1 This is the Bash FAQ, version 2.1, for Bash version 2.0.
3 This document contains a set of frequently-asked questions concerning
4 Bash, the GNU Bourne-Again Shell. Bash is a freely-available command
5 interpreter with advanced features for both interactive use and shell
8 Another good source of basic information about shells is the collection
9 of FAQ articles periodically posted to comp.unix.shell.
11 Questions and comments concerning this document should be sent to
14 This document is available for anonymous FTP with the URL
16 ftp://slc2.ins.cwru.edu/pub/bash/FAQ
24 2) What's the latest version?
25 3) Where can I get it?
26 4) On what machines will bash run?
27 5) How can I build bash with gcc?
28 6) How can I make bash my login shell?
29 7) I just changed my login shell to bash, and now I can't FTP into my
31 8) What's the `POSIX 1003.2 standard'?
32 9) What is the bash `posix mode'?
34 Section B: The latest version
36 10) What's new in version 2.0?
37 11) Are there any user-visible incompatibilities between bash-2.0 and
40 Section C: Differences from other Unix shells
42 12) How does bash differ from sh, the Bourne shell?
43 13) How does bash differ from the Korn shell, version ksh88?
44 14) Which new features in ksh-93 are not in bash, and which are?
46 Section D: Why does bash do some things differently than other Unix shells?
48 15) Why does bash run a different version of `command' than
49 `which command' says it will?
50 16) Why doesn't bash treat brace expansions exactly like csh?
51 17) Why doesn't bash have csh variable modifiers?
52 18) How can I make my csh aliases work when I convert to bash?
53 19) How can I pipe standard output and standard error from one command to
54 another, like csh does with `|&'?
55 20) Now that I've converted from ksh to bash, are there equivalents to
56 ksh features like autoloaded functions and the `whence' command?
58 Section E: How can I get bash to do certain things, and why does bash do
59 things the way it does?
61 21) Why is the bash builtin `test' slightly different from /bin/test?
62 22) Why does bash sometimes say `Broken pipe'?
63 23) How can I get bash to read and display eight-bit characters?
64 24) How do I write a function `x' to replace builtin command `x', but
65 still invoke the command from within the function?
66 25) When I have terminal escape sequences in my prompt, why does bash
67 wrap lines at the wrong column?
68 26) How can I find the value of a shell variable whose name is the value
69 of another shell variable?
70 27) If I pipe the output of a command into `read variable', why doesn't
71 the output show up in $variable when the read command finishes?
72 28) I have a bunch of shell scripts that use backslash-escaped characters
73 in arguments to `echo'. Bash doesn't interpret these characters. Why
74 not, and how can I make it understand them?
75 29) Why doesn't a while or for loop get suspended when I type ^Z?
76 30) How can I make the bash `time' reserved word print timing output that
77 looks like the output from my system's /usr/bin/time?
79 Section F: Things to watch out for on certain Unix versions
81 31) Why can't I use command line editing in my `cmdtool'?
82 32) I built bash on Solaris 2. Why do globbing expansions and filename
83 completion chop off the first few characters of each filename?
84 33) Why does bash dump core after I interrupt username completion or
85 `~user' tilde expansion on a machine running NIS?
86 34) I'm running SVR4.2. Why is the line erased every time I type `@'?
87 35) Why does bash report syntax errors when my C News scripts use a
88 redirection before a subshell command?
90 Section G: Where do I go from here?
92 36) How do I report bugs in bash, and where should I look for fixes and
94 37) What kind of bash documentation is there?
95 38) What's coming in future versions?
96 39) What's on the bash `wish list'?
97 40) When will the next release appear?
100 Section A: The Basics
104 Bash is a Unix command interpreter (shell). It is an implementation of
105 the Posix 1003.2 shell standard, and resembles the Korn and System V
108 Bash contains a number of enhancements over those shells, both
109 for interactive use and shell programming. Features geared
110 toward interactive use include command line editing, command
111 history, job control, aliases, and prompt expansion. Programming
112 features include additional variable expansions, shell
113 arithmetic, and a number of variables and options to control
116 Bash was originally written by Brian Fox of the Free Software
117 Foundation. The current developer and maintainer is Chet Ramey
118 of Case Western Reserve University.
120 2) What's the latest version?
122 The latest version is 2.0, first made available on December 23, 1996.
124 3) Where can I get it?
126 Bash is the GNU project's shell, and so is available from the
127 master GNU archive site, prep.ai.mit.edu, and its mirrors. The
128 latest version is also available for FTP from slc2.ins.cwru.edu,
129 the maintainer's machine. The following URLs tell how to get
132 ftp://prep.ai.mit.edu/pub/gnu/bash-2.0.tar.gz
133 ftp://slc2.ins.cwru.edu/pub/dist/bash-2.0.tar.gz
135 Formatted versions of the documentation are available with the URLs:
137 ftp://prep.ai.mit.edu/pub/gnu/bash-doc-2.0.tar.gz
138 ftp://slc2.ins.cwru.edu/pub/dist/bash-doc-2.0.tar.gz
140 4) On what machines will bash run?
