1 This is the Bash FAQ, version 3.31, for Bash version 3.1.
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://ftp.cwru.edu/pub/bash/FAQ
18 The Bash home page is http://cnswww.cns.cwru.edu/~chet/bash/bashtop.html
26 A2) What's the latest version?
27 A3) Where can I get it?
28 A4) On what machines will bash run?
29 A5) Will bash run on operating systems other than Unix?
30 A6) How can I build bash with gcc?
31 A7) How can I make bash my login shell?
32 A8) I just changed my login shell to bash, and now I can't FTP into my
34 A9) What's the `POSIX Shell and Utilities standard'?
35 A10) What is the bash `posix mode'?
37 Section B: The latest version
39 B1) What's new in version 3.1?
40 B2) Are there any user-visible incompatibilities between bash-3.1 and
43 Section C: Differences from other Unix shells
45 C1) How does bash differ from sh, the Bourne shell?
46 C2) How does bash differ from the Korn shell, version ksh88?
47 C3) Which new features in ksh-93 are not in bash, and which are?
49 Section D: Why does bash do some things differently than other Unix shells?
51 D1) Why does bash run a different version of `command' than
52 `which command' says it will?
53 D2) Why doesn't bash treat brace expansions exactly like csh?
54 D3) Why doesn't bash have csh variable modifiers?
55 D4) How can I make my csh aliases work when I convert to bash?
56 D5) How can I pipe standard output and standard error from one command to
57 another, like csh does with `|&'?
58 D6) Now that I've converted from ksh to bash, are there equivalents to
59 ksh features like autoloaded functions and the `whence' command?
61 Section E: Why does bash do certain things the way it does?
63 E1) Why is the bash builtin `test' slightly different from /bin/test?
64 E2) Why does bash sometimes say `Broken pipe'?
65 E3) When I have terminal escape sequences in my prompt, why does bash
66 wrap lines at the wrong column?
67 E4) If I pipe the output of a command into `read variable', why doesn't
68 the output show up in $variable when the read command finishes?
69 E5) I have a bunch of shell scripts that use backslash-escaped characters
70 in arguments to `echo'. Bash doesn't interpret these characters. Why
71 not, and how can I make it understand them?
72 E6) Why doesn't a while or for loop get suspended when I type ^Z?
73 E7) What about empty for loops in Makefiles?
74 E8) Why does the arithmetic evaluation code complain about `08'?
75 E9) Why does the pattern matching expression [A-Z]* match files beginning
76 with every letter except `z'?
77 E10) Why does `cd //' leave $PWD as `//'?
78 E11) If I resize my xterm while another program is running, why doesn't bash
80 E12) Why don't negative offsets in substring expansion work like I expect?
82 Section F: Things to watch out for on certain Unix versions
84 F1) Why can't I use command line editing in my `cmdtool'?
85 F2) I built bash on Solaris 2. Why do globbing expansions and filename
86 completion chop off the first few characters of each filename?
87 F3) Why does bash dump core after I interrupt username completion or
88 `~user' tilde expansion on a machine running NIS?
89 F4) I'm running SVR4.2. Why is the line erased every time I type `@'?
90 F5) Why does bash report syntax errors when my C News scripts use a
91 redirection before a subshell command?
92 F6) Why can't I use vi-mode editing on Red Hat Linux 6.1?
93 F7) Why do bash-2.05a and bash-2.05b fail to compile `printf.def' on
96 Section G: How can I get bash to do certain common things?
98 G1) How can I get bash to read and display eight-bit characters?
99 G2) How do I write a function `x' to replace builtin command `x', but
100 still invoke the command from within the function?
101 G3) How can I find the value of a shell variable whose name is the value
102 of another shell variable?
103 G4) How can I make the bash `time' reserved word print timing output that
104 looks like the output from my system's /usr/bin/time?
105 G5) How do I get the current directory into my prompt?
106 G6) How can I rename "*.foo" to "*.bar"?
107 G7) How can I translate a filename from uppercase to lowercase?
108 G8) How can I write a filename expansion (globbing) pattern that will match
109 all files in the current directory except "." and ".."?
