1 This is the Bash FAQ, version 2.5, for Bash version 2.01.
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) Will bash run on operating systems other than Unix?
28 6) How can I build bash with gcc?
29 7) How can I make bash my login shell?
30 8) I just changed my login shell to bash, and now I can't FTP into my
32 9) What's the `POSIX 1003.2 standard'?
33 10) What is the bash `posix mode'?
35 Section B: The latest version
37 11) What's new in version 2.01?
38 12) Are there any user-visible incompatibilities between bash-2.01 and
41 Section C: Differences from other Unix shells
43 13) How does bash differ from sh, the Bourne shell?
44 14) How does bash differ from the Korn shell, version ksh88?
45 15) Which new features in ksh-93 are not in bash, and which are?
47 Section D: Why does bash do some things differently than other Unix shells?
49 16) Why does bash run a different version of `command' than
50 `which command' says it will?
51 17) Why doesn't bash treat brace expansions exactly like csh?
52 18) Why doesn't bash have csh variable modifiers?
53 19) How can I make my csh aliases work when I convert to bash?
54 20) How can I pipe standard output and standard error from one command to
55 another, like csh does with `|&'?
56 21) Now that I've converted from ksh to bash, are there equivalents to
57 ksh features like autoloaded functions and the `whence' command?
59 Section E: How can I get bash to do certain things, and why does bash do
60 things the way it does?
62 22) Why is the bash builtin `test' slightly different from /bin/test?
63 23) Why does bash sometimes say `Broken pipe'?
64 24) How can I get bash to read and display eight-bit characters?
65 25) How do I write a function `x' to replace builtin command `x', but
66 still invoke the command from within the function?
67 26) When I have terminal escape sequences in my prompt, why does bash
68 wrap lines at the wrong column?
69 27) How can I find the value of a shell variable whose name is the value
70 of another shell variable?
71 28) If I pipe the output of a command into `read variable', why doesn't
72 the output show up in $variable when the read command finishes?
73 29) I have a bunch of shell scripts that use backslash-escaped characters
74 in arguments to `echo'. Bash doesn't interpret these characters. Why
75 not, and how can I make it understand them?
76 30) Why doesn't a while or for loop get suspended when I type ^Z?
77 31) How can I make the bash `time' reserved word print timing output that
78 looks like the output from my system's /usr/bin/time?
80 Section F: Things to watch out for on certain Unix versions
82 32) Why can't I use command line editing in my `cmdtool'?
83 33) I built bash on Solaris 2. Why do globbing expansions and filename
84 completion chop off the first few characters of each filename?
85 34) Why does bash dump core after I interrupt username completion or
86 `~user' tilde expansion on a machine running NIS?
87 35) I'm running SVR4.2. Why is the line erased every time I type `@'?
88 36) Why does bash report syntax errors when my C News scripts use a
89 redirection before a subshell command?
91 Section G: Where do I go from here?
93 37) How do I report bugs in bash, and where should I look for fixes and
95 38) What kind of bash documentation is there?
96 39) What's coming in future versions?
97 40) What's on the bash `wish list'?
98 41) When will the next release appear?
101 Section A: The Basics
105 Bash is a Unix command interpreter (shell). It is an implementation of
106 the Posix 1003.2 shell standard, and resembles the Korn and System V
109 Bash contains a number of enhancements over those shells, both
110 for interactive use and shell programming. Features geared
111 toward interactive use include command line editing, command
112 history, job control, aliases, and prompt expansion. Programming
113 features include additional variable expansions, shell
114 arithmetic, and a number of variables and options to control
117 Bash was originally written by Brian Fox of the Free Software
118 Foundation. The current developer and maintainer is Chet Ramey
119 of Case Western Reserve University.
121 2) What's the latest version?
123 The latest version is 2.01, first made available on June 6, 1997.
125 3) Where can I get it?
127 Bash is the GNU project's shell, and so is available from the
128 master GNU archive site, prep.ai.mit.edu, and its mirrors. The
129 latest version is also available for FTP from slc2.ins.cwru.edu,
130 the maintainer's machine. The following URLs tell how to get
133 ftp://prep.ai.mit.edu/pub/gnu/bash-2.01.tar.gz
134 ftp://slc2.ins.cwru.edu/pub/dist/bash-2.01.tar.gz
136 Formatted versions of the documentation are available with the URLs:
138 ftp://prep.ai.mit.edu/pub/gnu/bash-doc-2.01.tar.gz
139 ftp://slc2.ins.cwru.edu/pub/dist/bash-doc-2.01.tar.gz
141 4) On what machines will bash run?
143 Bash has been ported to nearly every version of UNIX. All you
144 should have to do to build it on a machine for which a port
145 exists is to type `configure' and then `make'. The build process
146 will attempt to discover the version of UNIX you have and tailor
147 itself accordingly, using a script created by GNU autoconf.
149 More information appears in the file `INSTALL' in the distribution.
151 5) Will bash run on operating systems other than Unix?
153 Configuration specifics for Unix-like systems such as QNX and
154 LynxOS are included in the distribution. Previous versions of
155 bash have been ported to Minix, but I don't believe anyone has
156 built bash-2.x on Minix yet.
