This is the Bash FAQ, version 2.5, for Bash version 2.01.
This document contains a set of frequently-asked questions concerning
Bash, the GNU Bourne-Again Shell. Bash is a freely-available command
interpreter with advanced features for both interactive use and shell
programming.
Another good source of basic information about shells is the collection
of FAQ articles periodically posted to comp.unix.shell.
Questions and comments concerning this document should be sent to
chet@po.cwru.edu.
This document is available for anonymous FTP with the URL
ftp://slc2.ins.cwru.edu/pub/bash/FAQ
-----------------------------------------------------------------------------------------------------------------------------------------------
-----------------------------------------------------------------------------------------------------------------------------------------------
Contents:
Section A: The Basics
1) What is it?
2) What's the latest version?
3) Where can I get it?
4) On what machines will bash run?
5) Will bash run on operating systems other than Unix?
6) How can I build bash with gcc?
7) How can I make bash my login shell?
8) I just changed my login shell to bash, and now I can't FTP into my
machine. Why not?
9) What's the `POSIX 1003.2 standard'?
10) What is the bash `posix mode'?
Section B: The latest version
11) What's new in version 2.01?
12) Are there any user-visible incompatibilities between bash-2.01 and
bash-1.14.7?
Section C: Differences from other Unix shells
13) How does bash differ from sh, the Bourne shell?
14) How does bash differ from the Korn shell, version ksh88?
15) Which new features in ksh-93 are not in bash, and which are?
Section D: Why does bash do some things differently than other Unix shells?
16) Why does bash run a different version of `command' than
`which command' says it will?
17) Why doesn't bash treat brace expansions exactly like csh?
18) Why doesn't bash have csh variable modifiers?
19) How can I make my csh aliases work when I convert to bash?
20) How can I pipe standard output and standard error from one command to
another, like csh does with `|&'?
21) Now that I've converted from ksh to bash, are there equivalents to
ksh features like autoloaded functions and the `whence' command?
Section E: How can I get bash to do certain things, and why does bash do
things the way it does?
22) Why is the bash builtin `test' slightly different from /bin/test?
23) Why does bash sometimes say `Broken pipe'?
24) How can I get bash to read and display eight-bit characters?
25) How do I write a function `x' to replace builtin command `x', but
still invoke the command from within the function?
26) When I have terminal escape sequences in my prompt, why does bash
wrap lines at the wrong column?
27) How can I find the value of a shell variable whose name is the value
of another shell variable?
28) If I pipe the output of a command into `read variable', why doesn't
the output show up in $variable when the read command finishes?
29) I have a bunch of shell scripts that use backslash-escaped characters
in arguments to `echo'. Bash doesn't interpret these characters. Why
not, and how can I make it understand them?
30) Why doesn't a while or for loop get suspended when I type ^Z?
31) How can I make the bash `time' reserved word print timing output that
looks like the output from my system's /usr/bin/time?
Section F: Things to watch out for on certain Unix versions
32) Why can't I use command line editing in my `cmdtool'?
33) I built bash on Solaris 2. Why do globbing expansions and filename
completion chop off the first few characters of each filename?
34) Why does bash dump core after I interrupt username completion or
`~user' tilde expansion on a machine running NIS?
35) I'm running SVR4.2. Why is the line erased every time I type `@'?
36) Why does bash report syntax errors when my C News scripts use a
redirection before a subshell command?
Section G: Where do I go from here?
37) How do I report bugs in bash, and where should I look for fixes and
advice?
38) What kind of bash documentation is there?
39) What's coming in future versions?
40) What's on the bash `wish list'?
41) When will the next release appear?
---------------------------------------------------------------------------------------------------------------------------------------------------
---------------------------------------------------------------------------------------------------------------------------------------------------
Section A: The Basics
1) What is it?
Bash is a Unix command interpreter (shell). It is an implementation of
the Posix 1003.2 shell standard, and resembles the Korn and System V
shells.