142 Bash has been ported to nearly every version of UNIX. All you
143 should have to do to build it on a machine for which a port
144 exists is to type `configure' and then `make'. The build process
145 will attempt to discover the version of UNIX you have and tailor
146 itself accordingly, using a script created by GNU autoconf.
148 More information appears in the file `INSTALL' in the distribution.
150 5) How can I build bash with gcc?
152 Bash configures to use gcc by default if it is available. Read the
153 file INSTALL in the distribution for more information.
155 6) How can I make bash my login shell?
157 Some machines let you use `chsh' to change your login shell. Other
158 systems use `passwd -s'. If one of these works for you, that's all
159 you need. Note that many systems require the full pathname to a shell
160 to appear in /etc/shells before you can make it your login shell. For
161 this, you may need the assistance of your friendly local system
164 If you cannot do this, you can still use bash as your login shell, but
165 you need to perform some tricks. The basic idea is to add a command
166 to your login shell's startup file to replace your login shell with
169 For example, if your login shell is csh or tcsh, and you have installed
170 bash in /usr/gnu/bin/bash, add the following line to ~/.login:
172 if ( -f /usr/gnu/bin/bash ) exec /usr/gnu/bin/bash --login
174 (the `--login' tells bash that it is a login shell).
176 It's not a good idea to put this command into ~/.cshrc, because every
177 csh you run without the `-f' option, even ones started to run csh scripts,
178 reads that file. If you must put the command in ~/.cshrc, use something
181 if ( $?prompt ) exec /usr/gnu/bin/bash --login
183 to ensure that bash is exec'd only when the csh is interactive.
185 If your login shell is sh or ksh, you have to do two things. First, add
186 a line similar to the above to ~/.profile:
188 [ -f /usr/gnu/bin/bash ] && exec /usr/gnu/bin/bash --login
190 Next, create an empty file in your home directory named `.bash_profile'.
191 The existence of this file will prevent the exec'd bash from trying to
192 read ~/.profile, and re-execing itself over and over again. ~/.bash_profile
193 is the file bash tries to read initialization commands from when it is
194 invoked as a login shell.
196 7) I just changed my login shell to bash, and now I can't FTP into my
199 You must add the full pathname to bash to the file /etc/shells. As
200 noted in the answer to the previous question, many systems require
201 this before you can make bash your login shell.
203 Most versions of ftpd use this file to prohibit `special' users
204 such as `uucp' and `news' from using FTP.
206 8) What's the `POSIX 1003.2 standard'?
208 POSIX is a name originally coined by Richard Stallman for a
209 family of open system standards based on UNIX. There are a
210 number of aspects of UNIX under consideration for
211 standardization, from the basic system services at the system
212 call and C library level to applications and tools to system
213 administration and management. Each area of standardization is
214 assigned to a working group in the 1003 series.
216 The POSIX Shell and Utilities standard has been developed by IEEE
217 Working Group 1003.2 (POSIX.2). It concentrates on the command
218 interpreter interface and utility programs commonly executed from
219 the command line or by other programs. An initial version of the
220 standard has been approved and published by the IEEE, and work is
221 currently underway to update it.
223 Bash is concerned with the aspects of the shell's behavior
224 defined by POSIX.2. The shell command language has of course
225 been standardized, including the basic flow control and program
226 execution constructs, I/O redirection and pipelining, argument
227 handling, variable expansion, and quoting.
229 The `special' builtins, which must be implemented as part of the
230 shell to provide the desired functionality, are specified as
231 being part of the shell; examples of these are `eval' and
232 `export'. Other utilities appear in the sections of POSIX.2 not
233 devoted to the shell which are commonly (and in some cases must
234 be) implemented as builtin commands, such as `read' and `test'.
235 POSIX.2 also specifies aspects of the shell's interactive
236 behavior as part of the UPE, including job control and command
237 line editing. Only vi-style line editing commands have been
238 standardized; emacs editing commands were left out due to
241 9) What is the bash `posix mode'?
243 Although bash is an implementation of the POSIX.2 shell
244 specification, there are areas where the bash default behavior
245 differs from that spec. The bash `posix mode' changes the bash
246 behavior in these areas so that it obeys the spec more closely.
248 Posix mode is entered by starting bash with the --posix option or
249 executing `set -o posix' after bash is running.
251 The specific aspects of bash which change when posix mode is
252 active are listed in the file CWRU/POSIX.NOTES in the bash
253 distribution. They are also listed in a section in the Bash
256 Section B: The latest version
258 10) What's new in version 2.0?
