path: root/doc/article.txt

                   Bash - The GNU shell*


                         Chet Ramey
              Case Western Reserve University
                      chet@po.cwru.edu






_1.  _I_n_t_r_o_d_u_c_t_i_o_n

     _B_a_s_h is the shell,  or  command  language  interpreter,
that  will  appear in the GNU operating system.  The name is
an acronym for the "Bourne-Again  SHell",  a  pun  on  Steve
Bourne,  the  author  of  the direct ancestor of the current
UNIX|- shell /_b_i_n/_s_h, which appeared in the  Seventh  Edition
Bell Labs Research version of UNIX.

     Bash is an sh-compatible shell that incorporates useful
features  from  the  Korn shell (ksh) and the C shell (csh),
described later in this article.  It is ultimately  intended
to  be  a  conformant implementation of the IEEE POSIX Shell
and Utilities specification (IEEE Working Group 1003.2).  It
offers  functional improvements over sh for both interactive
and programming use.

     While the GNU operating system will most likely include
a  version  of  the  Berkeley  shell  csh,  Bash will be the
default shell.  Like other GNU software, Bash is quite port-
able.  It currently runs on nearly every version of UNIX and
a few other operating systems -  an  independently-supported
port  exists  for OS/2, and there are rumors of ports to DOS
and Windows NT.  Ports to UNIX-like systems such as QNX  and
Minix are part of the distribution.

     The original author of Bash was Brian Fox, an  employee
of  the Free Software Foundation.  The current developer and
maintainer is Chet Ramey, a  volunteer  who  works  at  Case
Western Reserve University.

_2.  _W_h_a_t'_s _P_O_S_I_X, _a_n_y_w_a_y?

     _P_O_S_I_X is a name originally coined by  Richard  Stallman
_________________________
*An earlier version of this  article  appeared  in  The
Linux Journal.
|- UNIX is a trademark of Bell Laboratories.




                      October 28, 1994





                           - 2 -


for a family of open system standards based on UNIX.   There
are  a  number  of  aspects  of UNIX under consideration for
standardization, from the basic system services at the  sys-
tem  call  and  C library level to applications and tools to
system administration and management.  Each  area  of  stan-
dardization  is  assigned  to  a  working  group in the 1003
series.

     The  POSIX  Shell  and  Utilities  standard  has   been
developed by IEEE Working Group 1003.2 (POSIX.2).|=  It  con-
centrates  on  the command interpreter interface and utility
programs commonly executed from the command line or by other
programs.   An  initial  version  of  the  standard has been
approved and published by the IEEE, and  work  is  currently
underway to update it.  There are four primary areas of work
in the 1003.2 standard:

o+    Aspects of the shell's syntax and command language.   A
     number  of  special  builtins  such  as _c_d and _e_x_e_c are
     being specified as part of the shell, since their func-
     tionality  usually  cannot be implemented by a separate
     executable;

o+    A set of utilities to be called by  shell  scripts  and
     applications.   Examples are programs like _s_e_d, _t_r, and
     _a_w_k.  Utilities commonly implemented as shell  builtins
     are  described  in this section, such as _t_e_s_t and _k_i_l_l.
     An expansion of this section's scope, termed  the  User
     Portability   Extension,   or   UPE,  has  standardized
     interactive programs such as _v_i and _m_a_i_l_x;

o+    A group of functional interfaces to  services  provided
     by  the  shell,  such  as  the  traditional  system() C
     library function.  There are functions to perform shell
     word expansions, perform filename expansion (_g_l_o_b_b_i_n_g),
     obtain values of  POSIX.2  system  configuration  vari-
     ables,   retrieve   values   of  environment  variables
     (getenv()), _a_n_d _o_t_h_e_r _s_e_r_v_i_c_e_s;

o+    A suite of "development" utilities  such  as  _c_8_9  (the
     POSIX.2 version of _c_c), and _y_a_c_c.

     Bash is concerned  with  the  aspects  of  the  shell's
behavior defined by POSIX.2.  The shell command language has
of course been standardized, including the basic  flow  con-
trol  and  program execution constructs, I/O redirection and
pipelining, argument handling, variable expansion, and quot-
ing.   The  _s_p_e_c_i_a_l  builtins,  which must be implemented as
part of the shell to provide the desired functionality,  are
_________________________
|=IEEE, _I_E_E_E  _S_t_a_n_d_a_r_d  _f_o_r  _I_n_f_o_r_m_a_t_i_o_n  _T_e_c_h_n_o_l_o_g_y  --
_P_o_r_t_a_b_l_e  _O_p_e_r_a_t_i_n_g  _S_y_s_t_e_m  _I_n_t_e_r_f_a_c_e  (_P_O_S_I_X) _P_a_r_t _2:
_S_h_e_l_l _a_n_d _U_t_i_l_i_t_i_e_s, 1992.




                      October 28, 1994





                           - 3 -


specified as being part of the shell; examples of these  are
_e_v_a_l  and _e_x_p_o_r_t.  Other utilities appear in the sections of
POSIX.2 not devoted to the shell which are commonly (and  in
some cases must be) implemented as builtin commands, such as
_r_e_a_d and  _t_e_s_t.   POSIX.2  also  specifies  aspects  of  the
shell's  interactive  behavior as part of the UPE, including
job control and command line editing.  Interestingly enough,
only  _v_i-style line editing commands have been standardized;
_e_m_a_c_s editing commands were left out due to objections.

     While POSIX.2 includes much of what the shell has trad-
itionally  provided, some important things have been omitted
as being "beyond its scope."  There  is,  for  instance,  no
mention  of a difference between a _l_o_g_i_n shell and any other
interactive shell (since POSIX.2 does not  specify  a  login
program).   No fixed startup files are defined, either - the
standard does not mention ._p_r_o_f_i_l_e.