158 Bash has been ported to versions of Windows implementing the Win32
159 programming interface. This includes Windows 95 and Windows NT.
160 The port was done by Cygnus Solutions as part of their GNU-Win32
161 project. For more information about the project, look at the URL
163 http://www.cygnus.com/misc/gnu-win32
165 Cygnus has ported bash-1.14.7. Maybe someday they (or I) will port
166 bash-2.01 (or later) to the GNU-Win32 environment.
168 D. J. Delorie has ported bash-1.14.7 to run under MS-DOS, as part of
169 the DJGPP project. For more information on the project, see
171 http://www.delorie.com/djgpp/
173 I picked up a binary of bash-1.14.7 that is purported to work with
174 the DJGPP V2 environment from
176 ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh1147b.zip
178 The corresponding source is
180 ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh1147s.zip
182 A port of bash-1.12 is available for OS/2 from
184 ftp://hobbes.nmsu.edu/os2/unix/bash_112.zip
186 I haven't looked at it.
188 6) How can I build bash with gcc?
190 Bash configures to use gcc by default if it is available. Read the
191 file INSTALL in the distribution for more information.
193 7) How can I make bash my login shell?
195 Some machines let you use `chsh' to change your login shell. Other
196 systems use `passwd -s'. If one of these works for you, that's all
197 you need. Note that many systems require the full pathname to a shell
198 to appear in /etc/shells before you can make it your login shell. For
199 this, you may need the assistance of your friendly local system
202 If you cannot do this, you can still use bash as your login shell, but
203 you need to perform some tricks. The basic idea is to add a command
204 to your login shell's startup file to replace your login shell with
207 For example, if your login shell is csh or tcsh, and you have installed
208 bash in /usr/gnu/bin/bash, add the following line to ~/.login:
210 if ( -f /usr/gnu/bin/bash ) exec /usr/gnu/bin/bash --login
212 (the `--login' tells bash that it is a login shell).
214 It's not a good idea to put this command into ~/.cshrc, because every
215 csh you run without the `-f' option, even ones started to run csh scripts,
216 reads that file. If you must put the command in ~/.cshrc, use something
219 if ( $?prompt ) exec /usr/gnu/bin/bash --login
221 to ensure that bash is exec'd only when the csh is interactive.
223 If your login shell is sh or ksh, you have to do two things.
225 First, create an empty file in your home directory named `.bash_profile'.
226 The existence of this file will prevent the exec'd bash from trying to
227 read ~/.profile, and re-execing itself over and over again. ~/.bash_profile
228 is the first file bash tries to read initialization commands from when
229 it is invoked as a login shell.
231 Next, add a line similar to the above to ~/.profile:
233 [ -f /usr/gnu/bin/bash ] && exec /usr/gnu/bin/bash --login
235 This will cause login shells to replace themselves with bash running as
236 a login shell. Once you have this working, you can copy your initialization
237 code from ~/.profile to ~/.bash_profile.
239 8) I just changed my login shell to bash, and now I can't FTP into my
242 You must add the full pathname to bash to the file /etc/shells. As
243 noted in the answer to the previous question, many systems require
244 this before you can make bash your login shell.
246 Most versions of ftpd use this file to prohibit `special' users
247 such as `uucp' and `news' from using FTP.
249 9) What's the `POSIX 1003.2 standard'?
251 POSIX is a name originally coined by Richard Stallman for a
252 family of open system standards based on UNIX. There are a
253 number of aspects of UNIX under consideration for
254 standardization, from the basic system services at the system
255 call and C library level to applications and tools to system
256 administration and management. Each area of standardization is
257 assigned to a working group in the 1003 series.
259 The POSIX Shell and Utilities standard has been developed by IEEE
260 Working Group 1003.2 (POSIX.2). It concentrates on the command
261 interpreter interface and utility programs commonly executed from
262 the command line or by other programs. An initial version of the
263 standard has been approved and published by the IEEE, and work is
264 currently underway to update it.
266 Bash is concerned with the aspects of the shell's behavior
267 defined by POSIX.2. The shell command language has of course
268 been standardized, including the basic flow control and program
269 execution constructs, I/O redirection and pipelining, argument
270 handling, variable expansion, and quoting.
272 The `special' builtins, which must be implemented as part of the
273 shell to provide the desired functionality, are specified as
274 being part of the shell; examples of these are `eval' and
275 `export'. Other utilities appear in the sections of POSIX.2 not
276 devoted to the shell which are commonly (and in some cases must
277 be) implemented as builtin commands, such as `read' and `test'.
278 POSIX.2 also specifies aspects of the shell's interactive
279 behavior as part of the UPE, including job control and command
280 line editing. Only vi-style line editing commands have been
281 standardized; emacs editing commands were left out due to
284 10) What is the bash `posix mode'?
286 Although bash is an implementation of the POSIX.2 shell
287 specification, there are areas where the bash default behavior
288 differs from that spec. The bash `posix mode' changes the bash
289 behavior in these areas so that it obeys the spec more closely.
291 Posix mode is entered by starting bash with the --posix option or
292 executing `set -o posix' after bash is running.