260 This version contains extensive changes and new features. Here's a
263 new `time' reserved word to time pipelines, shell builtins, and
265 one-dimensional arrays with a new compound assignment statement,
266 appropriate expansion constructs and modifications to some
267 of the builtins (read, declare, etc.) to use them
268 new quoting syntaxes for ANSI-C string expansion and locale-specific
270 new expansions to do substring extraction, pattern replacement, and
271 indirect variable expansion
272 new builtins: `disown' and `shopt'
273 new variables: HISTIGNORE, SHELLOPTS, PIPESTATUS, DIRSTACK, GLOBIGNORE,
274 MACHTYPE, BASH_VERSINFO
275 special handling of many unused or redundant variables removed
276 (e.g., $notify, $glob_dot_filenames, $no_exit_on_failed_exec)
277 dynamic loading of new builtin commands; many loadable examples provided
278 new prompt expansions: \a, \e, \n, \H, \T, \@, \v, \V
279 history and aliases available in shell scripts
280 new readline variables: enable-keypad, mark-directories, input-meta,
281 visible-stats, disable-completion, comment-begin
282 new readline commands to manipulate the mark and operate on the region
283 new readline emacs mode commands and bindings for ksh-88 compatibility
284 updated and extended builtins
286 expanded (and now documented) restricted shell mode
288 implementation stuff:
289 autoconf-based configuration
290 nearly all of the bugs reported since version 1.14 have been fixed
291 most builtins converted to use builtin `getopt' for consistency
292 most builtins use -p option to display output in a reusable form
294 grammar tighter and smaller (66 reduce-reduce conflicts gone)
295 lots of code now smaller and faster
296 test suite greatly expanded
298 11) Are there any user-visible incompatibilities between bash-2.0 and
301 There are a few incompatibilities between version 1.14.7 and version 2.0.
302 They are detailed in the file COMPAT in the bash-2.0 distribution.
304 Section C: Differences from other Unix shells
306 12) How does bash differ from sh, the Bourne shell?
308 This is a non-comprehensive list of features that differentiate bash
309 from the SVR4.2 shell. The bash manual page explains these more
312 Things bash has that sh does not:
313 long invocation options
314 `!' reserved word to invert pipeline return value
315 `time' reserved word to time pipelines and shell builtins
316 the `function' reserved word
317 the select compound command and reserved word
318 new $'...' and $"..." quoting
319 the $(...) form of command substitution
320 the ${#param} parameter value length operator
321 the ${!param} indirect parameter expansion operator
322 the ${param:length[:offset]} parameter substring operator
323 the ${param/pat[/string]} parameter pattern substitution operator
324 expansions to perform substring removal (${p%[%]w}, ${p#[#]w})
325 expansion of positional parameters beyond $9 with ${num}
326 variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, REPLY,
327 TIMEFORMAT, PPID, PWD, OLDPWD, SHLVL, RANDOM, SECONDS,
328 LINENO, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE, HOSTNAME,
329 ENV, PS3, PS4, DIRSTACK, PIPESTATUS, HISTSIZE, HISTFILE,
330 HISTFILESIZE, HISTCONTROL, HISTIGNORE, GLOBIGNORE,
331 PROMPT_COMMAND, FCEDIT, FIGNORE, IGNOREEOF, INPUTRC,
332 SHELLOPTS, OPTERR, HOSTFILE, TMOUT, histchars, auto_resume
334 variable arrays with new compound assignment syntax
335 redirections: <>, &>, >|
336 prompt string special char translation and variable expansion
337 auto-export of modified values of variables in initial environment
338 command search finds functions before builtins
339 bash return builtin will exit a file sourced with `.'
340 builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E, hash -p.
341 export -n/-f/-p/name=value, pwd -L/-P, read -e/-p/-a,
342 readonly -a/-f/name=value, trap -l, set +o,
343 set -b/-m/-o option/-h/-p/-B/-C/-H/-P,
344 unset -f/-v, ulimit -m/-p/-u,
345 type -a/-p/-t, suspend -f, kill -n,
346 test -o optname/s1 == s2/s1 < s2/s1 > s2/-nt/-ot/-ef/-O/-G/-S
347 bash reads ~/.bashrc for interactive shells, $ENV for non-interactive
348 bash restricted shell mode is more extensive
349 bash allows functions and variables with the same name
352 arithmetic expansion with $((...)) and `let' builtin
354 aliases and alias/unalias builtins
355 local variables in functions and `local' builtin
356 readline and command-line editing
357 command history and history/fc builtins
358 csh-like history expansion
359 other new bash builtins: bind, command, builtin, declare/typeset,
360 dirs, enable, fc, help, history, logout,
361 popd, pushd, disown, shopt
363 filename generation when using output redirection (command >a*)
364 variable assignments preceding commands affect only that command,
365 even for builtins and functions
368 Things sh has that bash does not:
369 uses variable SHACCT to do shell accounting
370 includes `stop' builtin (bash can use alias stop='kill -s STOP')
372 turns on job control if called as `jsh'
373 ulimit attempts to set both soft & hard limits if -S/-H not given
374 $TIMEOUT (like bash $TMOUT)
375 `^' is a synonym for `|'
376 new SVR4.2 sh builtins: mldmode, priv
378 Implementation differences:
379 redirection to/from compound commands causes sh to create a subshell
380 bash does not allow unbalanced quotes; sh silently inserts them at EOF
381 bash does not mess with signal 11
382 sh sets (euid, egid) to (uid, gid) if -p not supplied and uid < 100
383 bash splits only the results of expansions on IFS, using POSIX.2
384 field splitting rules; sh splits all words on IFS
385 sh does not allow MAILCHECK to be unset (?)