_3.  _B_a_s_i_c _B_a_s_h _f_e_a_t_u_r_e_s

     Since the Bourne shell provides Bash with most  of  its
philosophical  underpinnings,  Bash  inherits  most  of  its
features and functionality from sh.  Bash implements all  of
the  traditional sh flow control constructs (_f_o_r, _i_f, _w_h_i_l_e,
etc.).  All of the Bourne shell  builtins,  including  those
not  specified  in  the  POSIX.2  standard,  appear in Bash.
Shell _f_u_n_c_t_i_o_n_s, introduced  in  the  SVR2  version  of  the
Bourne  shell, are similar to shell scripts, but are defined
using a special syntax and are executed in the same  process
as the calling shell.  Bash has shell functions which behave
in a fashion upward-compatible with sh functions.  There are
certain shell variables that Bash interprets in the same way
as sh, such as _P_S_1, _I_F_S, and _P_A_T_H.  Bash  implements  essen-
tially  the  same  grammar, parameter and variable expansion
semantics, redirection, and quoting  as  the  Bourne  shell.
Where  differences  appear  between the POSIX.2 standard and
traditional sh behavior, Bash follows POSIX.

     The Korn Shell (ksh) is  a  descendent  of  the  Bourne
shell  written at AT&T Bell Laboratories by David Korn|-.  It
provides a number of useful features  that  POSIX  and  Bash
have adopted.  Many of the interactive facilities in POSIX.2
have their roots in the ksh: for example, the POSIX and  ksh
job  control  facilities are nearly identical. Bash includes
features from the Korn Shell for both  interactive  use  and
shell programming.  For programming, Bash provides variables
such as _R_A_N_D_O_M and _R_E_P_L_Y, the _t_y_p_e_s_e_t builtin,  the  ability
to  remove  substrings from variables based on patterns, and
shell arithmetic.  _R_A_N_D_O_M expands to a  random  number  each
time it is referenced; assigning a value to _R_A_N_D_O_M seeds the
_________________________
|-Morris Bolsky and David Korn,  _T_h_e  _K_o_r_n_S_h_e_l_l  _C_o_m_m_a_n_d
_a_n_d _P_r_o_g_r_a_m_m_i_n_g _L_a_n_g_u_a_g_e, Prentice Hall, 1989.




                      October 28, 1994





                           - 4 -


random number generator.  _R_E_P_L_Y is the default variable used
by  the  _r_e_a_d builtin when no variable names are supplied as
arguments.  The _t_y_p_e_s_e_t builtin is used to define  variables
and  give them attributes such as readonly.  Bash arithmetic
allows the evaluation of an expression and the  substitution
of the result.  Shell variables may be used as operands, and
the result of an expression may be assigned to  a  variable.
Nearly  all  of the operators from the C language are avail-
able, with the same precedence rules:
9     $ echo $((3 + 5 * 32))
     163
9
For interactive use, Bash implements ksh-style  aliases  and
builtins  such  as  _f_c  (discussed  below)  and  _j_o_b_s.  Bash
aliases allow a string to be substituted for a command name.
They  can  be  used  to create a mnemonic for a UNIX command
name (alias del=rm), to expand a single word  to  a  complex
command (alias news='xterm -g 80x45 -title trn -e trn -e -S1
-N &'), or to ensure that a command is invoked with a  basic
set of options (alias ls="/bin/ls -F").

     The C shell (csh)|-,  originally  written  by  Bill  Joy
while  at Berkeley, is widely used and quite popular for its
interactive facilities.  Bash includes a csh-compatible his-
tory  expansion  mechanism  ("!  history"), brace expansion,
access to a stack of directories via the  _p_u_s_h_d,  _p_o_p_d,  and
_d_i_r_s  builtins, and tilde expansion, to generate users' home
directories.  Tilde expansion has also been adopted by  both
the Korn Shell and POSIX.2.

     There were certain areas in which  POSIX.2  felt  stan-
dardization  was  necessary,  but no existing implementation
provided the proper behavior.  The  working  group  invented
and  standardized  functionality  in these areas, which Bash
implements.  The _c_o_m_m_a_n_d builtin was invented so that  shell
functions could be written to replace builtins; it makes the
capabilities of the builtin available to the function.   The
reserved  word "!" was added to negate the return value of a
command or pipeline; it was nearly impossible to express "if
not  x" cleanly using the sh language.  There exist multiple
incompatible implementations  of  the  _t_e_s_t  builtin,  which
tests  files  for  type  and  other  attributes and performs
arithmetic and string comparisons.  POSIX considered none of
these  correct,  so  the  standard behavior was specified in
terms of the number of arguments to  the  command.   POSIX.2
dictates  exactly  what will happen when four or fewer argu-
ments are given to _t_e_s_t, and leaves the  behavior  undefined
when  more  arguments  are  supplied.  Bash uses the POSIX.2
_________________________
|-Bill Joy, An Introduction to the C Shell, _U_N_I_X  _U_s_e_r'_s
_S_u_p_p_l_e_m_e_n_t_a_r_y  _D_o_c_u_m_e_n_t_s,  University  of California at
Berkeley, 1986.




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                           - 5 -


algorithm, which was conceived by David Korn.

_3._1.  _F_e_a_t_u_r_e_s _n_o_t _i_n _t_h_e _B_o_u_r_n_e _S_h_e_l_l

     There are a number of minor  differences  between  Bash
and  the  version  of  sh  present on most other versions of
UNIX.  The majority of these are due to the POSIX  standard,
but some are the result of Bash adopting features from other
shells.  For instance, Bash includes the  new  "!"  reserved
word,  the  _c_o_m_m_a_n_d builtin, the ability of the _r_e_a_d builtin
to correctly return a line ending with a backslash, symbolic
arguments  to the _u_m_a_s_k builtin, variable substring removal,
a way to get the length of a variable, and the new algorithm
for  the  _t_e_s_t  builtin  from  the POSIX.2 standard, none of
which appear in sh.

