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-.\" $NetBSD: sh.1,v 1.78 2004/06/03 19:54:37 hubertf Exp $
-.\" Copyright (c) 1991, 1993
-.\" The Regents of the University of California. All rights reserved.
-.\"
-.\" This code is derived from software contributed to Berkeley by
-.\" Kenneth Almquist.
-.\"
-.\" Redistribution and use in source and binary forms, with or without
-.\" modification, are permitted provided that the following conditions
-.\" are met:
-.\" 1. Redistributions of source code must retain the above copyright
-.\" notice, this list of conditions and the following disclaimer.
-.\" 2. Redistributions in binary form must reproduce the above copyright
-.\" notice, this list of conditions and the following disclaimer in the
-.\" documentation and/or other materials provided with the distribution.
-.\" 3. Neither the name of the University nor the names of its contributors
-.\" may be used to endorse or promote products derived from this software
-.\" without specific prior written permission.
-.\"
-.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
-.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
-.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
-.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
-.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
-.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
-.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
-.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
-.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
-.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
-.\" SUCH DAMAGE.
-.\"
-.\" @(#)sh.1 8.6 (Berkeley) 5/4/95
-.\"
-.Dd April 17, 2004
-.Os
-.Dt SH 1
-.Sh NAME
-.Nm sh
-.Nd command interpreter (shell)
-.Sh SYNOPSIS
-.Nm
-.Bk -words
-.Op Fl aCefnuvxIimqVEb
-.Op Cm +aCefnuvxIimqVEb
-.Ek
-.Bk -words
-.Op Fl o Ar option_name
-.Op Cm +o Ar option_name
-.Ek
-.Bk -words
-.Op Ar command_file Oo Ar argument ... Oc
-.Ek
-.Nm
-.Fl c
-.Bk -words
-.Op Fl aCefnuvxIimqVEb
-.Op Cm +aCefnuvxIimqVEb
-.Ek
-.Bk -words
-.Op Fl o Ar option_name
-.Op Cm +o Ar option_name
-.Ek
-.Bk -words
-.Ar command_string
-.Op Ar command_name Oo Ar argument ... Oc
-.Ek
-.Nm
-.Fl s
-.Bk -words
-.Op Fl aCefnuvxIimqVEb
-.Op Cm +aCefnuvxIimqVEb
-.Ek
-.Bk -words
-.Op Fl o Ar option_name
-.Op Cm +o Ar option_name
-.Ek
-.Bk -words
-.Op Ar argument ...
-.Ek
-.Sh DESCRIPTION
-.Nm
-is the standard command interpreter for the system.
-The current version of
-.Nm
-is in the process of being changed to conform with the
-.Tn POSIX
-1003.2 and 1003.2a specifications for the shell.
-This version has many
-features which make it appear similar in some respects to the Korn shell,
-but it is not a Korn shell clone (see
-.Xr ksh 1 ) .
-Only features designated by
-.Tn POSIX ,
-plus a few Berkeley extensions, are being incorporated into this shell.
-.\" We expect
-.\" .Tn POSIX
-.\" conformance by the time 4.4 BSD is released.
-This man page is not intended
-to be a tutorial or a complete specification of the shell.
-.Ss Overview
-The shell is a command that reads lines from either a file or the
-terminal, interprets them, and generally executes other commands.
-It is the program that is running when a user logs into the system
-(although a user can select a different shell with the
-.Xr chsh 1
-command).
-The shell implements a language that has flow control
-constructs, a macro facility that provides a variety of features in
-addition to data storage, along with built in history and line editing
-capabilities.
-It incorporates many features to aid interactive use and
-has the advantage that the interpretative language is common to both
-interactive and non-interactive use (shell scripts).
-That is, commands
-can be typed directly to the running shell or can be put into a file and
-the file can be executed directly by the shell.
-.Ss Invocation
-If no args are present and if the standard input of the shell
-is connected to a terminal (or if the
-.Fl i
-flag is set),
-and the
-.Fl c
-option is not present, the shell is considered an interactive shell.
-An interactive shell generally prompts before each command and handles
-programming and command errors differently (as described below).
-When first starting,
-the shell inspects argument 0, and if it begins with a dash
-.Sq - ,
-the shell is also considered
-a login shell.
-This is normally done automatically by the system
-when the user first logs in.
-A login shell first reads commands
-from the files
-.Pa /etc/profile
-and
-.Pa .profile
-if they exist.
-If the environment variable
-.Ev ENV
-is set on entry to a shell, or is set in the
-.Pa .profile
-of a login shell, the shell next reads
-commands from the file named in
-.Ev ENV .
-Therefore, a user should place commands that are to be executed only at
-login time in the
-.Pa .profile
-file, and commands that are executed for every shell inside the
-.Ev ENV
-file.
-To set the
-.Ev ENV
-variable to some file, place the following line in your
-.Pa .profile
-of your home directory
-.Pp
-.Dl ENV=$HOME/.shinit; export ENV
-.Pp
-substituting for
-.Dq .shinit
-any filename you wish.
-Since the
-.Ev ENV
-file is read for every invocation of the shell, including shell scripts
-and non-interactive shells, the following paradigm is useful for
-restricting commands in the
-.Ev ENV
-file to interactive invocations.
-Place commands within the
-.Dq case
-and
-.Dq esac
-below (these commands are described later):
-.Pp
-.Bl -item -compact -offset indent
-.It
-.Li case $- in *i*)
-.Bl -item -compact -offset indent
-.It
-.Li # commands for interactive use only
-.It
-.Li ...
-.El
-.It
-.Li esac
-.El
-.Pp
-If command line arguments besides the options have been specified, then
-the shell treats the first argument as the name of a file from which to
-read commands (a shell script), and the remaining arguments are set as the
-positional parameters of the shell ($1, $2, etc).
-Otherwise, the shell
-reads commands from its standard input.
-.Ss Argument List Processing
-All of the single letter options have a corresponding name that can be
-used as an argument to the
-.Fl o
-option.
-The set
-.Fl o
-name is provided next to the single letter option in
-the description below.
-Specifying a dash
-.Dq -
-turns the option on, while using a plus
-.Dq +
-disables the option.
-The following options can be set from the command line or
-with the
-.Ic set
-builtin (described later).
-.Bl -tag -width aaaallexportfoo -offset indent
-.It Fl a Em allexport
-Export all variables assigned to.
-.It Fl c
-Read commands from the
-.Ar command_string
-operand instead of from the standard input.
-Special parameter 0 will be set from the
-.Ar command_name
-operand and the positional parameters ($1, $2, etc.)
-set from the remaining argument operands.
-.It Fl C Em noclobber
-Don't overwrite existing files with
-.Dq \*[Gt] .
-.It Fl e Em errexit
-If not interactive, exit immediately if any untested command fails.
-The exit status of a command is considered to be
-explicitly tested if the command is used to control an
-.Ic if ,
-.Ic elif ,
-.Ic while ,
-or
-.Ic until ;
-or if the command is the left hand operand of an
-.Dq \*[Am]\*[Am]
-or
-.Dq ||
-operator.
-.It Fl f Em noglob
-Disable pathname expansion.
-.It Fl n Em noexec
-If not interactive, read commands but do not execute them.
-This is useful for checking the syntax of shell scripts.
