. @node Definitions @chapter Definitions These definitions are used throughout the remainder of this manual. @table @code @item POSIX @cindex POSIX A family of open system standards based on Unix. Bash is primarily concerned with the Shell and Utilities portion of the @sc{posix} 1003.1 standard. @item blank A space or tab character. @item builtin @cindex builtin A command that is implemented internally by the shell itself, rather than by an executable program somewhere in the file system. @item control operator @cindex control operator A @code{token} that performs a control function. It is a @code{newline} or one of the following: @samp{||}, @samp{&&}, @samp{&}, @samp{;}, @samp{;;}, @samp{|}, @samp{|&}, @samp{(}, or @samp{)}. @item exit status @cindex exit status The value returned by a command to its caller. The value is restricted to eight bits, so the maximum value is 255. @item field @cindex field A unit of text that is the result of one of the shell expansions. After expansion, when executing a command, the resulting fields are used as the command name and arguments. @item filename @cindex filename A string of characters used to identify a file. @item job @cindex job A set of processes comprising a pipeline, and any processes descended from it, that are all in the same process group. @item job control @cindex job control A mechanism by which users can selectively stop (suspend) and restart (resume) execution of processes. @item metacharacter @cindex metacharacter A character that, when unquoted, separates words. A metacharacter is a @code{blank} or one of the following characters: @samp{|}, @samp{&}, @samp{;}, @samp{(}, @samp{)}, @samp{<}, or @samp{>}. @item name @cindex name @cindex identifier A @code{word} consisting solely of letters, numbers, and underscores, and beginning with a letter or underscore. @code{Name}s are used as shell variable and function names. Also referred to as an @code{identifier}. @item operator @cindex operator, shell A @code{control operator} or a @code{redirection operator}. @xref{Redirections}, for a list of redirection operators. Operators contain at least one unquoted @code{metacharacter}. @item process group @cindex process group A collection of related processes each having the same process group @sc{id}. @item process group ID @cindex process group ID A unique identifier that represents a @code{process group} during its lifetime. @item reserved word @cindex reserved word A @code{word} that has a special meaning to the shell. Most reserved words introduce shell flow control constructs, such as @code{for} and @code{while}. @item return status @cindex return status A synonym for @code{exit status}. @item signal @cindex signal A mechanism by which a process may be notified by the kernel of an event occurring in the system. @item special builtin @cindex special builtin A shell builtin command that has been classified as special by the @sc{posix} standard. @item token @cindex token A sequence of characters considered a single unit by the shell. It is either a @code{word} or an @code{operator}. @item word @cindex word A sequence of characters treated as a unit by the shell. Words may not include unquoted @code{metacharacters}. @end table @node Basic Shell Features @chapter Basic Shell Features @cindex Bourne shell Bash is an acronym for @samp{Bourne-Again SHell}. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, The rules for evaluation and quoting are taken from the @sc{posix} specification for the `standard' Unix shell. This chapter briefly summarizes the shell's `building blocks': commands, control structures, shell functions, shell @i{parameters}, shell expansions, @i{redirections}, which are a way to direct input and output from and to named files, and how the shell executes commands. @menu * Shell Syntax:: What your input means to the shell. * Shell Commands:: The types of commands you can use. * Shell Functions:: Grouping commands by name. * Shell Parameters:: How the shell stores values. * Shell Expansions:: How Bash expands parameters and the various expansions available. * Redirections:: A way to control where input and output go. * Executing Commands:: What happens when you run a command. * Shell Scripts:: Executing files of shell commands. @end menu @node Shell Syntax @section Shell Syntax @menu * Shell Operation:: The basic operation of the shell. * Quoting:: How to remove the special meaning from characters. * Comments:: How to specify comments. @end menu When the shell reads input, it proceeds through a sequence of operations. If the input indicates the beginning of a comment, the shell ignores the comment symbol (@samp{#}), and the rest of that line. Otherwise, roughly speaking, the shell reads its input and divides the input into words and operators, employing the quoting rules to select which meanings to assign various words and characters. The shell then parses these tokens into commands and other constructs, removes the special meaning of certain words or characters, expands others, redirects input and output as needed, executes the specified command, waits for the command's exit status, and makes that exit status available for further inspection or processing. @node Shell Operation @subsection Shell Operation The following is a brief description of the shell's operation when it reads and executes a command. Basically, the shell does the following: @enumerate @item Reads its input from a file (@pxref{Shell Scripts}), from a string supplied as an argument to the @option{-c} invocation option (@pxref{Invoking Bash}), or from the user's terminal. @item Breaks the input into words and operators, obeying the quoting rules described in @ref{Quoting}. These tokens are separated by @code{metacharacters}. Alias expansion is performed by this step (@pxref{Aliases}). @item Parses the tokens into simple and compound commands (@pxref{Shell Commands}). @item Performs the various shell expansions (@pxref{Shell Expansions}), breaking the expanded tokens into lists of filenames (@pxref{Filename Expansion}) and commands and arguments. @item Performs any necessary redirections (@pxref{Redirections}) and removes the redirection operators and their operands from the argument list. @item Executes the command (@pxref{Executing Commands}). @item Optionally waits for the command to complete and collects its exit status (@pxref{Exit Status}). @end enumerate @node Quoting @subsection Quoting @cindex quoting @menu * Escape Character:: How to remove the special meaning from a single character. * Single Quotes:: How to inhibit all interpretation of a sequence of characters. * Double Quotes:: How to suppress most of the interpretation of a sequence of characters. * ANSI-C Quoting:: How to expand ANSI-C sequences in quoted strings. * Locale Translation:: How to translate strings into different languages. @end menu Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion. Each of the shell metacharacters (@pxref{Definitions}) has special meaning to the shell and must be quoted if it is to represent itself. When the command history expansion facilities are being used (@pxref{History Interaction}), the @var{history expansion} character, usually @samp{!}, must be quoted to prevent history expansion. @xref{Bash History Facilities}, for more details concerning history expansion. There are three quoting mechanisms: the @var{escape character}, single quotes, and double quotes. @node Escape Character @subsubsection Escape Character A non-quoted backslash @samp{\} is the Bash escape character. It preserves the literal value of the next character that follows, with the exception of @code{newline}. If a @code{\newline} pair appears, and the backslash itself is not quoted, the @code{\newline} is treated as a line continuation (that is, it is removed from the input stream and effectively ignored). @node Single Quotes @subsubsection Single Quotes Enclosing characters in single quotes (@samp{'}) preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash. @node Double Quotes @subsubsection Double Quotes Enclosing characters in double quotes (@samp{"}) preserves the literal value of all characters within the quotes, with the exception of @samp{$}, @samp{`}, @samp{\}, and, when history expansion is enabled, @samp{!}. The characters @samp{$} and @samp{`} retain their special meaning within double quotes (@pxref{Shell Expansions}). The backslash retains its special meaning only when followed by one of the following characters: @samp{$}, @samp{`}, @samp{"}, @samp{\}, or @code{newline}. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an @samp{!} appearing in double quotes is escaped using a backslash. The backslash preceding the @samp{!} is not removed. The special parameters @samp{*} and @samp{@@} have special meaning when in double quotes (@pxref{Shell Parameter Expansion}). @node ANSI-C Quoting @subsubsection ANSI-C Quoting @cindex quoting, ANSI Words of the form @code{$'@var{string}'} are treated specially. The word expands to @var{string}, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows: @table @code @item \a alert (bell) @item \b backspace @item \e @itemx \E an escape character (not ANSI C) @item \f form feed @item \n newline @item \r carriage return @item \t horizontal tab @item \v vertical tab @item \\ backslash @item \' single quote @item \" double quote @item \@var{nnn} the eight-bit character whose value is the octal value @var{nnn} (one to three digits) @item \x@var{HH} the eight-bit character whose value is the hexadecimal value @var{HH} (one or two hex digits) @item \u@var{HHHH} the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value @var{HHHH} (one to four hex digits) @item \U@var{HHHHHHHH} the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value @var{HHHHHHHH} (one to eight hex digits) @item \c@var{x} a control-@var{x} character @end table @noindent The expanded result is single-quoted, as if the dollar sign had not been present. @node Locale Translation @subsubsection Locale-Specific Translation @cindex localization @cindex internationalization @cindex native languages @cindex translation, native languages A double-quoted string preceded by a dollar sign (@samp{$}) will cause the string to be translated according to the current locale. If the current locale is @code{C} or @code{POSIX}, the dollar sign is ignored. If the string is translated and replaced, the replacement is double-quoted. @vindex LC_MESSAGES @vindex TEXTDOMAIN @vindex TEXTDOMAINDIR Some systems use the message catalog selected by the @env{LC_MESSAGES} shell variable. Others create the name of the message catalog from the value of the @env{TEXTDOMAIN} shell variable, possibly adding a suffix of @samp{.mo}. If you use the @env{TEXTDOMAIN} variable, you may need to set the @env{TEXTDOMAINDIR} variable to the location of the message catalog files. Still others use both variables in this fashion: @env{TEXTDOMAINDIR}/@env{LC_MESSAGES}/LC_MESSAGES/@env{TEXTDOMAIN}.mo. @node Comments @subsection Comments @cindex comments, shell In a non-interactive shell, or an interactive shell in which the @code{interactive_comments} option to the @code{shopt} builtin is enabled (@pxref{The Shopt Builtin}), a word beginning with @samp{#} causes that word and all remaining characters on that line to be ignored. An interactive shell without the @code{interactive_comments} option enabled does not allow comments. The @code{interactive_comments} option is on by default in interactive shells. @xref{Interactive Shells}, for a description of what makes a shell interactive. @node Shell Commands @section Shell Commands @cindex commands, shell A simple shell command such as @code{echo a b c} consists of the command itself followed by arguments, separated by spaces. More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping. @menu * Simple Commands:: The most common type of command. * Pipelines:: Connecting the input and output of several commands. * Lists:: How to execute commands sequentially. * Compound Commands:: Shell commands for control flow. * Coprocesses:: Two-way communication between commands. * GNU Parallel:: Running commands in parallel. @end menu @node Simple Commands @subsection Simple Commands @cindex commands, simple A simple command is the kind of command encountered most often. It's just a sequence of words separated by @code{blank}s, terminated by one of the shell's control operators (@pxref{Definitions}). The first word generally specifies a command to be executed, with the rest of the words being that command's arguments. The return status (@pxref{Exit Status}) of a simple command is its exit status as provided by the @sc{posix} 1003.1 @code{waitpid} function, or 128+@var{n} if the command was terminated by signal @var{n}. @node Pipelines @subsection Pipelines @cindex pipeline @cindex commands, pipelines A @code{pipeline} is a sequence of simple commands separated by one of the control operators @samp{|} or @samp{|&}. @rwindex time @rwindex ! @cindex command timing The format for a pipeline is @example [@code{time} [@code{-p}]] [@code{!}] @var{command1} [ [@code{|} or @code{|&}] @var{command2} @dots{}] @end example @noindent The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command's output. This connection is performed before any redirections specified by the command. If @samp{|&} is used, the standard error of @var{command1} is connected to @var{command2}'s standard input through the pipe; it is shorthand for @code{2>&1 |}. This implicit redirection of the standard error is performed after any redirections specified by the command. The reserved word @code{time} causes timing statistics to be printed for the pipeline once it finishes. The statistics currently consist of elapsed (wall-clock) time and user and system time consumed by the command's execution. The @option{-p} option changes the output format to that specified by @sc{posix}. When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}), it does not recognize @code{time} as a reserved word if the next token begins with a @samp{-}. The @env{TIMEFORMAT} variable may be set to a format string that specifies how the timing information should be displayed. @xref{Bash Variables}, for a description of the available formats. The use of @code{time} as a reserved word permits the timing of shell builtins, shell functions, and pipelines. An external @code{time} command cannot time these easily. When the shell is in @sc{posix} mode (@pxref{Bash POSIX Mode}), @code{time} may be followed by a newline. In this case, the shell displays the total user and system time consumed by the shell and its children. The @env{TIMEFORMAT} variable may be used to specify the format of the time information. If the pipeline is not executed asynchronously (@pxref{Lists}), the shell waits for all commands in the pipeline to complete. Each command in a pipeline is executed in its own subshell (@pxref{Command Execution Environment}). The exit status of a pipeline is the exit status of the last command in the pipeline, unless the @code{pipefail} option is enabled (@pxref{The Set Builtin}). If @code{pipefail} is enabled, the pipeline's return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved word @samp{!} precedes the pipeline, the exit status is the logical negation of the exit status as described above. The shell waits for all commands in the pipeline to terminate before returning a value. @node Lists @subsection Lists of Commands @cindex commands, lists A @code{list} is a sequence of one or more pipelines separated by one of the operators @samp{;}, @samp{&}, @samp{&&}, or @samp{||}, and optionally terminated by one of @samp{;}, @samp{&}, or a @code{newline}. Of these list operators, @samp{&&} and @samp{||} have equal precedence, followed by @samp{;} and @samp{&}, which have equal precedence. A sequence of one or more newlines may appear in a @code{list} to delimit commands, equivalent to a semicolon. If a command is terminated by the control operator @samp{&}, the shell executes the command asynchronously in a subshell. This is known as executing the command in the @var{background}. The shell does not wait for the command to finish, and the return status is 0 (true). When job control is not active (@pxref{Job Control}), the standard input for asynchronous commands, in the absence of any explicit redirections, is redirected from @code{/dev/null}. Commands separated by a @samp{;} are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed. @sc{and} and @sc{or} lists are sequences of one or more pipelines separated by the control operators @samp{&&} and @samp{||}, respectively. @sc{and} and @sc{or} lists are executed with left associativity. An @sc{and} list has the form @example @var{command1} && @var{command2} @end example @noindent @var{command2} is executed if, and only if, @var{command1} returns an exit status of zero. An @sc{or} list has the form @example @var{command1} || @var{command2} @end example @noindent @var{command2} is executed if, and only if, @var{command1} returns a non-zero exit status. The return status of @sc{and} and @sc{or} lists is the exit status of the last command executed in the list. @node Compound Commands @subsection Compound Commands @cindex commands, compound @menu * Looping Constructs:: Shell commands for iterative action. * Conditional Constructs:: Shell commands for conditional execution. * Command Grouping:: Ways to group commands. @end menu Compound commands are the shell programming constructs. Each construct begins with a reserved word or control operator and is terminated by a corresponding reserved word or operator. Any redirections (@pxref{Redirections}) associated with a compound command apply to all commands within that compound command unless explicitly overridden. Bash provides looping constructs, conditional commands, and mechanisms to group commands and execute them as a unit. @node Looping Constructs @subsubsection Looping Constructs @cindex commands, looping Bash supports the following looping constructs. Note that wherever a @samp{;} appears in the description of a command's syntax, it may be replaced with one or more newlines. @table @code @item until @rwindex until @rwindex do @rwindex done The syntax of the @code{until} command is: @example until @var{test-commands}; do @var{consequent-commands}; done @end example Execute @var{consequent-commands} as long as @var{test-commands} has an exit status which is not zero. The return status is the exit status of the last command executed in @var{consequent-commands}, or zero if none was executed. @item while @rwindex while The syntax of the @code{while} command is: @example while @var{test-commands}; do @var{consequent-commands}; done @end example Execute @var{consequent-commands} as long as @var{test-commands} has an exit status of zero. The return status is the exit status of the last command executed in @var{consequent-commands}, or zero if none was executed. @item for @rwindex for The syntax of the @code{for} command is: @example for @var{name} [ [in [@var{words} @dots{}] ] ; ] do @var{commands}; done @end example Expand @var{words}, and execute @var{commands} once for each member in the resultant list, with @var{name} bound to the current member. If @samp{in @var{words}} is not present, the @code{for} command executes the @var{commands} once for each positional parameter that is set, as if @samp{in "$@@"} had been specified (@pxref{Special Parameters}). The return status is the exit status of the last command that executes. If there are no items in the expansion of @var{words}, no commands are executed, and the return status is zero. An alternate form of the @code{for} command is also supported: @example for (( @var{expr1} ; @var{expr2} ; @var{expr3} )) ; do @var{commands} ; done @end example First, the arithmetic expression @var{expr1} is evaluated according to the rules described below (@pxref{Shell Arithmetic}). The arithmetic expression @var{expr2} is then evaluated repeatedly until it evaluates to zero. Each time @var{expr2} evaluates to a non-zero value, @var{commands} are executed and the arithmetic expression @var{expr3} is evaluated. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in @var{commands} that is executed, or false if any of the expressions is invalid. @end table The @code{break} and @code{continue} builtins (@pxref{Bourne Shell Builtins}) may be used to control loop execution. @node Conditional Constructs @subsubsection Conditional Constructs @cindex commands, conditional @table @code @item if @rwindex if @rwindex then @rwindex else @rwindex elif @rwindex fi The syntax of the @code{if} command is: @example if @var{test-commands}; then @var{consequent-commands}; [elif @var{more-test-commands}; then @var{more-consequents};] [else @var{alternate-consequents};] fi @end example The @var{test-commands} list is executed, and if its return status is zero, the @var{consequent-commands} list is executed. If @var{test-commands} returns a non-zero status, each @code{elif} list is executed in turn, and if its exit status is zero, the corresponding @var{more-consequents} is executed and the command completes. If @samp{else @var{alternate-consequents}} is present, and the final command in the final @code{if} or @code{elif} clause has a non-zero exit status, then @var{alternate-consequents} is executed. The return status is the exit status of the last command executed, or zero if no condition tested true. @item case @rwindex case @rwindex in @rwindex esac The syntax of the @code{case} command is: @example @code{case @var{word} in [ [(] @var{pattern} [| @var{pattern}]@dots{}) @var{command-list} ;;]@dots{} esac} @end example @code{case} will selectively execute the @var{command-list} corresponding to the first @var{pattern} that matches @var{word}. If the shell option @code{nocasematch} (see the description of @code{shopt} in @ref{The Shopt Builtin}) is enabled, the match is performed without regard to the case of alphabetic characters. The @samp{|} is used to separate multiple patterns, and the @samp{)} operator terminates a pattern list. A list of patterns and an associated command-list is known as a @var{clause}. Each clause must be terminated with @samp{;;}, @samp{;&}, or @samp{;;&}. The @var{word} undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before matching is attempted. Each @var{pattern} undergoes tilde expansion, parameter expansion, command substitution, and arithmetic expansion. There may be an arbitrary number of @code{case} clauses, each terminated by a @samp{;;}, @samp{;&}, or @samp{;;&}. The first pattern that matches determines the command-list that is executed. Here is an example using @code{case} in a script that could be used to describe one interesting feature of an animal: @example echo -n "Enter the name of an animal: " read ANIMAL echo -n "The $ANIMAL has " case $ANIMAL in horse | dog | cat) echo -n "four";; man | kangaroo ) echo -n "two";; *) echo -n "an unknown number of";; esac echo " legs." @end example @noindent If the @samp{;;} operator is used, no subsequent matches are attempted after the first pattern match. Using @samp{;&} in place of @samp{;;} causes execution to continue with the @var{command-list} associated with the next clause, if any. Using @samp{;;&} in place of @samp{;;} causes the shell to test the patterns in the next clause, if any, and execute any associated @var{command-list} on a successful match. The return status is zero if no @var{pattern} is matched. Otherwise, the return status is the exit status of the @var{command-list} executed. @item select @rwindex select The @code{select} construct allows the easy generation of menus. It has almost the same syntax as the @code{for} command: @example select @var{name} [in @var{words} @dots{}]; do @var{commands}; done @end example The list of words following @code{in} is expanded, generating a list of items. The set of expanded words is printed on the standard error output stream, each preceded by a number. If the @samp{in @var{words}} is omitted, the positional parameters are printed, as if @samp{in "$@@"} had been specified. The @env{PS3} prompt is then displayed and a line is read from the standard input. If the line consists of a number corresponding to one of the displayed words, then the value of @var{name} is set to that word. If the line is empty, the words and prompt are displayed again. If @code{EOF} is read, the @code{select} command completes. Any other value read causes @var{name} to be set to null. The line read is saved in the variable @env{REPLY}. The @var{commands} are executed after each selection until a @code{break} command is executed, at which point the @code{select} command completes. Here is an example that allows the user to pick a filename from the current directory, and displays the name and index of the file selected. @example select fname in *; do echo you picked $fname \($REPLY\) break; done @end example @item ((@dots{})) @example (( @var{expression} )) @end example The arithmetic @var{expression} is evaluated according to the rules described below (@pxref{Shell Arithmetic}). If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to @example let "@var{expression}" @end example @noindent @xref{Bash Builtins}, for a full description of the @code{let} builtin. @item [[@dots{}]] @rwindex [[ @rwindex ]] @example [[ @var{expression} ]] @end example Return a status of 0 or 1 depending on the evaluation of the conditional expression @var{expression}. Expressions are composed of the primaries described below in @ref{Bash Conditional Expressions}. Word splitting and filename expansion are not performed on the words between the @samp{[[} and @samp{]]}; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as @samp{-f} must be unquoted to be recognized as primaries. When used with @samp{[[}, the @samp{<} and @samp{>} operators sort lexicographically using the current locale. When the @samp{==} and @samp{!=} operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below in @ref{Pattern Matching}. If the shell option @code{nocasematch} (see the description of @code{shopt} in @ref{The Shopt Builtin}) is enabled, the match is performed without regard to the case of alphabetic characters. The return value is 0 if the string matches (@samp{==}) or does not match (@samp{!=})the pattern, and 1 otherwise. Any part of the pattern may be quoted to force it to be matched as a string. An additional binary operator, @samp{=~}, is available, with the same precedence as @samp{==} and @samp{!=}. When it is used, the string to the right of the operator is considered an extended regular expression and matched accordingly (as in @i{regex}3)). The return value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the conditional expression's return value is 2. If the shell option @code{nocasematch} (see the description of @code{shopt} in @ref{The Shopt Builtin}) is enabled, the match is performed without regard to the case of alphabetic characters. Any part of the pattern may be quoted to force it to be matched as a string. Substrings matched by parenthesized subexpressions within the regular expression are saved in the array variable @code{BASH_REMATCH}. The element of @code{BASH_REMATCH} with index 0 is the portion of the string matching the entire regular expression. The element of @code{BASH_REMATCH} with index @var{n} is the portion of the string matching the @var{n}th parenthesized subexpression. Expressions may be combined using the following operators, listed in decreasing order of precedence: @table @code @item ( @var{expression} ) Returns the value of @var{expression}. This may be used to override the normal precedence of operators. @item ! @var{expression} True if @var{expression} is false. @item @var{expression1} && @var{expression2} True if both @var{expression1} and @var{expression2} are true. @item @var{expression1} || @var{expression2} True if either @var{expression1} or @var{expression2} is true. @end table @noindent The @code{&&} and @code{||} operators do not evaluate @var{expression2} if the value of @var{expression1} is sufficient to determine the return value of the entire conditional expression. @end table @node Command Grouping @subsubsection Grouping Commands @cindex commands, grouping Bash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream. @table @code @item () @example ( @var{list} ) @end example Placing a list of commands between parentheses causes a subshell environment to be created (@pxref{Command Execution Environment}), and each of the commands in @var{list} to be executed in that subshell. Since the @var{list} is executed in a subshell, variable assignments do not remain in effect after the subshell completes. @item @{@} @rwindex @{ @rwindex @} @example @{ @var{list}; @} @end example Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The semicolon (or newline) following @var{list} is required. @end table In addition to the creation of a subshell, there is a subtle difference between these two constructs due to historical reasons. The braces are @code{reserved words}, so they must be separated from the @var{list} by @code{blank}s or other shell metacharacters. The parentheses are @code{operators}, and are recognized as separate tokens by the shell even if they are not separated from the @var{list} by whitespace. The exit status of both of these constructs is the exit status of @var{list}. @node Coprocesses @subsection Coprocesses @cindex coprocess A @code{coprocess} is a shell command preceded by the @code{coproc} reserved word. A coprocess is executed asynchronously in a subshell, as if the command had been terminated with the @samp{&} control operator, with a two-way pipe established between the executing shell and the coprocess. The format for a coprocess is: @example @code{coproc} [@var{NAME}] @var{command} [@var{redirections}] @end example @noindent This creates a coprocess named @var{NAME}. If @var{NAME} is not supplied, the default name is @var{COPROC}. @var{NAME} must not be supplied if @var{command} is a simple command (@pxref{Simple Commands}); otherwise, it is interpreted as the first word of the simple command. When the coproc is executed, the shell creates an array variable (@pxref{Arrays}) named @var{NAME} in the context of the executing shell. The standard output of @var{command} is connected via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned to @var{NAME}[0]. The standard input of @var{command} is connected via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned to @var{NAME}[1]. This pipe is established before any redirections specified by the command (@pxref{Redirections}). The file descriptors can be utilized as arguments to shell commands and redirections using standard word expansions. The process ID of the shell spawned to execute the coprocess is available as the value of the variable @var{NAME}_PID. The @code{wait} builtin command may be used to wait for the coprocess to terminate. The return status of a coprocess is the exit status of @var{command}. @node GNU Parallel @subsection GNU Parallel GNU Parallel, as its name suggests, can be used to build and run commands in parallel. You may run the same command with different arguments, whether they are filenames, usernames, hostnames, or lines read from files. For a complete description, refer to the GNU Parallel documentation. A few examples should provide a brief introduction to its use. For example, it is easy to prefix each line in a text file with a specified string: @example cat file | parallel -k echo prefix_string @end example @noindent The @option{-k} option is required to preserve the lines' order. Similarly, you can append a specified string to each line in a text file: @example cat file | parallel -k echo @{@} append_string @end example You can use Parallel to move files from the current directory when the number of files is too large to process with one @code{mv} invocation: @example ls | parallel mv @{@} destdir @end example As you can see, the @{@} is replaced with each line read from standard input. This will run as many @code{mv} commands as there are files in the current directory. You can emulate a parallel @code{xargs} by adding the @option{-X} option: @example ls | parallel -X mv @{@} destdir @end example GNU Parallel can replace certain common idioms that operate on lines read from a file (in this case, filenames): @example for x in $(cat list); do do-something1 $x config-$x do-something2 < $x done | process-output @end example @noindent with a more compact syntax reminiscent of lambdas: @example cat list | parallel "do-something1 @{@} config-@{@} ; do-something2 < @{@}" | process-output @end example Parallel provides a built-in mechanism to remove filename extensions, which lends itself to batch file transformations or renaming: @example ls *.gz | parallel -j+0 "zcat @{@} | bzip2 >@{.