294 The specific aspects of bash which change when posix mode is
295 active are listed in the file CWRU/POSIX.NOTES in the bash
296 distribution. They are also listed in a section in the Bash
299 Section B: The latest version
301 11) What's new in version 2.01?
303 Bash-2.01 contains only a few new features.
305 new `GROUPS' builtin array variable containing the user's group list
306 new bindable readline commands: history-and-alias-expand-line and
309 Bash-2.0 contains extensive changes and new features from bash-1.14.7.
312 new `time' reserved word to time pipelines, shell builtins, and
314 one-dimensional arrays with a new compound assignment statement,
315 appropriate expansion constructs and modifications to some
316 of the builtins (read, declare, etc.) to use them
317 new quoting syntaxes for ANSI-C string expansion and locale-specific
319 new expansions to do substring extraction, pattern replacement, and
320 indirect variable expansion
321 new builtins: `disown' and `shopt'
322 new variables: HISTIGNORE, SHELLOPTS, PIPESTATUS, DIRSTACK, GLOBIGNORE,
323 MACHTYPE, BASH_VERSINFO
324 special handling of many unused or redundant variables removed
325 (e.g., $notify, $glob_dot_filenames, $no_exit_on_failed_exec)
326 dynamic loading of new builtin commands; many loadable examples provided
327 new prompt expansions: \a, \e, \n, \H, \T, \@, \v, \V
328 history and aliases available in shell scripts
329 new readline variables: enable-keypad, mark-directories, input-meta,
330 visible-stats, disable-completion, comment-begin
331 new readline commands to manipulate the mark and operate on the region
332 new readline emacs mode commands and bindings for ksh-88 compatibility
333 updated and extended builtins
335 expanded (and now documented) restricted shell mode
337 implementation stuff:
338 autoconf-based configuration
339 nearly all of the bugs reported since version 1.14 have been fixed
340 most builtins converted to use builtin `getopt' for consistency
341 most builtins use -p option to display output in a reusable form
343 grammar tighter and smaller (66 reduce-reduce conflicts gone)
344 lots of code now smaller and faster
345 test suite greatly expanded
347 12) Are there any user-visible incompatibilities between bash-2.01 and
350 There are a few incompatibilities between version 1.14.7 and version 2.01.
351 They are detailed in the file COMPAT in the bash-2.01 distribution.
353 Section C: Differences from other Unix shells
355 13) How does bash differ from sh, the Bourne shell?
357 This is a non-comprehensive list of features that differentiate bash
358 from the SVR4.2 shell. The bash manual page explains these more
361 Things bash has that sh does not:
362 long invocation options
363 `!' reserved word to invert pipeline return value
364 `time' reserved word to time pipelines and shell builtins
365 the `function' reserved word
366 the select compound command and reserved word
367 new $'...' and $"..." quoting
368 the $(...) form of command substitution
369 the ${#param} parameter value length operator
370 the ${!param} indirect parameter expansion operator
371 the ${param:length[:offset]} parameter substring operator
372 the ${param/pat[/string]} parameter pattern substitution operator
373 expansions to perform substring removal (${p%[%]w}, ${p#[#]w})
374 expansion of positional parameters beyond $9 with ${num}
375 variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, REPLY,
376 TIMEFORMAT, PPID, PWD, OLDPWD, SHLVL, RANDOM, SECONDS,
377 LINENO, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE, HOSTNAME,
378 ENV, PS3, PS4, DIRSTACK, PIPESTATUS, HISTSIZE, HISTFILE,
379 HISTFILESIZE, HISTCONTROL, HISTIGNORE, GLOBIGNORE,
380 PROMPT_COMMAND, FCEDIT, FIGNORE, IGNOREEOF, INPUTRC,
381 SHELLOPTS, OPTERR, HOSTFILE, TMOUT, histchars, auto_resume
383 variable arrays with new compound assignment syntax
384 redirections: <>, &>, >|
385 prompt string special char translation and variable expansion
386 auto-export of modified values of variables in initial environment
387 command search finds functions before builtins
388 bash return builtin will exit a file sourced with `.'
389 builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E, hash -p.
390 export -n/-f/-p/name=value, pwd -L/-P, read -e/-p/-a,
391 readonly -a/-f/name=value, trap -l, set +o,
392 set -b/-m/-o option/-h/-p/-B/-C/-H/-P,
393 unset -f/-v, ulimit -m/-p/-u,
394 type -a/-p/-t, suspend -f, kill -n,
395 test -o optname/s1 == s2/s1 < s2/s1 > s2/-nt/-ot/-ef/-O/-G/-S
396 bash reads ~/.bashrc for interactive shells, $ENV for non-interactive
397 bash restricted shell mode is more extensive
398 bash allows functions and variables with the same name
401 arithmetic expansion with $((...)) and `let' builtin
403 aliases and alias/unalias builtins
404 local variables in functions and `local' builtin
405 readline and command-line editing
406 command history and history/fc builtins
407 csh-like history expansion
408 other new bash builtins: bind, command, builtin, declare/typeset,
409 dirs, enable, fc, help, history, logout,
410 popd, pushd, disown, shopt
412 filename generation when using output redirection (command >a*)
413 variable assignments preceding commands affect only that command,
414 even for builtins and functions
417 Things sh has that bash does not:
418 uses variable SHACCT to do shell accounting
419 includes `stop' builtin (bash can use alias stop='kill -s STOP')
421 turns on job control if called as `jsh'
422 $TIMEOUT (like bash $TMOUT)
423 `^' is a synonym for `|'
424 new SVR4.2 sh builtins: mldmode, priv
426 Implementation differences:
427 redirection to/from compound commands causes sh to create a subshell
428 bash does not allow unbalanced quotes; sh silently inserts them at EOF
429 bash does not mess with signal 11
430 sh sets (euid, egid) to (uid, gid) if -p not supplied and uid < 100
431 bash splits only the results of expansions on IFS, using POSIX.2
432 field splitting rules; sh splits all words on IFS
433 sh does not allow MAILCHECK to be unset (?)