Bash contains a number of enhancements over those shells, both
for interactive use and shell programming. Features geared
toward interactive use include command line editing, command
history, job control, aliases, and prompt expansion. Programming
features include additional variable expansions, shell
arithmetic, and a number of variables and options to control
shell behavior.
Bash was originally written by Brian Fox of the Free Software
Foundation. The current developer and maintainer is Chet Ramey
of Case Western Reserve University.
2) What's the latest version?
The latest version is 2.01, first made available on June 6, 1997.
3) Where can I get it?
Bash is the GNU project's shell, and so is available from the
master GNU archive site, prep.ai.mit.edu, and its mirrors. The
latest version is also available for FTP from slc2.ins.cwru.edu,
the maintainer's machine. The following URLs tell how to get
version 2.01:
ftp://prep.ai.mit.edu/pub/gnu/bash-2.01.tar.gz
ftp://slc2.ins.cwru.edu/pub/dist/bash-2.01.tar.gz
Formatted versions of the documentation are available with the URLs:
ftp://prep.ai.mit.edu/pub/gnu/bash-doc-2.01.tar.gz
ftp://slc2.ins.cwru.edu/pub/dist/bash-doc-2.01.tar.gz
4) On what machines will bash run?
Bash has been ported to nearly every version of UNIX. All you
should have to do to build it on a machine for which a port
exists is to type `configure' and then `make'. The build process
will attempt to discover the version of UNIX you have and tailor
itself accordingly, using a script created by GNU autoconf.
More information appears in the file `INSTALL' in the distribution.
5) Will bash run on operating systems other than Unix?
Configuration specifics for Unix-like systems such as QNX and
LynxOS are included in the distribution. Previous versions of
bash have been ported to Minix, but I don't believe anyone has
built bash-2.x on Minix yet.
Bash has been ported to versions of Windows implementing the Win32
programming interface. This includes Windows 95 and Windows NT.
The port was done by Cygnus Solutions as part of their GNU-Win32
project. For more information about the project, look at the URL
http://www.cygnus.com/misc/gnu-win32
Cygnus has ported bash-1.14.7. Maybe someday they (or I) will port
bash-2.01 (or later) to the GNU-Win32 environment.
D. J. Delorie has ported bash-1.14.7 to run under MS-DOS, as part of
the DJGPP project. For more information on the project, see
http://www.delorie.com/djgpp/
I picked up a binary of bash-1.14.7 that is purported to work with
the DJGPP V2 environment from
ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh1147b.zip
The corresponding source is
ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh1147s.zip
A port of bash-1.12 is available for OS/2 from
ftp://hobbes.nmsu.edu/os2/unix/bash_112.zip
I haven't looked at it.
6) How can I build bash with gcc?
Bash configures to use gcc by default if it is available. Read the
file INSTALL in the distribution for more information.
7) How can I make bash my login shell?
Some machines let you use `chsh' to change your login shell. Other
systems use `passwd -s'. If one of these works for you, that's all
you need. Note that many systems require the full pathname to a shell
to appear in /etc/shells before you can make it your login shell. For
this, you may need the assistance of your friendly local system
administrator.
If you cannot do this, you can still use bash as your login shell, but
you need to perform some tricks. The basic idea is to add a command
to your login shell's startup file to replace your login shell with
bash.
For example, if your login shell is csh or tcsh, and you have installed
bash in /usr/gnu/bin/bash, add the following line to ~/.login:
if ( -f /usr/gnu/bin/bash ) exec /usr/gnu/bin/bash --login
(the `--login' tells bash that it is a login shell).
It's not a good idea to put this command into ~/.cshrc, because every
csh you run without the `-f' option, even ones started to run csh scripts,
reads that file. If you must put the command in ~/.cshrc, use something
like
if ( $?prompt ) exec /usr/gnu/bin/bash --login
to ensure that bash is exec'd only when the csh is interactive.
If your login shell is sh or ksh, you have to do two things.
First, create an empty file in your home directory named `.bash_profile'.
The existence of this file will prevent the exec'd bash from trying to
read ~/.profile, and re-execing itself over and over again. ~/.bash_profile
is the first file bash tries to read initialization commands from when
it is invoked as a login shell.