-.It Fl u Em nounset
-Write a message to standard error when attempting to expand a variable
-that is not set, and if the shell is not interactive, exit immediately.
-.It Fl v Em verbose
-The shell writes its input to standard error as it is read.
-Useful for debugging.
-.It Fl x Em xtrace
-Write each command to standard error (preceded by a
-.Sq +\ )
-before it is executed.
-Useful for debugging.
-.It Fl q Em quietprofile
-If the
-.Fl v
-or
-.Fl x
-options have been set, do not apply them when reading
-initialization files, these being
-.Pa /etc/profile ,
-.Pa .profile ,
-and the file specified by the
-.Ev ENV
-environment variable.
-.It Fl I Em ignoreeof
-Ignore EOF's from input when interactive.
-.It Fl i Em interactive
-Force the shell to behave interactively.
-.It Fl m Em monitor
-Turn on job control (set automatically when interactive).
-.It Fl s Em stdin
-Read commands from standard input (set automatically if no file arguments
-are present).
-This option has no effect when set after the shell has
-already started running (i.e. with
-.Ic set ) .
-.It Fl V Em vi
-Enable the built-in
-.Xr vi 1
-command line editor (disables
-.Fl E
-if it has been set).
-(See the
-.Sx Command Line Editing
-section below.)
-.It Fl E Em emacs
-Enable the built-in emacs style
-command line editor (disables
-.Fl V
-if it has been set).
-(See the
-.Sx Command Line Editing
-section below.)
-.It Fl b Em notify
-Enable asynchronous notification of background job completion.
-(UNIMPLEMENTED for 4.4alpha)
-.It "\ \ " Em cdprint
-Make an interactive shell always print the new directory name when
-changed by the
-.Ic cd
-command.
-.El
-.Ss Lexical Structure
-The shell reads input in terms of lines from a file and breaks it up into
-words at whitespace (blanks and tabs), and at certain sequences of
-characters that are special to the shell called
-.Dq operators .
-There are two types of operators: control operators and redirection
-operators (their meaning is discussed later).
-Following is a list of operators:
-.Bl -ohang -offset indent
-.It "Control operators:"
-.Dl \*[Am] \*[Am]\*[Am] \&( \&) \&; ;; | || \*[Lt]newline\*[Gt]
-.It "Redirection operators:"
-.Dl \*[Lt] \*[Gt] \*[Gt]| \*[Lt]\*[Lt] \*[Gt]\*[Gt] \*[Lt]\*[Am] \*[Gt]\*[Am] \*[Lt]\*[Lt]- \*[Lt]\*[Gt]
-.El
-.Ss Quoting
-Quoting is used to remove the special meaning of certain characters or
-words to the shell, such as operators, whitespace, or keywords.
-There are three types of quoting: matched single quotes,
-matched double quotes, and backslash.
-.Ss Backslash
-A backslash preserves the literal meaning of the following
-character, with the exception of
-.Aq newline .
-A backslash preceding a
-.Aq newline
-is treated as a line continuation.
-.Ss Single Quotes
-Enclosing characters in single quotes preserves the literal meaning of all
-the characters (except single quotes, making it impossible to put
-single-quotes in a single-quoted string).
-.Ss Double Quotes
-Enclosing characters within double quotes preserves the literal
-meaning of all characters except dollarsign
-.Pq $ ,
-backquote
-.Pq ` ,
-and backslash
-.Pq \e .
-The backslash inside double quotes is historically weird, and serves to
-quote only the following characters:
-.Dl $ ` \*q \e \*[Lt]newline\*[Gt] .
-Otherwise it remains literal.
-.Ss Reserved Words
-Reserved words are words that have special meaning to the
-shell and are recognized at the beginning of a line and
-after a control operator.
-The following are reserved words:
-.Bl -column while while while while while -offset indent
-.It ! Ta elif Ta fi Ta while Ta case
-.It else Ta for Ta then Ta { Ta }
-.It do Ta done Ta until Ta if Ta esac
-.El
-.Pp
-Their meaning is discussed later.
-.Ss Aliases
-An alias is a name and corresponding value set using the
-.Ic alias
-builtin command.
-Whenever a reserved word may occur (see above),
-and after checking for reserved words, the shell
-checks the word to see if it matches an alias.
-If it does, it replaces it in the input stream with its value.
-For example, if there is an alias called
-.Dq lf
-with the value
-.Dq "ls -F" ,
-then the input:
-.Pp
-.Dl lf foobar Aq return
-.Pp
-would become
-.Pp
-.Dl ls -F foobar Aq return
-.Pp
-Aliases provide a convenient way for naive users to create shorthands for
-commands without having to learn how to create functions with arguments.
-They can also be used to create lexically obscure code.
-This use is discouraged.
-.Ss Commands
-The shell interprets the words it reads according to a language, the
-specification of which is outside the scope of this man page (refer to the
-BNF in the
-.Tn POSIX
-1003.2 document).
-Essentially though, a line is read and if the first
-word of the line (or after a control operator) is not a reserved word,
-then the shell has recognized a simple command.
-Otherwise, a complex
-command or some other special construct may have been recognized.
-.Ss Simple Commands
-If a simple command has been recognized, the shell performs
-the following actions:
-.Bl -enum -offset indent
-.It
-Leading words of the form
-.Dq name=value
-are stripped off and assigned to the environment of the simple command.
-Redirection operators and their arguments (as described below) are
-stripped off and saved for processing.
-.It
-The remaining words are expanded as described in
-the section called
-.Dq Expansions ,
-and the first remaining word is considered the command name and the
-command is located.
-The remaining words are considered the arguments of the command.
-If no command name resulted, then the
-.Dq name=value
-variable assignments recognized in item 1 affect the current shell.
-.It
-Redirections are performed as described in the next section.
-.El
-.Ss Redirections
-Redirections are used to change where a command reads its input or sends
-its output.
-In general, redirections open, close, or duplicate an
-existing reference to a file.
-The overall format used for redirection is:
-.Pp
-.Dl [n] Va redir-op Ar file
-.Pp
-where
-.Va redir-op
-is one of the redirection operators mentioned previously.
-Following is a list of the possible redirections.
-The
-.Bq n
-is an optional number, as in
-.Sq 3
-(not
-.Sq Bq 3 ) ,
-that refers to a file descriptor.
-.Bl -tag -width aaabsfiles -offset indent
-.It [n] Ns \*[Gt] file
-Redirect standard output (or n) to file.
-.It [n] Ns \*[Gt]| file
-Same, but override the
-.Fl C
-option.
-.It [n] Ns \*[Gt]\*[Gt] file
-Append standard output (or n) to file.
-.It [n] Ns \*[Lt] file
-Redirect standard input (or n) from file.
-.It [n1] Ns \*[Lt]\*[Am] Ns n2
-Duplicate standard input (or n1) from file descriptor n2.
-.It [n] Ns \*[Lt]\*[Am]-
-Close standard input (or n).
-.It [n1] Ns \*[Gt]\*[Am] Ns n2
-Duplicate standard output (or n1) to n2.
-.It [n] Ns \*[Gt]\*[Am]-
-Close standard output (or n).
-.It [n] Ns \*[Lt]\*[Gt] file
-Open file for reading and writing on standard input (or n).