@}.bz2 && rm @{@}" @end example @noindent This will recompress all files in the current directory with names ending in .gz using bzip2, running one job per CPU (-j+0) in parallel. If a command generates output, you may want to preserve the input order in the output. For instance, the following command @example @{ echo foss.org.my ; echo debian.org; echo freenetproject.org; @} | parallel traceroute @end example @noindent will display as output the traceroute invocation that finishes first. Using the @option{-k} option, as we saw above @example @{ echo foss.org.my ; echo debian.org; echo freenetproject.org; @} | parallel -k traceroute @end example @noindent will ensure that the output of @code{traceroute foss.org.my} is displayed first. @node Shell Functions @section Shell Functions @cindex shell function @cindex functions, shell Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. Shell functions are executed in the current shell context; no new process is created to interpret them. Functions are declared using this syntax: @rwindex function @example @var{name} () @var{compound-command} [ @var{redirections} ]@*or@* @code{function} @var{name} [()] @var{compound-command} [ @var{redirections} ] @end example This defines a shell function named @var{name}. The reserved word @code{function} is optional. If the @code{function} reserved word is supplied, the parentheses are optional. The @var{body} of the function is the compound command @var{compound-command} (@pxref{Compound Commands}). That command is usually a @var{list} enclosed between @{ and @}, but may be any compound command listed above. @var{compound-command} is executed whenever @var{name} is specified as the name of a command. Any redirections (@pxref{Redirections}) associated with the shell function are performed when the function is executed. A function definition may be deleted using the @option{-f} option to the @code{unset} builtin (@pxref{Bourne Shell Builtins}). The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body. Note that for historical reasons, in the most common usage the curly braces that surround the body of the function must be separated from the body by @code{blank}s or newlines. This is because the braces are reserved words and are only recognized as such when they are separated from the command list by whitespace or another shell metacharacter. Also, when using the braces, the @var{list} must be terminated by a semicolon, a @samp{&}, or a newline. When a function is executed, the arguments to the function become the positional parameters during its execution (@pxref{Positional Parameters}). The special parameter @samp{#} that expands to the number of positional parameters is updated to reflect the change. Special parameter @code{0} is unchanged. The first element of the @env{FUNCNAME} variable is set to the name of the function while the function is executing. All other aspects of the shell execution environment are identical between a function and its caller with these exceptions: the @env{DEBUG} and @env{RETURN} traps are not inherited unless the function has been given the @code{trace} attribute using the @code{declare} builtin or the @code{-o functrace} option has been enabled with the @code{set} builtin, (in which case all functions inherit the @env{DEBUG} and @env{RETURN} traps), and the @env{ERR} trap is not inherited unless the @code{-o errtrace} shell option has been enabled. @xref{Bourne Shell Builtins}, for the description of the @code{trap} builtin. The @env{FUNCNEST} variable, if set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that exceed the limit cause the entire command to abort. If the builtin command @code{return} is executed in a function, the function completes and execution resumes with the next command after the function call. Any command associated with the @code{RETURN} trap is executed before execution resumes. When a function completes, the values of the positional parameters and the special parameter @samp{#} are restored to the values they had prior to the function's execution. If a numeric argument is given to @code{return}, that is the function's return status; otherwise the function's return status is the exit status of the last command executed before the @code{return}. Variables local to the function may be declared with the @code{local} builtin. These variables are visible only to the function and the commands it invokes. Function names and definitions may be listed with the @option{-f} option to the @code{declare} or @code{typeset} builtin commands (@pxref{Bash Builtins}). The @option{-F} option to @code{declare} or @code{typeset} will list the function names only (and optionally the source file and line number, if the @code{extdebug} shell option is enabled). Functions may be exported so that subshells automatically have them defined with the @option{-f} option to the @code{export} builtin (@pxref{Bourne Shell Builtins}). Note that shell functions and variables with the same name may result in multiple identically-named entries in the environment passed to the shell's children. Care should be taken in cases where this may cause a problem. Functions may be recursive. The @code{FUNCNEST} variable may be used to limit the depth of the function call stack and restrict the number of function invocations. By default, no limit is placed on the number of recursive calls. @node Shell Parameters @section Shell Parameters @cindex parameters @cindex variable, shell @cindex shell variable @menu * Positional Parameters:: The shell's command-line arguments. * Special Parameters:: Parameters denoted by special characters. @end menu A @var{parameter} is an entity that stores values. It can be a @code{name}, a number, or one of the special characters listed below. A @var{variable} is a parameter denoted by a @code{name}. A variable has a @var{value} and zero or more @var{attributes}. Attributes are assigned using the @code{declare} builtin command (see the description of the @code{declare} builtin in @ref{Bash Builtins}). A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the @code{unset} builtin command. A variable may be assigned to by a statement of the form @example @var{name}=[@var{value}] @end example @noindent If @var{value} is not given, the variable is assigned the null string. All @var{value}s undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal (detailed below). If the variable has its @code{integer} attribute set, then @var{value} is evaluated as an arithmetic expression even if the @code{$((@dots{}))} expansion is not used (@pxref{Arithmetic Expansion}). Word splitting is not performed, with the exception of @code{"$@@"} as explained below. Filename expansion is not performed. Assignment statements may also appear as arguments to the @code{alias}, @code{declare}, @code{typeset}, @code{export}, @code{readonly}, and @code{local} builtin commands. In the context where an assignment statement is assigning a value to a shell variable or array index (@pxref{Arrays}), the @samp{+=} operator can be used to append to or add to the variable's previous value. When @samp{+=} is applied to a variable for which the @var{integer} attribute has been set, @var{value} is evaluated as an arithmetic expression and added to the variable's current value, which is also evaluated. When @samp{+=} is applied to an array variable using compound assignment (@pxref{Arrays}), the variable's value is not unset (as it is when using @samp{=}), and new values are appended to the array beginning at one greater than the array's maximum index (for indexed arrays), or added as additional key-value pairs in an associative array. When applied to a string-valued variable, @var{value} is expanded and appended to the variable's value. @node Positional Parameters @subsection Positional Parameters @cindex parameters, positional A @var{positional parameter} is a parameter denoted by one or more digits, other than the single digit @code{0}. Positional parameters are assigned from the shell's arguments when it is invoked, and may be reassigned using the @code{set} builtin command. Positional parameter @code{N} may be referenced as @code{$@{N@}}, or as @code{$N} when @code{N} consists of a single digit. Positional parameters may not be assigned to with assignment statements. The @code{set} and @code{shift} builtins are used to set and unset them (@pxref{Shell Builtin Commands}). The positional parameters are temporarily replaced when a shell function is executed (@pxref{Shell Functions}). When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces. @node Special Parameters @subsection Special Parameters @cindex parameters, special The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed. @vtable @code @item * Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the @env{IFS} special variable. That is, @code{"$*"} is equivalent to @code{"$1@var{c}$2@var{c}@dots{}"}, where @var{c} is the first character of the value of the @code{IFS} variable. If @env{IFS} is unset, the parameters are separated by spaces. If @env{IFS} is null, the parameters are joined without intervening separators. @item @@ Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each parameter expands to a separate word. That is, @code{"$@@"} is equivalent to @code{"$1" "$2" @dots{}}. If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word. When there are no positional parameters, @code{"$@@"} and @code{$@@} expand to nothing (i.e., they are removed). @item # Expands to the number of positional parameters in decimal. @item ? Expands to the exit status of the most recently executed foreground pipeline. @item - (A hyphen.) Expands to the current option flags as specified upon invocation, by the @code{set} builtin command, or those set by the shell itself (such as the @option{-i} option). @item $ Expands to the process @sc{id} of the shell. In a @code{()} subshell, it expands to the process @sc{id} of the invoking shell, not the subshell. @item ! Expands to the process @sc{id} of the most recently executed background (asynchronous) command. @item 0 Expands to the name of the shell or shell script. This is set at shell initialization. If Bash is invoked with a file of commands (@pxref{Shell Scripts}), @code{$0} is set to the name of that file. If Bash is started with the @option{-c} option (@pxref{Invoking Bash}), then @code{$0} is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the filename used to invoke Bash, as given by argument zero. @item _ (An underscore.) At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, expands to the last argument to the previous command, after expansion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the mail file. @end vtable @node Shell Expansions @section Shell Expansions @cindex expansion Expansion is performed on the command line after it has been split into @code{token}s. There are seven kinds of expansion performed: @itemize @bullet @item brace expansion @item tilde expansion @item parameter and variable expansion @item command substitution @item arithmetic expansion @item word splitting @item filename expansion @end itemize @menu * Brace Expansion:: Expansion of expressions within braces. * Tilde Expansion:: Expansion of the ~ character. * Shell Parameter Expansion:: How Bash expands variables to their values. * Command Substitution:: Using the output of a command as an argument. * Arithmetic Expansion:: How to use arithmetic in shell expansions. * Process Substitution:: A way to write and read to and from a command. * Word Splitting:: How the results of expansion are split into separate arguments. * Filename Expansion:: A shorthand for specifying filenames matching patterns. * Quote Removal:: How and when quote characters are removed from words. @end menu The order of expansions is: brace expansion, tilde expansion, parameter, variable, and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and filename expansion. On systems that can support it, there is an additional expansion available: @var{process substitution}. This is performed at the same time as parameter, variable, and arithmetic expansion and command substitution. Only brace expansion, word splitting, and filename expansion can change the number of words of the expansion; other expansions expand a single word to a single word. The only exceptions to this are the expansions of @code{"$@@"} (@pxref{Special Parameters}) and @code{"$@{@var{name}[@@]@}"} (@pxref{Arrays}). After all expansions, @code{quote removal} (@pxref{Quote Removal}) is performed. @node Brace Expansion @subsection Brace Expansion @cindex brace expansion @cindex expansion, brace Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to @var{filename expansion} (@pxref{Filename Expansion}), but the file names generated need not exist. Patterns to be brace expanded take the form of an optional @var{preamble}, followed by either a series of comma-separated strings or a seqeunce expression between a pair of braces, followed by an optional @var{postscript}. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right. Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example, @example bash$ echo a@{d,c,b@}e ade ace abe @end example A sequence expression takes the form @code{@{@var{x}..@var{y}[..@var{incr}]@}}, where @var{x} and @var{y} are either integers or single characters, and @var{incr}, an optional increment, is an integer. When integers are supplied, the expression expands to each number between @var{x} and @var{y}, inclusive. Supplied integers may be prefixed with @samp{0} to force each term to have the same width. When either @var{x} or @var{y} begins with a zero, the shell attempts to force all generated terms to contain the same number of digits, zero-padding where necessary. When characters are supplied, the expression expands to each character lexicographically between @var{x} and @var{y}, inclusive. Note that both @var{x} and @var{y} must be of the same type. When the increment is supplied, it is used as the difference between each term. The default increment is 1 or -1 as appropriate. Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces. To avoid conflicts with parameter expansion, the string @samp{$@{} is not considered eligible for brace expansion. A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged. A @{ or @samp{,} may be quoted with a backslash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string @samp{$@{} is not considered eligible for brace expansion. This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example: @example mkdir /usr/local/src/bash/@{old,new,dist,bugs@} @end example or @example chown root /usr/@{ucb/@{ex,edit@},lib/@{ex?.?*,how_ex@}@} @end example @node Tilde Expansion @subsection Tilde Expansion @cindex tilde expansion @cindex expansion, tilde If a word begins with an unquoted tilde character (@samp{~}), all of the characters up to the first unquoted slash (or all characters, if there is no unquoted slash) are considered a @var{tilde-prefix}. If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following the tilde are treated as a possible @var{login name}. If this login name is the null string, the tilde is replaced with the value of the @env{HOME} shell variable. If @env{HOME} is unset, the home directory of the user executing the shell is substituted instead. Otherwise, the tilde-prefix is replaced with the home directory associated with the specified login name. If the tilde-prefix is @samp{~+}, the value of the shell variable @env{PWD} replaces the tilde-prefix. If the tilde-prefix is @samp{~-}, the value of the shell variable @env{OLDPWD}, if it is set, is substituted. If the characters following the tilde in the tilde-prefix consist of a number @var{N}, optionally prefixed by a @samp{+} or a @samp{-}, the tilde-prefix is replaced with the corresponding element from the directory stack, as it would be displayed by the @code{dirs} builtin invoked with the characters following tilde in the tilde-prefix as an argument (@pxref{The Directory Stack}). If the tilde-prefix, sans the tilde, consists of a number without a leading @samp{+} or @samp{-}, @samp{+} is assumed. If the login name is invalid, or the tilde expansion fails, the word is left unchanged. Each variable assignment is checked for unquoted tilde-prefixes immediately following a @samp{:} or the first @samp{=}. In these cases, tilde expansion is also performed. Consequently, one may use file names with tildes in assignments to @env{PATH}, @env{MAILPATH}, and @env{CDPATH}, and the shell assigns the expanded value. The following table shows how Bash treats unquoted tilde-prefixes: @table @code @item ~ The value of @code{$HOME} @item ~/foo @file{$HOME/foo} @item ~fred/foo The subdirectory @code{foo} of the home directory of the user @code{fred} @item ~+/foo @file{$PWD/foo} @item ~-/foo @file{$@{OLDPWD-'~-'@}/foo} @item ~@var{N} The string that would be displayed by @samp{dirs +@var{N}} @item ~+@var{N} The string that would be displayed by @samp{dirs +@var{N}} @item ~-@var{N} The string that would be displayed by @samp{dirs -@var{N}} @end table @node Shell Parameter Expansion @subsection Shell Parameter Expansion @cindex parameter expansion @cindex expansion, parameter The @samp{$} character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name. When braces are used, the matching ending brace is the first @samp{@}} not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion. The basic form of parameter expansion is $@{@var{parameter}@}. The value of @var{parameter} is substituted. The braces are required when @var{parameter} is a positional parameter with more than one digit, or when @var{parameter} is followed by a character that is not to be interpreted as part of its name. If the first character of @var{parameter} is an exclamation point (!), a level of variable indirection is introduced. Bash uses the value of the variable formed from the rest of @var{parameter} as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of @var{parameter} itself. This is known as @code{indirect expansion}. The exceptions to this are the expansions of $@{!@var{prefix}@*@} and $@{!@var{name}[@@]@} described below. The exclamation point must immediately follow the left brace in order to introduce indirection. In each of the cases below, @var{word} is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. When not performing substring expansion, using the form described below, Bash tests for a parameter that is unset or null. Omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both @var{parameter}'s existence and that its value is not null; if the colon is omitted, the operator tests only for existence. @table @code @item $@{@var{parameter}:@minus{}@var{word}@} If @var{parameter} is unset or null, the expansion of @var{word} is substituted. Otherwise, the value of @var{parameter} is substituted. @item $@{@var{parameter}:=@var{word}@} If @var{parameter} is unset or null, the expansion of @var{word} is assigned to @var{parameter}. The value of @var{parameter} is then substituted. Positional parameters and special parameters may not be assigned to in this way. @item $@{@var{parameter}:?@var{word}@} If @var{parameter} is null or unset, the expansion of @var{word} (or a message to that effect if @var{word} is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of @var{parameter} is substituted. @item $@{@var{parameter}:+@var{word}@} If @var{parameter} is null or unset, nothing is substituted, otherwise the expansion of @var{word} is substituted. @item $@{@var{parameter}:@var{offset}@} @itemx $@{@var{parameter}:@var{offset}:@var{length}@} Expands to up to @var{length} characters of @var{parameter} starting at the character specified by @var{offset}. If @var{length} is omitted, expands to the substring of @var{parameter} starting at the character specified by @var{offset}. @var{length} and @var{offset} are arithmetic expressions (@pxref{Shell Arithmetic}). This is referred to as Substring Expansion. If @var{offset} evaluates to a number less than zero, the value is used as an offset from the end of the value of @var{parameter}. If @var{length} evaluates to a number less than zero, and @var{parameter} is not @samp{@@} and not an indexed or associative array, it is interpreted as an offset from the end of the value of @var{parameter} rather than a number of characters, and the expansion is the characters between the two offsets. If @var{parameter} is @samp{@@}, the result is @var{length} positional parameters beginning at @var{offset}. If @var{parameter} is an indexed array name subscripted by @samp{@@} or @samp{*}, the result is the @var{length} members of the array beginning with @code{$@{@var{parameter}[@var{offset}]@}}. A negative @var{offset} is taken relative to one greater than the maximum index of the specified array. Substring expansion applied to an associative array produces undefined results. Note that a negative offset must be separated from the colon by at least one space to avoid being confused with the @samp{:-} expansion. Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1 by default. If @var{offset} is 0, and the positional parameters are used, @code{$@@} is prefixed to the list. @item $@{!@var{prefix}*@} @itemx $@{!@var{prefix}@@@} Expands to the names of variables whose names begin with @var{prefix}, separated by the first character of the @env{IFS} special variable. When @samp{@@} is used and the expansion appears within double quotes, each variable name expands to a separate word. @item $@{!@var{name}[@@]@} @itemx $@{!@var{name}[*]@} If @var{name} is an array variable, expands to the list of array indices (keys) assigned in @var{name}. If @var{name} is not an array, expands to 0 if @var{name} is set and null otherwise. When @samp{@@} is used and the expansion appears within double quotes, each key expands to a separate word. @item $@{#@var{parameter}@} The length in characters of the expanded value of @var{parameter} is substituted. If @var{parameter} is @samp{*} or @samp{@@}, the value substituted is the number of positional parameters. If @var{parameter} is an array name subscripted by @samp{*} or @samp{@@}, the value substituted is the number of elements in the array. @item $@{@var{parameter}#@var{word}@} @itemx $@{@var{parameter}##@var{word}@} The @var{word} is expanded to produce a pattern just as in filename expansion (@pxref{Filename Expansion}). If the pattern matches the beginning of the expanded value of @var{parameter}, then the result of the expansion is the expanded value of @var{parameter} with the shortest matching pattern (the @samp{#} case) or the longest matching pattern (the @samp{##} case) deleted. If @var{parameter} is @samp{@@} or @samp{*}, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If @var{parameter} is an array variable subscripted with @samp{@@} or @samp{*}, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list. @item $@{@var{parameter}%@var{word}@} @itemx $@{@var{parameter}%%@var{word}@} The @var{word} is expanded to produce a pattern just as in filename expansion. If the pattern matches a trailing portion of the expanded value of @var{parameter}, then the result of the expansion is the value of @var{parameter} with the shortest matching pattern (the @samp{%} case) or the longest matching pattern (the @samp{%%} case) deleted. If @var{parameter} is @samp{@@} or @samp{*}, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If @var{parameter} is an array variable subscripted with @samp{@@} or @samp{*}, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list. @item $@{@var{parameter}/@var{pattern}/@var{string}@} The @var{pattern} is expanded to produce a pattern just as in filename expansion. @var{Parameter} is expanded and the longest match of @var{pattern} against its value is replaced with @var{string}. If @var{pattern} begins with @samp{/}, all matches of @var{pattern} are replaced with @var{string}. Normally only the first match is replaced. If @var{pattern} begins with @samp{#}, it must match at the beginning of the expanded value of @var{parameter}. If @var{pattern} begins with @samp{%}, it must match at the end of the expanded value of @var{parameter}. If @var{string} is null, matches of @var{pattern} are deleted and the @code{/} following @var{pattern} may be omitted. If @var{parameter} is @samp{@@} or @samp{*}, the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If @var{parameter} is an array variable subscripted with @samp{@@} or @samp{*}, the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list. @item $@{@var{parameter}^@var{pattern}@} @itemx $@{@var{parameter}^^@var{pattern}@} @itemx $@{@var{parameter},@var{pattern}@} @itemx $@{@var{parameter},,@var{pattern}@} This expansion modifies the case of alphabetic characters in @var{parameter}. The @var{pattern} is expanded to produce a pattern just as in filename expansion. The @samp{^} operator converts lowercase letters matching @var{pattern} to uppercase; the @samp{,} operator converts matching uppercase letters to lowercase. The @samp{^^} and @samp{,,} expansions convert each matched character in the expanded value; the @samp{^} and @samp{,} expansions match and convert only the first character in the expanded value. If @var{pattern} is omitted, it is treated like a @samp{?}, which matches every character. If @var{parameter} is @samp{@@} or @samp{*}, the case modification operation is applied to each positional parameter in turn, and the expansion is the resultant list. If @var{parameter} is an array variable subscripted with @samp{@@} or @samp{*}, the case modification operation is applied to each member of the array in turn, and the expansion is the resultant list. @end table @node Command Substitution @subsection Command Substitution @cindex command substitution Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed as follows: @example $(@var{command}) @end example @noindent or @example `@var{command}` @end example @noindent Bash performs the expansion by executing @var{command} and replacing the command substitution with the standard output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word splitting. The command substitution @code{$(cat @var{file})} can be replaced by the equivalent but faster @code{$(< @var{file})}. When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by @samp{$}, @samp{`}, or @samp{\}. The first backquote not preceded by a backslash terminates the command substitution. When using the @code{$(@var{command})} form, all characters between the parentheses make up the command; none are treated specially. Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes. If the substitution appears within double quotes, word splitting and filename expansion are not performed on the results. @node Arithmetic Expansion @subsection Arithmetic Expansion @cindex expansion, arithmetic @cindex arithmetic expansion Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is: @example $(( @var{expression} )) @end example The expression is treated as if it were within double quotes, but a double quote inside the parentheses is not treated specially. All tokens in the expression undergo parameter expansion, command substitution, and quote removal. Arithmetic expansions may be nested. The evaluation is performed according to the rules listed below (@pxref{Shell Arithmetic}). If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs. @node Process Substitution @subsection Process Substitution @cindex process substitution Process substitution is supported on systems that support named pipes (@sc{fifo}s) or the @file{/dev/fd} method of naming open files. It takes the form of @example <(@var{list}) @end example @noindent or @example >(@var{list}) @end example @noindent The process @var{list} is run with its input or output connected to a @sc{fifo} or some file in @file{/dev/fd}. The name of this file is passed as an argument to the current command as the result of the expansion. If the @code{>(@var{list})} form is used, writing to the file will provide input for @var{list}. If the @code{<(@var{list})} form is used, the file passed as an argument should be read to obtain the output of @var{list}. Note that no space may appear between the @code{<} or @code{>} and the left parenthesis, otherwise the construct would be interpreted as a redirection. When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion. @node Word Splitting @subsection Word Splitting @cindex word splitting The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting. The shell treats each character of @env{$IFS} as a delimiter, and splits the results of the other expansions into words on these characters. If @env{IFS} is unset, or its value is exactly @code{}, the default, then sequences of @code{ }, @code{}, and @code{} at the beginning and end of the results of the previous expansions are ignored, and any sequence of @env{IFS} characters not at the beginning or end serves to delimit words. If @env{IFS} has a value other than the default, then sequences of the whitespace characters @code{space} and @code{tab} are ignored at the beginning and end of the word, as long as the whitespace character is in the value of @env{IFS} (an @env{IFS} whitespace character). Any character in @env{IFS} that is not @env{IFS} whitespace, along with any adjacent @env{IFS} whitespace characters, delimits a field. A sequence of @env{IFS} whitespace characters is also treated as a delimiter. If the value of @env{IFS} is null, no word splitting occurs. Explicit null arguments (@code{""} or @code{''}) are retained. Unquoted implicit null arguments, resulting from the expansion of parameters that have no values, are removed. If a parameter with no value is expanded within double quotes, a null argument results and is retained. Note that if no expansion occurs, no splitting is performed. @node Filename Expansion @subsection Filename Expansion @menu * Pattern Matching:: How the shell matches patterns. @end menu @cindex expansion, filename @cindex expansion, pathname @cindex filename expansion @cindex pathname expansion After word splitting, unless the @option{-f} option has been set (@pxref{The Set Builtin}), Bash scans each word for the characters @samp{*}, @samp{?}, and @samp{[}. If one of these characters appears, then the word is regarded as a @var{pattern}, and replaced with an alphabetically sorted list of file names matching the pattern. If no matching file names are found, and the shell option @code{nullglob} is disabled, the word is left unchanged. If the @code{nullglob} option is set, and no matches are found, the word is removed. If the @code{failglob} shell option is set, and no matches are found, an error message is printed and the command is not executed. If the shell option @code{nocaseglob} is enabled, the match is performed without regard to the case of alphabetic characters. When a pattern is used for filename expansion, the character @samp{.} at the start of a filename or immediately following a slash must be matched explicitly, unless the shell option @code{dotglob} is set. When matching a file name, the slash character must always be matched explicitly. In other cases, the @samp{.} character is not treated specially. See the description of @code{shopt} in @ref{The Shopt Builtin}, for a description of the @code{nocaseglob}, @code{nullglob}, @code{failglob}, and @code{dotglob} options. The @env{GLOBIGNORE} shell variable may be used to restrict the set of filenames matching a pattern. If @env{GLOBIGNORE} is set, each matching filename that also matches one of the patterns in @env{GLOBIGNORE} is removed from the list of matches. The filenames @file{.} and @file{..} are always ignored when @env{GLOBIGNORE} is set and not null. However, setting @env{GLOBIGNORE} to a non-null value has the effect of enabling the @code{dotglob} shell option, so all other filenames beginning with a @samp{.} will match. To get the old behavior of ignoring filenames beginning with a @samp{.}, make @samp{.*} one of the patterns in @env{GLOBIGNORE}. The @code{dotglob} option is disabled when @env{GLOBIGNORE} is unset. @node Pattern Matching @subsubsection Pattern Matching @cindex pattern matching @cindex matching, pattern Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The @sc{nul} character may not occur in a pattern. A backslash escapes the following character; the escaping backslash is discarded when matching. The special pattern characters must be quoted if they are to be matched literally. The special pattern characters have the following meanings: @table @code @item * Matches any string, including the null string. When the @code{globstar} shell option is enabled, and @samp{*} is used in a filename expansion context, two adjacent @samp{*}s used as a single pattern will match all files and zero or more directories and subdirectories. If followed by a @samp{/}, two adjacent @samp{*}s will match only directories and subdirectories. @item ? Matches any single character. @item [@dots{}] Matches any one of the enclosed characters. A pair of characters separated by a hyphen denotes a @var{range expression}; any character that sorts between those two characters, inclusive, using the current locale's collating sequence and character set, is matched. If the first character following the @samp{[} is a @samp{!} or a @samp{^} then any character not enclosed is matched. A @samp{@minus{}} may be matched by including it as the first or last character in the set. A @samp{]} may be matched by including it as the first character in the set. The sorting order of characters in range expressions is determined by the current locale and the value of the @env{LC_COLLATE} shell variable, if set. For example, in the default C locale, @samp{[a-dx-z]} is equivalent to @samp{[abcdxyz]}. Many locales sort characters in dictionary order, and in these locales @samp{[a-dx-z]} is typically not equivalent to @samp{[abcdxyz]}; it might be equivalent to @samp{[aBbCcDdxXyYz]}, for example. To obtain the traditional interpretation of ranges in bracket expressions, you can force the use of the C locale by setting the @env{LC_COLLATE} or @env{LC_ALL} environment variable to the value @samp{C}. Within @samp{[} and @samp{]}, @var{character classes} can be specified using the syntax @code{[:}@var{class}@code{:]}, where @var{class} is one of the following classes defined in the @sc{posix} standard: @example alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit @end example @noindent A character class matches any character belonging to that class. The @code{word} character class matches letters, digits, and the character @samp{_}. Within @samp{[} and @samp{]}, an @var{equivalence class} can be specified using the syntax @code{[=}@var{c}@code{=]}, which matches all characters with the same collation weight (as defined by the current locale) as the character @var{c}. Within @samp{[} and @samp{]}, the syntax @code{[.}@var{symbol}@code{.]} matches the collating symbol @var{symbol}. @end table If the @code{extglob} shell option is enabled using the @code{shopt} builtin, several extended pattern matching operators are recognized. In the following description, a @var{pattern-list} is a list of one or more patterns separated by a @samp{|}. Composite patterns may be formed using one or more of the following sub-patterns: @table @code @item ?(@var{pattern-list}) Matches zero or one occurrence of the given patterns. @item *(@var{pattern-list}) Matches zero or more occurrences of the given patterns. @item +(@var{pattern-list}) Matches one or more occurrences of the given patterns. @item @@(@var{pattern-list}) Matches one of the given patterns. @item !(@var{pattern-list}) Matches anything except one of the given patterns. @end table @node Quote Removal @subsection Quote Removal After the preceding expansions, all unquoted occurrences of the characters @samp{\}, @samp{'}, and @samp{"} that did not result from one of the above expansions are removed. @node Redirections @section Redirections @cindex redirection Before a command is executed, its input and output may be @var{redirected} using a special notation interpreted by the shell. Redirection may also be used to open and close files for the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right. Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form @{@var{varname}@}. In this case, for each redirection operator except >&- and <&-, the shell will allocate a file descriptor greater than 10 and assign it to @{@var{varname}@}. If >&- or <&- is preceded by @{@var{varname}@}, the value of @var{varname} defines the file descriptor to close. In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is @samp{<}, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is @samp{>}, the redirection refers to the standard output (file descriptor 1). The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error. Note that the order of redirections is significant. For example, the command @example ls > @var{dirlist} 2>&1 @end example @noindent directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file @var{dirlist}, while the command @example ls 2>&1 > @var{dirlist} @end example @noindent directs only the standard output to file @var{dirlist}, because the standard error was made a copy of the standard output before the standard output was redirected to @var{dirlist}. Bash handles several filenames specially when they are used in redirections, as described in the following table: @table @code @item /dev/fd/@var{fd} If @var{fd} is a valid integer, file descriptor @var{fd} is duplicated. @item /dev/stdin File descriptor 0 is duplicated. @item /dev/stdout File descriptor 1 is duplicated. @item /dev/stderr File descriptor 2 is duplicated. @item /dev/tcp/@var{host}/@var{port} If @var{host} is a valid hostname or Internet address, and @var{port} is an integer port number or service name, Bash attempts to open a TCP connection to the corresponding socket. @item /dev/udp/@var{host}/@var{port} If @var{host} is a valid hostname or Internet address, and @var{port} is an integer port number or service name, Bash attempts to open a UDP connection to the corresponding socket. @end table A failure to open or create a file causes the redirection to fail. Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally. @subsection Redirecting Input Redirection of input causes the file whose name results from the expansion of @var{word} to be opened for reading on file descriptor @code{n}, or the standard input (file descriptor 0) if @code{n} is not specified. The general format for redirecting input is: @example [@var{n}]<@var{word} @end example @subsection Redirecting Output Redirection of output causes the file whose name results from the expansion of @var{word} to be opened for writing on file descriptor @var{n}, or the standard output (file descriptor 1) if @var{n} is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size. The general format for redirecting output is: @example [@var{n}]>[|]@var{word} @end example If the redirection operator is @samp{>}, and the @code{noclobber} option to the @code{set} builtin has been enabled, the redirection will fail if the file whose name results from the expansion of @var{word} exists and is a regular file. If the redirection operator is @samp{>|}, or the redirection operator is @samp{>} and the @code{noclobber} option is not enabled, the redirection is attempted even if the file named by @var{word} exists. @subsection Appending Redirected Output Redirection of output in this fashion causes the file whose name results from the expansion of @var{word} to be opened for appending on file descriptor @var{n}, or the standard output (file descriptor 1) if @var{n} is not specified. If the file does not exist it is created. The general format for appending output is: @example [@var{n}]>>@var{word} @end example @subsection Redirecting Standard Output and Standard Error This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of @var{word}. There are two formats for redirecting standard output and standard error: @example &>@var{word} @end example @noindent and @example >&@var{word} @end example @noindent Of the two forms, the first is preferred. This is semantically equivalent to @example >@var{word} 2>&1 @end example @subsection Appending Standard Output and Standard Error This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be appended to the file whose name is the expansion of @var{word}. The format for appending standard output and standard error is: @example &>>@var{word} @end example @noindent This is semantically equivalent to @example >>@var{word} 2>&1 @end example @subsection Here Documents This type of redirection instructs the shell to read input from the current source until a line containing only @var{word} (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a command. The format of here-documents is: @example <<[@minus{}]@var{word} @var{here-document} @var{delimiter} @end example No parameter expansion, command substitution, arithmetic expansion, or filename expansion is performed on @var{word}. If any characters in @var{word} are quoted, the @var{delimiter} is the result of quote removal on @var{word}, and the lines in the here-document are not expanded. If @var{word} is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion. In the latter case, the character sequence @code{\newline} is ignored, and @samp{\} must be used to quote the characters @samp{\}, @samp{$}, and @samp{`}. If the redirection operator is @samp{<<-}, then all leading tab characters are stripped from input lines and the line containing @var{delimiter}. This allows here-documents within shell scripts to be indented in a natural fashion. @subsection Here Strings A variant of here documents, the format is: @example <<< @var{word} @end example The @var{word} is expanded and supplied to the command on its standard input. @subsection Duplicating File Descriptors The redirection operator @example [@var{n}]<&@var{word} @end example @noindent is used to duplicate input file descriptors. If @var{word} expands to one or more digits, the file descriptor denoted by @var{n} is made to be a copy of that file descriptor. If the digits in @var{word} do not specify a file descriptor open for input, a redirection error occurs. If @var{word} evaluates to @samp{-}, file descriptor @var{n} is closed. If @var{n} is not specified, the standard input (file descriptor 0) is used. The operator @example [@var{n}]>&@var{word} @end example @noindent is used similarly to duplicate output file descriptors. If @var{n} is not specified, the standard output (file descriptor 1) is used. If the digits in @var{word} do not specify a file descriptor open for output, a redirection error occurs. As a special case, if @var{n} is omitted, and @var{word} does not expand to one or more digits, the standard output and standard error are redirected as described previously. @subsection Moving File Descriptors The redirection operator @example [@var{n}]<&@var{digit}- @end example @noindent moves the file descriptor @var{digit} to file descriptor @var{n}, or the standard input (file descriptor 0) if @var{n} is not specified. @var{digit} is closed after being duplicated to @var{n}. Similarly, the redirection operator @example [@var{n}]>&@var{digit}- @end example @noindent moves the file descriptor @var{digit} to file descriptor @var{n}, or the standard output (file descriptor 1) if @var{n} is not specified. @subsection Opening File Descriptors for Reading and Writing The redirection operator @example [@var{n}]<>@var{word} @end example @noindent causes the file whose name is the expansion of @var{word} to be opened for both reading and writing on file descriptor @var{n}, or on file descriptor 0 if @var{n} is not specified. If the file does not exist, it is created. @node Executing Commands @section Executing Commands @menu * Simple Command Expansion:: How Bash expands simple commands before executing them. * Command Search and Execution:: How Bash finds commands and runs them. * Command Execution Environment:: The environment in which Bash executes commands that are not shell builtins. * Environment:: The environment given to a command. * Exit Status:: The status returned by commands and how Bash interprets it. * Signals:: What happens when Bash or a command it runs receives a signal. @end menu @node Simple Command Expansion @subsection Simple Command Expansion @cindex command expansion When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right. @enumerate @item The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing. @item The words that are not variable assignments or redirections are expanded (@pxref{Shell Expansions}). If any words remain after expansion, the first word is taken to be the name of the command and the remaining words are the arguments. @item Redirections are performed as described above (@pxref{Redirections}). @item The text after the @samp{=} in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable. @end enumerate If no command name results, the variable assignments affect the current shell environment. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status. If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status. If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. If there were no command substitutions, the command exits with a status of zero. @node Command Search and Execution @subsection Command Search and Execution @cindex command execution @cindex command search After a command has been split into words, if it results in a simple command and an optional list of arguments, the following actions are taken. @enumerate @item If the command name contains no slashes, the shell attempts to locate it. If there exists a shell function by that name, that function is invoked as described in @ref{Shell Functions}. @item If the name does not match a function, the shell searches for it in the list of shell builtins. If a match is found, that builtin is invoked. @item If the name is neither a shell function nor a builtin, and contains no slashes, Bash searches each element of @env{$PATH} for a directory containing an executable file by that name. Bash uses a hash table to remember the full pathnames of executable files to avoid multiple @env{PATH} searches (see the description of @code{hash} in @ref{Bourne Shell Builtins}). A full search of the directories in @env{$PATH} is performed only if the command is not found in the hash table. If the search is unsuccessful, the shell searches for a defined shell function named @code{command_not_found_handle}. If that function exists, it is invoked with the original command and the original command's arguments as its arguments, and the function's exit status becomes the exit status of the shell. If that function is not defined, the shell prints an error message and returns an exit status of 127. @item If the search is successful, or if the command name contains one or more slashes, the shell executes the named program in a separate execution environment. Argument 0 is set to the name given, and the remaining arguments to the command are set to the arguments supplied, if any. @item If this execution fails because the file is not in executable format, and the file is not a directory, it is assumed to be a @var{shell script} and the shell executes it as described in @ref{Shell Scripts}. @item If the command was not begun asynchronously, the shell waits for the command to complete and collects its exit status. @end enumerate @node Command Execution Environment @subsection Command Execution Environment @cindex execution environment The shell has an @var{execution environment}, which consists of the following: @itemize @bullet @item open files inherited by the shell at invocation, as modified by redirections supplied to the @code{exec} builtin @item the current working directory as set by @code{cd}, @code{pushd}, or @code{popd}, or inherited by the shell at invocation @item the file creation mode mask as set by @code{umask} or inherited from the shell's parent @item current traps set by @code{trap} @item shell parameters that are set by variable assignment or with @code{set} or inherited from the shell's parent in the environment @item shell functions defined during execution or inherited from the shell's parent in the environment @item options enabled at invocation (either by default or with command-line arguments) or by @code{set} @item options enabled by @code{shopt} (@pxref{The Shopt Builtin}) @item shell aliases defined with @code{alias} (@pxref{Aliases}) @item various process @sc{id}s, including those of background jobs (@pxref{Lists}), the value of @code{$$}, and the value of @env{$PPID} @end itemize When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell. @itemize @bullet @item the shell's open files, plus any modifications and additions specified by redirections to the command @item the current working directory @item the file creation mode mask @item shell variables and functions marked for export, along with variables exported for the command, passed in the environment (@pxref{Environment}) @item traps caught by the shell are reset to the values inherited from the shell's parent, and traps ignored by the shell are ignored @end itemize A command invoked in this separate environment cannot affect the shell's execution environment. Command substitution, commands grouped with parentheses, and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline are also executed in a subshell environment. Changes made to the subshell environment cannot affect the shell's execution environment. Subshells spawned to execute command substitutions inherit the value of the @option{-e} option from the parent shell. When not in @sc{posix} mode, Bash clears the @option{-e} option in such subshells. If a command is followed by a @samp{&} and job control is not active, the default standard input for the command is the empty file @file{/dev/null}. Otherwise, the invoked command inherits the file descriptors of the calling shell as modified by redirections. @node Environment @subsection Environment @cindex environment When a program is invoked it is given an array of strings called the @var{environment}. This is a list of name-value pairs, of the form @code{name=value}. Bash provides several ways to manipulate the environment. On invocation, the shell scans its own environment and creates a parameter for each name found, automatically marking it for @var{export} to child processes. Executed commands inherit the environment. The @code{export} and @samp{declare -x} commands allow parameters and functions to be added to and deleted from the environment. If the value of a parameter in the environment is modified, the new value becomes part of the environment, replacing the old. The environment inherited by any executed command consists of the shell's initial environment, whose values may be modified in the shell, less any pairs removed by the @code{unset} and @samp{export -n} commands, plus any additions via the @code{export} and @samp{declare -x} commands. The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments, as described in @ref{Shell Parameters}. These assignment statements affect only the environment seen by that command. If the @option{-k} option is set (@pxref{The Set Builtin}), then all parameter assignments are placed in the environment for a command, not just those that precede the command name. When Bash invokes an external command, the variable @samp{$_} is set to the full path name of the command and passed to that command in its environment. @node Exit Status @subsection Exit Status @cindex exit status The exit status of an executed command is the value returned by the @var{waitpid} system call or equivalent function. Exit statuses fall between 0 and 255, though, as explained below, the shell may use values above 125 specially. Exit statuses from shell builtins and compound commands are also limited to this range. Under certain circumstances, the shell will use special values to indicate specific failure modes. For the shell's purposes, a command which exits with a zero exit status has succeeded. A non-zero exit status indicates failure. This seemingly counter-intuitive scheme is used so there is one well-defined way to indicate success and a variety of ways to indicate various failure modes. When a command terminates on a fatal signal whose number is @var{N}, Bash uses the value 128+@var{N} as the exit status. If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126. If a command fails because of an error during expansion or redirection, the exit status is greater than zero. The exit status is used by the Bash conditional commands (@pxref{Conditional Constructs}) and some of the list constructs (@pxref{Lists}). All of the Bash builtins return an exit status of zero if they succeed and a non-zero status on failure, so they may be used by the conditional and list constructs. All builtins return an exit status of 2 to indicate incorrect usage. @node Signals @subsection Signals @cindex signal handling When Bash is interactive, in the absence of any traps, it ignores @code{SIGTERM} (so that @samp{kill 0} does not kill an interactive shell), and @code{SIGINT} is caught and handled (so that the @code{wait} builtin is interruptible). When Bash receives a @code{SIGINT}, it breaks out of any executing loops. In all cases, Bash ignores @code{SIGQUIT}. If job control is in effect (@pxref{Job Control}), Bash ignores @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}. Non-builtin commands started by Bash have signal handlers set to the values inherited by the shell from its parent. When job control is not in effect, asynchronous commands ignore @code{SIGINT} and @code{SIGQUIT} in addition to these inherited handlers. Commands run as a result of command substitution ignore the keyboard-generated job control signals @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}. The shell exits by default upon receipt of a @code{SIGHUP}. Before exiting, an interactive shell resends the @code{SIGHUP} to all jobs, running or stopped. Stopped jobs are sent @code{SIGCONT} to ensure that they receive the @code{SIGHUP}. To prevent the shell from sending the @code{SIGHUP} signal to a particular job, it should be removed from the jobs table with the @code{disown} builtin (@pxref{Job Control Builtins}) or marked to not receive @code{SIGHUP} using @code{disown -h}. If the @code{huponexit} shell option has been set with @code{shopt} (@pxref{The Shopt Builtin}), Bash sends a @code{SIGHUP} to all jobs when an interactive login shell exits. If Bash is waiting for a command to complete and receives a signal for which a trap has been set, the trap will not be executed until the command completes. When Bash is waiting for an asynchronous command via the @code{wait} builtin, the reception of a signal for which a trap has been set will cause the @code{wait} builtin to return immediately with an exit status greater than 128, immediately after which the trap is executed. @node Shell Scripts @section Shell Scripts @cindex shell script A shell script is a text file containing shell commands. When such a file is used as the first non-option argument when invoking Bash, and neither the @option{-c} nor @option{-s} option is supplied (@pxref{Invoking Bash}), Bash reads and executes commands from the file, then exits. This mode of operation creates a non-interactive shell. The shell first searches for the file in the current directory, and looks in the directories in @env{$PATH} if not found there. When Bash runs a shell script, it sets the special parameter @code{0} to the name of the file, rather than the name of the shell, and the positional parameters are set to the remaining arguments, if any are given. If no additional arguments are supplied, the positional parameters are unset. A shell script may be made executable by using the @code{chmod} command to turn on the execute bit. When Bash finds such a file while searching the @env{$PATH} for a command, it spawns a subshell to execute it. In other words, executing @example filename @var{arguments} @end example @noindent is equivalent to executing @example bash filename @var{arguments} @end example @noindent if @code{filename} is an executable shell script. This subshell reinitializes itself, so that the effect is as if a new shell had been invoked to interpret the script, with the exception that the locations of commands remembered by the parent (see the description of @code{hash} in @ref{Bourne Shell Builtins}) are retained by the child. Most versions of Unix make this a part of the operating system's command execution mechanism. If the first line of a script begins with the two characters @samp{#!}, the remainder of the line specifies an interpreter for the program. Thus, you can specify Bash, @code{awk}, Perl, or some other interpreter and write the rest of the script file in that language. The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the script file, followed by the name of the script file, followed by the rest of the arguments. Bash will perform this action on operating systems that do not handle it themselves. Note that some older versions of Unix limit the interpreter name and argument to a maximum of 32 characters. Bash scripts often begin with @code{#! /bin/bash} (assuming that Bash has been installed in @file{/bin}), since this ensures that Bash will be used to interpret the script, even if it is executed under another shell. @node Shell Builtin Commands @chapter Shell Builtin Commands @menu * Bourne Shell Builtins:: Builtin commands inherited from the Bourne Shell. * Bash Builtins:: Table of builtins specific to Bash. * Modifying Shell Behavior:: Builtins to modify shell attributes and optional behavior. * Special Builtins:: Builtin commands classified specially by POSIX. @end menu Builtin commands are contained within the shell itself. When the name of a builtin command is used as the first word of a simple command (@pxref{Simple Commands}), the shell executes the command directly, without invoking another program. Builtin commands are necessary to implement functionality impossible or inconvenient to obtain with separate utilities. This section briefly describes the builtins which Bash inherits from the Bourne Shell, as well as the builtin commands which are unique to or have been extended in Bash. Several builtin commands are described in other chapters: builtin commands which provide the Bash interface to the job control facilities (@pxref{Job Control Builtins}), the directory stack (@pxref{Directory Stack Builtins}), the command history (@pxref{Bash History Builtins}), and the programmable completion facilities (@pxref{Programmable Completion Builtins}). Many of the builtins have been extended by @sc{posix} or Bash. Unless otherwise noted, each builtin command documented as accepting options preceded by @samp{-} accepts @samp{--} to signify the end of the options. The @code{:}, @code{true}, @code{false}, and @code{test} builtins do not accept options and do not treat @samp{--} specially. The @code{exit}, @code{logout}, @code{break}, @code{continue}, @code{let}, and @code{shift} builtins accept and process arguments beginning with @samp{-} without requiring @samp{--}. Other builtins that accept arguments but are not specified as accepting options interpret arguments beginning with @samp{-} as invalid options and require @samp{--} to prevent this interpretation. @node Bourne Shell Builtins @section Bourne Shell Builtins The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the @sc{posix} standard. @table @code @item : @r{(a colon)} @btindex : @example : [@var{arguments}] @end example Do nothing beyond expanding @var{arguments} and performing redirections. The return status is zero. @item . @r{(a period)} @btindex . @example . @var{filename} [@var{arguments}] @end example Read and execute commands from the @var{filename} argument in the current shell context. If @var{filename} does not contain a slash, the @env{PATH} variable is used to find @var{filename}. When Bash is not in @sc{posix} mode, the current directory is searched if @var{filename} is not found in @env{$PATH}. If any @var{arguments} are supplied, they become the positional parameters when @var{filename} is executed. Otherwise the positional parameters are unchanged. The return status is the exit status of the last command executed, or zero if no commands are executed. If @var{filename} is not found, or cannot be read, the return status is non-zero. This builtin is equivalent to @code{source}. @item break @btindex break @example break [@var{n}] @end example Exit from a @code{for}, @code{while}, @code{until}, or @code{select} loop. If @var{n} is supplied, the @var{n}th enclosing loop is exited. @var{n} must be greater than or equal to 1. The return status is zero unless @var{n} is not greater than or equal to 1. @item cd @btindex cd @example cd [-L|[-P [-e]]] [@var{directory}] @end example Change the current working directory to @var{directory}. If @var{directory} is not given, the value of the @env{HOME} shell variable is used. If the shell variable @env{CDPATH} exists, it is used as a search path. If @var{directory} begins with a slash, @env{CDPATH} is not used. The @option{-P} option means to not follow symbolic links; symbolic links are followed by default or with the @option{-L} option. If the @option{-e} option is supplied with @option{-P} and the current working directory cannot be successfully determined after a successful directory change, @code{cd} will return an unsuccessful status. If @var{directory} is @samp{-}, it is equivalent to @env{$OLDPWD}. If a non-empty directory name from @env{CDPATH} is used, or if @samp{-} is the first argument, and the directory change is successful, the absolute pathname of the new working directory is written to the standard output. The return status is zero if the directory is successfully changed, non-zero otherwise. @item continue @btindex continue @example continue [@var{n}] @end example Resume the next iteration of an enclosing @code{for}, @code{while}, @code{until}, or @code{select} loop. If @var{n} is supplied, the execution of the @var{n}th enclosing loop is resumed. @var{n} must be greater than or equal to 1. The return status is zero unless @var{n} is not greater than or equal to 1. @item eval @btindex eval @example eval [@var{arguments}] @end example The arguments are concatenated together into a single command, which is then read and executed, and its exit status returned as the exit status of @code{eval}. If there are no arguments or only empty arguments, the return status is zero. @item exec @btindex exec @example exec [-cl] [-a @var{name}] [@var{command} [@var{arguments}]] @end example If @var{command} is supplied, it replaces the shell without creating a new process. If the @option{-l} option is supplied, the shell places a dash at the beginning of the zeroth argument passed to @var{command}. This is what the @code{login} program does. The @option{-c} option causes @var{command} to be executed with an empty environment. If @option{-a} is supplied, the shell passes @var{name} as the zeroth argument to @var{command}. If no @var{command} is specified, redirections may be used to affect the current shell environment. If there are no redirection errors, the return status is zero; otherwise the return status is non-zero. @item exit @btindex exit @example exit [@var{n}] @end example Exit the shell, returning a status of @var{n} to the shell's parent. If @var{n} is omitted, the exit status is that of the last command executed. Any trap on @code{EXIT} is executed before the shell terminates. @item export @btindex export @example export [-fn] [-p] [@var{name}[=@var{value}]] @end example Mark each @var{name} to be passed to child processes in the environment. If the @option{-f} option is supplied, the @var{name}s refer to shell functions; otherwise the names refer to shell variables. The @option{-n} option means to no longer mark each @var{name} for export. If no @var{names} are supplied, or if the @option{-p} option is given, a list of exported names is displayed. The @option{-p} option displays output in a form that may be reused as input. If a variable name is followed by =@var{value}, the value of the variable is set to @var{value}. The return status is zero unless an invalid option is supplied, one of the names is not a valid shell variable name, or @option{-f} is supplied with a name that is not a shell function. @item getopts @btindex getopts @example getopts @var{optstring} @var{name} [@var{args}] @end example @code{getopts} is used by shell scripts to parse positional parameters. @var{optstring} contains the option characters to be recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by white space. The colon (@samp{:}) and question mark (@samp{?}) may not be used as option characters. Each time it is invoked, @code{getopts} places the next option in the shell variable @var{name}, initializing @var{name} if it does not exist, and the index of the next argument to be processed into the variable @env{OPTIND}. @env{OPTIND} is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, @code{getopts} places that argument into the variable @env{OPTARG}. The shell does not reset @env{OPTIND} automatically; it must be manually reset between multiple calls to @code{getopts} within the same shell invocation if a new set of parameters is to be used. When the end of options is encountered, @code{getopts} exits with a return value greater than zero. @env{OPTIND} is set to the index of the first non-option argument, and @var{name} is set to @samp{?}. @code{getopts} normally parses the positional parameters, but if more arguments are given in @var{args}, @code{getopts} parses those instead. @code{getopts} can report errors in two ways. If the first character of @var{optstring} is a colon, @var{silent} error reporting is used. In normal operation diagnostic messages are printed when invalid options or missing option arguments are encountered. If the variable @env{OPTERR} is set to 0, no error messages will be displayed, even if the first character of @code{optstring} is not a colon. If an invalid option is seen, @code{getopts} places @samp{?} into @var{name} and, if not silent, prints an error message and unsets @env{OPTARG}. If @code{getopts} is silent, the option character found is placed in @env{OPTARG} and no diagnostic message is printed. If a required argument is not found, and @code{getopts} is not silent, a question mark (@samp{?}) is placed in @var{name}, @code{OPTARG} is unset, and a diagnostic message is printed. If @code{getopts} is silent, then a colon (@samp{:}) is placed in @var{name} and @env{OPTARG} is set to the option character found. @item hash @btindex hash @example hash [-r] [-p @var{filename}] [-dt] [@var{name}] @end example Each time @code{hash} is invoked, it remembers the full pathnames of the commands specified as @var{name} arguments, so they need not be searched for on subsequent invocations. The commands are found by searching through the directories listed in @env{$PATH}. Any previously-remembered pathname is discarded. The @option{-p} option inhibits the path search, and @var{filename} is used as the location of @var{name}. The @option{-r} option causes the shell to forget all remembered locations. The @option{-d} option causes the shell to forget the remembered location of each @var{name}. If the @option{-t} option is supplied, the full pathname to which each @var{name} corresponds is printed. If multiple @var{name} arguments are supplied with @option{-t} the @var{name} is printed before the hashed full pathname. The @option{-l} option causes output to be displayed in a format that may be reused as input. If no arguments are given, or if only @option{-l} is supplied, information about remembered commands is printed. The return status is zero unless a @var{name} is not found or an invalid option is supplied. @item pwd @btindex pwd @example pwd [-LP] @end example Print the absolute pathname of the current working directory. If the @option{-P} option is supplied, the pathname printed will not contain symbolic links. If the @option{-L} option is supplied, the pathname printed may contain symbolic links. The return status is zero unless an error is encountered while determining the name of the current directory or an invalid option is supplied. @item readonly @btindex readonly @example readonly [-aAf] [-p] [@var{name}[=@var{value}]] @dots{} @end example Mark each @var{name} as readonly. The values of these names may not be changed by subsequent assignment. If the @option{-f} option is supplied, each @var{name} refers to a shell function. The @option{-a} option means each @var{name} refers to an indexed array variable; the @option{-A} option means each @var{name} refers to an associative array variable. If both options are supplied, @option{-A} takes precedence. If no @var{name} arguments are given, or if the @option{-p} option is supplied, a list of all readonly names is printed. The other options may be used to restrict the output to a subset of the set of readonly names. The @option{-p} option causes output to be displayed in a format that may be reused as input. If a variable name is followed by =@var{value}, the value of the variable is set to @var{value}. The return status is zero unless an invalid option is supplied, one of the @var{name} arguments is not a valid shell variable or function name, or the @option{-f} option is supplied with a name that is not a shell function. @item return @btindex return @example return [@var{n}] @end example Cause a shell function to exit with the return value @var{n}. If @var{n} is not supplied, the return value is the exit status of the last command executed in the function. This may also be used to terminate execution of a script being executed with the @code{.} (or @code{source}) builtin, returning either @var{n} or the exit status of the last command executed within the script as the exit status of the script. Any command associated with the @code{RETURN} trap is executed before execution resumes after the function or script. The return status is non-zero if @code{return} is used outside a function and not during the execution of a script by @code{.