Next, add a line similar to the above to ~/.profile:
[ -f /usr/gnu/bin/bash ] && exec /usr/gnu/bin/bash --login
This will cause login shells to replace themselves with bash running as
a login shell. Once you have this working, you can copy your initialization
code from ~/.profile to ~/.bash_profile.
8) I just changed my login shell to bash, and now I can't FTP into my
machine. Why not?
You must add the full pathname to bash to the file /etc/shells. As
noted in the answer to the previous question, many systems require
this before you can make bash your login shell.
Most versions of ftpd use this file to prohibit `special' users
such as `uucp' and `news' from using FTP.
9) What's the `POSIX 1003.2 standard'?
POSIX is a name originally coined by Richard Stallman 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 system
call and C library level to applications and tools to system
administration and management. Each area of standardization 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 concentrates 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.
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 control and program
execution constructs, I/O redirection and pipelining, argument
handling, variable expansion, and quoting.
The `special' builtins, which must be implemented as part of the
shell to provide the desired functionality, are specified as
being part of the shell; examples of these are `eval' and
`export'. 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 `read' and `test'.
POSIX.2 also specifies aspects of the shell's interactive
behavior as part of the UPE, including job control and command
line editing. Only vi-style line editing commands have been
standardized; emacs editing commands were left out due to
objections.
10) What is the bash `posix mode'?
Although bash is an implementation of the POSIX.2 shell
specification, there are areas where the bash default behavior
differs from that spec. The bash `posix mode' changes the bash
behavior in these areas so that it obeys the spec more closely.
Posix mode is entered by starting bash with the --posix option or
executing `set -o posix' after bash is running.
The specific aspects of bash which change when posix mode is
active are listed in the file CWRU/POSIX.NOTES in the bash
distribution. They are also listed in a section in the Bash
Reference Manual.
Section B: The latest version
11) What's new in version 2.01?
Bash-2.01 contains only a few new features.
new `GROUPS' builtin array variable containing the user's group list
new bindable readline commands: history-and-alias-expand-line and
alias-expand-line
Bash-2.0 contains extensive changes and new features from bash-1.14.7.
Here's a short list:
new `time' reserved word to time pipelines, shell builtins, and
shell functions
one-dimensional arrays with a new compound assignment statement,
appropriate expansion constructs and modifications to some
of the builtins (read, declare, etc.) to use them
new quoting syntaxes for ANSI-C string expansion and locale-specific
string translation
new expansions to do substring extraction, pattern replacement, and
indirect variable expansion
new builtins: `disown' and `shopt'
new variables: HISTIGNORE, SHELLOPTS, PIPESTATUS, DIRSTACK, GLOBIGNORE,
MACHTYPE, BASH_VERSINFO
special handling of many unused or redundant variables removed
(e.g., $notify, $glob_dot_filenames, $no_exit_on_failed_exec)
dynamic loading of new builtin commands; many loadable examples provided
new prompt expansions: \a, \e, \n, \H, \T, \@, \v, \V
history and aliases available in shell scripts
new readline variables: enable-keypad, mark-directories, input-meta,
visible-stats, disable-completion, comment-begin
new readline commands to manipulate the mark and operate on the region
new readline emacs mode commands and bindings for ksh-88 compatibility
updated and extended builtins
new DEBUG trap
expanded (and now documented) restricted shell mode
implementation stuff:
autoconf-based configuration
nearly all of the bugs reported since version 1.14 have been fixed
most builtins converted to use builtin `getopt' for consistency
most builtins use -p option to display output in a reusable form
(for consistency)
grammar tighter and smaller (66 reduce-reduce conflicts gone)
lots of code now smaller and faster
test suite greatly expanded
12) Are there any user-visible incompatibilities between bash-2.01 and
bash-1.14.7?
There are a few incompatibilities between version 1.14.7 and version 2.01.
They are detailed in the file COMPAT in the bash-2.01 distribution.
Section C: Differences from other Unix shells
13) How does bash differ from sh, the Bourne shell?