386 sh does not allow traps on SIGALRM or SIGCHLD
387 bash allows multiple option arguments when invoked (e.g. -x -v);
388 sh allows only a single option argument (`sh -x -v' attempts
389 to open a file named `-v', and, on SunOS 4.1.4, dumps core)
390 sh exits a script if any builtin fails; bash exits only if one of
391 the POSIX.2 `special' builtins fails
393 13) How does bash differ from the Korn shell, version ksh88?
395 Things bash has or uses that ksh88 does not:
396 long invocation options
398 posix mode and posix conformance
400 tilde expansion for assignment statements that look like $PATH
401 process substitution with named pipes if /dev/fd is not available
402 the ${!param} indirect parameter expansion operator
403 the ${param:length[:offset]} parameter substring operator
404 the ${param/pat[/string]} parameter pattern substitution operator
405 variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, SHLVL,
406 TIMEFORMAT, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE,
407 HISTFILESIZE, HISTIGNORE, HISTCONTROL, PROMPT_COMMAND,
408 IGNOREEOF, FIGNORE, INPUTRC, HOSTFILE, DIRSTACK,
409 PIPESTATUS, HOSTNAME, OPTERR, SHELLOPTS, GLOBIGNORE,
410 histchars, auto_resume
411 prompt expansion with backslash escapes and command substitution
412 redirection: &> (stdout and stderr)
413 more extensive and extensible editing and completion
414 builtins: bind, builtin, command, declare, dirs, echo -e/-E, enable,
415 exec -l/-c/-a, fc -s, export -n/-f/-p, hash, help, history,
416 jobs -x/-r/-s, kill -s/-n/-l, local, logout, popd, pushd,
417 read -e/-p/-a, readonly -a/-n/-f/-p, set -o braceexpand/
418 -o histexpand/-o interactive-comments/-o notify/-o physical/
419 -o posix/-o hashall/-o onecmd/-h/-B/-C/-b/-H/-P, set +o,
420 suspend, trap -l, type, typeset -a/-F/-p, ulimit -u,
421 umask -S, alias -p, shopt, disown
422 `!' csh-style history expansion
424 Things ksh88 has or uses that bash does not:
425 new version of test: [[...]]
428 variables: ERRNO, FPATH, COLUMNS, LINES, EDITOR, VISUAL
429 extended pattern matching with egrep-style pattern lists
430 co-processes (|&, >&p, <&p)
431 weirdly-scoped functions
432 typeset +f to list all function names without definitions
433 text of command history kept in a file, not memory
434 builtins: alias -x, cd old new, fc -e -, newgrp, print,
435 read -p/-s/-u/var?prompt, set -A/-o gmacs/
436 -o bgnice/-o markdirs/-o nolog/-o trackall/-o viraw/-s,
437 typeset -H/-L/-R/-A/-ft/-fu/-fx/-l/-u/-t, whence
439 Implementation differences:
440 ksh runs last command of a pipeline in parent shell context
441 ksh ulimit sets hard and soft limits by default
442 bash has brace expansion by default (ksh88 compile-time option)
443 bash has fixed startup file for all interactive shells; ksh reads $ENV
444 bash has exported functions
445 bash command search finds functions before builtins
447 14) Which new features in ksh-93 are not in bash, and which are?
449 New things in ksh-93 not in bash-2.0:
451 floating point arithmetic
452 ++, --, comma arithmetic operators
453 math library functions
454 ${!name[sub]} name of subscript for associative array
455 ${!prefix*} and {!prefix@} variable name prefix expansions
456 `.' is allowed in variable names to create a hierarchical namespace
457 more extensive compound assignment syntax
459 `sleep' and `getconf' builtins (bash has loadable versions)
460 typeset -n and `nameref' variables
462 variables: .sh.edchar, .sh.edmode, .sh.edcol, .sh.edtext, HISTEDIT,
463 .sh.version, .sh.name, .sh.subscript, .sh.value
464 backreferences in pattern matching
465 print -f and printf (bash has loadable versions)
466 `fc' has been renamed to `hist'
468 `.' can execute shell functions
469 ENV processed only for interactive shells
471 New things in ksh-93 present in bash-2.0:
472 ?: arithmetic operator
473 expansions: ${!param}, ${param:len[:offset]}, ${param/pat[/str]}
474 compound array assignment
475 the `!' reserved word
476 loadable builtins -- but ksh uses `builtin' while bash uses `enable'
477 `command', `builtin', `disown' builtins
478 new $'...' and $"..." quoting
479 FIGNORE (but bash uses GLOBIGNORE), HISTCMD
481 changes to kill builtin
482 read -A (bash uses read -a)
485 `.' restores the positional parameters when it completes
489 command and arithmetic substitution performed on PS1, PS4, and ENV
490 command name completion
492 Section D: Why does bash do some things differently than other Unix shells?
494 15) Why does bash run a different version of `command' than
495 `which command' says it will?