434 sh does not allow traps on SIGALRM or SIGCHLD
435 bash allows multiple option arguments when invoked (e.g. -x -v);
436 sh allows only a single option argument (`sh -x -v' attempts
437 to open a file named `-v', and, on SunOS 4.1.4, dumps core.
438 On Solaris 2, sh goes into an infinite loop.)
439 sh exits a script if any builtin fails; bash exits only if one of
440 the POSIX.2 `special' builtins fails
442 14) How does bash differ from the Korn shell, version ksh88?
444 Things bash has or uses that ksh88 does not:
445 long invocation options
447 posix mode and posix conformance
449 tilde expansion for assignment statements that look like $PATH
450 process substitution with named pipes if /dev/fd is not available
451 the ${!param} indirect parameter expansion operator
452 the ${param:length[:offset]} parameter substring operator
453 the ${param/pat[/string]} parameter pattern substitution operator
454 variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, SHLVL,
455 TIMEFORMAT, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE,
456 HISTFILESIZE, HISTIGNORE, HISTCONTROL, PROMPT_COMMAND,
457 IGNOREEOF, FIGNORE, INPUTRC, HOSTFILE, DIRSTACK,
458 PIPESTATUS, HOSTNAME, OPTERR, SHELLOPTS, GLOBIGNORE,
459 histchars, auto_resume
460 prompt expansion with backslash escapes and command substitution
461 redirection: &> (stdout and stderr)
462 more extensive and extensible editing and completion
463 builtins: bind, builtin, command, declare, dirs, echo -e/-E, enable,
464 exec -l/-c/-a, fc -s, export -n/-f/-p, hash, help, history,
465 jobs -x/-r/-s, kill -s/-n/-l, local, logout, popd, pushd,
466 read -e/-p/-a, readonly -a/-n/-f/-p, set -o braceexpand/
467 -o histexpand/-o interactive-comments/-o notify/-o physical/
468 -o posix/-o hashall/-o onecmd/-h/-B/-C/-b/-H/-P, set +o,
469 suspend, trap -l, type, typeset -a/-F/-p, ulimit -u,
470 umask -S, alias -p, shopt, disown
471 `!' csh-style history expansion
473 Things ksh88 has or uses that bash does not:
474 new version of test: [[...]]
477 variables: ERRNO, FPATH, COLUMNS, LINES, EDITOR, VISUAL
478 extended pattern matching with egrep-style pattern lists
479 co-processes (|&, >&p, <&p)
480 weirdly-scoped functions
481 typeset +f to list all function names without definitions
482 text of command history kept in a file, not memory
483 builtins: alias -x, cd old new, fc -e -, newgrp, print,
484 read -p/-s/-u/var?prompt, set -A/-o gmacs/
485 -o bgnice/-o markdirs/-o nolog/-o trackall/-o viraw/-s,
486 typeset -H/-L/-R/-A/-ft/-fu/-fx/-l/-u/-t, whence
488 Implementation differences:
489 ksh runs last command of a pipeline in parent shell context
490 bash has brace expansion by default (ksh88 compile-time option)
491 bash has fixed startup file for all interactive shells; ksh reads $ENV
492 bash has exported functions
493 bash command search finds functions before builtins
495 15) Which new features in ksh-93 are not in bash, and which are?
497 New things in ksh-93 not in bash-2.01:
499 floating point arithmetic
500 ++, --, comma arithmetic operators
501 math library functions
502 ${!name[sub]} name of subscript for associative array
503 ${!prefix*} and {!prefix@} variable name prefix expansions
504 `.' is allowed in variable names to create a hierarchical namespace
505 more extensive compound assignment syntax
507 `sleep' and `getconf' builtins (bash has loadable versions)
508 typeset -n and `nameref' variables
510 variables: .sh.edchar, .sh.edmode, .sh.edcol, .sh.edtext, HISTEDIT,
511 .sh.version, .sh.name, .sh.subscript, .sh.value
512 backreferences in pattern matching
513 print -f and printf (bash has loadable versions)
514 `fc' has been renamed to `hist'
516 `.' can execute shell functions
518 New things in ksh-93 present in bash-2.01:
519 ?: arithmetic operator
520 expansions: ${!param}, ${param:offset[:len]}, ${param/pat[/str]}
521 compound array assignment
522 the `!' reserved word
523 loadable builtins -- but ksh uses `builtin' while bash uses `enable'
524 `command', `builtin', `disown' builtins
525 new $'...' and $"..." quoting
526 FIGNORE (but bash uses GLOBIGNORE), HISTCMD
528 changes to kill builtin
529 read -A (bash uses read -a)
532 `.' restores the positional parameters when it completes
536 command and arithmetic substitution performed on PS1, PS4, and ENV
537 command name completion
538 ENV processed only for interactive shells
540 Section D: Why does bash do some things differently than other Unix shells?
542 16) Why does bash run a different version of `command' than
543 `which command' says it will?