-.El
-.Pp
-The following redirection is often called a
-.Dq here-document .
-.Bl -item -offset indent
-.It
-.Li [n]\*[Lt]\*[Lt] delimiter
-.Dl here-doc-text ...
-.Li delimiter
-.El
-.Pp
-All the text on successive lines up to the delimiter is saved away and
-made available to the command on standard input, or file descriptor n if
-it is specified.
-If the delimiter as specified on the initial line is
-quoted, then the here-doc-text is treated literally, otherwise the text is
-subjected to parameter expansion, command substitution, and arithmetic
-expansion (as described in the section on
-.Dq Expansions ) .
-If the operator is
-.Dq \*[Lt]\*[Lt]-
-instead of
-.Dq \*[Lt]\*[Lt] ,
-then leading tabs in the here-doc-text are stripped.
-.Ss Search and Execution
-There are three types of commands: shell functions, builtin commands, and
-normal programs -- and the command is searched for (by name) in that order.
-They each are executed in a different way.
-.Pp
-When a shell function is executed, all of the shell positional parameters
-(except $0, which remains unchanged) are set to the arguments of the shell
-function.
-The variables which are explicitly placed in the environment of
-the command (by placing assignments to them before the function name) are
-made local to the function and are set to the values given.
-Then the command given in the function definition is executed.
-The positional parameters are restored to their original values
-when the command completes.
-This all occurs within the current shell.
-.Pp
-Shell builtins are executed internally to the shell, without spawning a
-new process.
-.Pp
-Otherwise, if the command name doesn't match a function or builtin, the
-command is searched for as a normal program in the file system (as
-described in the next section).
-When a normal program is executed, the shell runs the program,
-passing the arguments and the environment to the program.
-If the program is not a normal executable file (i.e., if it does
-not begin with the "magic number" whose
-.Tn ASCII
-representation is "#!", so
-.Xr execve 2
-returns
-.Er ENOEXEC
-then) the shell will interpret the program in a subshell.
-The child shell will reinitialize itself in this case,
-so that the effect will be as if a
-new shell had been invoked to handle the ad-hoc shell script, except that
-the location of hashed commands located in the parent shell will be
-remembered by the child.
-.Pp
-Note that previous versions of this document and the source code itself
-misleadingly and sporadically refer to a shell script without a magic
-number as a "shell procedure".
-.Ss Path Search
-When locating a command, the shell first looks to see if it has a shell
-function by that name.
-Then it looks for a builtin command by that name.
-If a builtin command is not found, one of two things happen:
-.Bl -enum
-.It
-Command names containing a slash are simply executed without performing
-any searches.
-.It
-The shell searches each entry in
-.Ev PATH
-in turn for the command.
-The value of the
-.Ev PATH
-variable should be a series of entries separated by colons.
-Each entry consists of a directory name.
-The current directory may be indicated
-implicitly by an empty directory name, or explicitly by a single period.
-.El
-.Ss Command Exit Status
-Each command has an exit status that can influence the behavior
-of other shell commands.
-The paradigm is that a command exits
-with zero for normal or success, and non-zero for failure,
-error, or a false indication.
-The man page for each command
-should indicate the various exit codes and what they mean.
-Additionally, the builtin commands return exit codes, as does
-an executed shell function.
-.Pp
-If a command consists entirely of variable assignments then the
-exit status of the command is that of the last command substitution
-if any, otherwise 0.
-.Ss Complex Commands
-Complex commands are combinations of simple commands with control
-operators or reserved words, together creating a larger complex command.
-More generally, a command is one of the following:
-.Bl -bullet
-.It
-simple command
-.It
-pipeline
-.It
-list or compound-list
-.It
-compound command
-.It
-function definition
-.El
-.Pp
-Unless otherwise stated, the exit status of a command is that of the last
-simple command executed by the command.
-.Ss Pipelines
-A pipeline is a sequence of one or more commands separated
-by the control operator |.
-The standard output of all but
-the last command is connected to the standard input
-of the next command.
-The standard output of the last
-command is inherited from the shell, as usual.
-.Pp
-The format for a pipeline is:
-.Pp
-.Dl [!] command1 [ | command2 ...]
-.Pp
-The standard output of command1 is connected to the standard input of
-command2.
-The standard input, standard output, or both of a command is
-considered to be assigned by the pipeline before any redirection specified
-by redirection operators that are part of the command.
-.Pp
-If the pipeline is not in the background (discussed later), the shell
-waits for all commands to complete.
-.Pp
-If the reserved word ! does not precede the pipeline, the exit status is
-the exit status of the last command specified in the pipeline.
-Otherwise, the exit status is the logical NOT of the exit status of the
-last command.
-That is, if the last command returns zero, the exit status
-is 1; if the last command returns greater than zero, the exit status is
-zero.
-.Pp
-Because pipeline assignment of standard input or standard output or both
-takes place before redirection, it can be modified by redirection.
-For example:
-.Pp
-.Dl $ command1 2\*[Gt]\*[Am]1 | command2
-.Pp
-sends both the standard output and standard error of command1
-to the standard input of command2.
-.Pp
-A ; or
-.Aq newline
-terminator causes the preceding AND-OR-list (described
-next) to be executed sequentially; a \*[Am] causes asynchronous execution of
-the preceding AND-OR-list.
-.Pp
-Note that unlike some other shells, each process in the pipeline is a
-child of the invoking shell (unless it is a shell builtin, in which case
-it executes in the current shell -- but any effect it has on the
-environment is wiped).
-.Ss Background Commands -- \*[Am]
-If a command is terminated by the control operator ampersand (\*[Am]), the
-shell executes the command asynchronously -- that is, the shell does not
-wait for the command to finish before executing the next command.
-.Pp
-The format for running a command in background is:
-.Pp
-.Dl command1 \*[Am] [command2 \*[Am] ...]
-.Pp
-If the shell is not interactive, the standard input of an asynchronous
-command is set to
-.Pa /dev/null .
-.Ss Lists -- Generally Speaking
-A list is a sequence of zero or more commands separated by newlines,
-semicolons, or ampersands, and optionally terminated by one of these three
-characters.
-The commands in a list are executed in the order they are written.
-If command is followed by an ampersand, the shell starts the
-command and immediately proceed onto the next command; otherwise it waits
-for the command to terminate before proceeding to the next one.
-.Ss Short-Circuit List Operators
-.Dq \*[Am]\*[Am]
-and
-.Dq ||
-are AND-OR list operators.
-.Dq \*[Am]\*[Am]
-executes the first command, and then executes the second command if and only
-if the exit status of the first command is zero.
-.Dq ||
-is similar, but executes the second command if and only if the exit status
-of the first command is nonzero.
-.Dq \*[Am]\*[Am]
-and
-.Dq ||
-both have the same priority.
-Note that these operators are left-associative, so
-.Dq true || echo bar && echo baz
-writes
-.Dq baz
-and nothing else.
-This is not the way it works in C.
-.Ss Flow-Control Constructs -- if, while, for, case
-The syntax of the if command is
-.Bd -literal -offset indent
-if list
-then list
-[ elif list
-then list ] ...
-[ else list ]
-fi
-.Ed
-.Pp
-The syntax of the while command is
-.Bd -literal -offset indent
-while list
-do list
-done
-.Ed
-.Pp
-The two lists are executed repeatedly while the exit status of the
-first list is zero.