     Bash also implements the "$(...)" command  substitution
syntax,  which  supersedes  the  sh  `...`  construct.   The
"$(...)" construct expands to the output of the command con-
tained   within  the  parentheses,  with  trailing  newlines
removed.  The sh syntax is accepted for  backwards  compati-
bility, but the "$(...)" form is preferred because its quot-
ing rules are much simpler and it is easier to nest.

     The Bourne shell does  not  provide  such  features  as
brace  expansion,  the  ability  to  define a variable and a
function with the same name, local variables in shell  func-
tions, the ability to enable and disable individual builtins
or write a function to replace a  builtin,  or  a  means  to
export a shell function to a child process.

     Bash has closed a long-standing shell security hole  by
not  using  the $_I_F_S variable to split each word read by the
shell, but splitting only the results of expansion (ksh  and
the  4.4  BSD  sh have fixed this as well).  Useful behavior
such as a means to abort execution of a script read with the
"."  command  using  the  return  builtin  or  automatically
exporting variables in the shell's environment  to  children
is  also  not  present in the Bourne shell.  Bash provides a
much more powerful environment for both interactive use  and
programming.

_4.  _B_a_s_h-_s_p_e_c_i_f_i_c _F_e_a_t_u_r_e_s

     This section details a few of the features  which  make
Bash unique.  Most of them provide improved interactive use,
but a few programming  improvements  are  present  as  well.
Full descriptions of these features can be found in the Bash
documentation.

_4._1.  _S_t_a_r_t_u_p _F_i_l_e_s

     Bash executes  startup  files  differently  than  other
shells.   The  Bash behavior is a compromise between the csh



                      October 28, 1994





                           - 6 -


principle of startup files with  fixed  names  executed  for
each shell and the sh "minimalist" behavior.  An interactive
instance of Bash started as a login shell reads and executes
~/._b_a_s_h__p_r_o_f_i_l_e  (the  file .bash_profile in the user's home
directory), if it exists.  An  interactive  non-login  shell
reads  and executes ~/._b_a_s_h_r_c.  A non-interactive shell (one
begun to execute a shell script, for example) reads no fixed
startup  file,  but  uses the value of the variable $_E_N_V, if
set, as the name of a startup file.   The  ksh  practice  of
reading  $_E_N_V  for every shell, with the accompanying diffi-
culty of defining the proper  variables  and  functions  for
interactive  and  non-interactive  shells or having the file
read only for interactive shells, was  considered  too  com-
plex.   Ease  of  use won out here.  Interestingly, the next
release of ksh will change to reading $_E_N_V only for interac-
tive shells.

_4._2.  _N_e_w _B_u_i_l_t_i_n _C_o_m_m_a_n_d_s

     There are a few builtins which are  new  or  have  been
extended  in  Bash.   The _e_n_a_b_l_e builtin allows builtin com-
mands to be turned on and off arbitrarily.  To use the  ver-
sion  of  _e_c_h_o found in a user's search path rather than the
Bash builtin, enable -n echo  suffices.   The  _h_e_l_p  builtin
provides  quick  synopses  of  the  shell facilities without
requiring access to a manual page.  _B_u_i_l_t_i_n  is  similar  to
_c_o_m_m_a_n_d  in  that  it  bypasses shell functions and directly
executes builtin commands.  Access to a csh-style  stack  of
directories  is provided via the _p_u_s_h_d, _p_o_p_d, and _d_i_r_s buil-
tins.  _P_u_s_h_d and _p_o_p_d insert and remove directories from the
stack,  respectively, and _d_i_r_s lists the stack contents.  On
systems that allow fine-grained control  of  resources,  the
_u_l_i_m_i_t  builtin  can be used to tune these settings.  _U_l_i_m_i_t
allows a user to control, among other things,  whether  core
dumps  are  to  be generated, how much memory the shell or a
child process is allowed to allocate, and how large  a  file
created  by  a  child process can grow.  The _s_u_s_p_e_n_d command
will stop the shell process when job control is active; most
other  shells  do  not  allow  themselves to be stopped like
that.  _T_y_p_e, the Bash answer to _w_h_i_c_h and _w_h_e_n_c_e, shows what
will happen when a word is typed as a command:
9     $ type export
     export is a shell builtin
     $ type -t export
     builtin
     $ type bash
     bash is /bin/bash
     $ type cd
     cd is a function
     cd ()
     {
         builtin cd ${1+"$@"} && xtitle $HOST: $PWD
     }
9


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                           - 7 -


Various modes tell what a command word  is  (reserved  word,
alias,  function,  builtin,  or  file) or which version of a
command will be executed based  on  a  user's  search  path.
Some  of  this functionality has been adopted by POSIX.2 and
folded into the _c_o_m_m_a_n_d utility.

_4._3.  _E_d_i_t_i_n_g _a_n_d _C_o_m_p_l_e_t_i_o_n

     One area in which Bash shines is command line  editing.
Bash  uses  the _r_e_a_d_l_i_n_e library to read and edit lines when
interactive.  Readline is  a  powerful  and  flexible  input
facility that a user can configure to individual tastes.  It
allows lines to be edited using either emacs or vi commands,
where  those  commands are appropriate.  The full capability
of emacs is not present - there is no way to execute a named
command  with  M-x, for instance - but the existing commands
are more than adequate.  The vi mode is compliant  with  the
command line editing standardized by POSIX.2.