545 `which' is actually a csh script that assumes you're running csh.
546 It reads the csh startup files from your home directory and uses
547 those to determine which `command' will be invoked. Since bash
548 doesn't use any of those startup files, there's a good chance
549 that your bash environment differs from your csh environment.
551 17) Why doesn't bash treat brace expansions exactly like csh?
553 The only difference between bash and csh brace expansion is that
554 bash requires a brace expression to contain at least one unquoted
555 comma if it is to be expanded. Any brace-surrounded word not
556 containing an unquoted comma is left unchanged by the brace
557 expansion code. This affords the greatest degree of sh
560 Bash, ksh, zsh, and pd-ksh all implement brace expansion this way.
562 18) Why doesn't bash have csh variable modifiers?
564 Posix has specified a more powerful, albeit somewhat more cryptic,
565 mechanism cribbed from ksh, and bash implements it.
568 Remove smallest suffix pattern. The WORD is expanded to produce
569 a pattern. It then expands to the value of PARAMETER, with the
570 smallest portion of the suffix matched by the pattern deleted.
578 Remove largest suffix pattern. The WORD is expanded to produce
579 a pattern. It then expands to the value of PARAMETER, with the
580 largest portion of the suffix matched by the pattern deleted.
587 Remove smallest prefix pattern. The WORD is expanded to produce
588 a pattern. It then expands to the value of PARAMETER, with the
589 smallest portion of the prefix matched by the pattern deleted.
596 Remove largest prefix pattern. The WORD is expanded to produce
597 a pattern. It then expands to the value of PARAMETER, with the
598 largest portion of the prefix matched by the pattern deleted.
617 19) How can I make my csh aliases work when I convert to bash?
619 Bash uses a different syntax to support aliases than csh does.
620 The details can be found in the documentation. We have provided
621 a shell script which does most of the work of conversion for you;
622 this script can be found in ./examples/misc/alias-conv.sh. Here is
625 Start csh in the normal way for you. (e.g., `csh')
627 Pipe the output of `alias' through `alias-conv.sh', saving the
628 results into `bash_aliases':
630 alias | alias-conv.sh >bash_aliases
632 Edit `bash_aliases', carefully reading through any created
633 functions. You will need to change the names of some csh specific
634 variables to the bash equivalents. The script converts $cwd to
635 $PWD, $term to $TERM, $home to $HOME, $user to $USER, and $prompt
636 to $PS1. You may also have to add quotes to avoid unwanted
639 For example, the csh alias:
641 alias cd 'cd \!*; echo $cwd'
643 is converted to the bash function:
645 cd () { command cd "$@"; echo $PWD ; }
647 The only thing that needs to be done is to quote $PWD:
649 cd () { command cd "$@"; echo "$PWD" ; }
651 Merge the edited file into your ~/.bashrc.
653 There is an additional, more ambitious, script in
654 examples/misc/cshtobash that attempts to convert your entire csh
655 environment to its bash equivalent. This script can be run as
656 simply `cshtobash' to convert your normal interactive
657 environment, or as `cshtobash ~/.login' to convert your login
660 20) How can I pipe standard output and standard error from one command to
661 another, like csh does with `|&'?
664 command 2>&1 | command2
666 The key is to remember that piping is performed before redirection, so
667 file descriptor 1 points to the pipe when it is duplicated onto file
670 21) Now that I've converted from ksh to bash, are there equivalents to
671 ksh features like autoloaded functions and the `whence' command?
673 There are features in ksh-88 that do not have direct bash equivalents.
674 Most, however, can be emulated with very little trouble.
676 ksh-88 feature Bash equivalent
677 -------------- ---------------
678 [[...]] can usually use [...]; minor differences (no
679 pattern matching, for one)
680 compiled-in aliases set up aliases in .bashrc; some ksh aliases are
681 bash builtins (hash, history, type)
683 extended patterns no good substitute
684 coprocesses named pipe pairs (one for read, one for write)
685 typeset +f declare -F
686 cd, print, whence function substitutes in examples/functions/kshenv
687 autoloaded functions examples/functions/autoload is the same as typeset -fu
688 read var?prompt read -p prompt var
690 Section E: How can I get bash to do certain things, and why does bash do
691 things the way it does?
693 22) Why is the bash builtin `test' slightly different from /bin/test?
695 The specific example used here is [ ! x -o x ], which is false.
697 Bash's builtin `test' implements the Posix.2 spec, which can be
698 summarized as follows (the wording is due to David Korn):
700 Here is the set of rules for processing test arguments.
703 1 Arg: True iff argument is not null.
704 2 Args: If first arg is !, True iff second argument is null.