} or @code{source}. @item shift @btindex shift @example shift [@var{n}] @end example Shift the positional parameters to the left by @var{n}. The positional parameters from @var{n}+1 @dots{} @code{$#} are renamed to @code{$1} @dots{} @code{$#}-@var{n}. Parameters represented by the numbers @code{$#} to @code{$#}-@var{n}+1 are unset. @var{n} must be a non-negative number less than or equal to @code{$#}. If @var{n} is zero or greater than @code{$#}, the positional parameters are not changed. If @var{n} is not supplied, it is assumed to be 1. The return status is zero unless @var{n} is greater than @code{$#} or less than zero, non-zero otherwise. @item test @itemx [ @btindex test @btindex [ Evaluate a conditional expression @var{expr}. Each operator and operand must be a separate argument. Expressions are composed of the primaries described below in @ref{Bash Conditional Expressions}. @code{test} does not accept any options, nor does it accept and ignore an argument of @option{--} as signifying the end of options. When the @code{[} form is used, the last argument to the command must be a @code{]}. Expressions may be combined using the following operators, listed in decreasing order of precedence. The evaluation depends on the number of arguments; see below. Operator precedence is used when there are five or more arguments. @table @code @item ! @var{expr} True if @var{expr} is false. @item ( @var{expr} ) Returns the value of @var{expr}. This may be used to override the normal precedence of operators. @item @var{expr1} -a @var{expr2} True if both @var{expr1} and @var{expr2} are true. @item @var{expr1} -o @var{expr2} True if either @var{expr1} or @var{expr2} is true. @end table The @code{test} and @code{[} builtins evaluate conditional expressions using a set of rules based on the number of arguments. @table @asis @item 0 arguments The expression is false. @item 1 argument The expression is true if and only if the argument is not null. @item 2 arguments If the first argument is @samp{!}, the expression is true if and only if the second argument is null. If the first argument is one of the unary conditional operators (@pxref{Bash Conditional Expressions}), the expression is true if the unary test is true. If the first argument is not a valid unary operator, the expression is false. @item 3 arguments The following conditions are applied in the order listed. If the second argument is one of the binary conditional operators (@pxref{Bash Conditional Expressions}), the result of the expression is the result of the binary test using the first and third arguments as operands. The @samp{-a} and @samp{-o} operators are considered binary operators when there are three arguments. If the first argument is @samp{!}, the value is the negation of the two-argument test using the second and third arguments. If the first argument is exactly @samp{(} and the third argument is exactly @samp{)}, the result is the one-argument test of the second argument. Otherwise, the expression is false. @item 4 arguments If the first argument is @samp{!}, the result is the negation of the three-argument expression composed of the remaining arguments. Otherwise, the expression is parsed and evaluated according to precedence using the rules listed above. @item 5 or more arguments The expression is parsed and evaluated according to precedence using the rules listed above. @end table When used with @code{test} or @samp{[}, the @samp{<} and @samp{>} operators sort lexicographically using ASCII ordering. @item times @btindex times @example times @end example Print out the user and system times used by the shell and its children. The return status is zero. @item trap @btindex trap @example trap [-lp] [@var{arg}] [@var{sigspec} @dots{}] @end example The commands in @var{arg} are to be read and executed when the shell receives signal @var{sigspec}. If @var{arg} is absent (and there is a single @var{sigspec}) or equal to @samp{-}, each specified signal's disposition is reset to the value it had when the shell was started. If @var{arg} is the null string, then the signal specified by each @var{sigspec} is ignored by the shell and commands it invokes. If @var{arg} is not present and @option{-p} has been supplied, the shell displays the trap commands associated with each @var{sigspec}. If no arguments are supplied, or only @option{-p} is given, @code{trap} prints the list of commands associated with each signal number in a form that may be reused as shell input. The @option{-l} option causes the shell to print a list of signal names and their corresponding numbers. Each @var{sigspec} is either a signal name or a signal number. Signal names are case insensitive and the @code{SIG} prefix is optional. If a @var{sigspec} is @code{0} or @code{EXIT}, @var{arg} is executed when the shell exits. If a @var{sigspec} is @code{DEBUG}, the command @var{arg} is executed before every simple command, @code{for} command, @code{case} command, @code{select} command, every arithmetic @code{for} command, and before the first command executes in a shell function. Refer to the description of the @code{extdebug} option to the @code{shopt} builtin (@pxref{The Shopt Builtin}) for details of its effect on the @code{DEBUG} trap. If a @var{sigspec} is @code{RETURN}, the command @var{arg} is executed each time a shell function or a script executed with the @code{.} or @code{source} builtins finishes executing. If a @var{sigspec} is @code{ERR}, the command @var{arg} is executed whenever a simple command has a non-zero exit status, subject to the following conditions. The @code{ERR} trap is not executed if the failed command is part of the command list immediately following an @code{until} or @code{while} keyword, part of the test following the @code{if} or @code{elif} reserved words, part of a command executed in a @code{&&} or @code{||} list, or if the command's return status is being inverted using @code{!}. These are the same conditions obeyed by the @code{errexit} option. Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals that are not being ignored are reset to their original values in a subshell or subshell environment when one is created. The return status is zero unless a @var{sigspec} does not specify a valid signal. @item umask @btindex umask @example umask [-p] [-S] [@var{mode}] @end example Set the shell process's file creation mask to @var{mode}. If @var{mode} begins with a digit, it is interpreted as an octal number; if not, it is interpreted as a symbolic mode mask similar to that accepted by the @code{chmod} command. If @var{mode} is omitted, the current value of the mask is printed. If the @option{-S} option is supplied without a @var{mode} argument, the mask is printed in a symbolic format. If the @option{-p} option is supplied, and @var{mode} is omitted, the output is in a form that may be reused as input. The return status is zero if the mode is successfully changed or if no @var{mode} argument is supplied, and non-zero otherwise. Note that when the mode is interpreted as an octal number, each number of the umask is subtracted from @code{7}. Thus, a umask of @code{022} results in permissions of @code{755}. @item unset @btindex unset @example unset [-fv] [@var{name}] @end example Each variable or function @var{name} is removed. If no options are supplied, or the @option{-v} option is given, each @var{name} refers to a shell variable. If the @option{-f} option is given, the @var{name}s refer to shell functions, and the function definition is removed. Readonly variables and functions may not be unset. The return status is zero unless a @var{name} is readonly. @end table @node Bash Builtins @section Bash Builtin Commands This section describes builtin commands which are unique to or have been extended in Bash. Some of these commands are specified in the @sc{posix} standard. @table @code @item alias @btindex alias @example alias [@code{-p}] [@var{name}[=@var{value}] @dots{}] @end example Without arguments or with the @option{-p} option, @code{alias} prints the list of aliases on the standard output in a form that allows them to be reused as input. If arguments are supplied, an alias is defined for each @var{name} whose @var{value} is given. If no @var{value} is given, the name and value of the alias is printed. Aliases are described in @ref{Aliases}. @item bind @btindex bind @example bind [-m @var{keymap}] [-lpsvPSV] bind [-m @var{keymap}] [-q @var{function}] [-u @var{function}] [-r @var{keyseq}] bind [-m @var{keymap}] -f @var{filename} bind [-m @var{keymap}] -x @var{keyseq:shell-command} bind [-m @var{keymap}] @var{keyseq:function-name} bind @var{readline-command} @end example Display current Readline (@pxref{Command Line Editing}) key and function bindings, bind a key sequence to a Readline function or macro, or set a Readline variable. Each non-option argument is a command as it would appear in a Readline initialization file (@pxref{Readline Init File}), but each binding or command must be passed as a separate argument; e.g., @samp{"\C-x\C-r":re-read-init-file}. Options, if supplied, have the following meanings: @table @code @item -m @var{keymap} Use @var{keymap} as the keymap to be affected by the subsequent bindings. Acceptable @var{keymap} names are @code{emacs}, @code{emacs-standard}, @code{emacs-meta}, @code{emacs-ctlx}, @code{vi}, @code{vi-move}, @code{vi-command}, and @code{vi-insert}. @code{vi} is equivalent to @code{vi-command}; @code{emacs} is equivalent to @code{emacs-standard}. @item -l List the names of all Readline functions. @item -p Display Readline function names and bindings in such a way that they can be used as input or in a Readline initialization file. @item -P List current Readline function names and bindings. @item -v Display Readline variable names and values in such a way that they can be used as input or in a Readline initialization file. @item -V List current Readline variable names and values. @item -s Display Readline key sequences bound to macros and the strings they output in such a way that they can be used as input or in a Readline initialization file. @item -S Display Readline key sequences bound to macros and the strings they output. @item -f @var{filename} Read key bindings from @var{filename}. @item -q @var{function} Query about which keys invoke the named @var{function}. @item -u @var{function} Unbind all keys bound to the named @var{function}. @item -r @var{keyseq} Remove any current binding for @var{keyseq}. @item -x @var{keyseq:shell-command} Cause @var{shell-command} to be executed whenever @var{keyseq} is entered. When @var{shell-command} is executed, the shell sets the @code{READLINE_LINE} variable to the contents of the Readline line buffer and the @code{READLINE_POINT} variable to the current location of the insertion point. If the executed command changes the value of @code{READLINE_LINE} or @code{READLINE_POINT}, those new values will be reflected in the editing state. @end table @noindent The return status is zero unless an invalid option is supplied or an error occurs. @item builtin @btindex builtin @example builtin [@var{shell-builtin} [@var{args}]] @end example Run a shell builtin, passing it @var{args}, and return its exit status. This is useful when defining a shell function with the same name as a shell builtin, retaining the functionality of the builtin within the function. The return status is non-zero if @var{shell-builtin} is not a shell builtin command. @item caller @btindex caller @example caller [@var{expr}] @end example Returns the context of any active subroutine call (a shell function or a script executed with the @code{.} or @code{source} builtins). Without @var{expr}, @code{caller} displays the line number and source filename of the current subroutine call. If a non-negative integer is supplied as @var{expr}, @code{caller} displays the line number, subroutine name, and source file corresponding to that position in the current execution call stack. This extra information may be used, for example, to print a stack trace. The current frame is frame 0. The return value is 0 unless the shell is not executing a subroutine call or @var{expr} does not correspond to a valid position in the call stack. @item command @btindex command @example command [-pVv] @var{command} [@var{arguments} @dots{}] @end example Runs @var{command} with @var{arguments} ignoring any shell function named @var{command}. Only shell builtin commands or commands found by searching the @env{PATH} are executed. If there is a shell function named @code{ls}, running @samp{command ls} within the function will execute the external command @code{ls} instead of calling the function recursively. The @option{-p} option means to use a default value for @env{PATH} that is guaranteed to find all of the standard utilities. The return status in this case is 127 if @var{command} cannot be found or an error occurred, and the exit status of @var{command} otherwise. If either the @option{-V} or @option{-v} option is supplied, a description of @var{command} is printed. The @option{-v} option causes a single word indicating the command or file name used to invoke @var{command} to be displayed; the @option{-V} option produces a more verbose description. In this case, the return status is zero if @var{command} is found, and non-zero if not. @item declare @btindex declare @example declare [-aAfFilrtux] [-p] [@var{name}[=@var{value}] @dots{}] @end example Declare variables and give them attributes. If no @var{name}s are given, then display the values of variables instead. The @option{-p} option will display the attributes and values of each @var{name}. When @option{-p} is used with @var{name} arguments, additional options are ignored. When @option{-p} is supplied without @var{name} arguments, @code{declare} will display the attributes and values of all variables having the attributes specified by the additional options. If no other options are supplied with @option{-p}, @code{declare} will display the attributes and values of all shell variables. The @option{-f} option will restrict the display to shell functions. The @option{-F} option inhibits the display of function definitions; only the function name and attributes are printed. If the @code{extdebug} shell option is enabled using @code{shopt} (@pxref{The Shopt Builtin}), the source file name and line number where the function is defined are displayed as well. @option{-F} implies @option{-f}. The @option{-g} option forces variables to be created or modified at the global scope, even when @code{declare} is executed in a shell function. It is ignored in all other cases. The following options can be used to restrict output to variables with the specified attributes or to give variables attributes: @table @code @item -a Each @var{name} is an indexed array variable (@pxref{Arrays}). @item -A Each @var{name} is an associative array variable (@pxref{Arrays}). @item -f Use function names only. @item -i The variable is to be treated as an integer; arithmetic evaluation (@pxref{Shell Arithmetic}) is performed when the variable is assigned a value. @item -l When the variable is assigned a value, all upper-case characters are converted to lower-case. The upper-case attribute is disabled. @item -r Make @var{name}s readonly. These names cannot then be assigned values by subsequent assignment statements or unset. @item -t Give each @var{name} the @code{trace} attribute. Traced functions inherit the @code{DEBUG} and @code{RETURN} traps from the calling shell. The trace attribute has no special meaning for variables. @item -u When the variable is assigned a value, all lower-case characters are converted to upper-case. The lower-case attribute is disabled. @item -x Mark each @var{name} for export to subsequent commands via the environment. @end table Using @samp{+} instead of @samp{-} turns off the attribute instead, with the exceptions that @samp{+a} may not be used to destroy an array variable and @samp{+r} will not remove the readonly attribute. When used in a function, @code{declare} makes each @var{name} local, as with the @code{local} command, unless the @samp{-g} option is used. If a variable name is followed by =@var{value}, the value of the variable is set to @var{value}. The return status is zero unless an invalid option is encountered, an attempt is made to define a function using @samp{-f foo=bar}, an attempt is made to assign a value to a readonly variable, an attempt is made to assign a value to an array variable without using the compound assignment syntax (@pxref{Arrays}), one of the @var{names} is not a valid shell variable name, an attempt is made to turn off readonly status for a readonly variable, an attempt is made to turn off array status for an array variable, or an attempt is made to display a non-existent function with @option{-f}. @item echo @btindex echo @example echo [-neE] [@var{arg} @dots{}] @end example Output the @var{arg}s, separated by spaces, terminated with a newline. The return status is always 0. If @option{-n} is specified, the trailing newline is suppressed. If the @option{-e} option is given, interpretation of the following backslash-escaped characters is enabled. The @option{-E} option disables the interpretation of these escape characters, even on systems where they are interpreted by default. The @code{xpg_echo} shell option may be used to dynamically determine whether or not @code{echo} expands these escape characters by default. @code{echo} does not interpret @option{--} to mean the end of options. @code{echo} interprets the following escape sequences: @table @code @item \a alert (bell) @item \b backspace @item \c suppress further output @item \e @itemx \E escape @item \f form feed @item \n new line @item \r carriage return @item \t horizontal tab @item \v vertical tab @item \\ backslash @item \0@var{nnn} the eight-bit character whose value is the octal value @var{nnn} (zero to three octal digits) @item \x@var{HH} the eight-bit character whose value is the hexadecimal value @var{HH} (one or two hex digits) @item \u@var{HHHH} the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value @var{HHHH} (one to four hex digits) @item \U@var{HHHHHHHH} the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value @var{HHHHHHHH} (one to eight hex digits) @end table @item enable @btindex enable @example enable [-a] [-dnps] [-f @var{filename}] [@var{name} @dots{}] @end example Enable and disable builtin shell commands. Disabling a builtin allows a disk command which has the same name as a shell builtin to be executed without specifying a full pathname, even though the shell normally searches for builtins before disk commands. If @option{-n} is used, the @var{name}s become disabled. Otherwise @var{name}s are enabled. For example, to use the @code{test} binary found via @env{$PATH} instead of the shell builtin version, type @samp{enable -n test}. If the @option{-p} option is supplied, or no @var{name} arguments appear, a list of shell builtins is printed. With no other arguments, the list consists of all enabled shell builtins. The @option{-a} option means to list each builtin with an indication of whether or not it is enabled. The @option{-f} option means to load the new builtin command @var{name} from shared object @var{filename}, on systems that support dynamic loading. The @option{-d} option will delete a builtin loaded with @option{-f}. If there are no options, a list of the shell builtins is displayed. The @option{-s} option restricts @code{enable} to the @sc{posix} special builtins. If @option{-s} is used with @option{-f}, the new builtin becomes a special builtin (@pxref{Special Builtins}). The return status is zero unless a @var{name} is not a shell builtin or there is an error loading a new builtin from a shared object. @item help @btindex help @example help [-dms] [@var{pattern}] @end example Display helpful information about builtin commands. If @var{pattern} is specified, @code{help} gives detailed help on all commands matching @var{pattern}, otherwise a list of the builtins is printed. Options, if supplied, have the following meanings: @table @code @item -d Display a short description of each @var{pattern} @item -m Display the description of each @var{pattern} in a manpage-like format @item -s Display only a short usage synopsis for each @var{pattern} @end table The return status is zero unless no command matches @var{pattern}. @item let @btindex let @example let @var{expression} [@var{expression}] @end example The @code{let} builtin allows arithmetic to be performed on shell variables. Each @var{expression} is evaluated according to the rules given below in @ref{Shell Arithmetic}. If the last @var{expression} evaluates to 0, @code{let} returns 1; otherwise 0 is returned. @item local @btindex local @example local [@var{option}] @var{name}[=@var{value}] @dots{} @end example For each argument, a local variable named @var{name} is created, and assigned @var{value}. The @var{option} can be any of the options accepted by @code{declare}. @code{local} can only be used within a function; it makes the variable @var{name} have a visible scope restricted to that function and its children. The return status is zero unless @code{local} is used outside a function, an invalid @var{name} is supplied, or @var{name} is a readonly variable. @item logout @btindex logout @example logout [@var{n}] @end example Exit a login shell, returning a status of @var{n} to the shell's parent. @item mapfile @btindex mapfile @example mapfile [-n @var{count}] [-O @var{origin}] [-s @var{count}] [-t] [-u @var{fd}] [ -C @var{callback}] [-c @var{quantum}] [@var{array}] @end example Read lines from the standard input into the indexed array variable @var{array}, or from file descriptor @var{fd} if the @option{-u} option is supplied. The variable @code{MAPFILE} is the default @var{array}. Options, if supplied, have the following meanings: @table @code @item -n Copy at most @var{count} lines. If @var{count} is 0, all lines are copied. @item -O Begin assigning to @var{array} at index @var{origin}. The default index is 0. @item -s Discard the first @var{count} lines read. @item -t Remove a trailing newline from each line read. @item -u Read lines from file descriptor @var{fd} instead of the standard input. @item -C Evaluate @var{callback} each time @var{quantum}P lines are read. The @option{-c} option specifies @var{quantum}. @item -c Specify the number of lines read between each call to @var{callback}. @end table If @option{-C} is specified without @option{-c}, the default quantum is 5000. When @var{callback} is evaluated, it is supplied the index of the next array element to be assigned and the line to be assigned to that element as additional arguments. @var{callback} is evaluated after the line is read but before the array element is assigned. If not supplied with an explicit origin, @code{mapfile} will clear @var{array} before assigning to it. @code{mapfile} returns successfully unless an invalid option or option argument is supplied, @var{array} is invalid or unassignable, or @var{array} is not an indexed array. @item printf @btindex printf @example printf [-v @var{var}] @var{format} [@var{arguments}] @end example Write the formatted @var{arguments} to the standard output under the control of the @var{format}. The @option{-v} option causes the output to be assigned to the variable @var{var} rather than being printed to the standard output. The @var{format} is a character string which contains three types of objects: plain characters, which are simply copied to standard output, character escape sequences, which are converted and copied to the standard output, and format specifications, each of which causes printing of the next successive @var{argument}. In addition to the standard @code{printf(1)} formats, @code{printf} interprets the following extensions: @table @code @item %b causes @code{printf} to expand backslash escape sequences in the corresponding @var{argument}, (except that @samp{\c} terminates output, backslashes in @samp{\'}, @samp{\"}, and @samp{\?} are not removed, and octal escapes beginning with @samp{\0} may contain up to four digits). @item %q causes @code{printf} to output the corresponding @var{argument} in a format that can be reused as shell input. @item %(@var{datefmt})T causes @code{printf} to output the date-time string resulting from using @var{datefmt} as a format string for @code{strftime}(3). The corresponding @var{argument} is an integer representing the number of seconds since the epoch. Two special argument values may be used: -1 represents the current time, and -2 represents the time the shell was invoked. @end table @noindent Arguments to non-string format specifiers are treated as C language constants, except that a leading plus or minus sign is allowed, and if the leading character is a single or double quote, the value is the ASCII value of the following character. The @var{format} is reused as necessary to consume all of the @var{arguments}. If the @var{format} requires more @var{arguments} than are supplied, the extra format specifications behave as if a zero value or null string, as appropriate, had been supplied. The return value is zero on success, non-zero on failure. @item read @btindex read @example read [-ers] [-a @var{aname}] [-d @var{delim}] [-i @var{text}] [-n @var{nchars}] [-N @var{nchars}] [-p @var{prompt}] [-t @var{timeout}] [-u @var{fd}] [@var{name} @dots{}] @end example One line is read from the standard input, or from the file descriptor @var{fd} supplied as an argument to the @option{-u} option, and the first word is assigned to the first @var{name}, the second word to the second @var{name}, and so on, with leftover words and their intervening separators assigned to the last @var{name}. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in the value of the @env{IFS} variable are used to split the line into words. The backslash character @samp{\} may be used to remove any special meaning for the next character read and for line continuation. If no names are supplied, the line read is assigned to the variable @env{REPLY}. The return code is zero, unless end-of-file is encountered, @code{read} times out (in which case the return code is greater than 128), or an invalid file descriptor is supplied as the argument to @option{-u}. Options, if supplied, have the following meanings: @table @code @item -a @var{aname} The words are assigned to sequential indices of the array variable @var{aname}, starting at 0. All elements are removed from @var{aname} before the assignment. Other @var{name} arguments are ignored. @item -d @var{delim} The first character of @var{delim} is used to terminate the input line, rather than newline. @item -e Readline (@pxref{Command Line Editing}) is used to obtain the line. Readline uses the current (or default, if line editing was not previously active) editing settings. @item -i @var{text} If Readline is being used to read the line, @var{text} is placed into the editing buffer before editing begins. @item -n @var{nchars} @code{read} returns after reading @var{nchars} characters rather than waiting for a complete line of input, but honor a delimiter if fewer than @var{nchars} characters are read before the delimiter. @item -N @var{nchars} @code{read} returns after reading exactly @var{nchars} characters rather than waiting for a complete line of input, unless EOF is encountered or @code{read} times out. Delimiter characters encountered in the input are not treated specially and do not cause @code{read} to return until @var{nchars} characters are read. @item -p @var{prompt} Display @var{prompt}, without a trailing newline, before attempting to read any input. The prompt is displayed only if input is coming from a terminal. @item -r If this option is given, backslash does not act as an escape character. The backslash is considered to be part of the line. In particular, a backslash-newline pair may not be used as a line continuation. @item -s Silent mode. If input is coming from a terminal, characters are not echoed. @item -t @var{timeout} Cause @code{read} to time out and return failure if a complete line of input is not read within @var{timeout} seconds. @var{timeout} may be a decimal number with a fractional portion following the decimal point. This option is only effective if @code{read} is reading input from a terminal, pipe, or other special file; it has no effect when reading from regular files. If @var{timeout} is 0, @code{read} returns success if input is available on the specified file descriptor, failure otherwise. The exit status is greater than 128 if the timeout is exceeded. @item -u @var{fd} Read input from file descriptor @var{fd}. @end table @item readarray @btindex readarray @example readarray [-n @var{count}] [-O @var{origin}] [-s @var{count}] [-t] [-u @var{fd}] [ -C @var{callback}] [-c @var{quantum}] [@var{array}] @end example Read lines from the standard input into the indexed array variable @var{array}, or from file descriptor @var{fd} if the @option{-u} option is supplied. A synonym for @code{mapfile}. @item source @btindex source @example source @var{filename} @end example A synonym for @code{.} (@pxref{Bourne Shell Builtins}). @item type @btindex type @example type [-afptP] [@var{name} @dots{}] @end example For each @var{name}, indicate how it would be interpreted if used as a command name. If the @option{-t} option is used, @code{type} prints a single word which is one of @samp{alias}, @samp{function}, @samp{builtin}, @samp{file} or @samp{keyword}, if @var{name} is an alias, shell function, shell builtin, disk file, or shell reserved word, respectively. If the @var{name} is not found, then nothing is printed, and @code{type} returns a failure status. If the @option{-p} option is used, @code{type} either returns the name of the disk file that would be executed, or nothing if @option{-t} would not return @samp{file}. The @option{-P} option forces a path search for each @var{name}, even if @option{-t} would not return @samp{file}. If a command is hashed, @option{-p} and @option{-P} print the hashed value, not necessarily the file that appears first in @code{$PATH}. If the @option{-a} option is used, @code{type} returns all of the places that contain an executable named @var{file}. This includes aliases and functions, if and only if the @option{-p} option is not also used. If the @option{-f} option is used, @code{type} does not attempt to find shell functions, as with the @code{command} builtin. The return status is zero if all of the @var{names} are found, non-zero if any are not found. @item typeset @btindex typeset @example typeset [-afFrxi] [-p] [@var{name}[=@var{value}] @dots{}] @end example The @code{typeset} command is supplied for compatibility with the Korn shell; however, it has been deprecated in favor of the @code{declare} builtin command. @item ulimit @btindex ulimit @example ulimit [-abcdefilmnpqrstuvxHST] [@var{limit}] @end example @code{ulimit} provides control over the resources available to processes started by the shell, on systems that allow such control. If an option is given, it is interpreted as follows: @table @code @item -S Change and report the soft limit associated with a resource. @item -H Change and report the hard limit associated with a resource. @item -a All current limits are reported. @item -b The maximum socket buffer size. @item -c The maximum size of core files created. @item -d The maximum size of a process's data segment. @item -e The maximum scheduling priority ("nice"). @item -f The maximum size of files written by the shell and its children. @item -i The maximum number of pending signals. @item -l The maximum size that may be locked into memory. @item -m The maximum resident set size (many systems do not honor this limit). @item -n The maximum number of open file descriptors (most systems do not allow this value to be set). @item -p The pipe buffer size. @item -q The maximum number of bytes in POSIX message queues. @item -r The maximum real-time scheduling priority. @item -s The maximum stack size. @item -t The maximum amount of cpu time in seconds. @item -u The maximum number of processes available to a single user. @item -v The maximum amount of virtual memory available to the shell, and, on some systems, to its children. @item -x The maximum number of file locks. @item -T The maximum number of threads. @end table If @var{limit} is given, it is the new value of the specified resource; the special @var{limit} values @code{hard}, @code{soft}, and @code{unlimited} stand for the current hard limit, the current soft limit, and no limit, respectively. A hard limit cannot be increased by a non-root user once it is set; a soft limit may be increased up to the value of the hard limit. Otherwise, the current value of the soft limit for the specified resource is printed, unless the @option{-H} option is supplied. When setting new limits, if neither @option{-H} nor @option{-S} is supplied, both the hard and soft limits are set. If no option is given, then @option{-f} is assumed. Values are in 1024-byte increments, except for @option{-t}, which is in seconds, @option{-p}, which is in units of 512-byte blocks, and @option{-n} and @option{-u}, which are unscaled values. The return status is zero unless an invalid option or argument is supplied, or an error occurs while setting a new limit. @item unalias @btindex unalias @example unalias [-a] [@var{name} @dots{} ] @end example Remove each @var{name} from the list of aliases. If @option{-a} is supplied, all aliases are removed. Aliases are described in @ref{Aliases}. @end table @node Modifying Shell Behavior @section Modifying Shell Behavior @menu * The Set Builtin:: Change the values of shell attributes and positional parameters. * The Shopt Builtin:: Modify shell optional behavior. @end menu @node The Set Builtin @subsection The Set Builtin This builtin is so complicated that it deserves its own section. @code{set} allows you to change the values of shell options and set the positional parameters, or to display the names and values of shell variables. @table @code @item set @btindex set @example set [--abefhkmnptuvxBCEHPT] [-o @var{option-name}] [@var{argument} @dots{}] set [+abefhkmnptuvxBCEHPT] [+o @var{option-name}] [@var{argument} @dots{}] @end example If no options or arguments are supplied, @code{set} displays the names and values of all shell variables and functions, sorted according to the current locale, in a format that may be reused as input for setting or resetting the currently-set variables. Read-only variables cannot be reset. In @sc{posix} mode, only shell variables are listed. When options are supplied, they set or unset shell attributes. Options, if specified, have the following meanings: @table @code @item -a Mark variables and function which are modified or created for export to the environment of subsequent commands. @item -b Cause the status of terminated background jobs to be reported immediately, rather than before printing the next primary prompt. @item -e Exit immediately if a pipeline (@pxref{Pipelines}), which may consist of a single simple command (@pxref{Simple Commands}), a subshell command enclosed in parentheses (@pxref{Command Grouping}), or one of the commands executed as part of a command list enclosed by braces (@pxref{Command Grouping}) returns a non-zero status. The shell does not exit if the command that fails is part of the command list immediately following a @code{while} or @code{until} keyword, part of the test in an @code{if} statement, part of any command executed in a @code{&&} or @code{||} list except the command following the final @code{&&} or @code{||}, any command in a pipeline but the last, or if the command's return status is being inverted with @code{!