     Readline is fully customizable.   In  addition  to  the
basic commands and key bindings, the library allows users to
define additional key bindings using a  startup  file.   The
_i_n_p_u_t_r_c file, which defaults to the file ~/._i_n_p_u_t_r_c, is read
each time readline initializes, permitting users to maintain
a  consistent  interface across a set of programs.  Readline
includes an extensible interface, so each program using  the
library  can  add  its  own  bindable  commands and program-
specific key bindings.  Bash uses this facility to add bind-
ings that perform history expansion or shell word expansions
on the current input line.

     Readline interprets a number of variables which further
tune  its  behavior.   Variables exist to control whether or
not eight-bit characters are directly read as input or  con-
verted  to  meta-prefixed key sequences (a meta-prefixed key
sequence consists of  the  character  with  the  eighth  bit
zeroed,  preceded  by  the  _m_e_t_a-_p_r_e_f_i_x  character,  usually
escape,  which  selects  an  alternate  keymap),  to  decide
whether  to  output  characters  with  the  eighth  bit  set
directly or as a meta-prefixed key sequence, whether or  not
to  wrap  to  a  new screen line when a line being edited is
longer than the screen width, the keymap to which subsequent
key  bindings  should apply, or even what happens when read-
line wants to ring the terminal's bell.  All of these  vari-
ables can be set in the inputrc file.

     The startup file understands a set of  C  preprocessor-
like  conditional  constructs  which  allow variables or key
bindings to be assigned based on the application using read-
line,  the  terminal  currently  being  used, or the editing
mode.  Users can add program-specific bindings to make their
lives easier:  I have bindings that let me edit the value of
$_P_A_T_H and double-quote the current or previous word:
9     # Macros that are convenient for shell interaction


9                      October 28, 1994





                           - 8 -


     $if Bash
     # edit the path
     "\C-xp": "PATH=${PATH}\e\C-e\C-a\ef\C-f"
     # prepare to type a quoted word -- insert open and close double
     # quotes and move to just after the open quote
     "\C-x\"": "\"\"\C-b"
     # Quote the current or previous word
     "\C-xq": "\eb\"\ef\""
     $endif
9
There is a readline command to re-read the  file,  so  users
can  edit  the  file, change some bindings, and begin to use
them almost immediately.

     Bash implements the _b_i_n_d builtin for more  dyamic  con-
trol  of  readline  than  the startup file permits.  _B_i_n_d is
used in several ways.  In _l_i_s_t  mode,  it  can  display  the
current  key  bindings, list all the readline editing direc-
tives available for binding, list which keys invoke a  given
directive,  or  output  the current set of key bindings in a
format that can be incorporated  directly  into  an  inputrc
file.   In  _b_a_t_c_h  mode,  it  reads a series of key bindings
directly from a file and passes them to  readline.   In  its
most  common usage, _b_i_n_d takes a single string and passes it
directly to readline, which interprets the line as if it had
just been read from the inputrc file.  Both key bindings and
variable assignments may appear in the string given to _b_i_n_d.

     The readline library also  provides  an  interface  for
_w_o_r_d  _c_o_m_p_l_e_t_i_o_n.   When  the  _c_o_m_p_l_e_t_i_o_n character (usually
TAB) is typed, readline looks at the  word  currently  being
entered  and  computes  the  set  of  filenames of which the
current word is a valid prefix.  If there is only one possi-
ble  completion,  the  rest  of  the characters are inserted
directly,  otherwise  the  common  prefix  of  the  set   of
filenames  is added to the current word.  A second TAB char-
acter entered  immediately  after  a  non-unique  completion
causes  readline  to list the possible completions; there is
an option to have the list displayed immediately.   Readline
provides  hooks  so  that  applications can provide specific
types of completion before the default  filename  completion
is attempted.  This is quite flexible, though it is not com-
pletely user-programmable.  Bash, for example, can  complete
filenames, command names (including aliases, builtins, shell
reserved words, shell functions, and  executables  found  in
the file system), shell variables, usernames, and hostnames.
It uses a set of heuristics that, while not perfect, is gen-
erally  quite good at determining what type of completion to
attempt.

_4._4.  _H_i_s_t_o_r_y

     Access to the list of commands previously entered  (the
_c_o_m_m_a_n_d  _h_i_s_t_o_r_y)  is  provided  jointly  by  Bash  and  the


9                      October 28, 1994





                           - 9 -


readline  library.   Bash  provides  variables   ($HISTFILE,
$HISTSIZE, and $HISTCONTROL) and the _h_i_s_t_o_r_y and _f_c builtins
to manipulate the history  list.   The  value  of  $_H_I_S_T_F_I_L_E
specifes  the  file where Bash writes the command history on
exit and reads it on startup.  $_H_I_S_T_S_I_Z_E is  used  to  limit
the  number  of commands saved in the history.  $_H_I_S_T_C_O_N_T_R_O_L
provides a crude form of control  over  which  commands  are
saved  on  the history list: a value of _i_g_n_o_r_e_s_p_a_c_e means to
not save commands which begin  with  a  space;  a  value  of
_i_g_n_o_r_e_d_u_p_s  means to not save commands identical to the last
command saved.  $HISTCONTROL was named  $history_control  in
earlier versions of Bash; the old name is still accepted for
backwards compatibility.  The _h_i_s_t_o_r_y command  can  read  or
write  files  containing  the  history  list and display the
current list contents.  The _f_c builtin, adopted from POSIX.2
and  the  Korn  Shell, allows display and re-execution, with
optional editing, of commands from the  history  list.   The
readline library offers a set of commands to search the his-
tory list for a portion of  the  current  input  line  or  a
string  typed  by  the  user.  Finally, the _h_i_s_t_o_r_y library,
generally incorporated directly into the  readline  library,
implements  a  facility  for  history recall, expansion, and
re-execution of previous commands very similar to csh ("bang
history", so called because the exclamation point introduces
a history substitution):
9     $ echo a b c d e
     a b c d e
     $ !! f g h i
     echo a b c d e f g h i
     a b c d e f g h i
     $ !-2
     echo a b c d e
     a b c d e
     $ echo !-2:1-4
     echo a b c d
     a b c d
9
The command history is only saved when the shell is interac-
tive, so it is not available for use by shell scripts.