-The until command is similar, but has the word
-until in place of while, which causes it to
-repeat until the exit status of the first list is zero.
-.Pp
-The syntax of the for command is
-.Bd -literal -offset indent
-for variable in word ...
-do list
-done
-.Ed
-.Pp
-The words are expanded, and then the list is executed repeatedly with the
-variable set to each word in turn.
-do and done may be replaced with
-.Dq {
-and
-.Dq } .
-.Pp
-The syntax of the break and continue command is
-.Bd -literal -offset indent
-break [ num ]
-continue [ num ]
-.Ed
-.Pp
-Break terminates the num innermost for or while loops.
-Continue continues with the next iteration of the innermost loop.
-These are implemented as builtin commands.
-.Pp
-The syntax of the case command is
-.Bd -literal -offset indent
-case word in
-pattern) list ;;
-\&...
-esac
-.Ed
-.Pp
-The pattern can actually be one or more patterns (see
-.Sx Shell Patterns
-described later), separated by
-.Dq \*(Ba
-characters.
-.Ss Grouping Commands Together
-Commands may be grouped by writing either
-.Pp
-.Dl (list)
-.Pp
-or
-.Pp
-.Dl { list; }
-.Pp
-The first of these executes the commands in a subshell.
-Builtin commands grouped into a (list) will not affect the current shell.
-The second form does not fork another shell so is slightly more efficient.
-Grouping commands together this way allows you to redirect
-their output as though they were one program:
-.Pp
-.Bd -literal -offset indent
-{ echo -n \*q hello \*q ; echo \*q world" ; } \*[Gt] greeting
-.Ed
-.Pp
-Note that
-.Dq }
-must follow a control operator (here,
-.Dq \&; )
-so that it is recognized as a reserved word and not as another command argument.
-.Ss Functions
-The syntax of a function definition is
-.Pp
-.Dl name ( ) command
-.Pp
-A function definition is an executable statement; when executed it
-installs a function named name and returns an exit status of zero.
-The command is normally a list enclosed between
-.Dq {
-and
-.Dq } .
-.Pp
-Variables may be declared to be local to a function by using a local
-command.
-This should appear as the first statement of a function, and the syntax is
-.Pp
-.Dl local [ variable | - ] ...
-.Pp
-Local is implemented as a builtin command.
-.Pp
-When a variable is made local, it inherits the initial value and exported
-and readonly flags from the variable with the same name in the surrounding
-scope, if there is one.
-Otherwise, the variable is initially unset.
-The shell uses dynamic scoping, so that if you make the variable x local to
-function f, which then calls function g, references to the variable x made
-inside g will refer to the variable x declared inside f, not to the global
-variable named x.
-.Pp
-The only special parameter that can be made local is
-.Dq - .
-Making
-.Dq -
-local any shell options that are changed via the set command inside the
-function to be restored to their original values when the function
-returns.
-.Pp
-The syntax of the return command is
-.Pp
-.Dl return [ exitstatus ]
-.Pp
-It terminates the currently executing function.
-Return is implemented as a builtin command.
-.Ss Variables and Parameters
-The shell maintains a set of parameters.
-A parameter denoted by a name is called a variable.
-When starting up, the shell turns all the environment
-variables into shell variables.
-New variables can be set using the form
-.Pp
-.Dl name=value
-.Pp
-Variables set by the user must have a name consisting solely of
-alphabetics, numerics, and underscores - the first of which must not be
-numeric.
-A parameter can also be denoted by a number or a special
-character as explained below.
-.Ss Positional Parameters
-A positional parameter is a parameter denoted by a number (n \*[Gt] 0).
-The shell sets these initially to the values of its command line arguments
-that follow the name of the shell script.
-The
-.Ic set
-builtin can also be used to set or reset them.
-.Ss Special Parameters
-A special parameter is a parameter denoted by one of the following special
-characters.
-The value of the parameter is listed next to its character.
-.Bl -tag -width thinhyphena
-.It *
-Expands to the positional parameters, starting from one.
-When the
-expansion occurs within a double-quoted string it expands to a single
-field with the value of each parameter separated by the first character of
-the
-.Ev IFS
-variable, or by a
-.Aq space
-if
-.Ev IFS
-is unset.
-.It @
-Expands to the positional parameters, starting from one.
-When the expansion occurs within double-quotes, each positional
-parameter expands as a separate argument.
-If there are no positional parameters, the
-expansion of @ generates zero arguments, even when @ is
-double-quoted.
-What this basically means, for example, is
-if $1 is
-.Dq abc
-and $2 is
-.Dq def ghi ,
-then
-.Qq $@
-expands to
-the two arguments:
-.Pp
-.Sm off
-.Dl \*q abc \*q \ \*q def\ ghi \*q
-.Sm on
-.It #
-Expands to the number of positional parameters.
-.It \&?
-Expands to the exit status of the most recent pipeline.
-.It - (Hyphen.)
-Expands to the current option flags (the single-letter
-option names concatenated into a string) as specified on
-invocation, by the set builtin command, or implicitly
-by the shell.
-.It $
-Expands to the process ID of the invoked shell.
-A subshell retains the same value of $ as its parent.
-.It \&!
-Expands to the process ID of the most recent background
-command executed from the current shell.
-For a pipeline, the process ID is that of the last command in the pipeline.
-.It 0 (Zero.)
-Expands to the name of the shell or shell script.
-.El
-.Ss Word Expansions
-This clause describes the various expansions that are performed on words.
-Not all expansions are performed on every word, as explained later.
-.Pp
-Tilde expansions, parameter expansions, command substitutions, arithmetic
-expansions, and quote removals that occur within a single word expand to a
-single field.
-It is only field splitting or pathname expansion that can
-create multiple fields from a single word.
-The single exception to this
-rule is the expansion of the special parameter @ within double-quotes, as
-was described above.
-.Pp
-The order of word expansion is:
-.Bl -enum
-.It
-Tilde Expansion, Parameter Expansion, Command Substitution,
-Arithmetic Expansion (these all occur at the same time).
-.It
-Field Splitting is performed on fields
-generated by step (1) unless the
-.Ev IFS
-variable is null.
-.It
-Pathname Expansion (unless set
-.Fl f
-is in effect).
-.It
-Quote Removal.
-.El
-.Pp
-The $ character is used to introduce parameter expansion, command
-substitution, or arithmetic evaluation.
-.Ss Tilde Expansion (substituting a user's home directory)
-A word beginning with an unquoted tilde character (~) is
-subjected to tilde expansion.
-All the characters up to
-a slash (/) or the end of the word are treated as a username
-and are replaced with the user's home directory.
-If the username is missing (as in
-.Pa ~/foobar ) ,
-the tilde is replaced with the value of the
-.Va HOME
-variable (the current user's home directory).
-.Ss Parameter Expansion
-The format for parameter expansion is as follows:
-.Pp
-.Dl ${expression}
-.Pp
-where expression consists of all characters until the matching
-.Dq } .
-Any
-.Dq }
-escaped by a backslash or within a quoted string, and characters in
-embedded arithmetic expansions, command substitutions, and variable
-expansions, are not examined in determining the matching
-.Dq } .