705 If first argument is unary, then true if unary test is true
707 3 Args: If second argument is a binary operator, do binary test of $1 $3
708 If first argument is !, negate two argument test of $2 $3
709 If first argument is `(' and third argument is `)', do the
710 one-argument test of the second argument.
712 4 Args: If first argument is !, negate three argument test of $2 $3 $4.
713 Otherwise unspecified
714 5 or more Args: unspecified. (Historical shells would use their
717 The operators -a and -o are considered binary operators for the purpose
720 As you can see, the test becomes (not (x or x)), which is false.
722 23) Why does bash sometimes say `Broken pipe'?
724 If a sequence of commands appears in a pipeline, and one of the
725 reading commands finishes before the writer has finished, the
726 writer receives a SIGPIPE signal. Many other shells special-case
727 SIGPIPE as an exit status in the pipeline and do not report it.
732 `head' can finish before `ps' writes all of its output, and ps
733 will try to write on a pipe without a reader. In that case, bash
734 will print `Broken pipe' to stderr when ps is killed by a
737 24) How can I get bash to read and display eight-bit characters?
739 This is a process requiring several steps.
741 First, you must ensure that the `physical' data path is a full eight
742 bits. For xterms, for example, the `vt100' resources `eightBitInput'
743 and `eightBitOutput' should be set to `true'.
745 Once you have set up an eight-bit path, you must tell the kernel and
746 tty driver to leave the eighth bit of characters alone when processing
747 keyboard input. Use `stty' to do this:
749 stty cs8 -istrip -parenb
751 For old BSD-style systems, you can use
759 Finally, you need to tell readline that you will be inputting and
760 displaying eight-bit characters. You use readline variables to do
761 this. These variables can be set in your .inputrc or using the bash
762 `bind' builtin. Here's an example using `bind':
764 bash$ bind 'set convert-meta off'
765 bash$ bind 'set meta-flag on'
766 bash$ bind 'set output-meta on'
768 The `set' commands between the single quotes may also be placed
771 25) How do I write a function `x' to replace builtin command `x', but
772 still invoke the command from within the function?
774 This is why the `command' and `builtin' builtins exist. The
775 `command' builtin executes the command supplied as its first
776 argument, skipping over any function defined with that name. The
777 `builtin' builtin executes the builtin command given as its first
780 For example, to write a function to replace `cd' that writes the
781 hostname and current directory to an xterm title bar, use
782 something like the following:
786 builtin cd "$@" && xtitle "$HOST: $PWD"
789 This could also be written using `command' instead of `builtin';
790 the version above is marginally more efficient.
792 26) When I have terminal escape sequences in my prompt, why does bash
793 wrap lines at the wrong column?
795 Readline, the line editing library that bash uses, does not know
796 that the terminal escape sequences do not take up space on the
797 screen. The redisplay code assumes, unless told otherwise, that
798 each character in the prompt is a `printable' character that
799 takes up one character position on the screen.
801 You can use the bash prompt expansion facility (see the PROMPTING
802 section in the manual page) to tell readline that sequences of
803 characters in the prompt strings take up no screen space.
805 Use the \[ escape to begin a sequence of non-printing characters,
806 and the \] escape to signal the end of such a sequence.
808 27) How can I find the value of a shell variable whose name is the value
809 of another shell variable?
811 Bash-2.01 supports this directly. You can use
815 For example, the following sequence of commands will echo `z':
821 For sh compatibility, use the `eval' builtin. The important
822 thing to remember is that `eval' expands the arguments you give
823 it again, so you need to quote the parts of the arguments that
824 you want `eval' to act on.
826 For example, this expression prints the value of the last positional
829 eval echo \"\$\{$#\}\"
831 The expansion of the quoted portions of this expression will be
832 deferred until `eval' runs, while the `$#' will be expanded
833 before `eval' is executed. In bash-2.01,
839 28) If I pipe the output of a command into `read variable', why doesn't
840 the output show up in $variable when the read command finishes?
842 This has to do with the parent-child relationship between Unix
845 Each element of a pipeline runs in a separate process, a child of
846 the shell running the pipeline. A subprocess cannot affect its
847 parent's environment. When the `read' command sets the variable
848 to the input, that variable is set only in the subshell, not the
849 parent shell. When the subshell exits, the value of the variable
852 Many pipelines that end with `read variable' can be converted
853 into command substitutions, which will capture the output of
854 a specified command. The output can then be assigned to a
857 grep ^gnu /usr/lib/news/active | wc -l | read ngroup
859 can be converted into
861 ngroup=$(grep ^gnu /usr/lib/news/active | wc -l)
863 This does not, unfortunately, work to split the text among
864 multiple variables, as read does when given multiple variable
865 arguments. If you need to do this, you can either use the
866 command substitution above to read the output into a variable
867 and chop up the variable using the bash pattern removal
868 expansion operators or use some variant of the following
871 Say /usr/local/bin/ipaddr is the following shell script:
874 host `hostname` | awk '/address/ {print $NF}'
878 /usr/local/bin/ipaddr | read A B C D
880 to break the local machine's IP address into separate octets, use
884 set -- $(/usr/local/bin/ipaddr)
886 A="$1" B="$2" C="$3" D="$4"
888 Beware, however, that this will change the shell's positional
889 parameters. If you need them, you should save them before doing
892 This is the general approach -- in most cases you will not need to
893 set $IFS to a different value.