_4._5.  _N_e_w _S_h_e_l_l _V_a_r_i_a_b_l_e_s

     There are a number of convenience variables  that  Bash
interprets  to  make  life  easier.   These include _F_I_G_N_O_R_E,
which is a set of filename  suffixes  identifying  files  to
exclude   when  completing  filenames;  _H_O_S_T_T_Y_P_E,  which  is
automatically  set  to  a  string  describing  the  type  of
hardware    on    which   Bash   is   currently   executing;
_c_o_m_m_a_n_d__o_r_i_e_n_t_e_d__h_i_s_t_o_r_y, which directs  Bash  to  save  all
lines of a multiple-line command such as a _w_h_i_l_e or _f_o_r loop
in a single history entry,  allowing  easy  re-editing;  and
_I_G_N_O_R_E_E_O_F,  whose  value indicates the number of consecutive
EOF characters that an interactive shell  will  read  before



                      October 28, 1994





                           - 10 -


exiting - an easy way to keep yourself from being logged out
accidentally.  The _a_u_t_o__r_e_s_u_m_e variable alters the  way  the
shell treats simple command names: if job control is active,
and  this  variable  is  set,  single-word  simple  commands
without  redirections  cause the shell to first look for and
restart a suspended job with that name before starting a new
process.

_4._6.  _B_r_a_c_e _E_x_p_a_n_s_i_o_n

     Since sh offers no convenient way to generate arbitrary
strings  that  share  a  common  prefix  or suffix (filename
expansion requires that the filenames  exist),  Bash  imple-
ments  _b_r_a_c_e  _e_x_p_a_n_s_i_o_n,  a  capability  picked up from csh.
Brace expansion is similar to filename  expansion,  but  the
strings  generated need not correspond to existing files.  A
brace expression consists of an optional _p_r_e_a_m_b_l_e,  followed
by  a  pair  of braces enclosing a series of comma-separated
strings,  and  an  optional  _p_o_s_t_a_m_b_l_e.   The  preamble   is
prepended  to each string within the braces, and the postam-
ble is then appended to each resulting string:
9     $ echo a{d,c,b}e
     ade ace abe
9
As this example demonstrates, the results of brace expansion
are not sorted, as they are by filename expansion.

_4._7.  _P_r_o_c_e_s_s _S_u_b_s_t_i_t_u_t_i_o_n

     On systems that can support it, Bash provides a  facil-
ity  known as _p_r_o_c_e_s_s _s_u_b_s_t_i_t_u_t_i_o_n.  Process substitution is
similar to command substitution in  that  its  specification
includes  a  command to execute, but the shell does not col-
lect the command's output and insert  it  into  the  command
line.   Rather,  Bash  opens a pipe to the command, which is
run in the background.  The shell uses named  pipes  (FIFOs)
or  the  /_d_e_v/_f_d  method  of naming open files to expand the
process substitution to a filename  which  connects  to  the
pipe  when  opened.  This filename becomes the result of the
expansion.  Process substitution can be used to compare  the
outputs  of two different versions of an application as part
of a regression test:
9     $ cmp <(old_prog) <(new_prog)
9
_4._8.  _P_r_o_m_p_t _C_u_s_t_o_m_i_z_a_t_i_o_n

     One of the more popular interactive features that  Bash
provides  is the ability to customize the prompt.  Both $_P_S_1
and $_P_S_2, the primary and secondary  prompts,  are  expanded
before being displayed.  Parameter and variable expansion is
performed when the prompt string is expanded, so  any  shell
variable  can  be  put  into the prompt (e.g., $_S_H_L_V_L, which



                      October 28, 1994





                           - 11 -


indicates how deeply the current  shell  is  nested).   Bash
specially  interprets  characters  in the prompt string pre-
ceded by a backslash.  Some of these backslash  escapes  are
replaced  with the current time, the date, the current work-
ing directory, the username, and the command number or  his-
tory  number  of the command being entered.  There is even a
backslash escape to cause the shell  to  change  its  prompt
when running as root after an _s_u.  Before printing each pri-
mary prompt, Bash expands the variable $_P_R_O_M_P_T__C_O_M_M_A_N_D  and,
if it has a value, executes the expanded value as a command,
allowing additional prompt customization.  For example, this
assignment  causes  the  current user, the current host, the
time, the last component of the current  working  directory,
the  level  of  shell nesting, and the history number of the
current command to be embedded into the primary prompt:
9     $ PS1='\u@\h [\t] \W($SHLVL:\!)\$ '
     chet@odin [21:03:44] documentation(2:636)$ cd ..
     chet@odin [21:03:54] src(2:637)$
9
The string being assigned is surrounded by single quotes  so
that  if it is exported, the value of $_S_H_L_V_L will be updated
by a child shell:
9     chet@odin [21:17:35] src(2:638)$ export PS1
     chet@odin [21:17:40] src(2:639)$ bash
     chet@odin [21:17:46] src(3:696)$
9
The \$ escape is displayed as "$" when running as  a  normal
user, but as "#" when running as root.