-.Pp
-The simplest form for parameter expansion is:
-.Pp
-.Dl ${parameter}
-.Pp
-The value, if any, of parameter is substituted.
-.Pp
-The parameter name or symbol can be enclosed in braces, which are
-optional except for positional parameters with more than one digit or
-when parameter is followed by a character that could be interpreted as
-part of the name.
-If a parameter expansion occurs inside double-quotes:
-.Bl -enum
-.It
-Pathname expansion is not performed on the results of the expansion.
-.It
-Field splitting is not performed on the results of the
-expansion, with the exception of @.
-.El
-.Pp
-In addition, a parameter expansion can be modified by using one of the
-following formats.
-.Bl -tag -width aaparameterwordaaaaa
-.It ${parameter:-word}
-Use Default Values.
-If parameter is unset or null, the expansion of word
-is substituted; otherwise, the value of parameter is substituted.
-.It ${parameter:=word}
-Assign Default Values.
-If parameter is unset or null, the expansion of
-word is assigned to parameter.
-In all cases, the final value of parameter is substituted.
-Only variables, not positional parameters or special
-parameters, can be assigned in this way.
-.It ${parameter:?[word]}
-Indicate Error if Null or Unset.
-If parameter is unset or null, the
-expansion of word (or a message indicating it is unset if word is omitted)
-is written to standard error and the shell exits with a nonzero exit status.
-Otherwise, the value of parameter is substituted.
-An interactive shell need not exit.
-.It ${parameter:+word}
-Use Alternative Value.
-If parameter is unset or null, null is
-substituted; otherwise, the expansion of word is substituted.
-.El
-.Pp
-In the parameter expansions shown previously, use of the colon in the
-format results in a test for a parameter that is unset or null; omission
-of the colon results in a test for a parameter that is only unset.
-.Bl -tag -width aaparameterwordaaaaa
-.It ${#parameter}
-String Length.
-The length in characters of the value of parameter.
-.El
-.Pp
-The following four varieties of parameter expansion provide for substring
-processing.
-In each case, pattern matching notation (see
-.Sx Shell Patterns ) ,
-rather than regular expression notation, is used to evaluate the patterns.
-If parameter is * or @, the result of the expansion is unspecified.
-Enclosing the full parameter expansion string in double-quotes does not
-cause the following four varieties of pattern characters to be quoted,
-whereas quoting characters within the braces has this effect.
-.Bl -tag -width aaparameterwordaaaaa
-.It ${parameter%word}
-Remove Smallest Suffix Pattern.
-The word is expanded to produce a pattern.
-The parameter expansion then results in parameter, with the
-smallest portion of the suffix matched by the pattern deleted.
-.It ${parameter%%word}
-Remove Largest Suffix Pattern.
-The word is expanded to produce a pattern.
-The parameter expansion then results in parameter, with the largest
-portion of the suffix matched by the pattern deleted.
-.It ${parameter#word}
-Remove Smallest Prefix Pattern.
-The word is expanded to produce a pattern.
-The parameter expansion then results in parameter, with the
-smallest portion of the prefix matched by the pattern deleted.
-.It ${parameter##word}
-Remove Largest Prefix Pattern.
-The word is expanded to produce a pattern.
-The parameter expansion then results in parameter, with the largest
-portion of the prefix matched by the pattern deleted.
-.El
-.Ss Command Substitution
-Command substitution allows the output of a command to be substituted in
-place of the command name itself.
-Command substitution occurs when the command is enclosed as follows:
-.Pp
-.Dl $(command)
-.Pp
-or
-.Po
-.Dq backquoted
-version
-.Pc :
-.Pp
-.Dl `command`
-.Pp
-The shell expands the command substitution by executing command in a
-subshell environment and replacing the command substitution with the
-standard output of the command, removing sequences of one or more
-.Ao newline Ac Ns s
-at the end of the substitution.
-(Embedded
-.Ao newline Ac Ns s
-before
-the end of the output are not removed; however, during field splitting,
-they may be translated into
-.Ao space Ac Ns s ,
-depending on the value of
-.Ev IFS
-and quoting that is in effect.)
-.Ss Arithmetic Expansion
-Arithmetic expansion provides a mechanism for evaluating an arithmetic
-expression and substituting its value.
-The format for arithmetic expansion is as follows:
-.Pp
-.Dl $((expression))
-.Pp
-The expression is treated as if it were in double-quotes, except
-that a double-quote inside the expression is not treated specially.
-The shell expands all tokens in the expression for parameter expansion,
-command substitution, and quote removal.
-.Pp
-Next, the shell treats this as an arithmetic expression and
-substitutes the value of the expression.
-.Ss White Space Splitting (Field Splitting)
-After parameter expansion, command substitution, and
-arithmetic expansion the shell scans the results of
-expansions and substitutions that did not occur in double-quotes for
-field splitting and multiple fields can result.
-.Pp
-The shell treats each character of the
-.Ev IFS
-as a delimiter and use the delimiters to split the results of parameter
-expansion and command substitution into fields.
-.Ss Pathname Expansion (File Name Generation)
-Unless the
-.Fl f
-flag is set, file name generation is performed after word splitting is
-complete.
-Each word is viewed as a series of patterns, separated by slashes.
-The process of expansion replaces the word with the names of all
-existing files whose names can be formed by replacing each pattern with a
-string that matches the specified pattern.
-There are two restrictions on
-this: first, a pattern cannot match a string containing a slash, and
-second, a pattern cannot match a string starting with a period unless the
-first character of the pattern is a period.
-The next section describes the
-patterns used for both Pathname Expansion and the
-.Ic case
-command.
-.Ss Shell Patterns
-A pattern consists of normal characters, which match themselves,
-and meta-characters.
-The meta-characters are
-.Dq \&! ,
-.Dq * ,
-.Dq \&? ,
-and
-.Dq \&[ .
-These characters lose their special meanings if they are quoted.
-When command or variable substitution is performed
-and the dollar sign or back quotes are not double quoted,
-the value of the variable or the output of
-the command is scanned for these characters and they are turned into
-meta-characters.
-.Pp
-An asterisk
-.Pq Dq *
-matches any string of characters.
-A question mark matches any single character.
-A left bracket
-.Pq Dq \&[
-introduces a character class.
-The end of the character class is indicated by a
-.Pq Dq \&] ;
-if the
-.Dq \&]
-is missing then the
-.Dq \&[
-matches a
-.Dq \&[
-rather than introducing a character class.
-A character class matches any of the characters between the square brackets.
-A range of characters may be specified using a minus sign.
-The character class may be complemented
-by making an exclamation point the first character of the character class.
-.Pp
-To include a
-.Dq \&]
-in a character class, make it the first character listed (after the
-.Dq \&! ,
-if any).
-To include a minus sign, make it the first or last character listed.
-.Ss Builtins
-This section lists the builtin commands which are builtin because they
-need to perform some operation that can't be performed by a separate
-process.
-In addition to these, there are several other commands that may
-be builtin for efficiency (e.g.
-.Xr printf 1 ,
-.Xr echo 1 ,
-.Xr test 1 ,
-etc).
-.Bl -tag -width 5n
-.It :
-A null command that returns a 0 (true) exit value.
-.It \&. file
-The commands in the specified file are read and executed by the shell.