}. A trap on @code{ERR}, if set, is executed before the shell exits. This option applies to the shell environment and each subshell environment separately (@pxref{Command Execution Environment}), and may cause subshells to exit before executing all the commands in the subshell. @item -f Disable filename expansion (globbing). @item -h Locate and remember (hash) commands as they are looked up for execution. This option is enabled by default. @item -k All arguments in the form of assignment statements are placed in the environment for a command, not just those that precede the command name. @item -m Job control is enabled (@pxref{Job Control}). @item -n Read commands but do not execute them; this may be used to check a script for syntax errors. This option is ignored by interactive shells. @item -o @var{option-name} Set the option corresponding to @var{option-name}: @table @code @item allexport Same as @code{-a}. @item braceexpand Same as @code{-B}. @item emacs Use an @code{emacs}-style line editing interface (@pxref{Command Line Editing}). This also affects the editing interface used for @code{read -e}. @item errexit Same as @code{-e}. @item errtrace Same as @code{-E}. @item functrace Same as @code{-T}. @item hashall Same as @code{-h}. @item histexpand Same as @code{-H}. @item history Enable command history, as described in @ref{Bash History Facilities}. This option is on by default in interactive shells. @item ignoreeof An interactive shell will not exit upon reading EOF. @item keyword Same as @code{-k}. @item monitor Same as @code{-m}. @item noclobber Same as @code{-C}. @item noexec Same as @code{-n}. @item noglob Same as @code{-f}. @item nolog Currently ignored. @item notify Same as @code{-b}. @item nounset Same as @code{-u}. @item onecmd Same as @code{-t}. @item physical Same as @code{-P}. @item pipefail If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled by default. @item posix Change the behavior of Bash where the default operation differs from the @sc{posix} standard to match the standard (@pxref{Bash POSIX Mode}). This is intended to make Bash behave as a strict superset of that standard. @item privileged Same as @code{-p}. @item verbose Same as @code{-v}. @item vi Use a @code{vi}-style line editing interface. This also affects the editing interface used for @code{read -e}. @item xtrace Same as @code{-x}. @end table @item -p Turn on privileged mode. In this mode, the @env{$BASH_ENV} and @env{$ENV} files are not processed, shell functions are not inherited from the environment, and the @env{SHELLOPTS}, @env{BASHOPTS}, @env{CDPATH} and @env{GLOBIGNORE} variables, if they appear in the environment, are ignored. If the shell is started with the effective user (group) id not equal to the real user (group) id, and the @code{-p} option is not supplied, these actions are taken and the effective user id is set to the real user id. If the @code{-p} option is supplied at startup, the effective user id is not reset. Turning this option off causes the effective user and group ids to be set to the real user and group ids. @item -t Exit after reading and executing one command. @item -u Treat unset variables and parameters other than the special parameters @samp{@@} or @samp{*} as an error when performing parameter expansion. An error message will be written to the standard error, and a non-interactive shell will exit. @item -v Print shell input lines as they are read. @item -x Print a trace of simple commands, @code{for} commands, @code{case} commands, @code{select} commands, and arithmetic @code{for} commands and their arguments or associated word lists after they are expanded and before they are executed. The value of the @env{PS4} variable is expanded and the resultant value is printed before the command and its expanded arguments. @item -B The shell will perform brace expansion (@pxref{Brace Expansion}). This option is on by default. @item -C Prevent output redirection using @samp{>}, @samp{>&}, and @samp{<>} from overwriting existing files. @item -E If set, any trap on @code{ERR} is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The @code{ERR} trap is normally not inherited in such cases. @item -H Enable @samp{!} style history substitution (@pxref{History Interaction}). This option is on by default for interactive shells. @item -P If set, do not follow symbolic links when performing commands such as @code{cd} which change the current directory. The physical directory is used instead. By default, Bash follows the logical chain of directories when performing commands which change the current directory. For example, if @file{/usr/sys} is a symbolic link to @file{/usr/local/sys} then: @example $ cd /usr/sys; echo $PWD /usr/sys $ cd ..; pwd /usr @end example @noindent If @code{set -P} is on, then: @example $ cd /usr/sys; echo $PWD /usr/local/sys $ cd ..; pwd /usr/local @end example @item -T If set, any trap on @code{DEBUG} and @code{RETURN} are inherited by shell functions, command substitutions, and commands executed in a subshell environment. The @code{DEBUG} and @code{RETURN} traps are normally not inherited in such cases. @item -- If no arguments follow this option, then the positional parameters are unset. Otherwise, the positional parameters are set to the @var{arguments}, even if some of them begin with a @samp{-}. @item - Signal the end of options, cause all remaining @var{arguments} to be assigned to the positional parameters. The @option{-x} and @option{-v} options are turned off. If there are no arguments, the positional parameters remain unchanged. @end table Using @samp{+} rather than @samp{-} causes these options to be turned off. The options can also be used upon invocation of the shell. The current set of options may be found in @code{$-}. The remaining N @var{arguments} are positional parameters and are assigned, in order, to @code{$1}, @code{$2}, @dots{} @code{$N}. The special parameter @code{#} is set to N. The return status is always zero unless an invalid option is supplied. @end table @node The Shopt Builtin @subsection The Shopt Builtin This builtin allows you to change additional shell optional behavior. @table @code @item shopt @btindex shopt @example shopt [-pqsu] [-o] [@var{optname} @dots{}] @end example Toggle the values of variables controlling optional shell behavior. With no options, or with the @option{-p} option, a list of all settable options is displayed, with an indication of whether or not each is set. The @option{-p} option causes output to be displayed in a form that may be reused as input. Other options have the following meanings: @table @code @item -s Enable (set) each @var{optname}. @item -u Disable (unset) each @var{optname}. @item -q Suppresses normal output; the return status indicates whether the @var{optname} is set or unset. If multiple @var{optname} arguments are given with @option{-q}, the return status is zero if all @var{optnames} are enabled; non-zero otherwise. @item -o Restricts the values of @var{optname} to be those defined for the @option{-o} option to the @code{set} builtin (@pxref{The Set Builtin}). @end table If either @option{-s} or @option{-u} is used with no @var{optname} arguments, the display is limited to those options which are set or unset, respectively. Unless otherwise noted, the @code{shopt} options are disabled (off) by default. The return status when listing options is zero if all @var{optnames} are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an @var{optname} is not a valid shell option. The list of @code{shopt} options is: @table @code @item autocd If set, a command name that is the name of a directory is executed as if it were the argument to the @code{cd} command. This option is only used by interactive shells. @item cdable_vars If this is set, an argument to the @code{cd} builtin command that is not a directory is assumed to be the name of a variable whose value is the directory to change to. @item cdspell If set, minor errors in the spelling of a directory component in a @code{cd} command will be corrected. The errors checked for are transposed characters, a missing character, and a character too many. If a correction is found, the corrected path is printed, and the command proceeds. This option is only used by interactive shells. @item checkhash If this is set, Bash checks that a command found in the hash table exists before trying to execute it. If a hashed command no longer exists, a normal path search is performed. @item checkjobs If set, Bash lists the status of any stopped and running jobs before exiting an interactive shell. If any jobs are running, this causes the exit to be deferred until a second exit is attempted without an intervening command (@pxref{Job Control}). The shell always postpones exiting if any jobs are stopped. @item checkwinsize If set, Bash checks the window size after each command and, if necessary, updates the values of @env{LINES} and @env{COLUMNS}. @item cmdhist If set, Bash attempts to save all lines of a multiple-line command in the same history entry. This allows easy re-editing of multi-line commands. @item compat31 If set, Bash changes its behavior to that of version 3.1 with respect to quoted arguments to the conditional command's @samp{=~} operator. @item compat32 If set, Bash changes its behavior to that of version 3.2 with respect to locale-specific string comparison when using the @samp{[[} conditional command's @samp{<} and @samp{>} operators. Bash versions prior to bash-4.0 use ASCII collation and strcmp(3); bash-4.1 and later use the current locale's collation sequence and strcoll(3). @item compat40 If set, Bash changes its behavior to that of version 4.0 with respect to locale-specific string comparison when using the @samp{[[} conditional command's @samp{<} and @samp{>} operators (see previous item) and the effect of interrupting a command list. @item compat41 If set, Bash, when in posix mode, treats a single quote in a double-quoted parameter expansion as a special character. The single quotes must match (an even number) and the characters between the single quotes are considered quoted. This is the behavior of @sc{posix} mode through version 4.1. The default Bash behavior remains as in previous versions. @item dirspell If set, Bash attempts spelling correction on directory names during word completion if the directory name initially supplied does not exist. @item dotglob If set, Bash includes filenames beginning with a `.' in the results of filename expansion. @item execfail If this is set, a non-interactive shell will not exit if it cannot execute the file specified as an argument to the @code{exec} builtin command. An interactive shell does not exit if @code{exec} fails. @item expand_aliases If set, aliases are expanded as described below under Aliases, @ref{Aliases}. This option is enabled by default for interactive shells. @item extdebug If set, behavior intended for use by debuggers is enabled: @enumerate @item The @option{-F} option to the @code{declare} builtin (@pxref{Bash Builtins}) displays the source file name and line number corresponding to each function name supplied as an argument. @item If the command run by the @code{DEBUG} trap returns a non-zero value, the next command is skipped and not executed. @item If the command run by the @code{DEBUG} trap returns a value of 2, and the shell is executing in a subroutine (a shell function or a shell script executed by the @code{.} or @code{source} builtins), a call to @code{return} is simulated. @item @code{BASH_ARGC} and @code{BASH_ARGV} are updated as described in their descriptions (@pxref{Bash Variables}). @item Function tracing is enabled: command substitution, shell functions, and subshells invoked with @code{( @var{command} )} inherit the @code{DEBUG} and @code{RETURN} traps. @item Error tracing is enabled: command substitution, shell functions, and subshells invoked with @code{( @var{command} )} inherit the @code{ERR} trap. @end enumerate @item extglob If set, the extended pattern matching features described above (@pxref{Pattern Matching}) are enabled. @item extquote If set, @code{$'@var{string}'} and @code{$"@var{string}"} quoting is performed within @code{$@{@var{parameter}@}} expansions enclosed in double quotes. This option is enabled by default. @item failglob If set, patterns which fail to match filenames during filename expansion result in an expansion error. @item force_fignore If set, the suffixes specified by the @env{FIGNORE} shell variable cause words to be ignored when performing word completion even if the ignored words are the only possible completions. @xref{Bash Variables}, for a description of @env{FIGNORE}. This option is enabled by default. @item globstar If set, the pattern @samp{**} used in a filename expansion context will match all files and zero or more directories and subdirectories. If the pattern is followed by a @samp{/}, only directories and subdirectories match. @item gnu_errfmt If set, shell error messages are written in the standard @sc{gnu} error message format. @item histappend If set, the history list is appended to the file named by the value of the @env{HISTFILE} variable when the shell exits, rather than overwriting the file. @item histreedit If set, and Readline is being used, a user is given the opportunity to re-edit a failed history substitution. @item histverify If set, and Readline is being used, the results of history substitution are not immediately passed to the shell parser. Instead, the resulting line is loaded into the Readline editing buffer, allowing further modification. @item hostcomplete If set, and Readline is being used, Bash will attempt to perform hostname completion when a word containing a @samp{@@} is being completed (@pxref{Commands For Completion}). This option is enabled by default. @item huponexit If set, Bash will send @code{SIGHUP} to all jobs when an interactive login shell exits (@pxref{Signals}). @item interactive_comments Allow a word beginning with @samp{#} to cause that word and all remaining characters on that line to be ignored in an interactive shell. This option is enabled by default. @item lastpipe If set, and job control is not active, the shell runs the last command of a pipeline not executed in the background in the current shell environment. @item lithist If enabled, and the @code{cmdhist} option is enabled, multi-line commands are saved to the history with embedded newlines rather than using semicolon separators where possible. @item login_shell The shell sets this option if it is started as a login shell (@pxref{Invoking Bash}). The value may not be changed. @item mailwarn If set, and a file that Bash is checking for mail has been accessed since the last time it was checked, the message @code{"The mail in @var{mailfile} has been read"} is displayed. @item no_empty_cmd_completion If set, and Readline is being used, Bash will not attempt to search the @env{PATH} for possible completions when completion is attempted on an empty line. @item nocaseglob If set, Bash matches filenames in a case-insensitive fashion when performing filename expansion. @item nocasematch If set, Bash matches patterns in a case-insensitive fashion when performing matching while executing @code{case} or @code{[[} conditional commands. @item nullglob If set, Bash allows filename patterns which match no files to expand to a null string, rather than themselves. @item progcomp If set, the programmable completion facilities (@pxref{Programmable Completion}) are enabled. This option is enabled by default. @item promptvars If set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote removal after being expanded as described below (@pxref{Printing a Prompt}). This option is enabled by default. @item restricted_shell The shell sets this option if it is started in restricted mode (@pxref{The Restricted Shell}). The value may not be changed. This is not reset when the startup files are executed, allowing the startup files to discover whether or not a shell is restricted. @item shift_verbose If this is set, the @code{shift} builtin prints an error message when the shift count exceeds the number of positional parameters. @item sourcepath If set, the @code{source} builtin uses the value of @env{PATH} to find the directory containing the file supplied as an argument. This option is enabled by default. @item xpg_echo If set, the @code{echo} builtin expands backslash-escape sequences by default. @end table @noindent The return status when listing options is zero if all @var{optnames} are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an @var{optname} is not a valid shell option. @end table @node Special Builtins @section Special Builtins @cindex special builtin For historical reasons, the @sc{posix} standard has classified several builtin commands as @emph{special}. When Bash is executing in @sc{posix} mode, the special builtins differ from other builtin commands in three respects: @enumerate @item Special builtins are found before shell functions during command lookup. @item If a special builtin returns an error status, a non-interactive shell exits. @item Assignment statements preceding the command stay in effect in the shell environment after the command completes. @end enumerate When Bash is not executing in @sc{posix} mode, these builtins behave no differently than the rest of the Bash builtin commands. The Bash @sc{posix} mode is described in @ref{Bash POSIX Mode}. These are the @sc{posix} special builtins: @example @w{break : . continue eval exec exit export readonly return set} @w{shift trap unset} @end example @node Shell Variables @chapter Shell Variables @menu * Bourne Shell Variables:: Variables which Bash uses in the same way as the Bourne Shell. * Bash Variables:: List of variables that exist in Bash. @end menu This chapter describes the shell variables that Bash uses. Bash automatically assigns default values to a number of variables. @node Bourne Shell Variables @section Bourne Shell Variables Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable. @vtable @code @item CDPATH A colon-separated list of directories used as a search path for the @code{cd} builtin command. @item HOME The current user's home directory; the default for the @code{cd} builtin command. The value of this variable is also used by tilde expansion (@pxref{Tilde Expansion}). @item IFS A list of characters that separate fields; used when the shell splits words as part of expansion. @item MAIL If this parameter is set to a filename or directory name and the @env{MAILPATH} variable is not set, Bash informs the user of the arrival of mail in the specified file or Maildir-format directory. @item MAILPATH A colon-separated list of filenames which the shell periodically checks for new mail. Each list entry can specify the message that is printed when new mail arrives in the mail file by separating the file name from the message with a @samp{?}. When used in the text of the message, @code{$_} expands to the name of the current mail file. @item OPTARG The value of the last option argument processed by the @code{getopts} builtin. @item OPTIND The index of the last option argument processed by the @code{getopts} builtin. @item PATH A colon-separated list of directories in which the shell looks for commands. A zero-length (null) directory name in the value of @code{PATH} indicates the current directory. A null directory name may appear as two adjacent colons, or as an initial or trailing colon. @item PS1 The primary prompt string. The default value is @samp{\s-\v\$ }. @xref{Printing a Prompt}, for the complete list of escape sequences that are expanded before @env{PS1} is displayed. @item PS2 The secondary prompt string. The default value is @samp{> }. @end vtable @node Bash Variables @section Bash Variables These variables are set or used by Bash, but other shells do not normally treat them specially. A few variables used by Bash are described in different chapters: variables for controlling the job control facilities (@pxref{Job Control Variables}). @vtable @code @item BASH The full pathname used to execute the current instance of Bash. @item BASHOPTS A colon-separated list of enabled shell options. Each word in the list is a valid argument for the @option{-s} option to the @code{shopt} builtin command (@pxref{The Shopt Builtin}). The options appearing in @env{BASHOPTS} are those reported as @samp{on} by @samp{shopt}. If this variable is in the environment when Bash starts up, each shell option in the list will be enabled before reading any startup files. This variable is readonly. @item BASHPID Expands to the process ID of the current Bash process. This differs from @code{$$} under certain circumstances, such as subshells that do not require Bash to be re-initialized. @item BASH_ALIASES An associative array variable whose members correspond to the internal list of aliases as maintained by the @code{alias} builtin. (@pxref{Bourne Shell Builtins}). Elements added to this array appear in the alias list; unsetting array elements cause aliases to be removed from the alias list. @item BASH_ARGC An array variable whose values are the number of parameters in each frame of the current bash execution call stack. The number of parameters to the current subroutine (shell function or script executed with @code{.} or @code{source}) is at the top of the stack. When a subroutine is executed, the number of parameters passed is pushed onto @code{BASH_ARGC}. The shell sets @code{BASH_ARGC} only when in extended debugging mode (see @ref{The Shopt Builtin} for a description of the @code{extdebug} option to the @code{shopt} builtin). @item BASH_ARGV An array variable containing all of the parameters in the current bash execution call stack. The final parameter of the last subroutine call is at the top of the stack; the first parameter of the initial call is at the bottom. When a subroutine is executed, the parameters supplied are pushed onto @code{BASH_ARGV}. The shell sets @code{BASH_ARGV} only when in extended debugging mode (see @ref{The Shopt Builtin} for a description of the @code{extdebug} option to the @code{shopt} builtin). @item BASH_CMDS An associative array variable whose members correspond to the internal hash table of commands as maintained by the @code{hash} builtin (@pxref{Bourne Shell Builtins}). Elements added to this array appear in the hash table; unsetting array elements cause commands to be removed from the hash table. @item BASH_COMMAND The command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap. @item BASH_ENV If this variable is set when Bash is invoked to execute a shell script, its value is expanded and used as the name of a startup file to read before executing the script. @xref{Bash Startup Files}. @item BASH_EXECUTION_STRING The command argument to the @option{-c} invocation option. @item BASH_LINENO An array variable whose members are the line numbers in source files where each corresponding member of @var{FUNCNAME} was invoked. @code{$@{BASH_LINENO[$i]@}} is the line number in the source file (@code{$@{BASH_SOURCE[$i+1]@}}) where @code{$@{FUNCNAME[$i]@}} was called (or @code{$@{BASH_LINENO[$i-1]@}} if referenced within another shell function). Use @code{LINENO} to obtain the current line number. @item BASH_REMATCH An array variable whose members are assigned by the @samp{=~} binary operator to the @code{[[} conditional command (@pxref{Conditional Constructs}). The element with index 0 is the portion of the string matching the entire regular expression. The element with index @var{n} is the portion of the string matching the @var{n}th parenthesized subexpression. This variable is read-only. @item BASH_SOURCE An array variable whose members are the source filenames where the corresponding shell function names in the @code{FUNCNAME} array variable are defined. The shell function @code{$@{FUNCNAME[$i]@}} is defined in the file @code{$@{BASH_SOURCE[$i]@}} and called from @code{$@{BASH_SOURCE[$i+1]@}} @item BASH_SUBSHELL Incremented by one each time a subshell or subshell environment is spawned. The initial value is 0. @item BASH_VERSINFO A readonly array variable (@pxref{Arrays}) whose members hold version information for this instance of Bash. The values assigned to the array members are as follows: @table @code @item BASH_VERSINFO[0] The major version number (the @var{release}). @item BASH_VERSINFO[1] The minor version number (the @var{version}). @item BASH_VERSINFO[2] The patch level. @item BASH_VERSINFO[3] The build version. @item BASH_VERSINFO[4] The release status (e.g., @var{beta1}). @item BASH_VERSINFO[5] The value of @env{MACHTYPE}. @end table @item BASH_VERSION The version number of the current instance of Bash. @item BASH_XTRACEFD If set to an integer corresponding to a valid file descriptor, Bash will write the trace output generated when @samp{set -x} is enabled to that file descriptor. This allows tracing output to be separated from diagnostic and error messages. The file descriptor is closed when @code{BASH_XTRACEFD} is unset or assigned a new value. Unsetting @code{BASH_XTRACEFD} or assigning it the empty string causes the trace output to be sent to the standard error. Note that setting @code{BASH_XTRACEFD} to 2 (the standard error file descriptor) and then unsetting it will result in the standard error being closed. @item COLUMNS Used by the @code{select} command to determine the terminal width when printing selection lists. Automatically set upon receipt of a @code{SIGWINCH}. @item COMP_CWORD An index into @env{$@{COMP_WORDS@}} of the word containing the current cursor position. This variable is available only in shell functions invoked by the programmable completion facilities (@pxref{Programmable Completion}). @item COMP_LINE The current command line. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (@pxref{Programmable Completion}). @item COMP_POINT The index of the current cursor position relative to the beginning of the current command. If the current cursor position is at the end of the current command, the value of this variable is equal to @code{$@{#COMP_LINE@}}. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (@pxref{Programmable Completion}). @item COMP_TYPE Set to an integer value corresponding to the type of completion attempted that caused a completion function to be called: @var{TAB}, for normal completion, @samp{?}, for listing completions after successive tabs, @samp{!}, for listing alternatives on partial word completion, @samp{@@}, to list completions if the word is not unmodified, or @samp{%}, for menu completion. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (@pxref{Programmable Completion}). @item COMP_KEY The key (or final key of a key sequence) used to invoke the current completion function. @item COMP_WORDBREAKS The set of characters that the Readline library treats as word separators when performing word completion. If @code{COMP_WORDBREAKS} is unset, it loses its special properties, even if it is subsequently reset. @item COMP_WORDS An array variable consisting of the individual words in the current command line. The line is split into words as Readline would split it, using @code{COMP_WORDBREAKS} as described above. This variable is available only in shell functions invoked by the programmable completion facilities (@pxref{Programmable Completion}). @item COMPREPLY An array variable from which Bash reads the possible completions generated by a shell function invoked by the programmable completion facility (@pxref{Programmable Completion}). @item COPROC An array variable created to hold the file descriptors for output from and input to an unnamed coprocess (@pxref{Coprocesses}). @item DIRSTACK An array variable containing the current contents of the directory stack. Directories appear in the stack in the order they are displayed by the @code{dirs} builtin. Assigning to members of this array variable may be used to modify directories already in the stack, but the @code{pushd} and @code{popd} builtins must be used to add and remove directories. Assignment to this variable will not change the current directory. If @env{DIRSTACK} is unset, it loses its special properties, even if it is subsequently reset. @item EMACS If Bash finds this variable in the environment when the shell starts with value @samp{t}, it assumes that the shell is running in an Emacs shell buffer and disables line editing. @item ENV Similar to @code{BASH_ENV}; used when the shell is invoked in @sc{posix} Mode (@pxref{Bash POSIX Mode}). @item EUID The numeric effective user id of the current user. This variable is readonly. @item FCEDIT The editor used as a default by the @option{-e} option to the @code{fc} builtin command. @item FIGNORE A colon-separated list of suffixes to ignore when performing filename completion. A file name whose suffix matches one of the entries in @env{FIGNORE} is excluded from the list of matched file names. A sample value is @samp{.o:~} @item FUNCNAME An array variable containing the names of all shell functions currently in the execution call stack. The element with index 0 is the name of any currently-executing shell function. The bottom-most element (the one with the highest index) is @code{"main"}. This variable exists only when a shell function is executing. Assignments to @env{FUNCNAME} have no effect and return an error status. If @env{FUNCNAME} is unset, it loses its special properties, even if it is subsequently reset. This variable can be used with @code{BASH_LINENO} and @code{BASH_SOURCE}. Each element of @code{FUNCNAME} has corresponding elements in @code{BASH_LINENO} and @code{BASH_SOURCE} to describe the call stack. For instance, @code{$@{FUNCNAME[$i]@}} was called from the file @code{$@{BASH_SOURCE[$i+1]@}} at line number @code{$@{BASH_LINENO[$i]@}}. The @code{caller} builtin displays the current call stack using this information. @item FUNCNEST If set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that exceed this nesting level will cause the current command to abort. @item GLOBIGNORE A colon-separated list of patterns defining the set of filenames to be ignored by filename expansion. If a filename matched by a filename expansion pattern also matches one of the patterns in @env{GLOBIGNORE}, it is removed from the list of matches. @item GROUPS An array variable containing the list of groups of which the current user is a member. Assignments to @env{GROUPS} have no effect and return an error status. If @env{GROUPS} is unset, it loses its special properties, even if it is subsequently reset. @item histchars Up to three characters which control history expansion, quick substitution, and tokenization (@pxref{History Interaction}). The first character is the @var{history expansion} character, that is, the character which signifies the start of a history expansion, normally @samp{!}. The second character is the character which signifies `quick substitution' when seen as the first character on a line, normally @samp{^}. The optional third character is the character which indicates that the remainder of the line is a comment when found as the first character of a word, usually @samp{#}. The history comment character causes history substitution to be skipped for the remaining words on the line. It does not necessarily cause the shell parser to treat the rest of the line as a comment. @item HISTCMD The history number, or index in the history list, of the current command. If @env{HISTCMD} is unset, it loses its special properties, even if it is subsequently reset. @item HISTCONTROL A colon-separated list of values controlling how commands are saved on the history list. If the list of values includes @samp{ignorespace}, lines which begin with a space character are not saved in the history list. A value of @samp{ignoredups} causes lines which match the previous history entry to not be saved. A value of @samp{ignoreboth} is shorthand for @samp{ignorespace} and @samp{ignoredups}. A value of @samp{erasedups} causes all previous lines matching the current line to be removed from the history list before that line is saved. Any value not in the above list is ignored. If @env{HISTCONTROL} is unset, or does not include a valid value, all lines read by the shell parser are saved on the history list, subject to the value of @env{HISTIGNORE}. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of @env{HISTCONTROL}. @item HISTFILE The name of the file to which the command history is saved. The default value is @file{~/.bash_history}. @item HISTFILESIZE The maximum number of lines contained in the history file. When this variable is assigned a value, the history file is truncated, if necessary, by removing the oldest entries, to contain no more than that number of lines. The history file is also truncated to this size after writing it when an interactive shell exits. The default value is 500. @item HISTIGNORE A colon-separated list of patterns used to decide which command lines should be saved on the history list. Each pattern is anchored at the beginning of the line and must match the complete line (no implicit @samp{*} is appended). Each pattern is tested against the line after the checks specified by @env{HISTCONTROL} are applied. In addition to the normal shell pattern matching characters, @samp{&} matches the previous history line. @samp{&} may be escaped using a backslash; the backslash is removed before attempting a match. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of @env{HISTIGNORE}. @env{HISTIGNORE} subsumes the function of @env{HISTCONTROL}. A pattern of @samp{&} is identical to @code{ignoredups}, and a pattern of @samp{[ ]*} is identical to @code{ignorespace}. Combining these two patterns, separating them with a colon, provides the functionality of @code{ignoreboth}. @item HISTSIZE The maximum number of commands to remember on the history list. The default value is 500. @item HISTTIMEFORMAT If this variable is set and not null, its value is used as a format string for @var{strftime} to print the time stamp associated with each history entry displayed by the @code{history} builtin. If this variable is set, time stamps are written to the history file so they may be preserved across shell sessions. This uses the history comment character to distinguish timestamps from other history lines. @item HOSTFILE Contains the name of a file in the same format as @file{/etc/hosts} that should be read when the shell needs to complete a hostname. The list of possible hostname completions may be changed while the shell is running; the next time hostname completion is attempted after the value is changed, Bash adds the contents of the new file to the existing list. If @env{HOSTFILE} is set, but has no value, or does not name a readable file, Bash attempts to read @file{/etc/hosts} to obtain the list of possible hostname completions. When @env{HOSTFILE} is unset, the hostname list is cleared. @item HOSTNAME The name of the current host. @item HOSTTYPE A string describing the machine Bash is running on. @item IGNOREEOF Controls the action of the shell on receipt of an @code{EOF} character as the sole input. If set, the value denotes the number of consecutive @code{EOF} characters that can be read as the first character on an input line before the shell will exit. If the variable exists but does not have a numeric value (or has no value) then the default is 10. If the variable does not exist, then @code{EOF} signifies the end of input to the shell. This is only in effect for interactive shells. @item INPUTRC The name of the Readline initialization file, overriding the default of @file{~/.inputrc}. @item LANG Used to determine the locale category for any category not specifically selected with a variable starting with @code{LC_}. @item LC_ALL This variable overrides the value of @env{LANG} and any other @code{LC_} variable specifying a locale category. @item LC_COLLATE This variable determines the collation order used when sorting the results of filename expansion, and determines the behavior of range expressions, equivalence classes, and collating sequences within filename expansion and pattern matching (@pxref{Filename Expansion}). @item LC_CTYPE This variable determines the interpretation of characters and the behavior of character classes within filename expansion and pattern matching (@pxref{Filename Expansion}). @item LC_MESSAGES This variable determines the locale used to translate double-quoted strings preceded by a @samp{$} (@pxref{Locale Translation}). @item LC_NUMERIC This variable determines the locale category used for number formatting. @item LINENO The line number in the script or shell function currently executing. @item LINES Used by the @code{select} command to determine the column length for printing selection lists. Automatically set upon receipt of a @code{SIGWINCH}. @item MACHTYPE A string that fully describes the system type on which Bash is executing, in the standard @sc{gnu} @var{cpu-company-system} format. @item MAILCHECK How often (in seconds) that the shell should check for mail in the files specified in the @env{MAILPATH} or @env{MAIL} variables. The default is 60 seconds. When it is time to check for mail, the shell does so before displaying the primary prompt. If this variable is unset, or set to a value that is not a number greater than or equal to zero, the shell disables mail checking. @item MAPFILE An array variable created to hold the text read by the @code{mapfile} builtin when no variable name is supplied. @item OLDPWD The previous working directory as set by the @code{cd} builtin. @item OPTERR If set to the value 1, Bash displays error messages generated by the @code{getopts} builtin command. @item OSTYPE A string describing the operating system Bash is running on. @item PIPESTATUS An array variable (@pxref{Arrays}) containing a list of exit status values from the processes in the most-recently-executed foreground pipeline (which may contain only a single command). @item POSIXLY_CORRECT If this variable is in the environment when @code{bash} starts, the shell enters @sc{posix} mode (@pxref{Bash POSIX Mode}) before reading the startup files, as if the @option{--posix} invocation option had been supplied. If it is set while the shell is running, @code{bash} enables @sc{posix} mode, as if the command @example @code{set -o posix} @end example @noindent had been executed. @item PPID The process @sc{id} of the shell's parent process. This variable is readonly. @item PROMPT_COMMAND If set, the value is interpreted as a command to execute before the printing of each primary prompt (@env{$PS1}). @item PROMPT_DIRTRIM If set to a number greater than zero, the value is used as the number of trailing directory components to retain when expanding the @code{\w} and @code{\W} prompt string escapes (@pxref{Printing a Prompt}). Characters removed are replaced with an ellipsis. @item PS3 The value of this variable is used as the prompt for the @code{select} command. If this variable is not set, the @code{select} command prompts with @samp{#? } @item PS4 The value is the prompt printed before the command line is echoed when the @option{-x} option is set (@pxref{The Set Builtin}). The first character of @env{PS4} is replicated multiple times, as necessary, to indicate multiple levels of indirection. The default is @samp{+ }. @item PWD The current working directory as set by the @code{cd} builtin. @item RANDOM Each time this parameter is referenced, a random integer between 0 and 32767 is generated. Assigning a value to this variable seeds the random number generator. @item READLINE_LINE The contents of the Readline line buffer, for use with @samp{bind -x} (@pxref{Bash Builtins}). @item READLINE_POINT The position of the insertion point in the Readline line buffer, for use with @samp{bind -x} (@pxref{Bash Builtins}). @item REPLY The default variable for the @code{read} builtin. @item SECONDS This variable expands to the number of seconds since the shell was started. Assignment to this variable resets the count to the value assigned, and the expanded value becomes the value assigned plus the number of seconds since the assignment. @item SHELL The full pathname to the shell is kept in this environment variable. If it is not set when the shell starts, Bash assigns to it the full pathname of the current user's login shell. @item SHELLOPTS A colon-separated list of enabled shell options. Each word in the list is a valid argument for the @option{-o} option to the @code{set} builtin command (@pxref{The Set Builtin}). The options appearing in @env{SHELLOPTS} are those reported as @samp{on} by @samp{set -o}. If this variable is in the environment when Bash starts up, each shell option in the list will be enabled before reading any startup files. This variable is readonly. @item SHLVL Incremented by one each time a new instance of Bash is started. This is intended to be a count of how deeply your Bash shells are nested. @item TIMEFORMAT The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the @code{time} reserved word should be displayed. The @samp{%} character introduces an escape sequence that is expanded to a time value or other information. The escape sequences and their meanings are as follows; the braces denote optional portions. @table @code @item %% A literal @samp{%}. @item %[@var{p}][l]R The elapsed time in seconds. @item %[@var{p}][l]U The number of CPU seconds spent in user mode. @item %[@var{p}][l]S The number of CPU seconds spent in system mode. @item %P The CPU percentage, computed as (%U + %S) / %R. @end table The optional @var{p} is a digit specifying the precision, the number of fractional digits after a decimal point. A value of 0 causes no decimal point or fraction to be output. At most three places after the decimal point may be specified; values of @var{p} greater than 3 are changed to 3. If @var{p} is not specified, the value 3 is used. The optional @code{l} specifies a longer format, including minutes, of the form @var{MM}m@var{SS}.@var{FF}s. The value of @var{p} determines whether or not the fraction is included. If this variable is not set, Bash acts as if it had the value @example @code{$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'} @end example If the value is null, no timing information is displayed. A trailing newline is added when the format string is displayed. @item TMOUT If set to a value greater than zero, @code{TMOUT} is treated as the default timeout for the @code{read} builtin (@pxref{Bash Builtins}). The @code{select} command (@pxref{Conditional Constructs}) terminates if input does not arrive after @code{TMOUT} seconds when input is coming from a terminal. In an interactive shell, the value is interpreted as the number of seconds to wait for input after issuing the primary prompt when the shell is interactive. Bash terminates after that number of seconds if input does not arrive. @item TMPDIR If set, Bash uses its value as the name of a directory in which Bash creates temporary files for the shell's use. @item UID The numeric real user id of the current user. This variable is readonly. @end vtable @node Bash Features @chapter Bash Features This section describes features unique to Bash. @menu * Invoking Bash:: Command line options that you can give to Bash. * Bash Startup Files:: When and how Bash executes scripts. * Interactive Shells:: What an interactive shell is. * Bash Conditional Expressions:: Primitives used in composing expressions for the @code{test} builtin. * Shell Arithmetic:: Arithmetic on shell variables. * Aliases:: Substituting one command for another. * Arrays:: Array Variables. * The Directory Stack:: History of visited directories. * Printing a Prompt:: Controlling the PS1 string. * The Restricted Shell:: A more controlled mode of shell execution. * Bash POSIX Mode:: Making Bash behave more closely to what the POSIX standard specifies. @end menu @node Invoking Bash @section Invoking Bash @example bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o @var{option}] [-O @var{shopt_option}] [@var{argument} @dots{}] bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o @var{option}] [-O @var{shopt_option}] -c @var{string} [@var{argument} @dots{}] bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o @var{option}] [-O @var{shopt_option}] [@var{argument} @dots{}] @end example All of the single-character options used with the @code{set} builtin (@pxref{The Set Builtin}) can be used as options when the shell is invoked. In addition, there are several multi-character options that you can use. These options must appear on the command line before the single-character options to be recognized. @table @code @item --debugger Arrange for the debugger profile to be executed before the shell starts. Turns on extended debugging mode (see @ref{The Shopt Builtin} for a description of the @code{extdebug} option to the @code{shopt} builtin). @item --dump-po-strings A list of all double-quoted strings preceded by @samp{$} is printed on the standard output in the @sc{gnu} @code{gettext} PO (portable object) file format. Equivalent to @option{-D} except for the output format. @item --dump-strings Equivalent to @option{-D}. @item --help Display a usage message on standard output and exit successfully. @item --init-file @var{filename} @itemx --rcfile @var{filename} Execute commands from @var{filename} (instead of @file{~/.bashrc}) in an interactive shell. @item --login Equivalent to @option{-l}. @item --noediting Do not use the @sc{gnu} Readline library (@pxref{Command Line Editing}) to read command lines when the shell is interactive. @item --noprofile Don't load the system-wide startup file @file{/etc/profile} or any of the personal initialization files @file{~/.bash_profile}, @file{~/.bash_login}, or @file{~/.profile} when Bash is invoked as a login shell. @item --norc Don't read the @file{~/.bashrc} initialization file in an interactive shell. This is on by default if the shell is invoked as @code{sh}. @item --posix Change the behavior of Bash where the default operation differs from the @sc{posix} standard to match the standard. This is intended to make Bash behave as a strict superset of that standard. @xref{Bash POSIX Mode}, for a description of the Bash @sc{posix} mode. @item --restricted Make the shell a restricted shell (@pxref{The Restricted Shell}). @item --verbose Equivalent to @option{-v}. Print shell input lines as they're read. @item --version Show version information for this instance of Bash on the standard output and exit successfully. @end table There are several single-character options that may be supplied at invocation which are not available with the @code{set} builtin. @table @code @item -c @var{string} Read and execute commands from @var{string} after processing the options, then exit. Any remaining arguments are assigned to the positional parameters, starting with @code{$0}. @item -i Force the shell to run interactively. Interactive shells are described in @ref{Interactive Shells}. @item -l Make this shell act as if it had been directly invoked by login. When the shell is interactive, this is equivalent to starting a login shell with @samp{exec -l bash}. When the shell is not interactive, the login shell startup files will be executed. @samp{exec bash -l} or @samp{exec bash --login} will replace the current shell with a Bash login shell. @xref{Bash Startup Files}, for a description of the special behavior of a login shell. @item -r Make the shell a restricted shell (@pxref{The Restricted Shell}). @item -s If this option is present, or if no arguments remain after option processing, then commands are read from the standard input. This option allows the positional parameters to be set when invoking an interactive shell. @item -D A list of all double-quoted strings preceded by @samp{$} is printed on the standard output. These are the strings that are subject to language translation when the current locale is not @code{C} or @code{POSIX} (@pxref{Locale Translation}). This implies the @option{-n} option; no commands will be executed. @item [-+]O [@var{shopt_option}] @var{shopt_option} is one of the shell options accepted by the @code{shopt} builtin (@pxref{The Shopt Builtin}). If @var{shopt_option} is present, @option{-O} sets the value of that option; @option{+O} unsets it. If @var{shopt_option} is not supplied, the names and values of the shell options accepted by @code{shopt} are printed on the standard output. If the invocation option is @option{+O}, the output is displayed in a format that may be reused as input. @item -- A @code{--} signals the end of options and disables further option processing. Any arguments after the @code{--} are treated as filenames and arguments. @end table @cindex login shell A @emph{login} shell is one whose first character of argument zero is @samp{-}, or one invoked with the @option{--login} option. @cindex interactive shell An @emph{interactive} shell is one started without non-option arguments, unless @option{-s} is specified, without specifying the @option{-c} option, and whose input and output are both connected to terminals (as determined by @code{isatty(3)}), or one started with the @option{-i} option. @xref{Interactive Shells}, for more information. If arguments remain after option processing, and neither the @option{-c} nor the @option{-s} option has been supplied, the first argument is assumed to be the name of a file containing shell commands (@pxref{Shell Scripts}). When Bash is invoked in this fashion, @code{$0} is set to the name of the file, and the positional parameters are set to the remaining arguments. Bash reads and executes commands from this file, then exits. Bash's exit status is the exit status of the last command executed in the script. If no commands are executed, the exit status is 0. @node Bash Startup Files @section Bash Startup Files @cindex startup files This section describes how Bash executes its startup files. If any of the files exist but cannot be read, Bash reports an error. Tildes are expanded in file names as described above under Tilde Expansion (@pxref{Tilde Expansion}). Interactive shells are described in @ref{Interactive Shells}. @subsubheading Invoked as an interactive login shell, or with @option{--login} When Bash is invoked as an interactive login shell, or as a non-interactive shell with the @option{--login} option, it first reads and executes commands from the file @file{/etc/profile}, if that file exists. After reading that file, it looks for @file{~/.bash_profile}, @file{~/.bash_login}, and @file{~/.profile}, in that order, and reads and executes commands from the first one that exists and is readable. The @option{--noprofile} option may be used when the shell is started to inhibit this behavior. When a login shell exits, Bash reads and executes commands from the file @file{~/.bash_logout}, if it exists. @subsubheading Invoked as an interactive non-login shell When an interactive shell that is not a login shell is started, Bash reads and executes commands from @file{~/.bashrc}, if that file exists. This may be inhibited by using the @option{--norc} option. The @option{--rcfile @var{file}} option will force Bash to read and execute commands from @var{file} instead of @file{~/.bashrc}. So, typically, your @file{~/.bash_profile} contains the line @example @code{if [ -f ~/.bashrc ]; then . ~/.bashrc; fi} @end example @noindent after (or before) any login-specific initializations. @subsubheading Invoked non-interactively When Bash is started non-interactively, to run a shell script, for example, it looks for the variable @env{BASH_ENV} in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command were executed: @example @code{if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi} @end example @noindent but the value of the @env{PATH} variable is not used to search for the file name. As noted above, if a non-interactive shell is invoked with the @option{--login} option, Bash attempts to read and execute commands from the login shell startup files. @subsubheading Invoked with name @code{sh} If Bash is invoked with the name @code{sh}, it tries to mimic the startup behavior of historical versions of @code{sh} as closely as possible, while conforming to the @sc{posix} standard as well. When invoked as an interactive login shell, or as a non-interactive shell with the @option{--login} option, it first attempts to read and execute commands from @file{/etc/profile} and @file{~/.profile}, in that order. The @option{--noprofile} option may be used to inhibit this behavior. When invoked as an interactive shell with the name @code{sh}, Bash looks for the variable @env{ENV}, expands its value if it is defined, and uses the expanded value as the name of a file to read and execute. Since a shell invoked as @code{sh} does not attempt to read and execute commands from any other startup files, the @option{--rcfile} option has no effect. A non-interactive shell invoked with the name @code{sh} does not attempt to read any other startup files. When invoked as @code{sh}, Bash enters @sc{posix} mode after the startup files are read. @subsubheading Invoked in @sc{posix} mode When Bash is started in @sc{posix} mode, as with the @option{--posix} command line option, it follows the @sc{posix} standard for startup files. In this mode, interactive shells expand the @env{ENV} variable and commands are read and executed from the file whose name is the expanded value. No other startup files are read. @subsubheading Invoked by remote shell daemon Bash attempts to determine when it is being run with its standard input connected to a network connection, as when executed by the remote shell daemon, usually @code{rshd}, or the secure shell daemon @code{sshd}. If Bash determines it is being run in this fashion, it reads and executes commands from @file{~/.bashrc}, if that file exists and is readable. It will not do this if invoked as @code{sh}. The @option{--norc} option may be used to inhibit this behavior, and the @option{--rcfile} option may be used to force another file to be read, but @code{rshd} does not generally invoke the shell with those options or allow them to be specified. @subsubheading Invoked with unequal effective and real @sc{uid/gid}s If Bash is started with the effective user (group) id not equal to the real user (group) id, and the @code{-p} option is not supplied, no startup files are read, shell functions are not inherited from the environment, the @env{SHELLOPTS}, @env{BASHOPTS}, @env{CDPATH}, and @env{GLOBIGNORE} variables, if they appear in the environment, are ignored, and the effective user id is set to the real user id. If the @code{-p} option is supplied at invocation, the startup behavior is the same, but the effective user id is not reset. @node Interactive Shells @section Interactive Shells @cindex interactive shell @cindex shell, interactive @menu * What is an Interactive Shell?:: What determines whether a shell is Interactive. * Is this Shell Interactive?:: How to tell if a shell is interactive. * Interactive Shell Behavior:: What changes in a interactive shell? @end menu @node What is an Interactive Shell? @subsection What is an Interactive Shell? An interactive shell is one started without non-option arguments, unless @option{-s} is specified, without specifying the @option{-c} option, and whose input and error output are both connected to terminals (as determined by @code{isatty(3)}), or one started with the @option{-i} option. An interactive shell generally reads from and writes to a user's terminal. The @option{-s} invocation option may be used to set the positional parameters when an interactive shell is started. @node Is this Shell Interactive? @subsection Is this Shell Interactive? To determine within a startup script whether or not Bash is running interactively, test the value of the @samp{-} special parameter. It contains @code{i} when the shell is interactive. For example: @example case "$-" in *i*) echo This shell is interactive ;; *) echo This shell is not interactive ;; esac @end example Alternatively, startup scripts may examine the variable @env{PS1}; it is unset in non-interactive shells, and set in interactive shells. Thus: @example if [ -z "$PS1" ]; then echo This shell is not interactive else echo This shell is interactive fi @end example @node Interactive Shell Behavior @subsection Interactive Shell Behavior When the shell is running interactively, it changes its behavior in several ways. @enumerate @item Startup files are read and executed as described in @ref{Bash Startup Files}. @item Job Control (@pxref{Job Control}) is enabled by default. When job control is in effect, Bash ignores the keyboard-generated job control signals @code{SIGTTIN}, @code{SIGTTOU}, and @code{SIGTSTP}. @item Bash expands and displays @env{PS1} before reading the first line of a command, and expands and displays @env{PS2} before reading the second and subsequent lines of a multi-line command. @item Bash executes the value of the @env{PROMPT_COMMAND} variable as a command before printing the primary prompt, @env{$PS1} (@pxref{Bash Variables}). @item Readline (@pxref{Command Line Editing}) is used to read commands from the user's terminal. @item Bash inspects the value of the @code{ignoreeof} option to @code{set -o} instead of exiting immediately when it receives an @code{EOF} on its standard input when reading a command (@pxref{The Set Builtin}). @item Command history (@pxref{Bash History Facilities}) and history expansion (@pxref{History Interaction}) are enabled by default. Bash will save the command history to the file named by @env{$HISTFILE} when an interactive shell exits. @item Alias expansion (@pxref{Aliases}) is performed by default. @item In the absence of any traps, Bash ignores @code{SIGTERM} (@pxref{Signals}). @item In the absence of any traps, @code{SIGINT} is caught and handled ((@pxref{Signals}). @code{SIGINT} will interrupt some shell builtins. @item An interactive login shell sends a @code{SIGHUP} to all jobs on exit if the @code{huponexit} shell option has been enabled (@pxref{Signals}). @item The @option{-n} invocation option is ignored, and @samp{set -n} has no effect (@pxref{The Set Builtin}). @item Bash will check for mail periodically, depending on the values of the @env{MAIL}, @env{MAILPATH}, and @env{MAILCHECK} shell variables (@pxref{Bash Variables}). @item Expansion errors due to references to unbound shell variables after @samp{set -u} has been enabled will not cause the shell to exit (@pxref{The Set Builtin}). @item The shell will not exit on expansion errors caused by @var{var} being unset or null in @code{$@{@var{var}:?@var{word}@}} expansions (@pxref{Shell Parameter Expansion}). @item Redirection errors encountered by shell builtins will not cause the shell to exit. @item When running in @sc{posix} mode, a special builtin returning an error status will not cause the shell to exit (@pxref{Bash POSIX Mode}). @item A failed @code{exec} will not cause the shell to exit (@pxref{Bourne Shell Builtins}). @item Parser syntax errors will not cause the shell to exit. @item Simple spelling correction for directory arguments to the @code{cd} builtin is enabled by default (see the description of the @code{cdspell} option to the @code{shopt} builtin in @ref{The Shopt Builtin}). @item The shell will check the value of the @env{TMOUT} variable and exit if a command is not read within the specified number of seconds after printing @env{$PS1} (@pxref{Bash Variables}). @end enumerate @node Bash Conditional Expressions @section Bash Conditional Expressions @cindex expressions, conditional Conditional expressions are used by the @code{[[} compound command and the @code{test} and @code{[} builtin commands. Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. If the @var{file} argument to one of the primaries is of the form @file{/dev/fd/@var{N}}, then file descriptor @var{N} is checked. If the @var{file} argument to one of the primaries is one of @file{/dev/stdin}, @file{/dev/stdout}, or @file{/dev/stderr}, file descriptor 0, 1, or 2, respectively, is checked. When used with @samp{[[}, the @samp{<} and @samp{>} operators sort lexicographically using the current locale. The @code{test} command uses ASCII ordering. Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather than the link itself. @table @code @item -a @var{file} True if @var{file} exists. @item -b @var{file} True if @var{file} exists and is a block special file. @item -c @var{file} True if @var{file} exists and is a character special file. @item -d @var{file} True if @var{file} exists and is a directory. @item -e @var{file} True if @var{file} exists. @item -f @var{file} True if @var{file} exists and is a regular file. @item -g @var{file} True if @var{file} exists and its set-group-id bit is set. @item -h @var{file} True if @var{file} exists and is a symbolic link. @item -k @var{file} True if @var{file} exists and its "sticky" bit is set. @item -p @var{file} True if @var{file} exists and is a named pipe (FIFO). @item -r @var{file} True if @var{file} exists and is readable. @item -s @var{file} True if @var{file} exists and has a size greater than zero. @item -t @var{fd} True if file descriptor @var{fd} is open and refers to a terminal. @item -u @var{file} True if @var{file} exists and its set-user-id bit is set. @item -w @var{file} True if @var{file} exists and is writable. @item -x @var{file} True if @var{file} exists and is executable. @item -G @var{file} True if @var{file} exists and is owned by the effective group id. @item -L @var{file} True if @var{file} exists and is a symbolic link. @item -N @var{file} True if @var{file} exists and has been modified since it was last read. @item -O @var{file} True if @var{file} exists and is owned by the effective user id. @item -S @var{file} True if @var{file} exists and is a socket. @item @var{file1} -ef @var{file2} True if @var{file1} and @var{file2} refer to the same device and inode numbers. @item @var{file1} -nt @var{file2} True if @var{file1} is newer (according to modification date) than @var{file2}, or if @var{file1} exists and @var{file2} does not. @item @var{file1} -ot @var{file2} True if @var{file1} is older than @var{file2}, or if @var{file2} exists and @var{file1} does not. @item -o @var{optname} True if the shell option @var{optname} is enabled. The list of options appears in the description of the @option{-o} option to the @code{set} builtin (@pxref{The Set Builtin}). @item -v @var{varname} True if the shell variable @var{varname} is set (has been assigned a value). @item -z @var{string} True if the length of @var{string} is zero. @item -n @var{string} @itemx @var{string} True if the length of @var{string} is non-zero. @item @var{string1} == @var{string2} @itemx @var{string1} = @var{string2} True if the strings are equal. @samp{=} should be used with the @code{test} command for @sc{posix} conformance. @item @var{string1} != @var{string2} True if the strings are not equal. @item @var{string1} < @var{string2} True if @var{string1} sorts before @var{string2} lexicographically. @item @var{string1} > @var{string2} True if @var{string1} sorts after @var{string2} lexicographically. @item @var{arg1} OP @var{arg2} @code{OP} is one of @samp{-eq}, @samp{-ne}, @samp{-lt}, @samp{-le}, @samp{-gt}, or @samp{-ge}. These arithmetic binary operators return true if @var{arg1} is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to @var{arg2}, respectively. @var{Arg1} and @var{arg2} may be positive or negative integers. @end table @node Shell Arithmetic @section Shell Arithmetic @cindex arithmetic, shell @cindex shell arithmetic @cindex expressions, arithmetic @cindex evaluation, arithmetic @cindex arithmetic evaluation The shell allows arithmetic expressions to be evaluated, as one of the shell expansions or by the @code{let} and the @option{-i} option to the @code{declare} builtins. Evaluation is done in fixed-width integers with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence, associativity, and values are the same as in the C language. The following list of operators is grouped into levels of equal-precedence operators. The levels are listed in order of decreasing precedence. @table @code @item @var{id}++ @var{id}-- variable post-increment and post-decrement @item ++@var{id} --@var{id} variable pre-increment and pre-decrement @item - + unary minus and plus @item ! ~ logical and bitwise negation @item ** exponentiation @item * / % multiplication, division, remainder @item + - addition, subtraction @item << >> left and right bitwise shifts @item <= >= < > comparison @item == != equality and inequality @item & bitwise AND @item ^ bitwise exclusive OR @item | bitwise OR @item && logical AND @item || logical OR @item expr ? expr : expr conditional operator @item = *= /= %= += -= <<= >>= &= ^= |= assignment @item expr1 , expr2 comma @end table Shell variables are allowed as operands; parameter expansion is performed before the expression is evaluated. Within an expression, shell variables may also be referenced by name without using the parameter expansion syntax. A shell variable that is null or unset evaluates to 0 when referenced by name without using the parameter expansion syntax. The value of a variable is evaluated as an arithmetic expression when it is referenced, or when a variable which has been given the @var{integer} attribute using @samp{declare -i} is assigned a value. A null value evaluates to 0. A shell variable need not have its @var{integer} attribute turned on to be used in an expression. Constants with a leading 0 are interpreted as octal numbers. A leading @samp{0x} or @samp{0X} denotes hexadecimal. Otherwise, numbers take the form [@var{base}@code{#}]@var{n}, where the optional @var{base} is a decimal number between 2 and 64 representing the arithmetic base, and @var{n} is a number in that base. If @var{base}@code{#} is omitted, then base 10 is used. The digits greater than 9 are represented by the lowercase letters, the uppercase letters, @samp{@@}, and @samp{_}, in that order. If @var{base} is less than or equal to 36, lowercase and uppercase letters may be used interchangeably to represent numbers between 10 and 35. Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first and may override the precedence rules above. @node Aliases @section Aliases @cindex alias expansion @var{Aliases} allow a string to be substituted for a word when it is used as the first word of a simple command. The shell maintains a list of aliases that may be set and unset with the @code{alias} and @code{unalias} builtin commands. The first word of each simple command, if unquoted, is checked to see if it has an alias. If so, that word is replaced by the text of the alias. The characters @samp{/}, @samp{$}, @samp{`}, @samp{=} and any of the shell metacharacters or quoting characters listed above may not appear in an alias name. The replacement text may contain any valid shell input, including shell metacharacters. The first word of the replacement text is tested for aliases, but a word that is identical to an alias being expanded is not expanded a second time. This means that one may alias @code{ls} to @code{"ls -F"}, for instance, and Bash does not try to recursively expand the replacement text. If the last character of the alias value is a space or tab character, then the next command word following the alias is also checked for alias expansion. Aliases are created and listed with the @code{alias} command, and removed with the @code{unalias} command. There is no mechanism for using arguments in the replacement text, as in @code{csh}. If arguments are needed, a shell function should be used (@pxref{Shell Functions}). Aliases are not expanded when the shell is not interactive, unless the @code{expand_aliases} shell option is set using @code{shopt} (@pxref{The Shopt Builtin}). The rules concerning the definition and use of aliases are somewhat confusing. Bash always reads at least one complete line of input before executing any of the commands on that line. Aliases are expanded when a command is read, not when it is executed. Therefore, an alias definition appearing on the same line as another command does not take effect until the next line of input is read. The commands following the alias definition on that line are not affected by the new alias. This behavior is also an issue when functions are executed. Aliases are expanded when a function definition is read, not when the function is executed, because a function definition is itself a compound command. As a consequence, aliases defined in a function are not available until after that function is executed. To be safe, always put alias definitions on a separate line, and do not use @code{alias} in compound commands. For almost every purpose, shell functions are preferred over aliases. @node Arrays @section Arrays @cindex arrays Bash provides one-dimensional indexed and associative array variables. Any variable may be used as an indexed array; the @code{declare} builtin will explicitly declare an array. There is no maximum limit on the size of an array, nor any requirement that members be indexed or assigned contiguously. Indexed arrays are referenced using integers (including arithmetic expressions (@pxref{Shell Arithmetic}) and are zero-based; associative arrays use arbitrary strings. An indexed array is created automatically if any variable is assigned to using the syntax @example name[@var{subscript}]=@var{value} @end example @noindent The @var{subscript} is treated as an arithmetic expression that must evaluate to a number. If @var{subscript} evaluates to a number less than zero, it is used as an offset from one greater than the array's maximum index (so a subcript of -1 refers to the last element of the array). To explicitly declare an array, use @example declare -a @var{name} @end example @noindent The syntax @example declare -a @var{name}[@var{subscript}] @end example @noindent is also accepted; the @var{subscript} is ignored. Associative arrays are created using @example declare -A @var{name}. @end example Attributes may be specified for an array variable using the @code{declare} and @code{readonly} builtins. Each attribute applies to all members of an array. Arrays are assigned to using compound assignments of the form @example name=(value@var{1} @dots{} value@var{n}) @end example @noindent where each @var{value} is of the form @code{[@var{subscript}]=}@var{string}. Indexed array assignments do not require the bracket and subscript. When assigning to indexed arrays, if the optional subscript is supplied, that index is assigned to; otherwise the index of the element assigned is the last index assigned to by the statement plus one. Indexing starts at zero. When assigning to an associative array, the subscript is required. This syntax is also accepted by the @code{declare} builtin. Individual array elements may be assigned to using the @code{name[}@var{subscript}@code{]=}@var{value} syntax introduced above. Any element of an array may be referenced using @code{$@{name[}@var{subscript}@code{]@}}. The braces are required to avoid conflicts with the shell's filename expansion operators. If the @var{subscript} is @samp{@@} or @samp{*}, the word expands to all members of the array @var{name}. These subscripts differ only when the word appears within double quotes. If the word is double-quoted, @code{$@{name[*]@}} expands to a single word with the value of each array member separated by the first character of the @env{IFS} variable, and @code{$@{name[@@]@}} expands each element of @var{name} to a separate word. When there are no array members, @code{$@{name[@@]@}} expands to nothing. If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word. This is analogous to the expansion of the special parameters @samp{@@} and @samp{*}. @code{$@{#name[}@var{subscript}@code{]@}} expands to the length of @code{$@{name[}@var{subscript}@code{]@}}. If @var{subscript} is @samp{@@} or @samp{*}, the expansion is the number of elements in the array. Referencing an array variable without a subscript is equivalent to referencing with a subscript of 0. An array variable is considered set if a subscript has been assigned a value. The null string is a valid value. The @code{unset} builtin is used to destroy arrays. @code{unset} @var{name}[@var{subscript}] destroys the array element at index @var{subscript}. Care must be taken to avoid unwanted side effects caused by filename expansion. @code{unset} @var{name}, where @var{name} is an array, removes the entire array. A subscript of @samp{*} or @samp{@@} also removes the entire array. The @code{declare}, @code{local}, and @code{readonly} builtins each accept a @option{-a} option to specify an indexed array and a @option{-A} option to specify an associative array. If both options are supplied, @option{-A} takes precedence. The @code{read} builtin accepts a @option{-a} option to assign a list of words read from the standard input to an array, and can read values from the standard input into individual array elements. The @code{set} and @code{declare} builtins display array values in a way that allows them to be reused as input. @node The Directory Stack @section The Directory Stack @cindex directory stack @menu * Directory Stack Builtins:: Bash builtin commands to manipulate the directory stack. @end menu The directory stack is a list of recently-visited directories. The @code{pushd} builtin adds directories to the stack as it changes the current directory, and the @code{popd} builtin removes specified directories from the stack and changes the current directory to the directory removed. The @code{dirs} builtin displays the contents of the directory stack. The contents of the directory stack are also visible as the value of the @env{DIRSTACK} shell variable. @node Directory Stack Builtins @subsection Directory Stack Builtins @table @code @item dirs @btindex dirs @example dirs [+@var{N} | -@var{N}] [-clpv] @end example Display the list of currently remembered directories. Directories are added to the list with the @code{pushd} command; the @code{popd} command removes directories from the list. @table @code @item +@var{N} Displays the @var{N}th directory (counting from the left of the list printed by @code{dirs} when invoked without options), starting with zero. @item -@var{N} Displays the @var{N}th directory (counting from the right of the list printed by @code{dirs} when invoked without options), starting with zero. @item -c Clears the directory stack by deleting all of the elements. @item -l Produces a longer listing; the default listing format uses a tilde to denote the home directory. @item -p Causes @code{dirs} to print the directory stack with one entry per line. @item -v Causes @code{dirs} to print the directory stack with one entry per line, prefixing each entry with its index in the stack. @end table @item popd @btindex popd @example popd [+@var{N} | -@var{N}] [-n] @end example Remove the top entry from the directory stack, and @code{cd} to the new top directory. When no arguments are given, @code{popd} removes the top directory from the stack and performs a @code{cd} to the new top directory. The elements are numbered from 0 starting at the first directory listed with @code{dirs}; i.e., @code{popd} is equivalent to @code{popd +0}. @table @code @item +@var{N} Removes the @var{N}th directory (counting from the left of the list printed by @code{dirs}), starting with zero. @item -@var{N} Removes the @var{N}th directory (counting from the right of the list printed by @code{dirs}), starting with zero. @item -n Suppresses the normal change of directory when removing directories from the stack, so that only the stack is manipulated. @end table @btindex pushd @item pushd @example pushd [-n] [@var{+N} | @var{-N} | @var{dir} ] @end example Save the current directory on the top of the directory stack and then @code{cd} to @var{dir}. With no arguments, @code{pushd} exchanges the top two directories. @table @code @item -n Suppresses the normal change of directory when adding directories to the stack, so that only the stack is manipulated. @item +@var{N} Brings the @var{N}th directory (counting from the left of the list printed by @code{dirs}, starting with zero) to the top of the list by rotating the stack. @item -@var{N} Brings the @var{N}th directory (counting from the right of the list printed by @code{dirs}, starting with zero) to the top of the list by rotating the stack. @item @var{dir} Makes the current working directory be the top of the stack, and then executes the equivalent of `@code{cd} @var{dir}'. @code{cd}s to @var{dir}. @end table @end table @node Printing a Prompt @section Controlling the Prompt @cindex prompting The value of the variable @env{PROMPT_COMMAND} is examined just before Bash prints each primary prompt. If @env{PROMPT_COMMAND} is set and has a non-null value, then the value is executed just as if it had been typed on the command line. In addition, the following table describes the special characters which can appear in the prompt variables: @table @code @item \a A bell character. @item \d The date, in "Weekday Month Date" format (e.g., "Tue May 26"). @item \D@{@var{format}@} The @var{format} is passed to @code{strftime}(3) and the result is inserted into the prompt string; an empty @var{format} results in a locale-specific time representation. The braces are required. @item \e An escape character. @item \h The hostname, up to the first `.'. @item \H The hostname. @item \j The number of jobs currently managed by the shell. @item \l The basename of the shell's terminal device name. @item \n A newline. @item \r A carriage return. @item \s The name of the shell, the basename of @code{$0} (the portion following the final slash). @item \t The time, in 24-hour HH:MM:SS format. @item \T The time, in 12-hour HH:MM:SS format. @item \@@ The time, in 12-hour am/pm format. @item \A The time, in 24-hour HH:MM format. @item \u The username of the current user. @item \v The version of Bash (e.g., 2.00) @item \V The release of Bash, version + patchlevel (e.g., 2.00.0) @item \w The current working directory, with @env{$HOME} abbreviated with a tilde (uses the @env{$PROMPT_DIRTRIM} variable). @item \W The basename of @env{$PWD}, with @env{$HOME} abbreviated with a tilde. @item \! The history number of this command. @item \# The command number of this command. @item \$ If the effective uid is 0, @code{#}, otherwise @code{$}. @item \@var{nnn} The character whose ASCII code is the octal value @var{nnn}. @item \\ A backslash. @item \[ Begin a sequence of non-printing characters. This could be used to embed a terminal control sequence into the prompt. @item \] End a sequence of non-printing characters. @end table The command number and the history number are usually different: the history number of a command is its position in the history list, which may include commands restored from the history file (@pxref{Bash History Facilities}), while the command number is the position in the sequence of commands executed during the current shell session. After the string is decoded, it is expanded via parameter expansion, command substitution, arithmetic expansion, and quote removal, subject to the value of the @code{promptvars} shell option (@pxref{Bash Builtins}). @node The Restricted Shell @section The Restricted Shell @cindex restricted shell If Bash is started with the name @code{rbash}, or the @option{--restricted} or @option{-r} option is supplied at invocation, the shell becomes restricted. A restricted shell is used to set up an environment more controlled than the standard shell. A restricted shell behaves identically to @code{bash} with the exception that the following are disallowed or not performed: @itemize @bullet @item Changing directories with the @code{cd} builtin. @item Setting or unsetting the values of the @env{SHELL}, @env{PATH}, @env{ENV}, or @env{BASH_ENV} variables. @item Specifying command names containing slashes. @item Specifying a filename containing a slash as an argument to the @code{.} builtin command. @item Specifying a filename containing a slash as an argument to the @option{-p} option to the @code{hash} builtin command. @item Importing function definitions from the shell environment at startup. @item Parsing the value of @env{SHELLOPTS} from the shell environment at startup. @item Redirecting output using the @samp{>}, @samp{>|}, @samp{<>}, @samp{>&}, @samp{&>}, and @samp{>>} redirection operators. @item Using the @code{exec} builtin to replace the shell with another command. @item Adding or deleting builtin commands with the @option{-f} and @option{-d} options to the @code{enable} builtin. @item Using the @code{enable} builtin command to enable disabled shell builtins. @item Specifying the @option{-p} option to the @code{command} builtin. @item Turning off restricted mode with @samp{set +r} or @samp{set +o restricted}. @end itemize These restrictions are enforced after any startup files are read. When a command that is found to be a shell script is executed (@pxref{Shell Scripts}), @code{rbash} turns off any restrictions in the shell spawned to execute the script. @node Bash POSIX Mode @section Bash POSIX Mode @cindex POSIX Mode Starting Bash with the @option{--posix} command-line option or executing @samp{set -o posix} while Bash is running will cause Bash to conform more closely to the @sc{posix} standard by changing the behavior to match that specified by @sc{posix} in areas where the Bash default differs. When invoked as @code{sh}, Bash enters @sc{posix} mode after reading the startup files. The following list is what's changed when `@sc{posix} mode' is in effect: @enumerate @item When a command in the hash table no longer exists, Bash will re-search @env{$PATH} to find the new location. This is also available with @samp{shopt -s checkhash}. @item The message printed by the job control code and builtins when a job exits with a non-zero status is `Done(status)'. @item The message printed by the job control code and builtins when a job is stopped is `Stopped(@var{signame})', where @var{signame} is, for example, @code{SIGTSTP}. @item The @code{bg} builtin uses the required format to describe each job placed in the background, which does not include an indication of whether the job is the current or previous job. @item Reserved words appearing in a context where reserved words are recognized do not undergo alias expansion. @item The @sc{posix} @env{PS1} and @env{PS2} expansions of @samp{!} to the history number and @samp{!!} to @samp{!} are enabled, and parameter expansion is performed on the values of @env{PS1} and @env{PS2} regardless of the setting of the @code{promptvars} option. @item The @sc{posix} startup files are executed (@env{$ENV}) rather than the normal Bash files. @item Tilde expansion is only performed on assignments preceding a command name, rather than on all assignment statements on the line. @item The default history file is @file{~/.sh_history} (this is the default value of @env{$HISTFILE}). @item The output of @samp{kill -l} prints all the signal names on a single line, separated by spaces, without the @samp{SIG} prefix. @item The @code{kill} builtin does not accept signal names with a @samp{SIG} prefix. @item Non-interactive shells exit if @var{filename} in @code{.} @var{filename} is not found. @item Non-interactive shells exit if a syntax error in an arithmetic expansion results in an invalid expression. @item Non-interactive shells exit if there is a syntax error in a script read with the @code{.} or @code{source} builtins, or in a string processed by the @code{eval} builtin. @item Redirection operators do not perform filename expansion on the word in the redirection unless the shell is interactive. @item Redirection operators do not perform word splitting on the word in the redirection. @item Function names must be valid shell @code{name}s. That is, they may not contain characters other than letters, digits, and underscores, and may not start with a digit. Declaring a function with an invalid name causes a fatal syntax error in non-interactive shells. @item @sc{posix} special builtins are found before shell functions during command lookup. @item The @code{time} reserved word may be used by itself as a command. When used in this way, it displays timing statistics for the shell and its completed children. The @env{TIMEFORMAT} variable controls the format of the timing information. @item When parsing and expanding a $@{@dots{}@} expansion that appears within double quotes, single quotes are no longer special and cannot be used to quote a closing brace or other special character, unless the operator is one of those defined to perform pattern removal. In this case, they do not have to appear as matched pairs. @item The parser does not recognize @code{time} as a reserved word if the next token begins with a @samp{-}. @item If a @sc{posix} special builtin returns an error status, a non-interactive shell exits. The fatal errors are those listed in the @sc{posix} standard, and include things like passing incorrect options, redirection errors, variable assignment errors for assignments preceding the command name, and so on. @item A non-interactive shell exits with an error status if a variable assignment error occurs when no command name follows the assignment statements. A variable assignment error occurs, for example, when trying to assign a value to a readonly variable. @item A non-interactive shell exists with an error status if a variable assignment error occurs in an assignment statement preceding a special builtin, but not with any other simple command. @item A non-interactive shell exits with an error status if the iteration variable in a @code{for} statement or the selection variable in a @code{select} statement is a readonly variable. @item Process substitution is not available. @item Assignment statements preceding @sc{posix} special builtins persist in the shell environment after the builtin completes. @item Assignment statements preceding shell function calls persist in the shell environment after the function returns, as if a @sc{posix} special builtin command had been executed. @item The @code{export} and @code{readonly} builtin commands display their output in the format required by @sc{posix}. @item The @code{trap} builtin displays signal names without the leading @code{SIG}. @item The @code{trap} builtin doesn't check the first argument for a possible signal specification and revert the signal handling to the original disposition if it is, unless that argument consists solely of digits and is a valid signal number. If users want to reset the handler for a given signal to the original disposition, they should use @samp{-} as the first argument. @item The @code{.} and @code{source} builtins do not search the current directory for the filename argument if it is not found by searching @env{PATH}. @item Subshells spawned to execute command substitutions inherit the value of the @option{-e} option from the parent shell. When not in @sc{posix} mode, Bash clears the @option{-e} option in such subshells. @item Alias expansion is always enabled, even in non-interactive shells. @item When the @code{alias} builtin displays alias definitions, it does not display them with a leading @samp{alias } unless the @option{-p} option is supplied. @item When the @code{set} builtin is invoked without options, it does not display shell function names and definitions. @item When the @code{set} builtin is invoked without options, it displays variable values without quotes, unless they contain shell metacharacters, even if the result contains nonprinting characters. @item When the @code{cd} builtin is invoked in @var{logical} mode, and the pathname constructed from @code{$PWD} and the directory name supplied as an argument does not refer to an existing directory, @code{cd} will fail instead of falling back to @var{physical} mode. @item The @code{pwd} builtin verifies that the value it prints is the same as the current directory, even if it is not asked to check the file system with the @option{-P} option. @item When listing the history, the @code{fc} builtin does not include an indication of whether or not a history entry has been modified. @item The default editor used by @code{fc} is @code{ed}. @item The @code{type} and @code{command} builtins will not report a non-executable file as having been found, though the shell will attempt to execute such a file if it is the only so-named file found in @code{$PATH}. @item The @code{vi} editing mode will invoke the @code{vi} editor directly when the @samp{v} command is run, instead of checking @code{$VISUAL} and @code{$EDITOR}. @item When the @code{xpg_echo} option is enabled, Bash does not attempt to interpret any arguments to @code{echo} as options. Each argument is displayed, after escape characters are converted. @item The @code{ulimit} builtin uses a block size of 512 bytes for the @option{-c} and @option{-f} options. @item The arrival of @code{SIGCHLD} when a trap is set on @code{SIGCHLD} does not interrupt the @code{wait} builtin and cause it to return immediately. The trap command is run once for each child that exits. @end enumerate There is other @sc{posix} behavior that Bash does not implement by default even when in @sc{posix} mode. Specifically: @enumerate @item The @code{fc} builtin checks @code{$EDITOR} as a program to edit history entries if @code{FCEDIT} is unset, rather than defaulting directly to @code{ed}. @code{fc} uses @code{ed} if @code{EDITOR} is unset. @item As noted above, Bash requires the @code{xpg_echo} option to be enabled for the @code{echo} builtin to be fully conformant. @end enumerate Bash can be configured to be @sc{posix}-conformant by default, by specifying the @option{--enable-strict-posix-default} to @code{configure} when building (@pxref{Optional Features}). @node Job Control @chapter Job Control This chapter discusses what job control is, how it works, and how Bash allows you to access its facilities. @menu * Job Control Basics:: How job control works. * Job Control Builtins:: Bash builtin commands used to interact with job control. * Job Control Variables:: Variables Bash uses to customize job control. @end menu @node Job Control Basics @section Job Control Basics @cindex job control @cindex foreground @cindex background @cindex suspending jobs Job control refers to the ability to selectively stop (suspend) the execution of processes and continue (resume) their execution at a later point. A user typically employs this facility via an interactive interface supplied jointly by the operating system kernel's terminal driver and Bash. The shell associates a @var{job} with each pipeline. It keeps a table of currently executing jobs, which may be listed with the @code{jobs} command. When Bash starts a job asynchronously, it prints a line that looks like: @example [1] 25647 @end example @noindent indicating that this job is job number 1 and that the process @sc{id} of the last process in the pipeline associated with this job is 25647. All of the processes in a single pipeline are members of the same job. Bash uses the @var{job} abstraction as the basis for job control. To facilitate the implementation of the user interface to job control, the operating system maintains the notion of a current terminal process group @sc{id}. Members of this process group (processes whose process group @sc{id} is equal to the current terminal process group @sc{id}) receive keyboard-generated signals such as @code{SIGINT}. These processes are said to be in the foreground. Background processes are those whose process group @sc{id} differs from the terminal's; such processes are immune to keyboard-generated signals. Only foreground processes are allowed to read from or, if the user so specifies with @code{stty tostop}, write to the terminal. Background processes which attempt to read from (write to when @code{stty tostop} is in effect) the terminal are sent a @code{SIGTTIN} (@code{SIGTTOU}) signal by the kernel's terminal driver, which, unless caught, suspends the process. If the operating system on which Bash is running supports job control, Bash contains facilities to use it. Typing the @var{suspend} character (typically @samp{^Z}, Control-Z) while a process is running causes that process to be stopped and returns control to Bash. Typing the @var{delayed suspend} character (typically @samp{^Y}, Control-Y) causes the process to be stopped when it attempts to read input from the terminal, and control to be returned to Bash. The user then manipulates the state of this job, using the @code{bg} command to continue it in the background, the @code{fg} command to continue it in the foreground, or the @code{kill} command to kill it. A @samp{^Z} takes effect immediately, and has the additional side effect of causing pending output and typeahead to be discarded. There are a number of ways to refer to a job in the shell. The character @samp{%} introduces a job specification (@var{jobspec}). Job number @code{n} may be referred to as @samp{%n}. The symbols @samp{%%} and @samp{%+} refer to the shell's notion of the current job, which is the last job stopped while it was in the foreground or started in the background. A single @samp{%} (with no accompanying job specification) also refers to the current job. The previous job may be referenced using @samp{%-}. If there is only a single job, @samp{%+} and @samp{%-} can both be used to refer to that job. In output pertaining to jobs (e.g., the output of the @code{jobs} command), the current job is always flagged with a @samp{+}, and the previous job with a @samp{-}. A job may also be referred to using a prefix of the name used to start it, or using a substring that appears in its command line. For example, @samp{%ce} refers to a stopped @code{ce} job. Using @samp{%?ce}, on the other hand, refers to any job containing the string @samp{ce} in its command line. If the prefix or substring matches more than one job, Bash reports an error. Simply naming a job can be used to bring it into the foreground: @samp{%1} is a synonym for @samp{fg %1}, bringing job 1 from the background into the foreground. Similarly, @samp{%1 &} resumes job 1 in the background, equivalent to @samp{bg %1} The shell learns immediately whenever a job changes state. Normally, Bash waits until it is about to print a prompt before reporting changes in a job's status so as to not interrupt any other output. If the @option{-b} option to the @code{set} builtin is enabled, Bash reports such changes immediately (@pxref{The Set Builtin}). Any trap on @code{SIGCHLD} is executed for each child process that exits. If an attempt to exit Bash is made while jobs are stopped, (or running, if the @code{checkjobs} option is enabled -- see @ref{The Shopt Builtin}), the shell prints a warning message, and if the @code{checkjobs} option is enabled, lists the jobs and their statuses. The @code{jobs} command may then be used to inspect their status. If a second attempt to exit is made without an intervening command, Bash does not print another warning, and any stopped jobs are terminated. @node Job Control Builtins @section Job Control Builtins @table @code @item bg @btindex bg @example bg [@var{jobspec} @dots{}] @end example Resume each suspended job @var{jobspec} in the background, as if it had been started with @samp{&}. If @var{jobspec} is not supplied, the current job is used. The return status is zero unless it is run when job control is not enabled, or, when run with job control enabled, any @var{jobspec} was not found or specifies a job that was started without job control. @item fg @btindex fg @example fg [@var{jobspec}] @end example Resume the job @var{jobspec} in the foreground and make it the current job. If @var{jobspec} is not supplied, the current job is used. The return status is that of the command placed into the foreground, or non-zero if run when job control is disabled or, when run with job control enabled, @var{jobspec} does not specify a valid job or @var{jobspec} specifies a job that was started without job control. @item jobs @btindex jobs @example jobs [-lnprs] [@var{jobspec}] jobs -x @var{command} [@var{arguments}] @end example The first form lists the active jobs. The options have the following meanings: @table @code @item -l List process @sc{id}s in addition to the normal information. @item -n Display information only about jobs that have changed status since the user was last notified of their status. @item -p List only the process @sc{id} of the job's process group leader. @item -r Restrict output to running jobs. @item -s Restrict output to stopped jobs. @end table If @var{jobspec} is given, output is restricted to information about that job. If @var{jobspec} is not supplied, the status of all jobs is listed. If the @option{-x} option is supplied, @code{jobs} replaces any @var{jobspec} found in @var{command} or @var{arguments} with the corresponding process group @sc{id}, and executes @var{command}, passing it @var{argument}s, returning its exit status. @item kill @btindex kill @example kill [-s @var{sigspec}] [-n @var{signum}] [-@var{sigspec}] @var{jobspec} or @var{pid} kill -l [@var{exit_status}] @end example Send a signal specified by @var{sigspec} or @var{signum} to the process named by job specification @var{jobspec} or process @sc{id} @var{pid}. @var{sigspec} is either a case-insensitive signal name such as @code{SIGINT} (with or without the @code{SIG} prefix) or a signal number; @var{signum} is a signal number. If @var{sigspec} and @var{signum} are not present, @code{SIGTERM} is used. The @option{-l} option lists the signal names. If any arguments are supplied when @option{-l} is given, the names of the signals corresponding to the arguments are listed, and the return status is zero. @var{exit_status} is a number specifying a signal number or the exit status of a process terminated by a signal. The return status is zero if at least one signal was successfully sent, or non-zero if an error occurs or an invalid option is encountered. @item wait @btindex wait @example wait [@var{jobspec} or @var{pid} ...] @end example Wait until the child process specified by each process @sc{id} @var{pid} or job specification @var{jobspec} exits and return the exit status of the last command waited for. If a job spec is given, all processes in the job are waited for. If no arguments are given, all currently active child processes are waited for, and the return status is zero. If neither @var{jobspec} nor @var{pid} specifies an active child process of the shell, the return status is 127. @item disown @btindex disown @example disown [-ar] [-h] [@var{jobspec} @dots{}] @end example Without options, each @var{jobspec} is removed from the table of active jobs. If the @option{-h} option is given, the job is not removed from the table, but is marked so that @code{SIGHUP} is not sent to the job if the shell receives a @code{SIGHUP}. If @var{jobspec} is not present, and neither the @option{-a} nor @option{-r} option is supplied, the current job is used. If no @var{jobspec} is supplied, the @option{-a} option means to remove or mark all jobs; the @option{-r} option without a @var{jobspec} argument restricts operation to running jobs. @item suspend @btindex suspend @example suspend [-f] @end example Suspend the execution of this shell until it receives a @code{SIGCONT} signal. A login shell cannot be suspended; the @option{-f} option can be used to override this and force the suspension. @end table When job control is not active, the @code{kill} and @code{wait} builtins do not accept @var{jobspec} arguments. They must be supplied process @sc{id}s. @node Job Control Variables @section Job Control Variables @vtable @code @item auto_resume This variable controls how the shell interacts with the user and job control. If this variable exists then single word simple commands without redirections are treated as candidates for resumption of an existing job. There is no ambiguity allowed; if there is more than one job beginning with the string typed, then the most recently accessed job will be selected. The name of a stopped job, in this context, is the command line used to start it. If this variable is set to the value @samp{exact}, the string supplied must match the name of a stopped job exactly; if set to @samp{substring}, the string supplied needs to match a substring of the name of a stopped job. The @samp{substring} value provides functionality analogous to the @samp{%?} job @sc{id} (@pxref{Job Control Basics}). If set to any other value, the supplied string must be a prefix of a stopped job's name; this provides functionality analogous to the @samp{%} job @sc{id}. @end vtable @set readline-appendix @set history-appendix @cindex Readline, how to use @include rluser.texi @cindex History, how to use @include hsuser.texi @clear readline-appendix @clear history-appendix @node Installing Bash @chapter Installing Bash This chapter provides basic instructions for installing Bash on the various supported platforms. The distribution supports the @sc{gnu} operating systems, nearly every version of Unix, and several non-Unix systems such as BeOS and Interix. Other independent ports exist for @sc{ms-dos}, @sc{os/2}, and Windows platforms. @menu * Basic Installation:: Installation instructions. * Compilers and Options:: How to set special options for various systems. * Compiling For Multiple Architectures:: How to compile Bash for more than one kind of system from the same source tree. * Installation Names:: How to set the various paths used by the installation. * Specifying the System Type:: How to configure Bash for a particular system. * Sharing Defaults:: How to share default configuration values among GNU programs. * Operation Controls:: Options recognized by the configuration program. * Optional Features:: How to enable and disable optional features when building Bash. @end menu @node Basic Installation @section Basic Installation @cindex installation @cindex configuration @cindex Bash installation @cindex Bash configuration These are installation instructions for Bash. The simplest way to compile Bash is: @enumerate @item @code{cd} to the directory containing the source code and type @samp{./configure} to configure Bash for your system. If you're using @code{csh} on an old version of System V, you might need to type @samp{sh ./configure} instead to prevent @code{csh} from trying to execute @code{configure} itself. Running @code{configure} takes some time. While running, it prints messages telling which features it is checking for. @item Type @samp{make} to compile Bash and build the @code{bashbug} bug reporting script. @item Optionally, type @samp{make tests} to run the Bash test suite. @item Type @samp{make install} to install @code{bash} and @code{bashbug}. This will also install the manual pages and Info file. @end enumerate The @code{configure} shell script attempts to guess correct values for various system-dependent variables used during compilation. It uses those values to create a @file{Makefile} in each directory of the package (the top directory, the @file{builtins}, @file{doc}, and @file{support} directories, each directory under @file{lib}, and several others). It also creates a @file{config.h} file containing system-dependent definitions. Finally, it creates a shell script named @code{config.status} that you can run in the future to recreate the current configuration, a file @file{config.cache} that saves the results of its tests to speed up reconfiguring, and a file @file{config.log} containing compiler output (useful mainly for debugging @code{configure}). If at some point @file{config.cache} contains results you don't want to keep, you may remove or edit it. To find out more about the options and arguments that the @code{configure} script understands, type @example bash-2.04$ ./configure --help @end example @noindent at the Bash prompt in your Bash source directory. If you need to do unusual things to compile Bash, please try to figure out how @code{configure} could check whether or not to do them, and mail diffs or instructions to @email{bash-maintainers@@gnu.org} so they can be considered for the next release. The file @file{configure.in} is used to create @code{configure} by a program called Autoconf. You only need @file{configure.in} if you want to change it or regenerate @code{configure} using a newer version of Autoconf. If you do this, make sure you are using Autoconf version 2.50 or newer. You can remove the program binaries and object files from the source code directory by typing @samp{make clean}. To also remove the files that @code{configure} created (so you can compile Bash for a different kind of computer), type @samp{make distclean}. @node Compilers and Options @section Compilers and Options Some systems require unusual options for compilation or linking that the @code{configure} script does not know about. You can give @code{configure} initial values for variables by setting them in the environment. Using a Bourne-compatible shell, you can do that on the command line like this: @example CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure @end example On systems that have the @code{env} program, you can do it like this: @example env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure @end example The configuration process uses GCC to build Bash if it is available. @node Compiling For Multiple Architectures @section Compiling For Multiple Architectures You can compile Bash for more than one kind of computer at the same time, by placing the object files for each architecture in their own directory. To do this, you must use a version of @code{make} that supports the @code{VPATH} variable, such as GNU @code{make}. @code{cd} to the directory where you want the object files and executables to go and run the @code{configure} script from the source directory. You may need to supply the @option{--srcdir=PATH} argument to tell @code{configure} where the source files are. @code{configure} automatically checks for the source code in the directory that @code{configure} is in and in `..'