_4._9.  _F_i_l_e _S_y_s_t_e_m _V_i_e_w_s

     Since Berkeley introduced symbolic links  in  4.2  BSD,
one of their most annoying properties has been the "warping"
to a completely different area of the file system when using
_c_d,  and  the  resultant  non-intuitive behavior of "cd ..".
The UNIX kernel treats symbolic links _p_h_y_s_i_c_a_l_l_y.  When  the
kernel is translating a pathname in which one component is a
symbolic link, it replaces all or part of the pathname while
processing  the  link.  If the contents of the symbolic link
begin  with  a  slash,  the  kernel  replaces  the  pathname
entirely; if not, the link contents replace the current com-
ponent.  In either case, the symbolic link is  visible.   If
the  link value is an absolute pathname, the user finds him-
self in a completely different part of the file system.

     Bash provides a _l_o_g_i_c_a_l view of the  file  system.   In
this default mode, command and filename completion and buil-
tin commands such as _c_d and _p_u_s_h_d which change  the  current
working  directory transparently follow symbolic links as if
they were directories.  The $_P_W_D variable, which  holds  the
shell's  idea  of  the current working directory, depends on
the path  used  to  reach  the  directory  rather  than  its



                      October 28, 1994





                           - 12 -


physical  location  in the local file system hierarchy.  For
example:
9     $ cd /usr/local/bin
     $ echo $PWD
     /usr/local/bin
     $ pwd
     /usr/local/bin
     $ /bin/pwd
     /net/share/sun4/local/bin
     $ cd ..
     $ pwd
     /usr/local
     $ /bin/pwd
     /net/share/sun4/local
     $ cd ..
     $ pwd
     /usr
     $ /bin/pwd
     /usr
9
One problem with this, of course, arises when programs  that
do  not  understand  the  shell's logical notion of the file
system interpret ".." differently.  This  generally  happens
when Bash completes filenames containing ".." according to a
logical hierarchy which does not correspond to their  physi-
cal  location.  For  users  who  find  this  troublesome,  a
corresponding _p_h_y_s_i_c_a_l view of the file system is available:
9     $ cd /usr/local/bin
     $ pwd
     /usr/local/bin
     $ set -o physical
     $ pwd
     /net/share/sun4/local/bin
9
_4._1_0.  _I_n_t_e_r_n_a_t_i_o_n_a_l_i_z_a_t_i_o_n

     One of the most  significant  improvements  in  version
1.13  of  Bash  was  the  change to "eight-bit cleanliness".
Previous versions used the eighth bit of characters to  mark
whether  or not they were quoted when performing word expan-
sions.  While this did not affect  the  majority  of  users,
most  of  whom  used  only  seven-bit ASCII characters, some
found it  confining.   Beginning  with  version  1.13,  Bash
implemented a different quoting mechanism that did not alter
the eighth bit of characters.  This allowed Bash to  manipu-
late  files  with  "odd"  characters in their names, but did
nothing to help users enter those  names,  so  version  1.13
introduced  changes to readline that made it eight-bit clean
as well.  Options exist that force  readline  to  attach  no
special  significance  to characters with the eighth bit set
(the default behavior is  to  convert  these  characters  to
meta-prefixed  key sequences) and to output these characters



                      October 28, 1994





                           - 13 -


without  conversion  to  meta-prefixed   sequences.    These
changes, along with the expansion of keymaps to a full eight
bits, enable readline to work with most of the ISO-8859 fam-
ily of character sets, used by many European countries.

_4._1_1.  _P_O_S_I_X _M_o_d_e

     Although Bash is intended  to  be  POSIX.2  conformant,
there  are areas in which the default behavior is not compa-
tible with the standard.  For users who wish to operate in a
strict  POSIX.2  environment,  Bash implements a _P_O_S_I_X _m_o_d_e.
When this mode is active, Bash modifies its  default  opera-
tion  where  it  differs from POSIX.2 to match the standard.
POSIX mode is entered when Bash is started with  the  -_p_o_s_i_x
option.   This feature is also available as an option to the
set builtin, set -o posix.  For compatibility with other GNU
software  that  attempts  to be POSIX.2 compliant, Bash also
enters POSIX mode if the variable  $_P_O_S_I_X_L_Y__C_O_R_R_E_C_T  is  set
when  Bash  is started or assigned a value during execution.
$_P_O_S_I_X__P_E_D_A_N_T_I_C is accepted as well, to be  compatible  with
some  older  GNU  utilities.   When Bash is started in POSIX
mode, for example, it sources the file named by the value of
$_E_N_V  rather  than  the "normal" startup files, and does not
allow reserved words to be aliased.

_5.  _N_e_w _F_e_a_t_u_r_e_s _a_n_d _F_u_t_u_r_e _P_l_a_n_s

     There are several features introduced  in  the  current
version of Bash, version 1.14, and a number under considera-
tion for future releases.  This section will briefly  detail
the  new  features  in  version  1.14  and  describe several
features that may appear in later versions.

_5._1.  _N_e_w _F_e_a_t_u_r_e_s _i_n _B_a_s_h-_1._1_4

     The new features available in Bash-1.14 answer  several
of the most common requests for enhancements.  Most notably,
there is a mechanism  for  including  non-visible  character
sequences  in  prompts, such as those which cause a terminal
to print characters in different colors or in standout mode.
There  was  nothing preventing the use of these sequences in
earlier  versions,  but  the  readline  redisplay  algorithm
assumed  each  character  occupied physical screen space and
would wrap lines prematurely.

     Readline has a few new variables, several new  bindable
commands,  and  some additional emacs mode default key bind-
ings.  A new history search mode has  been  implemented:  in
this mode, readline searches the history for lines beginning
with the characters between the  beginning  of  the  current
line  and  the  cursor.   The  existing readline incremental
search commands no longer match identical  lines  more  than
once.   Filename  completion now expands variables in direc-
tory names.  The history expansion facilities are now nearly



                      October 28, 1994





                           - 14 -


completely csh-compatible: missing modifiers have been added
and history substitution has been extended.