-.It alias Op Ar name Ns Op Ar "=string ..."
-If
-.Ar name=string
-is specified, the shell defines the alias
-.Ar name
-with value
-.Ar string .
-If just
-.Ar name
-is specified, the value of the alias
-.Ar name
-is printed.
-With no arguments, the
-.Ic alias
-builtin prints the
-names and values of all defined aliases (see
-.Ic unalias ) .
-.It bg [ Ar job ] ...
-Continue the specified jobs (or the current job if no
-jobs are given) in the background.
-.It Xo command
-.Op Fl p
-.Op Fl v
-.Op Fl V
-.Ar command
-.Op Ar arg ...
-.Xc
-Execute the specified command but ignore shell functions when searching
-for it.
-(This is useful when you
-have a shell function with the same name as a builtin command.)
-.Bl -tag -width 5n
-.It Fl p
-search for command using a
-.Ev PATH
-that guarantees to find all the standard utilities.
-.It Fl V
-Do not execute the command but
-search for the command and print the resolution of the
-command search.
-This is the same as the type builtin.
-.It Fl v
-Do not execute the command but
-search for the command and print the absolute pathname
-of utilities, the name for builtins or the expansion of aliases.
-.El
-.It cd Op Ar directory Op Ar replace
-Switch to the specified directory (default
-.Ev $HOME ) .
-If
-.Ar replace
-is specified, then the new directory name is generated by replacing
-the first occurrence of
-.Ar directory
-in the current directory name with
-.Ar replace .
-Otherwise if an entry for
-.Ev CDPATH
-appears in the environment of the
-.Ic cd
-command or the shell variable
-.Ev CDPATH
-is set and the directory name does not begin with a slash, then the
-directories listed in
-.Ev CDPATH
-will be searched for the specified directory.
-The format of
-.Ev CDPATH
-is the same as that of
-.Ev PATH .
-In an interactive shell, the
-.Ic cd
-command will print out the name of the
-directory that it actually switched to if this is different from the name
-that the user gave.
-These may be different either because the
-.Ev CDPATH
-mechanism was used or because a symbolic link was crossed.
-.It eval Ar string ...
-Concatenate all the arguments with spaces.
-Then re-parse and execute the command.
-.It exec Op Ar command arg ...
-Unless command is omitted, the shell process is replaced with the
-specified program (which must be a real program, not a shell builtin or
-function).
-Any redirections on the
-.Ic exec
-command are marked as permanent, so that they are not undone when the
-.Ic exec
-command finishes.
-.It exit Op Ar exitstatus
-Terminate the shell process.
-If
-.Ar exitstatus
-is given it is used as the exit status of the shell; otherwise the
-exit status of the preceding command is used.
-.It export Ar name ...
-.It export Fl p
-The specified names are exported so that they will appear in the
-environment of subsequent commands.
-The only way to un-export a variable is to unset it.
-The shell allows the value of a variable to be set at the
-same time it is exported by writing
-.Pp
-.Dl export name=value
-.Pp
-With no arguments the export command lists the names of all exported variables.
-With the
-.Fl p
-option specified the output will be formatted suitably for non-interactive use.
-.It Xo fc Op Fl e Ar editor
-.Op Ar first Op Ar last
-.Xc
-.It Xo fc Fl l
-.Op Fl nr
-.Op Ar first Op Ar last
-.Xc
-.It Xo fc Fl s Op Ar old=new
-.Op Ar first
-.Xc
-The
-.Ic fc
-builtin lists, or edits and re-executes, commands previously entered
-to an interactive shell.
-.Bl -tag -width 5n
-.It Fl e No editor
-Use the editor named by editor to edit the commands.
-The editor string is a command name, subject to search via the
-.Ev PATH
-variable.
-The value in the
-.Ev FCEDIT
-variable is used as a default when
-.Fl e
-is not specified.
-If
-.Ev FCEDIT
-is null or unset, the value of the
-.Ev EDITOR
-variable is used.
-If
-.Ev EDITOR
-is null or unset,
-.Xr ed 1
-is used as the editor.
-.It Fl l No (ell)
-List the commands rather than invoking an editor on them.
-The commands are written in the sequence indicated by
-the first and last operands, as affected by
-.Fl r ,
-with each command preceded by the command number.
-.It Fl n
-Suppress command numbers when listing with -l.
-.It Fl r
-Reverse the order of the commands listed (with
-.Fl l )
-or edited (with neither
-.Fl l
-nor
-.Fl s ) .
-.It Fl s
-Re-execute the command without invoking an editor.
-.It first
-.It last
-Select the commands to list or edit.
-The number of previous commands that
-can be accessed are determined by the value of the
-.Ev HISTSIZE
-variable.
-The value of first or last or both are one of the following:
-.Bl -tag -width 5n
-.It [+]number
-A positive number representing a command number; command numbers can be
-displayed with the
-.Fl l
-option.
-.It Fl number
-A negative decimal number representing the command that was executed
-number of commands previously.
-For example, \-1 is the immediately previous command.
-.El
-.It string
-A string indicating the most recently entered command that begins with
-that string.
-If the old=new operand is not also specified with
-.Fl s ,
-the string form of the first operand cannot contain an embedded equal sign.
-.El
-.Pp
-The following environment variables affect the execution of fc:
-.Bl -tag -width HISTSIZE
-.It Ev FCEDIT
-Name of the editor to use.
-.It Ev HISTSIZE
-The number of previous commands that are accessible.
-.El
-.It fg Op Ar job
-Move the specified job or the current job to the foreground.
-.It getopts Ar optstring var
-The
-.Tn POSIX
-.Ic getopts
-command, not to be confused with the
-.Em Bell Labs
--derived
-.Xr getopt 1 .
-.Pp
-The first argument should be a series of letters, each of which may be
-optionally followed by a colon to indicate that the option requires an
-argument.
-The variable specified is set to the parsed option.
-.Pp
-The
-.Ic getopts
-command deprecates the older
-.Xr getopt 1
-utility due to its handling of arguments containing whitespace.
-.Pp
-The
-.Ic getopts
-builtin may be used to obtain options and their arguments
-from a list of parameters.
-When invoked,
-.Ic getopts
-places the value of the next option from the option string in the list in
-the shell variable specified by
-.Va var
-and its index in the shell variable
-.Ev OPTIND .
-When the shell is invoked,
-.Ev OPTIND
-is initialized to 1.
-For each option that requires an argument, the
-.Ic getopts
-builtin will place it in the shell variable
-.Ev OPTARG .
-If an option is not allowed for in the
-.Va optstring ,
-then
-.Ev OPTARG
-will be unset.
-.Pp
-.Va optstring
-is a string of recognized option letters (see
-.Xr getopt 3 ) .
-If a letter is followed by a colon, the option is expected to have an
-argument which may or may not be separated from it by white space.
-If an option character is not found where expected,
-.Ic getopts
-will set the variable
-.Va var
-to a
-.Dq \&? ;
-.Ic getopts
-will then unset
-.Ev OPTARG
-and write output to standard error.
-By specifying a colon as the first character of
-.Va optstring
-all errors will be ignored.
-.Pp
-A nonzero value is returned when the last option is reached.