This is a non-comprehensive list of features that differentiate bash
from the SVR4.2 shell. The bash manual page explains these more
completely.
Things bash has that sh does not:
long invocation options
`!' reserved word to invert pipeline return value
`time' reserved word to time pipelines and shell builtins
the `function' reserved word
the select compound command and reserved word
new $'...' and $"..." quoting
the $(...) form of command substitution
the ${#param} parameter value length operator
the ${!param} indirect parameter expansion operator
the ${param:length[:offset]} parameter substring operator
the ${param/pat[/string]} parameter pattern substitution operator
expansions to perform substring removal (${p%[%]w}, ${p#[#]w})
expansion of positional parameters beyond $9 with ${num}
variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, REPLY,
TIMEFORMAT, PPID, PWD, OLDPWD, SHLVL, RANDOM, SECONDS,
LINENO, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE, HOSTNAME,
ENV, PS3, PS4, DIRSTACK, PIPESTATUS, HISTSIZE, HISTFILE,
HISTFILESIZE, HISTCONTROL, HISTIGNORE, GLOBIGNORE,
PROMPT_COMMAND, FCEDIT, FIGNORE, IGNOREEOF, INPUTRC,
SHELLOPTS, OPTERR, HOSTFILE, TMOUT, histchars, auto_resume
DEBUG trap
variable arrays with new compound assignment syntax
redirections: <>, &>, >|
prompt string special char translation and variable expansion
auto-export of modified values of variables in initial environment
command search finds functions before builtins
bash return builtin will exit a file sourced with `.'
builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E, hash -p.
export -n/-f/-p/name=value, pwd -L/-P, read -e/-p/-a,
readonly -a/-f/name=value, trap -l, set +o,
set -b/-m/-o option/-h/-p/-B/-C/-H/-P,
unset -f/-v, ulimit -m/-p/-u,
type -a/-p/-t, suspend -f, kill -n,
test -o optname/s1 == s2/s1 < s2/s1 > s2/-nt/-ot/-ef/-O/-G/-S
bash reads ~/.bashrc for interactive shells, $ENV for non-interactive
bash restricted shell mode is more extensive
bash allows functions and variables with the same name
brace expansion
tilde expansion
arithmetic expansion with $((...)) and `let' builtin
process substitution
aliases and alias/unalias builtins
local variables in functions and `local' builtin
readline and command-line editing
command history and history/fc builtins
csh-like history expansion
other new bash builtins: bind, command, builtin, declare/typeset,
dirs, enable, fc, help, history, logout,
popd, pushd, disown, shopt
exported functions
filename generation when using output redirection (command >a*)
variable assignments preceding commands affect only that command,
even for builtins and functions
posix mode
Things sh has that bash does not:
uses variable SHACCT to do shell accounting
includes `stop' builtin (bash can use alias stop='kill -s STOP')
`newgrp' builtin
turns on job control if called as `jsh'
$TIMEOUT (like bash $TMOUT)
`^' is a synonym for `|'
new SVR4.2 sh builtins: mldmode, priv
Implementation differences:
redirection to/from compound commands causes sh to create a subshell
bash does not allow unbalanced quotes; sh silently inserts them at EOF
bash does not mess with signal 11
sh sets (euid, egid) to (uid, gid) if -p not supplied and uid < 100
bash splits only the results of expansions on IFS, using POSIX.2
field splitting rules; sh splits all words on IFS
sh does not allow MAILCHECK to be unset (?)
sh does not allow traps on SIGALRM or SIGCHLD
bash allows multiple option arguments when invoked (e.g. -x -v);
sh allows only a single option argument (`sh -x -v' attempts
to open a file named `-v', and, on SunOS 4.1.4, dumps core.
On Solaris 2, sh goes into an infinite loop.)
sh exits a script if any builtin fails; bash exits only if one of
the POSIX.2 `special' builtins fails
14) How does bash differ from the Korn shell, version ksh88?