497 `which' is actually a csh script that assumes you're running csh.
498 It reads the csh startup files from your home directory and uses
499 those to determine which `command' will be invoked. Since bash
500 doesn't use any of those startup files, there's a good chance
501 that your bash environment differs from your csh environment.
503 16) Why doesn't bash treat brace expansions exactly like csh?
505 The only difference between bash and csh brace expansion is that
506 bash requires a brace expression to contain at least one unquoted
507 comma if it is to be expanded. Any brace-surrounded word not
508 containing an unquoted comma is left unchanged by the brace
509 expansion code. This affords the greatest degree of sh
512 Bash, ksh, zsh, and pd-ksh all implement brace expansion this way.
514 17) Why doesn't bash have csh variable modifiers?
516 Posix has specified a more powerful, albeit somewhat more cryptic,
517 mechanism cribbed from ksh, and bash implements it.
520 Remove smallest suffix pattern. The WORD is expanded to produce
521 a pattern. It then expands to the value of PARAMETER, with the
522 smallest portion of the suffix matched by the pattern deleted.
530 Remove largest suffix pattern. The WORD is expanded to produce
531 a pattern. It then expands to the value of PARAMETER, with the
532 largest portion of the suffix matched by the pattern deleted.
539 Remove smallest prefix pattern. The WORD is expanded to produce
540 a pattern. It then expands to the value of PARAMETER, with the
541 smallest portion of the prefix matched by the pattern deleted.
548 Remove largest prefix pattern. The WORD is expanded to produce
549 a pattern. It then expands to the value of PARAMETER, with the
550 largest portion of the prefix matched by the pattern deleted.
569 18) How can I make my csh aliases work when I convert to bash?
571 Bash uses a different syntax to support aliases than csh does.
572 The details can be found in the documentation. We have provided
573 a shell script which does most of the work of conversion for you;
574 this script can be found in ./examples/misc/alias-conv.sh. Here is
577 Start csh in the normal way for you. (e.g., `csh')
579 Pipe the output of `alias' through `alias-conv.sh', saving the
580 results into `bash_aliases':
582 alias | alias-conv.sh >bash_aliases
584 Edit `bash_aliases', carefully reading through any created
585 functions. You will need to change the names of some csh specific
586 variables to the bash equivalents. The script converts $cwd to
587 $PWD, $term to $TERM, $home to $HOME, $user to $USER, and $prompt
588 to $PS1. You may also have to add quotes to avoid unwanted
591 For example, the csh alias:
593 alias cd 'cd \!*; echo $cwd'
595 is converted to the bash function:
597 cd () { command cd "$@"; echo $PWD ; }
599 The only thing that needs to be done is to quote $PWD:
601 cd () { command cd "$@"; echo "$PWD" ; }
603 Merge the edited file into your ~/.bashrc.
605 There is an additional, more ambitious, script in
606 examples/misc/cshtobash that attempts to convert your entire csh
607 environment to its bash equivalent. This script can be run as
608 simply `cshtobash' to convert your normal interactive
609 environment, or as `cshtobash ~/.login' to convert your login
612 19) How can I pipe standard output and standard error from one command to
613 another, like csh does with `|&'?
616 command 2>&1 | command2
618 The key is to remember that piping is performed before redirection, so
619 file descriptor 1 points to the pipe when it is duplicated onto file
622 20) Now that I've converted from ksh to bash, are there equivalents to
623 ksh features like autoloaded functions and the `whence' command?
625 There are features in ksh-88 that do not have direct bash equivalents.
626 Most, however, can be emulated with very little trouble.
628 ksh-88 feature Bash equivalent
629 -------------- ---------------
630 [[...]] can usually use [...]; minor differences (no
631 pattern matching, for one)
632 compiled-in aliases set up aliases in .bashrc; some ksh aliases are
633 bash builtins (hash, history, type)
635 extended patterns no good substitute
636 coprocesses named pipe pairs (one for read, one for write)
637 typeset +f declare -F
638 cd, print, whence function substitutes in examples/functions/kshenv
639 autoloaded functions examples/functions/autoload is the same as typeset -fu
640 read var?prompt read -p prompt var
642 Section E: How can I get bash to do certain things, and why does bash do
643 things the way it does?
645 21) Why is the bash builtin `test' slightly different from /bin/test?
647 The specific example used here is [ ! x -o x ], which is false.
649 Bash's builtin `test' implements the Posix.2 spec, which can be
650 summarized as follows (the wording is due to David Korn):
652 Here is the set of rules for processing test arguments.
655 1 Arg: True iff argument is not null.
656 2 Args: If first arg is !, True iff second argument is null.
657 If first argument is unary, then true if unary test is true
659 3 Args: If second argument is a binary operator, do binary test of $1 $3
660 If first argument is !, negate two argument test of $2 $3
661 If first argument is `(' and third argument is `)', do the
662 one-argument test of the second argument.