. If you have to use a @code{make} that does not supports the @code{VPATH} variable, you can compile Bash for one architecture at a time in the source code directory. After you have installed Bash for one architecture, use @samp{make distclean} before reconfiguring for another architecture. Alternatively, if your system supports symbolic links, you can use the @file{support/mkclone} script to create a build tree which has symbolic links back to each file in the source directory. Here's an example that creates a build directory in the current directory from a source directory @file{/usr/gnu/src/bash-2.0}: @example bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 . @end example @noindent The @code{mkclone} script requires Bash, so you must have already built Bash for at least one architecture before you can create build directories for other architectures. @node Installation Names @section Installation Names By default, @samp{make install} will install into @file{/usr/local/bin}, @file{/usr/local/man}, etc. You can specify an installation prefix other than @file{/usr/local} by giving @code{configure} the option @option{--prefix=@var{PATH}}, or by specifying a value for the @code{DESTDIR} @samp{make} variable when running @samp{make install}. You can specify separate installation prefixes for architecture-specific files and architecture-independent files. If you give @code{configure} the option @option{--exec-prefix=@var{PATH}}, @samp{make install} will use @var{PATH} as the prefix for installing programs and libraries. Documentation and other data files will still use the regular prefix. @node Specifying the System Type @section Specifying the System Type There may be some features @code{configure} can not figure out automatically, but need to determine by the type of host Bash will run on. Usually @code{configure} can figure that out, but if it prints a message saying it can not guess the host type, give it the @option{--host=TYPE} option. @samp{TYPE} can either be a short name for the system type, such as @samp{sun4}, or a canonical name with three fields: @samp{CPU-COMPANY-SYSTEM} (e.g., @samp{i386-unknown-freebsd4.2}). See the file @file{support/config.sub} for the possible values of each field. @node Sharing Defaults @section Sharing Defaults If you want to set default values for @code{configure} scripts to share, you can create a site shell script called @code{config.site} that gives default values for variables like @code{CC}, @code{cache_file}, and @code{prefix}. @code{configure} looks for @file{PREFIX/share/config.site} if it exists, then @file{PREFIX/etc/config.site} if it exists. Or, you can set the @code{CONFIG_SITE} environment variable to the location of the site script. A warning: the Bash @code{configure} looks for a site script, but not all @code{configure} scripts do. @node Operation Controls @section Operation Controls @code{configure} recognizes the following options to control how it operates. @table @code @item --cache-file=@var{file} Use and save the results of the tests in @var{file} instead of @file{./config.cache}. Set @var{file} to @file{/dev/null} to disable caching, for debugging @code{configure}. @item --help Print a summary of the options to @code{configure}, and exit. @item --quiet @itemx --silent @itemx -q Do not print messages saying which checks are being made. @item --srcdir=@var{dir} Look for the Bash source code in directory @var{dir}. Usually @code{configure} can determine that directory automatically. @item --version Print the version of Autoconf used to generate the @code{configure} script, and exit. @end table @code{configure} also accepts some other, not widely used, boilerplate options. @samp{configure --help} prints the complete list. @node Optional Features @section Optional Features The Bash @code{configure} has a number of @option{--enable-@var{feature}} options, where @var{feature} indicates an optional part of Bash. There are also several @option{--with-@var{package}} options, where @var{package} is something like @samp{bash-malloc} or @samp{purify}. To turn off the default use of a package, use @option{--without-@var{package}}. To configure Bash without a feature that is enabled by default, use @option{--disable-@var{feature}}. Here is a complete list of the @option{--enable-} and @option{--with-} options that the Bash @code{configure} recognizes. @table @code @item --with-afs Define if you are using the Andrew File System from Transarc. @item --with-bash-malloc Use the Bash version of @code{malloc} in the directory @file{lib/malloc}. This is not the same @code{malloc} that appears in @sc{gnu} libc, but an older version originally derived from the 4.2 @sc{bsd} @code{malloc}. This @code{malloc} is very fast, but wastes some space on each allocation. This option is enabled by default. The @file{NOTES} file contains a list of systems for which this should be turned off, and @code{configure} disables this option automatically for a number of systems. @item --with-curses Use the curses library instead of the termcap library. This should be supplied if your system has an inadequate or incomplete termcap database. @item --with-gnu-malloc A synonym for @code{--with-bash-malloc}. @item --with-installed-readline[=@var{PREFIX}] Define this to make Bash link with a locally-installed version of Readline rather than the version in @file{lib/readline}. This works only with Readline 5.0 and later versions. If @var{PREFIX} is @code{yes} or not supplied, @code{configure} uses the values of the make variables @code{includedir} and @code{libdir}, which are subdirectories of @code{prefix} by default, to find the installed version of Readline if it is not in the standard system include and library directories. If @var{PREFIX} is @code{no}, Bash links with the version in @file{lib/readline}. If @var{PREFIX} is set to any other value, @code{configure} treats it as a directory pathname and looks for the installed version of Readline in subdirectories of that directory (include files in @var{PREFIX}/@code{include} and the library in @var{PREFIX}/@code{lib}). @item --with-purify Define this to use the Purify memory allocation checker from Rational Software. @item --enable-minimal-config This produces a shell with minimal features, close to the historical Bourne shell. @end table There are several @option{--enable-} options that alter how Bash is compiled and linked, rather than changing run-time features. @table @code @item --enable-largefile Enable support for @uref{http://www.sas.com/standards/large_file/x_open.20Mar96.html, large files} if the operating system requires special compiler options to build programs which can access large files. This is enabled by default, if the operating system provides large file support. @item --enable-profiling This builds a Bash binary that produces profiling information to be processed by @code{gprof} each time it is executed. @item --enable-static-link This causes Bash to be linked statically, if @code{gcc} is being used. This could be used to build a version to use as root's shell. @end table The @samp{minimal-config} option can be used to disable all of the following options, but it is processed first, so individual options may be enabled using @samp{enable-@var{feature}}. All of the following options except for @samp{disabled-builtins} and @samp{xpg-echo-default} are enabled by default, unless the operating system does not provide the necessary support. @table @code @item --enable-alias Allow alias expansion and include the @code{alias} and @code{unalias} builtins (@pxref{Aliases}). @item --enable-arith-for-command Include support for the alternate form of the @code{for} command that behaves like the C language @code{for} statement (@pxref{Looping Constructs}). @item --enable-array-variables Include support for one-dimensional array shell variables (@pxref{Arrays}). @item --enable-bang-history Include support for @code{csh}-like history substitution (@pxref{History Interaction}). @item --enable-brace-expansion Include @code{csh}-like brace expansion ( @code{b@{a,b@}c} @expansion{} @code{bac bbc} ). See @ref{Brace Expansion}, for a complete description. @item --enable-casemod-attributes Include support for case-modifying attributes in the @code{declare} builtin and assignment statements. Variables with the @var{uppercase} attribute, for example, will have their values converted to uppercase upon assignment. @item --enable-casemod-expansion Include support for case-modifying word expansions. @item --enable-command-timing Include support for recognizing @code{time} as a reserved word and for displaying timing statistics for the pipeline following @code{time} (@pxref{Pipelines}). This allows pipelines as well as shell builtins and functions to be timed. @item --enable-cond-command Include support for the @code{[[} conditional command. (@pxref{Conditional Constructs}). @item --enable-cond-regexp Include support for matching @sc{posix} regular expressions using the @samp{=~} binary operator in the @code{[[} conditional command. (@pxref{Conditional Constructs}). @item --enable-coprocesses Include support for coprocesses and the @code{coproc} reserved word (@pxref{Pipelines}). @item --enable-debugger Include support for the bash debugger (distributed separately). @item --enable-directory-stack Include support for a @code{csh}-like directory stack and the @code{pushd}, @code{popd}, and @code{dirs} builtins (@pxref{The Directory Stack}). @item --enable-disabled-builtins Allow builtin commands to be invoked via @samp{builtin xxx} even after @code{xxx} has been disabled using @samp{enable -n xxx}. See @ref{Bash Builtins}, for details of the @code{builtin} and @code{enable} builtin commands. @item --enable-dparen-arithmetic Include support for the @code{((@dots{}))} command (@pxref{Conditional Constructs}). @item --enable-extended-glob Include support for the extended pattern matching features described above under @ref{Pattern Matching}. @item --enable-extended-glob-default Set the default value of the @var{extglob} shell option described above under @ref{The Shopt Builtin} to be enabled. @item --enable-help-builtin Include the @code{help} builtin, which displays help on shell builtins and variables (@pxref{Bash Builtins}). @item --enable-history Include command history and the @code{fc} and @code{history} builtin commands (@pxref{Bash History Facilities}). @item --enable-job-control This enables the job control features (@pxref{Job Control}), if the operating system supports them. @item --enable-multibyte This enables support for multibyte characters if the operating system provides the necessary support. @item --enable-net-redirections This enables the special handling of filenames of the form @code{/dev/tcp/@var{host}/@var{port}} and @code{/dev/udp/@var{host}/@var{port}} when used in redirections (@pxref{Redirections}). @item --enable-process-substitution This enables process substitution (@pxref{Process Substitution}) if the operating system provides the necessary support. @item --enable-progcomp Enable the programmable completion facilities (@pxref{Programmable Completion}). If Readline is not enabled, this option has no effect. @item --enable-prompt-string-decoding Turn on the interpretation of a number of backslash-escaped characters in the @env{$PS1}, @env{$PS2}, @env{$PS3}, and @env{$PS4} prompt strings. See @ref{Printing a Prompt}, for a complete list of prompt string escape sequences. @item --enable-readline Include support for command-line editing and history with the Bash version of the Readline library (@pxref{Command Line Editing}). @item --enable-restricted Include support for a @dfn{restricted shell}. If this is enabled, Bash, when called as @code{rbash}, enters a restricted mode. See @ref{The Restricted Shell}, for a description of restricted mode. @item --enable-select Include the @code{select} compound command, which allows the generation of simple menus (@pxref{Conditional Constructs}). @item --enable-separate-helpfiles Use external files for the documentation displayed by the @code{help} builtin instead of storing the text internally. @item --enable-single-help-strings Store the text displayed by the @code{help} builtin as a single string for each help topic. This aids in translating the text to different languages. You may need to disable this if your compiler cannot handle very long string literals. @item --enable-strict-posix-default Make Bash @sc{posix}-conformant by default (@pxref{Bash POSIX Mode}). @item --enable-usg-echo-default A synonym for @code{--enable-xpg-echo-default}. @item --enable-xpg-echo-default Make the @code{echo} builtin expand backslash-escaped characters by default, without requiring the @option{-e} option. This sets the default value of the @code{xpg_echo} shell option to @code{on}, which makes the Bash @code{echo} behave more like the version specified in the Single Unix Specification, version 3. @xref{Bash Builtins}, for a description of the escape sequences that @code{echo} recognizes. @end table The file @file{config-top.h} contains C Preprocessor @samp{#define} statements for options which are not settable from @code{configure}. Some of these are not meant to be changed; beware of the consequences if you do. Read the comments associated with each definition for more information about its effect. @node Reporting Bugs @appendix Reporting Bugs Please report all bugs you find in Bash. But first, you should make sure that it really is a bug, and that it appears in the latest version of Bash. The latest version of Bash is always available for FTP from @uref{ftp://ftp.gnu.org/pub/gnu/bash/}. Once you have determined that a bug actually exists, use the @code{bashbug} command to submit a bug report. If you have a fix, you are encouraged to mail that as well! Suggestions and `philosophical' bug reports may be mailed to @email{bug-bash@@gnu.org} or posted to the Usenet newsgroup @code{gnu.bash.bug}. All bug reports should include: @itemize @bullet @item The version number of Bash. @item The hardware and operating system. @item The compiler used to compile Bash. @item A description of the bug behaviour. @item A short script or `recipe' which exercises the bug and may be used to reproduce it. @end itemize @noindent @code{bashbug} inserts the first three items automatically into the template it provides for filing a bug report. Please send all reports concerning this manual to @email{chet.ramey@@case.edu}. @node Major Differences From The Bourne Shell @appendix Major Differences From The Bourne Shell Bash implements essentially the same grammar, parameter and variable expansion, redirection, and quoting as the Bourne Shell. Bash uses the @sc{posix} standard as the specification of how these features are to be implemented. There are some differences between the traditional Bourne shell and Bash; this section quickly details the differences of significance. A number of these differences are explained in greater depth in previous sections. This section uses the version of @code{sh} included in SVR4.2 (the last version of the historical Bourne shell) as the baseline reference. @itemize @bullet @item Bash is @sc{posix}-conformant, even where the @sc{posix} specification differs from traditional @code{sh} behavior (@pxref{Bash POSIX Mode}). @item Bash has multi-character invocation options (@pxref{Invoking Bash}). @item Bash has command-line editing (@pxref{Command Line Editing}) and the @code{bind} builtin. @item Bash provides a programmable word completion mechanism (@pxref{Programmable Completion}), and builtin commands @code{complete}, @code{compgen}, and @code{compopt}, to manipulate it. @item Bash has command history (@pxref{Bash History Facilities}) and the @code{history} and @code{fc} builtins to manipulate it. The Bash history list maintains timestamp information and uses the value of the @code{HISTTIMEFORMAT} variable to display it. @item Bash implements @code{csh}-like history expansion (@pxref{History Interaction}). @item Bash has one-dimensional array variables (@pxref{Arrays}), and the appropriate variable expansions and assignment syntax to use them. Several of the Bash builtins take options to act on arrays. Bash provides a number of built-in array variables. @item The @code{$'@dots{}'} quoting syntax, which expands ANSI-C backslash-escaped characters in the text between the single quotes, is supported (@pxref{ANSI-C Quoting}). @item Bash supports the @code{$"@dots{}"} quoting syntax to do locale-specific translation of the characters between the double quotes. The @option{-D}, @option{--dump-strings}, and @option{--dump-po-strings} invocation options list the translatable strings found in a script (@pxref{Locale Translation}). @item Bash implements the @code{!} keyword to negate the return value of a pipeline (@pxref{Pipelines}). Very useful when an @code{if} statement needs to act only if a test fails. The Bash @samp{-o pipefail} option to @code{set} will cause a pipeline to return a failure status if any command fails. @item Bash has the @code{time} reserved word and command timing (@pxref{Pipelines}). The display of the timing statistics may be controlled with the @env{TIMEFORMAT} variable. @item Bash implements the @code{for (( @var{expr1} ; @var{expr2} ; @var{expr3} ))} arithmetic for command, similar to the C language (@pxref{Looping Constructs}). @item Bash includes the @code{select} compound command, which allows the generation of simple menus (@pxref{Conditional Constructs}). @item Bash includes the @code{[[} compound command, which makes conditional testing part of the shell grammar (@pxref{Conditional Constructs}), including optional regular expression matching. @item Bash provides optional case-insensitive matching for the @code{case} and @code{[[} constructs. @item Bash includes brace expansion (@pxref{Brace Expansion}) and tilde expansion (@pxref{Tilde Expansion}). @item Bash implements command aliases and the @code{alias} and @code{unalias} builtins (@pxref{Aliases}). @item Bash provides shell arithmetic, the @code{((} compound command (@pxref{Conditional Constructs}), and arithmetic expansion (@pxref{Shell Arithmetic}). @item Variables present in the shell's initial environment are automatically exported to child processes. The Bourne shell does not normally do this unless the variables are explicitly marked using the @code{export} command. @item Bash supports the @samp{+=} assignment operator, which appends to the value of the variable named on the left hand side. @item Bash includes the @sc{posix} pattern removal @samp{%}, @samp{#}, @samp{%%} and @samp{##} expansions to remove leading or trailing substrings from variable values (@pxref{Shell Parameter Expansion}). @item The expansion @code{$@{#xx@}}, which returns the length of @code{$@{xx@}}, is supported (@pxref{Shell Parameter Expansion}). @item The expansion @code{$@{var:}@var{offset}@code{[:}@var{length}@code{]@}}, which expands to the substring of @code{var}'s value of length @var{length}, beginning at @var{offset}, is present (@pxref{Shell Parameter Expansion}). @item The expansion @code{$@{var/[/]}@var{pattern}@code{[/}@var{replacement}@code{]@}}, which matches @var{pattern} and replaces it with @var{replacement} in the value of @code{var}, is available (@pxref{Shell Parameter Expansion}). @item The expansion @code{$@{!@var{prefix@}*}} expansion, which expands to the names of all shell variables whose names begin with @var{prefix}, is available (@pxref{Shell Parameter Expansion}). @item Bash has @var{indirect} variable expansion using @code{$@{!word@}} (@pxref{Shell Parameter Expansion}). @item Bash can expand positional parameters beyond @code{$9} using @code{$@{@var{num}@}}. @item The @sc{posix} @code{$()} form of command substitution is implemented (@pxref{Command Substitution}), and preferred to the Bourne shell's @code{``} (which is also implemented for backwards compatibility). @item Bash has process substitution (@pxref{Process Substitution}). @item Bash automatically assigns variables that provide information about the current user (@env{UID}, @env{EUID}, and @env{GROUPS}), the current host (@env{HOSTTYPE}, @env{OSTYPE}, @env{MACHTYPE}, and @env{HOSTNAME}), and the instance of Bash that is running (@env{BASH}, @env{BASH_VERSION}, and @env{BASH_VERSINFO}). @xref{Bash Variables}, for details. @item The @env{IFS} variable is used to split only the results of expansion, not all words (@pxref{Word Splitting}). This closes a longstanding shell security hole. @item Bash implements the full set of @sc{posix} filename expansion operators, including @var{character classes}, @var{equivalence classes}, and @var{collating symbols} (@pxref{Filename Expansion}). @item Bash implements extended pattern matching features when the @code{extglob} shell option is enabled (@pxref{Pattern Matching}). @item It is possible to have a variable and a function with the same name; @code{sh} does not separate the two name spaces. @item Bash functions are permitted to have local variables using the @code{local} builtin, and thus useful recursive functions may be written (@pxref{Bash Builtins}). @item Variable assignments preceding commands affect only that command, even builtins and functions (@pxref{Environment}). In @code{sh}, all variable assignments preceding commands are global unless the command is executed from the file system. @item Bash performs filename expansion on filenames specified as operands to input and output redirection operators (@pxref{Redirections}). @item Bash contains the @samp{<>} redirection operator, allowing a file to be opened for both reading and writing, and the @samp{&>} redirection operator, for directing standard output and standard error to the same file (@pxref{Redirections}). @item Bash includes the @samp{<<<} redirection operator, allowing a string to be used as the standard input to a command. @item Bash implements the @samp{[n]<&@var{word}} and @samp{[n]>&@var{word}} redirection operators, which move one file descriptor to another. @item Bash treats a number of filenames specially when they are used in redirection operators (@pxref{Redirections}). @item Bash can open network connections to arbitrary machines and services with the redirection operators (@pxref{Redirections}). @item The @code{noclobber} option is available to avoid overwriting existing files with output redirection (@pxref{The Set Builtin}). The @samp{>|} redirection operator may be used to override @code{noclobber}. @item The Bash @code{cd} and @code{pwd} builtins (@pxref{Bourne Shell Builtins}) each take @option{-L} and @option{-P} options to switch between logical and physical modes. @item Bash allows a function to override a builtin with the same name, and provides access to that builtin's functionality within the function via the @code{builtin} and @code{command} builtins (@pxref{Bash Builtins}). @item The @code{command} builtin allows selective disabling of functions when command lookup is performed (@pxref{Bash Builtins}). @item Individual builtins may be enabled or disabled using the @code{enable} builtin (@pxref{Bash Builtins}). @item The Bash @code{exec} builtin takes additional options that allow users to control the contents of the environment passed to the executed command, and what the zeroth argument to the command is to be (@pxref{Bourne Shell Builtins}). @item Shell functions may be exported to children via the environment using @code{export -f} (@pxref{Shell Functions}). @item The Bash @code{export}, @code{readonly}, and @code{declare} builtins can take a @option{-f} option to act on shell functions, a @option{-p} option to display variables with various attributes set in a format that can be used as shell input, a @option{-n} option to remove various variable attributes, and @samp{name=value} arguments to set variable attributes and values simultaneously. @item The Bash @code{hash} builtin allows a name to be associated with an arbitrary filename, even when that filename cannot be found by searching the @env{$PATH}, using @samp{hash -p} (@pxref{Bourne Shell Builtins}). @item Bash includes a @code{help} builtin for quick reference to shell facilities (@pxref{Bash Builtins}). @item The @code{printf} builtin is available to display formatted output (@pxref{Bash Builtins}). @item The Bash @code{read} builtin (@pxref{Bash Builtins}) will read a line ending in @samp{\} with the @option{-r} option, and will use the @env{REPLY} variable as a default if no non-option arguments are supplied. The Bash @code{read} builtin also accepts a prompt string with the @option{-p} option and will use Readline to obtain the line when given the @option{-e} option. The @code{read} builtin also has additional options to control input: the @option{-s} option will turn off echoing of input characters as they are read, the @option{-t} option will allow @code{read} to time out if input does not arrive within a specified number of seconds, the @option{-n} option will allow reading only a specified number of characters rather than a full line, and the @option{-d} option will read until a particular character rather than newline. @item The @code{return} builtin may be used to abort execution of scripts executed with the @code{.} or @code{source} builtins (@pxref{Bourne Shell Builtins}). @item Bash includes the @code{shopt} builtin, for finer control of shell optional capabilities (@pxref{The Shopt Builtin}), and allows these options to be set and unset at shell invocation (@pxref{Invoking Bash}). @item Bash has much more optional behavior controllable with the @code{set} builtin (@pxref{The Set Builtin}). @item The @samp{-x} (@code{xtrace}) option displays commands other than simple commands when performing an execution trace (@pxref{The Set Builtin}). @item The @code{test} builtin (@pxref{Bourne Shell Builtins}) is slightly different, as it implements the @sc{posix} algorithm, which specifies the behavior based on the number of arguments. @item Bash includes the @code{caller} builtin, which displays the context of any active subroutine call (a shell function or a script executed with the @code{.} or @code{source} builtins). This supports the bash debugger. @item The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a @code{DEBUG} pseudo-signal specification, similar to @code{EXIT}. Commands specified with a @code{DEBUG} trap are executed before every simple command, @code{for} command, @code{case} command, @code{select} command, every arithmetic @code{for} command, and before the first command executes in a shell function. The @code{DEBUG} trap is not inherited by shell functions unless the function has been given the @code{trace} attribute or the @code{functrace} option has been enabled using the @code{shopt} builtin. The @code{extdebug} shell option has additional effects on the @code{DEBUG} trap. The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows an @code{ERR} pseudo-signal specification, similar to @code{EXIT} and @code{DEBUG}. Commands specified with an @code{ERR} trap are executed after a simple command fails, with a few exceptions. The @code{ERR} trap is not inherited by shell functions unless the @code{-o errtrace} option to the @code{set} builtin is enabled. The @code{trap} builtin (@pxref{Bourne Shell Builtins}) allows a @code{RETURN} pseudo-signal specification, similar to @code{EXIT} and @code{DEBUG}. Commands specified with an @code{RETURN} trap are executed before execution resumes after a shell function or a shell script executed with @code{.} or @code{source} returns. The @code{RETURN} trap is not inherited by shell functions unless the function has been given the @code{trace} attribute or the @code{functrace} option has been enabled using the @code{shopt} builtin. @item The Bash @code{type} builtin is more extensive and gives more information about the names it finds (@pxref{Bash Builtins}). @item The Bash @code{umask} builtin permits a @option{-p} option to cause the output to be displayed in the form of a @code{umask} command that may be reused as input (@pxref{Bourne Shell Builtins}). @item Bash implements a @code{csh}-like directory stack, and provides the @code{pushd}, @code{popd}, and @code{dirs} builtins to manipulate it (@pxref{The Directory Stack}). Bash also makes the directory stack visible as the value of the @env{DIRSTACK} shell variable. @item Bash interprets special backslash-escaped characters in the prompt strings when interactive (@pxref{Printing a Prompt}). @item The Bash restricted mode is more useful (@pxref{The Restricted Shell}); the SVR4.2 shell restricted mode is too limited. @item The @code{disown} builtin can remove a job from the internal shell job table (@pxref{Job Control Builtins}) or suppress the sending of @code{SIGHUP} to a job when the shell exits as the result of a @code{SIGHUP}. @item Bash includes a number of features to support a separate debugger for shell scripts. @item The SVR4.2 shell has two privilege-related builtins (@code{mldmode} and @code{priv}) not present in Bash. @item Bash does not have the @code{stop} or @code{newgrp} builtins. @item Bash does not use the @env{SHACCT} variable or perform shell accounting. @item The SVR4.2 @code{sh} uses a @env{TIMEOUT} variable like Bash uses @env{TMOUT}. @end itemize @noindent More features unique to Bash may be found in @ref{Bash Features}. @appendixsec Implementation Differences From The SVR4.2 Shell Since Bash is a completely new implementation, it does not suffer from many of the limitations of the SVR4.2 shell. For instance: @itemize @bullet @item Bash does not fork a subshell when redirecting into or out of a shell control structure such as an @code{if} or @code{while} statement. @item Bash does not allow unbalanced quotes. The SVR4.2 shell will silently insert a needed closing quote at @code{EOF} under certain circumstances. This can be the cause of some hard-to-find errors. @item The SVR4.2 shell uses a baroque memory management scheme based on trapping @code{SIGSEGV}. If the shell is started from a process with @code{SIGSEGV} blocked (e.g., by using the @code{system()} C library function call), it misbehaves badly. @item In a questionable attempt at security, the SVR4.2 shell, when invoked without the @option{-p} option, will alter its real and effective @sc{uid} and @sc{gid} if they are less than some magic threshold value, commonly 100. This can lead to unexpected results. @item The SVR4.2 shell does not allow users to trap @code{SIGSEGV}, @code{SIGALRM}, or @code{SIGCHLD}. @item The SVR4.2 shell does not allow the @env{IFS}, @env{MAILCHECK}, @env{PATH}, @env{PS1}, or @env{PS2} variables to be unset. @item The SVR4.2 shell treats @samp{^} as the undocumented equivalent of @samp{|}. @item Bash allows multiple option arguments when it is invoked (@code{-x -v}); the SVR4.2 shell allows only one option argument (@code{-xv}). In fact, some versions of the shell dump core if the second argument begins with a @samp{-}. @item The SVR4.2 shell exits a script if any builtin fails; Bash exits a script only if one of the @sc{posix} special builtins fails, and only for certain failures, as enumerated in the @sc{posix} standard. @item The SVR4.2 shell behaves differently when invoked as @code{jsh} (it turns on job control). @end itemize @node GNU Free Documentation License @appendix GNU Free Documentation License @include fdl.texi @node Indexes @appendix Indexes @menu * Builtin Index:: Index of Bash builtin commands. * Reserved Word Index:: Index of Bash reserved words. * Variable Index:: Quick reference helps you find the variable you want. * Function Index:: Index of bindable Readline functions. * Concept Index:: General index for concepts described in this manual. @end menu @node Builtin Index @appendixsec Index of Shell Builtin Commands @printindex bt @node Reserved Word Index @appendixsec Index of Shell Reserved Words @printindex rw @node Variable Index @appendixsec Parameter and Variable Index @printindex vr @node Function Index @appendixsec Function Index @printindex fn @node Concept Index @appendixsec Concept Index @printindex cp @bye