     Several of the features described earlier, such as  _s_e_t
-_o  _p_o_s_i_x  and  $_P_O_S_I_X__P_E_D_A_N_T_I_C,  are  new  in version 1.14.
There is a new shell variable, _O_S_T_Y_P_E, to which Bash assigns
a  value that identifies the version of UNIX it's running on
(great for putting architecture-specific binary  directories
into the $PATH).  Two variables have been renamed: $_H_I_S_T_C_O_N_-
_T_R_O_L  replaces  $_h_i_s_t_o_r_y__c_o_n_t_r_o_l,  and  $_H_O_S_T_F_I_L_E   replaces
$_h_o_s_t_n_a_m_e__c_o_m_p_l_e_t_i_o_n__f_i_l_e.  In both cases, the old names are
accepted for backwards compatibility.  The ksh  _s_e_l_e_c_t  con-
struct,  which  allows  the  generation of simple menus, has
been implemented.   New  capabilities  have  been  added  to
existing  variables:  $_a_u_t_o__r_e_s_u_m_e  can  now  take values of
_e_x_a_c_t or _s_u_b_s_t_r_i_n_g, and $_H_I_S_T_C_O_N_T_R_O_L understands  the  value
_i_g_n_o_r_e_b_o_t_h,  which  combines  the  two previously acceptable
values.  The _d_i_r_s builtin has acquired options to print  out
specific members of the directory stack.  The $_n_o_l_i_n_k_s vari-
able, which forces a physical view of the file  system,  has
been  superseded  by  the  -_P  option  to  the  _s_e_t  builtin
(equivalent to set -o physical); the  variable  is  retained
for  backwards  compatibility.  The version string contained
in $_B_A_S_H__V_E_R_S_I_O_N now includes an  indication  of  the  patch
level  as  well  as  the  "build version".  Some little-used
features have been removed:  the _b_y_e synonym  for  _e_x_i_t  and
the  $_N_O__P_R_O_M_P_T__V_A_R_S  variable  are  gone.   There is now an
organized test suite that can be run as  a  regression  test
when building a new version of Bash.

     The documentation has been thoroughly overhauled: there
is  a  new  manual page on the readline library and the _i_n_f_o
file has been updated to reflect the  current  version.   As
always,  as  many bugs as possible have been fixed, although
some surely remain.

_5._2.  _O_t_h_e_r _F_e_a_t_u_r_e_s

     There are a few features that  I  hope  to  include  in
later Bash releases.  Some are based on work already done in
other shells.

     In addition to simple variables, a  future  release  of
Bash  will  include  one-dimensional  arrays,  using the ksh
implementation of arrays as a model.  Additions to  the  ksh
syntax,  such  as  _v_a_r_n_a_m_e=( ... ) to assign a list of words
directly to an array and a mechanism to allow the _r_e_a_d buil-
tin  to  read a list of values directly into an array, would
be desirable.  Given those extensions, the ksh _s_e_t -_A syntax
may not be worth supporting (the -_A option assigns a list of
values to an array, but is a rather peculiar special case).

     Some shells include a means of _p_r_o_g_r_a_m_m_a_b_l_e  word  com-
pletion, where the user specifies on a per-command basis how



                      October 28, 1994





                           - 15 -


the arguments of the command are to be treated when  comple-
tion  is  attempted:  as  filenames,  hostnames,  executable
files, and so on.  The other aspects  of  the  current  Bash
implementation  could  remain as-is; the existing heuristics
would still be valid.  Only when completing the arguments to
a  simple  command  would  the programmable completion be in
effect.

     It would also be nice to give  the  user  finer-grained
control over which commands are saved onto the history list.
One proposal is for a variable,  tentatively  named  _H_I_S_T_I_G_-
_N_O_R_E,  which  would  contain  a colon-separated list of com-
mands.  Lines beginning with these commands, after the  res-
trictions  of  $_H_I_S_T_C_O_N_T_R_O_L  have been applied, would not be
placed onto the history list.   The  shell  pattern-matching
capabilities  could  also  be  available when specifying the
contents of $_H_I_S_T_I_G_N_O_R_E.

     One thing that newer shells such as _w_k_s_h (also known as
_d_t_k_s_h)  provide is a command to dynamically load code imple-
menting additional builtin commands into  a  running  shell.
This new builtin would take an object file or shared library
implementing the "body" of the  builtin  (_x_x_x__b_u_i_l_t_i_n()  for
those familiar with Bash internals) and a structure contain-
ing the name of the new command, the function to  call  when
the new builtin is invoked (presumably defined in the shared
object specified as an argument), and the  documentation  to
be  printed  by  the  _h_e_l_p  command (possibly present in the
shared object as well).  It  would  manage  the  details  of
extending the internal table of builtins.

     A few other builtins would also be desirable:  two  are
the POSIX.2 _g_e_t_c_o_n_f command, which prints the values of sys-
tem configuration variables defined by POSIX.2, and a _d_i_s_o_w_n
builtin,  which  causes  a  shell  running  with job control
active to "forget about" one or more background jobs in  its
internal  jobs  table.   Using  _g_e_t_c_o_n_f, for example, a user
could retrieve a value for $_P_A_T_H guaranteed to find  all  of
the POSIX standard utilities, or find out how long filenames
may be in the file system containing a specified directory.

     There are no implementation timetables for any of these
features,  nor are there concrete plans to include them.  If
anyone has comments on these proposals, feel free to send me
electronic mail.