664 4 Args: If first argument is !, negate three argument test of $2 $3 $4.
665 Otherwise unspecified
666 5 or more Args: unspecified. (Historical shells would use their
669 The operators -a and -o are considered binary operators for the purpose
672 As you can see, the test becomes (not (x or x)), which is false.
674 22) Why does bash sometimes say `Broken pipe'?
676 If a sequence of commands appears in a pipeline, and one of the
677 reading commands finishes before the writer has finished, the
678 writer receives a SIGPIPE signal. Many other shells special-case
679 SIGPIPE as an exit status in the pipeline and do not report it.
684 `head' can finish before `ps' writes all of its output, and ps
685 will try to write on a pipe without a reader. In that case, bash
686 will print `Broken pipe' to stderr when ps is killed by a
689 23) How can I get bash to read and display eight-bit characters?
691 This is a process requiring several steps.
693 First, you must ensure that the `physical' data path is a full eight
694 bits. For xterms, for example, the `vt100' resources `eightBitInput'
695 and `eightBitOutput' should be set to `true'.
697 Once you have set up an eight-bit path, you must tell the kernel and
698 tty driver to leave the eighth bit of characters alone when processing
699 keyboard input. Use `stty' to do this:
701 stty cs8 -istrip -parenb
703 For old BSD-style systems, you can use
711 Finally, you need to tell readline that you will be inputting and
712 displaying eight-bit characters. You use readline variables to do
713 this. These variables can be set in your .inputrc or using the bash
714 `bind' builtin. Here's an example using `bind':
716 bash$ bind 'set convert-meta off'
717 bash$ bind 'set meta-flag on'
718 bash$ bind 'set output-meta on'
720 The `set' commands between the single quotes may also be placed
723 24) How do I write a function `x' to replace builtin command `x', but
724 still invoke the command from within the function?
726 This is why the `command' and `builtin' builtins exist. The
727 `command' builtin executes the command supplied as its first
728 argument, skipping over any function defined with that name. The
729 `builtin' builtin executes the builtin command given as its first
732 For example, to write a function to replace `cd' that writes the
733 hostname and current directory to an xterm title bar, use
734 something like the following:
738 builtin cd "$@" && xtitle "$HOST: $PWD"
741 This could also be written using `command' instead of `builtin';
742 the version above is marginally more efficient.
744 25) When I have terminal escape sequences in my prompt, why does bash
745 wrap lines at the wrong column?
747 Readline, the line editing library that bash uses, does not know
748 that the terminal escape sequences do not take up space on the
749 screen. The redisplay code assumes, unless told otherwise, that
750 each character in the prompt is a `printable' character that
751 takes up one character position on the screen.
753 You can use the bash prompt expansion facility (see the PROMPTING
754 section in the manual page) to tell readline that sequences of
755 characters in the prompt strings take up no screen space.
757 Use the \[ escape to begin a sequence of non-printing characters,
758 and the \] escape to signal the end of such a sequence.
760 26) How can I find the value of a shell variable whose name is the value
761 of another shell variable?
763 Bash-2.0 supports this directly. You can use
767 For example, the following sequence of commands will echo `z':
773 For sh compatibility, use the `eval' builtin. The important
774 thing to remember is that `eval' expands the arguments you give
775 it again, so you need to quote the parts of the arguments that
776 you want `eval' to act on.
778 For example, this expression prints the value of the last positional
781 eval echo \"\$\{$#\}\"
783 The expansion of the quoted portions of this expression will be
784 deferred until `eval' runs, while the `$#' will be expanded
785 before `eval' is executed. In bash-2.0,
791 27) If I pipe the output of a command into `read variable', why doesn't
792 the output show up in $variable when the read command finishes?
794 This has to do with the parent-child relationship between Unix
797 Each element of a pipeline runs in a separate process, a child of
798 the shell running the pipeline. A subprocess cannot affect its
799 parent's environment. When the `read' command sets the variable
800 to the input, that variable is set only in the subshell, not the
801 parent shell. When the subshell exits, the value of the variable
804 Many pipelines that end with `read variable' can be converted
805 into command substitutions, which will capture the output of
806 a specified command. The output can then be assigned to a
809 grep ^gnu /usr/lib/news/active | wc -l | read ngroup
811 can be converted into
813 ngroup=$(grep ^gnu /usr/lib/news/active | wc -l)
815 This does not, unfortunately, work to split the text among
816 multiple variables, as read does when given multiple variable
817 arguments. If you need to do this, you can either use the
818 command substitution above to read the output into a variable
819 and chop up the variable using the bash pattern removal
820 expansion operators or use some variant of the following
823 Say /usr/local/bin/ipaddr is the following shell script:
826 host `hostname` | awk '/address/ {print $NF}'
830 /usr/local/bin/ipaddr | read A B C D
832 to break the local machine's IP address into separate octets, use
836 set -- $(/usr/local/bin/ipaddr)
838 A="$1" B="$2" C="$3" D="$4"
840 Beware, however, that this will change the shell's positional
841 parameters. If you need them, you should save them before doing
844 This is the general approach -- in most cases you will not need to
845 set $IFS to a different value.