Things bash has or uses that ksh88 does not:
long invocation options
`!' reserved word
posix mode and posix conformance
command hashing
tilde expansion for assignment statements that look like $PATH
process substitution with named pipes if /dev/fd is not available
the ${!param} indirect parameter expansion operator
the ${param:length[:offset]} parameter substring operator
the ${param/pat[/string]} parameter pattern substitution operator
variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, SHLVL,
TIMEFORMAT, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE,
HISTFILESIZE, HISTIGNORE, HISTCONTROL, PROMPT_COMMAND,
IGNOREEOF, FIGNORE, INPUTRC, HOSTFILE, DIRSTACK,
PIPESTATUS, HOSTNAME, OPTERR, SHELLOPTS, GLOBIGNORE,
histchars, auto_resume
prompt expansion with backslash escapes and command substitution
redirection: &> (stdout and stderr)
more extensive and extensible editing and completion
builtins: bind, builtin, command, declare, dirs, echo -e/-E, enable,
exec -l/-c/-a, fc -s, export -n/-f/-p, hash, help, history,
jobs -x/-r/-s, kill -s/-n/-l, local, logout, popd, pushd,
read -e/-p/-a, readonly -a/-n/-f/-p, set -o braceexpand/
-o histexpand/-o interactive-comments/-o notify/-o physical/
-o posix/-o hashall/-o onecmd/-h/-B/-C/-b/-H/-P, set +o,
suspend, trap -l, type, typeset -a/-F/-p, ulimit -u,
umask -S, alias -p, shopt, disown
`!' csh-style history expansion
Things ksh88 has or uses that bash does not:
new version of test: [[...]]
tracked aliases
$(&p, <&p)
weirdly-scoped functions
typeset +f to list all function names without definitions
text of command history kept in a file, not memory
builtins: alias -x, cd old new, fc -e -, newgrp, print,
read -p/-s/-u/var?prompt, set -A/-o gmacs/
-o bgnice/-o markdirs/-o nolog/-o trackall/-o viraw/-s,
typeset -H/-L/-R/-A/-ft/-fu/-fx/-l/-u/-t, whence
Implementation differences:
ksh runs last command of a pipeline in parent shell context
bash has brace expansion by default (ksh88 compile-time option)
bash has fixed startup file for all interactive shells; ksh reads $ENV
bash has exported functions
bash command search finds functions before builtins
15) Which new features in ksh-93 are not in bash, and which are?
New things in ksh-93 not in bash-2.01:
associative arrays
floating point arithmetic
++, --, comma arithmetic operators
math library functions
${!name[sub]} name of subscript for associative array
${!prefix*} and {!prefix@} variable name prefix expansions
`.' is allowed in variable names to create a hierarchical namespace
more extensive compound assignment syntax
discipline functions
`sleep' and `getconf' builtins (bash has loadable versions)
typeset -n and `nameref' variables
KEYBD trap
variables: .sh.edchar, .sh.edmode, .sh.edcol, .sh.edtext, HISTEDIT,
.sh.version, .sh.name, .sh.subscript, .sh.value
backreferences in pattern matching
print -f and printf (bash has loadable versions)
`fc' has been renamed to `hist'
read -t/-d
`.' can execute shell functions
New things in ksh-93 present in bash-2.01:
?: arithmetic operator
expansions: ${!param}, ${param:offset[:len]}, ${param/pat[/str]}
compound array assignment
the `!' reserved word
loadable builtins -- but ksh uses `builtin' while bash uses `enable'
`command', `builtin', `disown' builtins
new $'...' and $"..." quoting
FIGNORE (but bash uses GLOBIGNORE), HISTCMD
set -o notify/-C
changes to kill builtin
read -A (bash uses read -a)
trap -p
exec -c/-a
`.' restores the positional parameters when it completes
POSIX.2 `test'
umask -S
unalias -a
command and arithmetic substitution performed on PS1, PS4, and ENV
command name completion
ENV processed only for interactive shells
Section D: Why does bash do some things differently than other Unix shells?
16) Why does bash run a different version of `command' than
`which command' says it will?
`which' is actually a csh script that assumes you're running csh.