_6.  _R_e_f_l_e_c_t_i_o_n_s _a_n_d _L_e_s_s_o_n_s _L_e_a_r_n_e_d

     The lesson that has been  repeated  most  often  during
Bash  development  is  that  there  are  dark corners in the
Bourne shell, and people use all of them.  In  the  original
description of the Bourne shell, quoting and the shell gram-
mar are both poorly  specified  and  incomplete;  subsequent
descriptions have not helped much.  The grammar presented in



                      October 28, 1994





                           - 16 -


Bourne's paper describing the  shell  distributed  with  the
Seventh  Edition  of  UNIX|-  is  so far off that it does not
allow the command who|wc.  In fact, as Tom Duff states:

     Nobody really knows what the Bourne shell's  gram-
     mar  is.   Even  examination of the source code is
     little help.|=

The POSIX.2 standard includes  a  _y_a_c_c  grammar  that  comes
close  to  capturing  the  Bourne  shell's  behavior, but it
disallows some constructs which sh accepts without complaint
-  and there are scripts out there that use them.  It took a
few versions and several bug reports before Bash implemented
sh-compatible  quoting,  and there are still some "legal" sh
constructs which Bash flags as syntax errors.   Complete  sh
compatibility is a tough nut.

     The shell is bigger  and  slower  than  I  would  like,
though the current version is substantially faster than pre-
viously.  The readline library  could  stand  a  substantial
rewrite.   A  hand-written  parser  to  replace  the current
_y_a_c_c-generated one would probably result in a  speedup,  and
would solve one glaring problem:  the shell could parse com-
mands in "$(...)" constructs as  they  are  entered,  rather
than reporting errors when the construct is expanded.

     As always, there is some chaff to go  with  the  wheat.
Areas  of  duplicated  functionality  need to be cleaned up.
There are several cases where Bash treats  a  variable  spe-
cially   to   enable  functionality  available  another  way
($notify vs.  set -o notify and $nolinks vs. set  -o  physi-
cal,  for  instance);  the special treatment of the variable
name should probably be removed.  A few  more  things  could
stand    removal;    the    $_a_l_l_o_w__n_u_l_l__g_l_o_b__e_x_p_a_n_s_i_o_n   and
$_g_l_o_b__d_o_t__f_i_l_e_n_a_m_e_s variables are of particularly  question-
able  value.   The  $[...]  arithmetic  evaluation syntax is
redundant now that the POSIX-mandated $((...)) construct has
been implemented, and could be deleted.  It would be nice if
the text output by the _h_e_l_p builtin  were  external  to  the
shell rather than compiled into it.  The behavior enabled by
$_c_o_m_m_a_n_d__o_r_i_e_n_t_e_d__h_i_s_t_o_r_y, which causes the shell to attempt
to  save  all lines of a multi-line command in a single his-
tory entry, should be made  the  default  and  the  variable
removed.


_________________________
|-S. R. Bourne, "UNIX  Time-Sharing  System:   The  UNIX
Shell",  _B_e_l_l  _S_y_s_t_e_m  _T_e_c_h_n_i_c_a_l  _J_o_u_r_n_a_l, 57(6), July-
August, 1978, pp. 1971-1990.
|=Tom Duff, "Rc - A Shell for Plan 9 and UNIX  systems",
_P_r_o_c. _o_f _t_h_e _S_u_m_m_e_r _1_9_9_0 _E_U_U_G _C_o_n_f_e_r_e_n_c_e, London, July,
1990, pp. 21-33.




                      October 28, 1994





                           - 17 -


_7.  _A_v_a_i_l_a_b_i_l_i_t_y

     As with all other GNU software, Bash is  available  for
anonymous  FTP  from _p_r_e_p._a_i._m_i_t._e_d_u:/_p_u_b/_g_n_u and from other
GNU software mirror sites.  The current version is in  _b_a_s_h-
_1._1_4._1._t_a_r._g_z  in  that  directory.   Use _a_r_c_h_i_e to find the
nearest archive site.  The latest version is  always  avail-
able  for FTP from _b_a_s_h._C_W_R_U._E_d_u:/_p_u_b/_d_i_s_t.  Bash documenta-
tion is available for FTP from _b_a_s_h._C_W_R_U._E_d_u:/_p_u_b/_b_a_s_h.

     The Free Software Foundation sells  tapes  and  CD-ROMs
containing Bash; send electronic mail to gnu@prep.ai.mit.edu
or call +1-617-876-3296 for more information.

     Bash is  also  distributed  with  several  versions  of
UNIX-compatible  systems.   It  is  included  as /bin/sh and
/bin/bash on several Linux  distributions  (more  about  the
difference  in  a  moment),  and  as contributed software in
BSDI's BSD/386* and FreeBSD.

     The Linux distribution deserves special mention.  There
are two configurations included in the standard Bash distri-
bution: a "normal" configuration, in which all of the  stan-
dard  features  are included, and a "minimal" configuration,
which omits job control, aliases, history and  command  line
editing,  the  directory  stack and _p_u_s_h_d/_p_o_p_d/_d_i_r_s, process
substitution, prompt string special character decoding,  and
the  _s_e_l_e_c_t  construct.  This minimal version is designed to
be a drop-in replacement for the traditional  UNIX  /bin/sh,
and is included as the Linux /bin/sh in several packagings.

_8.  _C_o_n_c_l_u_s_i_o_n

     Bash is a worthy successor to sh.  It  is  sufficiently
portable to run on nearly every version of UNIX from 4.3 BSD
to SVR4.2, and several UNIX workalikes.  It is robust enough
to  replace  sh  on most of those systems, and provides more
functionality.  It has several thousand regular  users,  and
their  feedback has helped to make it as good as it is today
- a testament to the benefits of free software.










_________________________
*BSD/386 is a trademark of  Berkeley  Software  Design,
Inc.




                      October 28, 1994