847 28) I have a bunch of shell scripts that use backslash-escaped characters
848 in arguments to `echo'. Bash doesn't interpret these characters. Why
849 not, and how can I make it understand them?
851 This is the behavior of echo on most Unix System V machines.
853 The bash builtin `echo' is modelled after the 9th Edition
854 Research Unix version of `echo'. It does not interpret
855 backslash-escaped characters in its argument strings by default;
856 it requires the use of the -e option to enable the
857 interpretation. The System V echo provides no way to disable the
858 special characters; the bash echo has a -E option to disable
861 There is a configuration option that will make bash behave like
862 the System V echo and interpret things like `\t' by default. Run
863 configure with the --enable-usg-echo-default option to turn this
864 on. Be aware that this will cause some of the tests run when you
865 type `make tests' to fail.
867 29) Why doesn't a while or for loop get suspended when I type ^Z?
869 This is a consequence of how job control works on Unix. The only
870 thing that can be suspended is the process group. This is a single
871 command or pipeline of commands that the shell forks and executes.
873 When you run a while or for loop, the only thing that the shell forks
874 and executes are any commands in the while loop test and commands in
875 the loop bodies. These, therefore, are the only things that can be
876 suspended when you type ^Z.
878 If you want to be able to stop the entire loop, you need to put it
879 within parentheses, which will force the loop into a subshell that
880 may be stopped (and subsequently restarted) as a single unit.
882 30) How can I make the bash `time' reserved word print timing output that
883 looks like the output from my system's /usr/bin/time?
885 The bash command timing code looks for a variable `TIMEFORMAT' and
886 uses its value as a format string to decide how to display the
889 The value of TIMEFORMAT is a string with `%' escapes expanded in a
890 fashion similar in spirit to printf(3). The manual page explains
891 the meanings of the escape sequences in the format string.
893 If TIMEFORMAT is not set, bash acts as if the following assignment had
896 TIMEFORMAT=$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
898 The POSIX.2 default time format (used by `time -p command') is
900 TIMEFORMAT=$'real %2R\nuser %2U\nsys %2S'
902 The BSD /usr/bin/time format can be emulated with:
904 TIMEFORMAT=$'\t%1R real\t%1U user\t%1S sys'
906 The System V /usr/bin/time format can be emulated with:
908 TIMEFORMAT=$'\nreal\t%1R\nuser\t%1U\nsys\t%1S'
910 The ksh format can be emulated with:
912 TIMEFORMAT=$'\nreal\t%2lR\nuser\t%2lU\nsys\t%2lS'
914 Section F: Things to watch out for on certain Unix versions
916 31) Why can't I use command line editing in my `cmdtool'?
918 The problem is `cmdtool' and bash fighting over the input. When
919 scrolling is enabled in a cmdtool window, cmdtool puts the tty in
920 `raw mode' to permit command-line editing using the mouse for
921 applications that cannot do it themselves. As a result, bash and
922 cmdtool each try to read keyboard input immediately, with neither
923 getting enough of it to be useful.
925 This mode also causes cmdtool to not implement many of the
926 terminal functions and control sequences appearing in the
927 `sun-cmd' termcap entry. For a more complete explanation, see
928 that file examples/suncmd.termcap in the bash distribution.
930 `xterm' is a better choice, and gets along with bash much more
933 If you must use cmdtool, you can use the termcap description in
934 examples/suncmd.termcap. Set the TERMCAP variable to the terminal
935 description contained in that file, i.e.
937 TERMCAP='Mu|sun-cmd:am:bs:km:pt:li#34:co#80:cl=^L:ce=\E[K:cd=\E[J:rs=\E[s:'
939 Then export TERMCAP and start a new cmdtool window from that shell.
940 The bash command-line editing should behave better in the new
941 cmdtool. If this works, you can put the assignment to TERMCAP
944 32) I built bash on Solaris 2. Why do globbing expansions and filename
945 completion chop off the first few characters of each filename?
947 This is the consequence of building bash on SunOS 5 and linking
948 with the libraries in /usr/ucblib, but using the definitions
949 and structures from files in /usr/include.
951 The actual conflict is between the dirent structure in
952 /usr/include/dirent.h and the struct returned by the version of
953 `readdir' in libucb.a (a 4.3-BSD style `struct direct').
955 Make sure you've got /usr/ccs/bin ahead of /usr/ucb in your $PATH
956 when configuring and building bash. This will ensure that you
957 use /usr/ccs/bin/cc or acc instead of /usr/ucb/cc and that you
958 link with libc before libucb.
960 If you have installed the Sun C compiler, you may also need to
961 put /usr/ccs/bin and /opt/SUNWspro/bin into your $PATH before
964 33) Why does bash dump core after I interrupt username completion or
965 `~user' tilde expansion on a machine running NIS?
967 This is a famous and long-standing bug in the SunOS YP (sorry, NIS)
968 client library, which is part of libc.