895 29) I have a bunch of shell scripts that use backslash-escaped characters
896 in arguments to `echo'. Bash doesn't interpret these characters. Why
897 not, and how can I make it understand them?
899 This is the behavior of echo on most Unix System V machines.
901 The bash builtin `echo' is modelled after the 9th Edition
902 Research Unix version of `echo'. It does not interpret
903 backslash-escaped characters in its argument strings by default;
904 it requires the use of the -e option to enable the
905 interpretation. The System V echo provides no way to disable the
906 special characters; the bash echo has a -E option to disable
909 There is a configuration option that will make bash behave like
910 the System V echo and interpret things like `\t' by default. Run
911 configure with the --enable-usg-echo-default option to turn this
912 on. Be aware that this will cause some of the tests run when you
913 type `make tests' to fail.
915 30) Why doesn't a while or for loop get suspended when I type ^Z?
917 This is a consequence of how job control works on Unix. The only
918 thing that can be suspended is the process group. This is a single
919 command or pipeline of commands that the shell forks and executes.
921 When you run a while or for loop, the only thing that the shell forks
922 and executes are any commands in the while loop test and commands in
923 the loop bodies. These, therefore, are the only things that can be
924 suspended when you type ^Z.
926 If you want to be able to stop the entire loop, you need to put it
927 within parentheses, which will force the loop into a subshell that
928 may be stopped (and subsequently restarted) as a single unit.
930 31) How can I make the bash `time' reserved word print timing output that
931 looks like the output from my system's /usr/bin/time?
933 The bash command timing code looks for a variable `TIMEFORMAT' and
934 uses its value as a format string to decide how to display the
937 The value of TIMEFORMAT is a string with `%' escapes expanded in a
938 fashion similar in spirit to printf(3). The manual page explains
939 the meanings of the escape sequences in the format string.
941 If TIMEFORMAT is not set, bash acts as if the following assignment had
944 TIMEFORMAT=$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
946 The POSIX.2 default time format (used by `time -p command') is
948 TIMEFORMAT=$'real %2R\nuser %2U\nsys %2S'
950 The BSD /usr/bin/time format can be emulated with:
952 TIMEFORMAT=$'\t%1R real\t%1U user\t%1S sys'
954 The System V /usr/bin/time format can be emulated with:
956 TIMEFORMAT=$'\nreal\t%1R\nuser\t%1U\nsys\t%1S'
958 The ksh format can be emulated with:
960 TIMEFORMAT=$'\nreal\t%2lR\nuser\t%2lU\nsys\t%2lS'
962 Section F: Things to watch out for on certain Unix versions
964 32) Why can't I use command line editing in my `cmdtool'?
966 The problem is `cmdtool' and bash fighting over the input. When
967 scrolling is enabled in a cmdtool window, cmdtool puts the tty in
968 `raw mode' to permit command-line editing using the mouse for
969 applications that cannot do it themselves. As a result, bash and
970 cmdtool each try to read keyboard input immediately, with neither
971 getting enough of it to be useful.
973 This mode also causes cmdtool to not implement many of the
974 terminal functions and control sequences appearing in the
975 `sun-cmd' termcap entry. For a more complete explanation, see
976 that file examples/suncmd.termcap in the bash distribution.
978 `xterm' is a better choice, and gets along with bash much more
981 If you must use cmdtool, you can use the termcap description in
982 examples/suncmd.termcap. Set the TERMCAP variable to the terminal
983 description contained in that file, i.e.
985 TERMCAP='Mu|sun-cmd:am:bs:km:pt:li#34:co#80:cl=^L:ce=\E[K:cd=\E[J:rs=\E[s:'
987 Then export TERMCAP and start a new cmdtool window from that shell.
988 The bash command-line editing should behave better in the new
989 cmdtool. If this works, you can put the assignment to TERMCAP
992 33) I built bash on Solaris 2. Why do globbing expansions and filename
993 completion chop off the first few characters of each filename?
995 This is the consequence of building bash on SunOS 5 and linking
996 with the libraries in /usr/ucblib, but using the definitions
997 and structures from files in /usr/include.
999 The actual conflict is between the dirent structure in
1000 /usr/include/dirent.h and the struct returned by the version of
1001 `readdir' in libucb.a (a 4.3-BSD style `struct direct').
1003 Make sure you've got /usr/ccs/bin ahead of /usr/ucb in your $PATH
1004 when configuring and building bash. This will ensure that you
1005 use /usr/ccs/bin/cc or acc instead of /usr/ucb/cc and that you
1006 link with libc before libucb.
1008 If you have installed the Sun C compiler, you may also need to
1009 put /usr/ccs/bin and /opt/SUNWspro/bin into your $PATH before
1012 34) Why does bash dump core after I interrupt username completion or
1013 `~user' tilde expansion on a machine running NIS?
1015 This is a famous and long-standing bug in the SunOS YP (sorry, NIS)
1016 client library, which is part of libc.
1018 The YP library code keeps static state -- a pointer into the data
1019 returned from the server. When YP initializes itself (setpwent),
1020 it looks at this pointer and calls free on it if it's non-null.