It reads the csh startup files from your home directory and uses
those to determine which `command' will be invoked. Since bash
doesn't use any of those startup files, there's a good chance
that your bash environment differs from your csh environment.
17) Why doesn't bash treat brace expansions exactly like csh?
The only difference between bash and csh brace expansion is that
bash requires a brace expression to contain at least one unquoted
comma if it is to be expanded. Any brace-surrounded word not
containing an unquoted comma is left unchanged by the brace
expansion code. This affords the greatest degree of sh
compatibility.
Bash, ksh, zsh, and pd-ksh all implement brace expansion this way.
18) Why doesn't bash have csh variable modifiers?
Posix has specified a more powerful, albeit somewhat more cryptic,
mechanism cribbed from ksh, and bash implements it.
${parameter%word}
Remove smallest suffix pattern. The WORD is expanded to produce
a pattern. It then expands to the value of PARAMETER, with the
smallest portion of the suffix matched by the pattern deleted.
x=file.c
echo ${x%.c}.o
-->file.o
${parameter%%word}
Remove largest suffix pattern. The WORD is expanded to produce
a pattern. It then expands to the value of PARAMETER, with the
largest portion of the suffix matched by the pattern deleted.
x=posix/src/std
echo ${x%%/*}
-->posix
${parameter#word}
Remove smallest prefix pattern. The WORD is expanded to produce
a pattern. It then expands to the value of PARAMETER, with the
smallest portion of the prefix matched by the pattern deleted.
x=$HOME/src/cmd
echo ${x#$HOME}
-->/src/cmd
${parameter##word}
Remove largest prefix pattern. The WORD is expanded to produce
a pattern. It then expands to the value of PARAMETER, with the
largest portion of the prefix matched by the pattern deleted.
x=/one/two/three
echo ${x##*/}
-->three
Given
a=/a/b/c/d
b=b.xxx
csh bash result
--- ---- ------
$a:h ${a%/*} /a/b/c
$a:t ${a##*/} d
$b:r ${b%.*} b
$b:e ${b##*.} xxx
19) How can I make my csh aliases work when I convert to bash?
Bash uses a different syntax to support aliases than csh does.
The details can be found in the documentation. We have provided
a shell script which does most of the work of conversion for you;
this script can be found in ./examples/misc/alias-conv.sh. Here is
how you use it:
Start csh in the normal way for you. (e.g., `csh')
Pipe the output of `alias' through `alias-conv.sh', saving the
results into `bash_aliases':
alias | alias-conv.sh >bash_aliases
Edit `bash_aliases', carefully reading through any created
functions. You will need to change the names of some csh specific
variables to the bash equivalents. The script converts $cwd to
$PWD, $term to $TERM, $home to $HOME, $user to $USER, and $prompt
to $PS1. You may also have to add quotes to avoid unwanted
expansion.
For example, the csh alias:
alias cd 'cd \!*; echo $cwd'
is converted to the bash function:
cd () { command cd "$@"; echo $PWD ; }
The only thing that needs to be done is to quote $PWD:
cd () { command cd "$@"; echo "$PWD" ; }
Merge the edited file into your ~/.bashrc.
There is an additional, more ambitious, script in
examples/misc/cshtobash that attempts to convert your entire csh
environment to its bash equivalent. This script can be run as
simply `cshtobash' to convert your normal interactive
environment, or as `cshtobash ~/.login' to convert your login
environment.
20) How can I pipe standard output and standard error from one command to
another, like csh does with `|&'?
Use
command 2>&1 | command2
The key is to remember that piping is performed before redirection, so
file descriptor 1 points to the pipe when it is duplicated onto file
descriptor 2.
21) Now that I've converted from ksh to bash, are there equivalents to
ksh features like autoloaded functions and the `whence' command?
There are features in ksh-88 that do not have direct bash equivalents.
Most, however, can be emulated with very little trouble.
ksh-88 feature Bash equivalent
-------------- ---------------
[[...]] can usually use [...]; minor differences (no
pattern matching, for one)
compiled-in aliases set up aliases in .bashrc; some ksh aliases are
bash builtins (hash, history, type)
$(