-If there are no remaining arguments,
-.Ic getopts
-will set
-.Va var
-to the special option,
-.Dq -- ,
-otherwise, it will set
-.Va var
-to
-.Dq \&? .
-.Pp
-The following code fragment shows how one might process the arguments
-for a command that can take the options
-.Op a
-and
-.Op b ,
-and the option
-.Op c ,
-which requires an argument.
-.Pp
-.Bd -literal -offset indent
-while getopts abc: f
-do
- case $f in
- a | b) flag=$f;;
- c) carg=$OPTARG;;
- \\?) echo $USAGE; exit 1;;
- esac
-done
-shift `expr $OPTIND - 1`
-.Ed
-.Pp
-This code will accept any of the following as equivalent:
-.Pp
-.Bd -literal -offset indent
-cmd \-acarg file file
-cmd \-a \-c arg file file
-cmd \-carg -a file file
-cmd \-a \-carg \-\- file file
-.Ed
-.It hash Fl rv Ar command ...
-The shell maintains a hash table which remembers the
-locations of commands.
-With no arguments whatsoever,
-the
-.Ic hash
-command prints out the contents of this table.
-Entries which have not been looked at since the last
-.Ic cd
-command are marked with an asterisk; it is possible for these entries
-to be invalid.
-.Pp
-With arguments, the
-.Ic hash
-command removes the specified commands from the hash table (unless
-they are functions) and then locates them.
-With the
-.Fl v
-option, hash prints the locations of the commands as it finds them.
-The
-.Fl r
-option causes the hash command to delete all the entries in the hash table
-except for functions.
-.It inputrc Ar file
-Read the
-.Va file
-to set keybindings as defined by
-.Xr editrc 5 .
-.It jobid Op Ar job
-Print the process id's of the processes in the job.
-If the
-.Ar job
-argument is omitted, the current job is used.
-.It jobs
-This command lists out all the background processes
-which are children of the current shell process.
-.It pwd Op Fl LP
-Print the current directory.
-If
-.Fl L
-is specified the cached value (initially set from
-.Ev PWD )
-is checked to see if it refers to the current directory, if it does
-the value is printed.
-Otherwise the current directory name is found using
-.Xr getcwd(3) .
-The environment variable
-.Ev PWD
-is set to printed value.
-.Pp
-The default is
-.Ic pwd
-.Fl L ,
-but note that the builtin
-.Ic cd
-command doesn't currently support
-.Fl L
-or
-.Fl P
-and will cache (almost) the absolute path.
-If
-.Ic cd
-is changed,
-.Ic pwd
-may be changed to default to
-.Ic pwd
-.Fl P .
-.Pp
-If the current directory is renamed and replaced by a symlink to the
-same directory, or the initial
-.Ev PWD
-value followed a symbolic link, then the cached value may not
-be the absolute path.
-.Pp
-The builtin command may differ from the program of the same name because
-the program will use
-.Ev PWD
-and the builtin uses a separately cached value.
-.It Xo read Op Fl p Ar prompt
-.Op Fl r
-.Ar variable
-.Op Ar ...
-.Xc
-The prompt is printed if the
-.Fl p
-option is specified and the standard input is a terminal.
-Then a line is read from the standard input.
-The trailing newline is deleted from the
-line and the line is split as described in the section on word splitting
-above, and the pieces are assigned to the variables in order.
-If there are more pieces than variables, the remaining pieces
-(along with the characters in
-.Ev IFS
-that separated them) are assigned to the last variable.
-If there are more variables than pieces,
-the remaining variables are assigned the null string.
-The
-.Ic read
-builtin will indicate success unless EOF is encountered on input, in
-which case failure is returned.
-.Pp
-By default, unless the
-.Fl r
-option is specified, the backslash
-.Dq \e
-acts as an escape character, causing the following character to be treated
-literally.
-If a backslash is followed by a newline, the backslash and the
-newline will be deleted.
-.It readonly Ar name ...
-.It readonly Fl p
-The specified names are marked as read only, so that they cannot be
-subsequently modified or unset.
-The shell allows the value of a variable
-to be set at the same time it is marked read only by writing
-.Pp
-.Dl readonly name=value
-.Pp
-With no arguments the readonly command lists the names of all read only
-variables.
-With the
-.Fl p
-option specified the output will be formatted suitably for non-interactive use.
-.Pp
-.It Xo set
-.Oo {
-.Fl options | Cm +options | Cm -- }
-.Oc Ar arg ...
-.Xc
-The
-.Ic set
-command performs three different functions.
-.Pp
-With no arguments, it lists the values of all shell variables.
-.Pp
-If options are given, it sets the specified option
-flags, or clears them as described in the section called
-.Sx Argument List Processing .
-.Pp
-The third use of the set command is to set the values of the shell's
-positional parameters to the specified args.
-To change the positional
-parameters without changing any options, use
-.Dq --
-as the first argument to set.
-If no args are present, the set command
-will clear all the positional parameters (equivalent to executing
-.Dq shift $# . )
-.It setvar Ar variable Ar value
-Assigns value to variable.
-(In general it is better to write
-variable=value rather than using
-.Ic setvar .
-.Ic setvar
-is intended to be used in
-functions that assign values to variables whose names are passed as
-parameters.)
-.It shift Op Ar n
-Shift the positional parameters n times.
-A
-.Ic shift
-sets the value of
-.Va $1
-to the value of
-.Va $2 ,
-the value of
-.Va $2
-to the value of
-.Va $3 ,
-and so on, decreasing
-the value of
-.Va $#
-by one.
-If there are zero positional parameters,
-.Ic shift
-does nothing.
-.It Xo trap
-.Op Fl l
-.Xc
-.It Xo trap
-.Op Ar action
-.Ar signal ...
-.Xc
-Cause the shell to parse and execute action when any of the specified
-signals are received.
-The signals are specified by signal number or as the name of the signal.
-If
-.Ar signal
-is
-.Li 0 ,
-the action is executed when the shell exits.
-.Ar action
-may be null, which cause the specified signals to be ignored.
-With
-.Ar action
-omitted or set to `-' the specified signals are set to their default action.
-When the shell forks off a subshell, it resets trapped (but not ignored)
-signals to the default action.
-The
-.Ic trap
-command has no effect on signals that were
-ignored on entry to the shell.
-Issuing
-.Ic trap
-with option
-.Ar -l
-will print a list of valid signal names.
-.Ic trap
-without any arguments cause it to write a list of signals and their
-associated action to the standard output in a format that is suitable
-as an input to the shell that achieves the same trapping results.
-.Pp
-Examples:
-.Pp
-.Dl trap
-.Pp
-List trapped signals and their corresponding action
-.Pp
-.Dl trap -l
-.Pp
-Print a list of valid signals
-.Pp
-.Dl trap '' INT QUIT tstp 30
-.Pp
-Ignore signals INT QUIT TSTP USR1
-.Pp
-.Dl trap date INT
-.Pp
-Print date upon receiving signal INT
-.It type Op Ar name ...
-Interpret each name as a command and print the resolution of the command
-search.
-Possible resolutions are:
-shell keyword, alias, shell builtin,
-command, tracked alias and not found.
-For aliases the alias expansion is
-printed; for commands and tracked aliases the complete pathname of the
-command is printed.