970 The YP library code keeps static state -- a pointer into the data
971 returned from the server. When YP initializes itself (setpwent),
972 it looks at this pointer and calls free on it if it's non-null.
975 If one of the YP functions is interrupted during getpwent (the
976 exact function is interpretwithsave()), and returns NULL, the
977 pointer is freed without being reset to NULL, and the function
978 returns. The next time getpwent is called, it sees that this
979 pointer is non-null, calls free, and the bash free() blows up
980 because it's being asked to free freed memory.
982 The traditional Unix mallocs allow memory to be freed multiple
983 times; that's probably why this has never been fixed. You can
984 run configure with the `--without-gnu-malloc' option to use
985 the C library malloc and avoid the problem.
987 34) I'm running SVR4.2. Why is the line erased every time I type `@'?
989 The `@' character is the default `line kill' character in most
990 versions of System V, including SVR4.2. You can change this
991 character to whatever you want using `stty'. For example, to
992 change the line kill character to control-u, type
996 where the `^' and `U' can be two separate characters.
998 35) Why does bash report syntax errors when my C News scripts use a
999 redirection before a subshell command?
1001 The actual command in question is something like
1005 According to the grammar given in the POSIX.2 standard, this construct
1006 is, in fact, a syntax error. Redirections may only precede `simple
1007 commands'. A subshell construct such as the above is one of the shell's
1008 `compound commands'. A redirection may only follow a compound command.
1010 The file CWRU/sh-redir-hack in the bash-2.0 distribution is an
1011 (unofficial) patch to parse.y that will modify the grammar to
1012 support this construct. It will not apply with `patch'; you must
1013 modify parse.y by hand. Note that if you apply this, you must
1014 recompile with -DREDIRECTION_HACK. This introduces a large
1015 number of reduce/reduce conflicts into the shell grammar.
1017 Section G: Where do I go from here?
1019 36) How do I report bugs in bash, and where should I look for fixes and
1022 Use the `bashbug' script to report bugs. It is built and
1023 installed at the same time as bash. It provides a standard
1024 template for reporting a problem and automatically includes
1025 information about your configuration and build environment.
1027 `bashbug' sends its reports to bug-bash@prep.ai.mit.edu, which
1028 is a large mailing list gatewayed to the usenet newsgroup gnu.bash.bug.
1030 Bug fixes, answers to questions, and announcements of new releases
1031 are all posted to gnu.bash.bug. Discussions concerning bash features
1032 and problems also take place there.
1034 To reach the bash maintainers directly, send mail to
1035 bash-maintainers@prep.ai.mit.edu.
1037 37) What kind of bash documentation is there?
1039 First, look in the doc directory in the bash distribution. It should
1040 contain at least the following files:
1042 bash.1 an extensive, thorough Unix-style manual page
1043 builtins.1 a manual page covering just bash builtin commands
1044 bashref.texi a reference manual in GNU info format
1045 bash.html an HTML version of the manual page
1046 bashref.html an HTML version of the reference manual
1048 article.ms text of an article written for The Linux Journal
1049 readline.3 a man page describing readline
1051 Postscript files created from the above source are available in
1052 the documentation distribution.
1054 There is additional documentation available for anonymous FTP from host
1055 slc2.ins.cwru.edu in the `pub/bash' directory.
1057 Cameron Newham and Bill Rosenblatt have written a book on bash, published
1058 by O'Reilly and Associates. The book is based on Bill Rosenblatt's Korn
1059 Shell book. The title is ``Learning the Bash Shell''. The ISBN number is
1060 1-56592-147-X. Look for it in fine bookstores near you. This book covers
1061 bash-1.14, but has an appendix describing some of the new features in
1064 38) What's coming in future versions?
1066 These are features I plan to include in a future version of bash.
1068 POSIX.2-style globbing character classes ([:alpha:], [:alnum:], etc.)
1069 a bash debugger (an incomplete, untested version is included with bash-2.0)
1071 39) What's on the bash `wish list' for future versions?
1073 These are features that may or may not appear in a future version of bash.
1075 Programmable completion a la zsh
1076 menu completion a la tcsh
1077 the ksh [[...]] extended test command
1078 the ksh egrep-style extended pattern matching operators
1079 associative arrays (not really all that hard)
1080 breaking some of the shell functionality into embeddable libraries
1081 better internationalization using GNU `gettext'
1082 an option to use external files for the long `help' text
1083 timeouts for the `read' builtin
1084 the ksh-93 ${!prefix*} and ${!prefix@} operators
1085 arithmetic ++ and -- prefix and postfix operators
1087 40) When will the next release appear?
1089 The next version will appear sometime in 1997. Never make predictions.
1092 This document is Copyright 1995, 1996 by Chester Ramey.
1094 Permission is hereby granted, without written agreement and
1095 without license or royalty fees, to use, copy, and distribute
1096 this document for any purpose, provided that the above copyright
1097 notice appears in all copies of this document and that the
1098 contents of this document remain unaltered.