1023 If one of the YP functions is interrupted during getpwent (the
1024 exact function is interpretwithsave()), and returns NULL, the
1025 pointer is freed without being reset to NULL, and the function
1026 returns. The next time getpwent is called, it sees that this
1027 pointer is non-null, calls free, and the bash free() blows up
1028 because it's being asked to free freed memory.
1030 The traditional Unix mallocs allow memory to be freed multiple
1031 times; that's probably why this has never been fixed. You can
1032 run configure with the `--without-gnu-malloc' option to use
1033 the C library malloc and avoid the problem.
1035 35) I'm running SVR4.2. Why is the line erased every time I type `@'?
1037 The `@' character is the default `line kill' character in most
1038 versions of System V, including SVR4.2. You can change this
1039 character to whatever you want using `stty'. For example, to
1040 change the line kill character to control-u, type
1044 where the `^' and `U' can be two separate characters.
1046 36) Why does bash report syntax errors when my C News scripts use a
1047 redirection before a subshell command?
1049 The actual command in question is something like
1053 According to the grammar given in the POSIX.2 standard, this construct
1054 is, in fact, a syntax error. Redirections may only precede `simple
1055 commands'. A subshell construct such as the above is one of the shell's
1056 `compound commands'. A redirection may only follow a compound command.
1058 The file CWRU/sh-redir-hack in the bash-2.01 distribution is an
1059 (unofficial) patch to parse.y that will modify the grammar to
1060 support this construct. It will not apply with `patch'; you must
1061 modify parse.y by hand. Note that if you apply this, you must
1062 recompile with -DREDIRECTION_HACK. This introduces a large
1063 number of reduce/reduce conflicts into the shell grammar.
1065 Section G: Where do I go from here?
1067 37) How do I report bugs in bash, and where should I look for fixes and
1070 Use the `bashbug' script to report bugs. It is built and
1071 installed at the same time as bash. It provides a standard
1072 template for reporting a problem and automatically includes
1073 information about your configuration and build environment.
1075 `bashbug' sends its reports to bug-bash@prep.ai.mit.edu, which
1076 is a large mailing list gatewayed to the usenet newsgroup gnu.bash.bug.
1078 Bug fixes, answers to questions, and announcements of new releases
1079 are all posted to gnu.bash.bug. Discussions concerning bash features
1080 and problems also take place there.
1082 To reach the bash maintainers directly, send mail to
1083 bash-maintainers@prep.ai.mit.edu.
1085 38) What kind of bash documentation is there?
1087 First, look in the doc directory in the bash distribution. It should
1088 contain at least the following files:
1090 bash.1 an extensive, thorough Unix-style manual page
1091 builtins.1 a manual page covering just bash builtin commands
1092 bashref.texi a reference manual in GNU info format
1093 bash.html an HTML version of the manual page
1094 bashref.html an HTML version of the reference manual
1096 article.ms text of an article written for The Linux Journal
1097 readline.3 a man page describing readline
1099 Postscript files created from the above source are available in
1100 the documentation distribution.
1102 There is additional documentation available for anonymous FTP from host
1103 slc2.ins.cwru.edu in the `pub/bash' directory.
1105 Cameron Newham and Bill Rosenblatt have written a book on bash, published
1106 by O'Reilly and Associates. The book is based on Bill Rosenblatt's Korn
1107 Shell book. The title is ``Learning the Bash Shell'', and the ISBN number
1108 is 1-56592-147-X. Look for it in fine bookstores near you. This book
1109 covers bash-1.14, but has an appendix describing some of the new features
1110 in bash-2.0. There are rumors of a second edition of this book, describing
1111 bash-2.0 (and 2.01). I do not know what ORA's publication schedule for
1114 39) What's coming in future versions?
1116 These are features I plan to include in a future version of bash.
1118 POSIX.2-style globbing character classes ([:alpha:], [:alnum:], etc.)
1119 POSIX.2-style globbing equivalence classes
1120 POSIX.2-style globbing collating symbols
1121 a bash debugger (a minimally-tested version is included with bash-2.01)
1123 40) What's on the bash `wish list' for future versions?
1125 These are features that may or may not appear in a future version of bash.
1127 Programmable completion a la zsh
1128 menu completion a la tcsh
1129 the ksh [[...]] extended test command
1130 the ksh egrep-style extended pattern matching operators
1131 associative arrays (not really all that hard)
1132 breaking some of the shell functionality into embeddable libraries
1133 better internationalization using GNU `gettext'
1134 an option to use external files for the long `help' text
1135 timeouts for the `read' builtin
1136 the ksh-93 ${!prefix*} and ${!prefix@} operators
1137 arithmetic ++ and -- prefix and postfix operators
1139 41) When will the next release appear?
1141 The next version will appear sometime in 1997. Never make predictions.
1144 This document is Copyright 1995, 1996 by Chester Ramey.
1146 Permission is hereby granted, without written agreement and
1147 without license or royalty fees, to use, copy, and distribute
1148 this document for any purpose, provided that the above copyright
1149 notice appears in all copies of this document and that the
1150 contents of this document remain unaltered.