-.It ulimit Xo
-.Op Fl H \*(Ba Fl S
-.Op Fl a \*(Ba Fl tfdscmlpn Op Ar value
-.Xc
-Inquire about or set the hard or soft limits on processes or set new
-limits.
-The choice between hard limit (which no process is allowed to
-violate, and which may not be raised once it has been lowered) and soft
-limit (which causes processes to be signaled but not necessarily killed,
-and which may be raised) is made with these flags:
-.Bl -tag -width Fl
-.It Fl H
-set or inquire about hard limits
-.It Fl S
-set or inquire about soft limits.
-If neither
-.Fl H
-nor
-.Fl S
-is specified, the soft limit is displayed or both limits are set.
-If both are specified, the last one wins.
-.El
-.Pp
-.Bl -tag -width Fl
-The limit to be interrogated or set, then, is chosen by specifying
-any one of these flags:
-.It Fl a
-show all the current limits
-.It Fl b
-show or set the limit on the socket buffer size of a process (in bytes)
-.It Fl t
-show or set the limit on CPU time (in seconds)
-.It Fl f
-show or set the limit on the largest file that can be created
-(in 512-byte blocks)
-.It Fl d
-show or set the limit on the data segment size of a process (in kilobytes)
-.It Fl s
-show or set the limit on the stack size of a process (in kilobytes)
-.It Fl c
-show or set the limit on the largest core dump size that can be produced
-(in 512-byte blocks)
-.It Fl m
-show or set the limit on the total physical memory that can be
-in use by a process (in kilobytes)
-.It Fl l
-show or set the limit on how much memory a process can lock with
-.Xr mlock 2
-(in kilobytes)
-.It Fl p
-show or set the limit on the number of processes this user can
-have at one time
-.It Fl n
-show or set the limit on the number of files a process can have open at once
-.El
-.Pp
-If none of these is specified, it is the limit on file size that is shown
-or set.
-If value is specified, the limit is set to that number; otherwise
-the current limit is displayed.
-.Pp
-Limits of an arbitrary process can be displayed or set using the
-.Xr sysctl 8
-utility.
-.Pp
-.It umask Op Ar mask
-Set the value of umask (see
-.Xr umask 2 )
-to the specified octal value.
-If the argument is omitted, the umask value is printed.
-.It unalias Xo
-.Op Fl a
-.Op Ar name
-.Xc
-If
-.Ar name
-is specified, the shell removes that alias.
-If
-.Fl a
-is specified, all aliases are removed.
-.It unset Ar name ...
-The specified variables and functions are unset and unexported.
-If a given name corresponds to both a variable and a function, both
-the variable and the function are unset.
-.It wait Op Ar job
-Wait for the specified job to complete and return the exit status of the
-last process in the job.
-If the argument is omitted, wait for all jobs to
-complete and then return an exit status of zero.
-.El
-.Ss Command Line Editing
-When
-.Nm
-is being used interactively from a terminal, the current command
-and the command history (see
-.Ic fc
-in
-.Sx Builtins )
-can be edited using emacs-mode or vi-mode command-line editing.
-The command
-.Ql set -o emacs
-enables emacs-mode editing.
-The command
-.Ql set -o vi
-enables vi-mode editing and places sh into vi insert mode.
-(See the
-.Sx Argument List Processing
-section above.)
-.Pp
-The vi mode uses commands similar to a subset of those described in the
-.Xr vi 1
-man page.
-With vi-mode
-enabled, sh can be switched between insert mode and command mode.
-It's similar to vi: typing
-.Aq ESC
-will throw you into command VI command mode.
-Hitting
-.Aq return
-while in command mode will pass the line to the shell.
-.Pp
-The emacs mode uses commands similar to a subset available in
-the emacs editor.
-With emacs-mode enabled, special keys can be used to modify the text
-in the buffer using the control key.
-.Pp
-.Nm
-uses the
-.Xr editline 3
-library.
-.Sh EXIT STATUS
-Errors that are detected by the shell, such as a syntax error, will cause the
-shell to exit with a non-zero exit status.
-If the shell is not an
-interactive shell, the execution of the shell file will be aborted.
-Otherwise
-the shell will return the exit status of the last command executed, or
-if the exit builtin is used with a numeric argument, it will return the
-argument.
-.Sh ENVIRONMENT
-.Bl -tag -width MAILCHECK
-.It Ev HOME
-Set automatically by
-.Xr login 1
-from the user's login directory in the password file
-.Pq Xr passwd 5 .
-This environment variable also functions as the default argument for the
-cd builtin.
-.It Ev PATH
-The default search path for executables.
-See the above section
-.Sx Path Search .
-.It Ev CDPATH
-The search path used with the cd builtin.
-.It Ev LANG
-The string used to specify localization information that allows users
-to work with different culture-specific and language conventions.
-See
-.Xr nls 7 .
-.It Ev MAIL
-The name of a mail file, that will be checked for the arrival of new mail.
-Overridden by
-.Ev MAILPATH .
-.It Ev MAILCHECK
-The frequency in seconds that the shell checks for the arrival of mail
-in the files specified by the
-.Ev MAILPATH
-or the
-.Ev MAIL
-file.
-If set to 0, the check will occur at each prompt.
-.It Ev MAILPATH
-A colon
-.Dq \&:
-separated list of file names, for the shell to check for incoming mail.
-This environment setting overrides the
-.Ev MAIL
-setting.
-There is a maximum of 10 mailboxes that can be monitored at once.
-.It Ev PS1
-The primary prompt string, which defaults to
-.Dq $ \ ,
-unless you are the superuser, in which case it defaults to
-.Dq # \ .
-.It Ev PS2
-The secondary prompt string, which defaults to
-.Dq \*[Gt] \ .
-.It Ev PS4
-Output before each line when execution trace (set -x) is enabled,
-defaults to
-.Dq + \ .
-.It Ev IFS
-Input Field Separators.
-This is normally set to
-.Aq space ,
-.Aq tab ,
-and
-.Aq newline .
-See the
-.Sx White Space Splitting
-section for more details.
-.It Ev TERM
-The default terminal setting for the shell.
-This is inherited by
-children of the shell, and is used in the history editing modes.
-.It Ev HISTSIZE
-The number of lines in the history buffer for the shell.
-.El
-.Sh FILES
-.Bl -item -width HOMEprofilexxxx
-.It
-.Pa $HOME/.profile
-.It
-.Pa /etc/profile
-.El
-.Sh SEE ALSO
-.Xr csh 1 ,
-.Xr echo 1 ,
-.Xr getopt 1 ,
-.Xr ksh 1 ,
-.Xr login 1 ,
-.Xr printf 1 ,
-.Xr test 1 ,
-.Xr editline 3 ,
-.Xr getopt 3 ,
-.\" .Xr profile 4 ,
-.Xr editrc 5 ,
-.Xr passwd 5 ,
-.Xr environ 7 ,
-.Xr nls 7 ,
-.Xr sysctl 8
-.Sh HISTORY
-A
-.Nm
-command appeared in
-.At v1 .
-It was, however, unmaintainable so we wrote this one.
-.Sh BUGS
-Setuid shell scripts should be avoided at all costs, as they are a
-significant security risk.
-.Pp
-PS1, PS2, and PS4 should be subject to parameter expansion before
-being displayed.