111 Section H: Where do I go from here?
113 H1) How do I report bugs in bash, and where should I look for fixes and
115 H2) What kind of bash documentation is there?
116 H3) What's coming in future versions?
117 H4) What's on the bash `wish list'?
118 H5) When will the next release appear?
121 Section A: The Basics
125 Bash is a Unix command interpreter (shell). It is an implementation of
126 the Posix 1003.2 shell standard, and resembles the Korn and System V
129 Bash contains a number of enhancements over those shells, both
130 for interactive use and shell programming. Features geared
131 toward interactive use include command line editing, command
132 history, job control, aliases, and prompt expansion. Programming
133 features include additional variable expansions, shell
134 arithmetic, and a number of variables and options to control
137 Bash was originally written by Brian Fox of the Free Software
138 Foundation. The current developer and maintainer is Chet Ramey
139 of Case Western Reserve University.
141 A2) What's the latest version?
143 The latest version is 3.1, first made available on xx December, 2005.
145 A3) Where can I get it?
147 Bash is the GNU project's shell, and so is available from the
148 master GNU archive site, ftp.gnu.org, and its mirrors. The
149 latest version is also available for FTP from ftp.cwru.edu.
150 The following URLs tell how to get version 3.1:
152 ftp://ftp.gnu.org/pub/gnu/bash/bash-3.1.tar.gz
153 ftp://ftp.cwru.edu/pub/bash/bash-3.1.tar.gz
155 Formatted versions of the documentation are available with the URLs:
157 ftp://ftp.gnu.org/pub/gnu/bash/bash-doc-3.1.tar.gz
158 ftp://ftp.cwru.edu/pub/bash/bash-doc-3.1.tar.gz
160 Any patches for the current version are available with the URL:
162 ftp://ftp.cwru.edu/pub/bash/bash-3.1-patches/
164 A4) On what machines will bash run?
166 Bash has been ported to nearly every version of Unix. All you
167 should have to do to build it on a machine for which a port
168 exists is to type `configure' and then `make'. The build process
169 will attempt to discover the version of Unix you have and tailor
170 itself accordingly, using a script created by GNU autoconf.
172 More information appears in the file `INSTALL' in the distribution.
174 The Bash web page (http://cnswww.cns.cwru.edu/~chet/bash/bashtop.html)
175 explains how to obtain binary versions of bash for most of the major
176 commercial Unix systems.
178 A5) Will bash run on operating systems other than Unix?
180 Configuration specifics for Unix-like systems such as QNX and
181 LynxOS are included in the distribution. Bash-2.05 and later
182 versions should compile and run on Minix 2.0 (patches were
183 contributed), but I don't believe anyone has built bash-2.x on
184 earlier Minix versions yet.
186 Bash has been ported to versions of Windows implementing the Win32
187 programming interface. This includes Windows 95 and Windows NT.
188 The port was done by Cygnus Solutions (now part of Red Hat) as part
189 of their CYGWIN project. For more information about the project, see
190 http://www.cygwin.com/.
192 Cygnus originally ported bash-1.14.7, and that port was part of their
193 early GNU-Win32 (the original name) releases. Cygnus has also done
194 ports of bash-2.05b and bash-3.0 to the CYGWIN environment, and both
195 are available as part of their current release. Bash-3.1 is currently
196 being tested and should be available soon.
198 Bash-2.05b and later versions should require no local Cygnus changes to
199 build and run under CYGWIN.
201 DJ Delorie has a port of bash-2.x which runs under MS-DOS, as part
202 of the DJGPP project. For more information on the project, see
204 http://www.delorie.com/djgpp/
206 I have been told that the original DJGPP port was done by Daisuke Aoyama.
208 Mark Elbrecht <snowball3@bigfoot.com> has sent me notice that bash-2.04
209 is available for DJGPP V2. The files are available as:
211 ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh204b.zip binary
212 ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh204d.zip documentation
213 ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh204s.zip source
215 Mark began to work with bash-2.05, but I don't know the current status.
217 Bash-3.0 compiles and runs with no modifications under Microsoft's Services
218 for Unix (SFU), once known as Interix. I do not anticipate any problems
219 with building bash-3.1.
221 A6) How can I build bash with gcc?
223 Bash configures to use gcc by default if it is available. Read the
224 file INSTALL in the distribution for more information.
226 A7) How can I make bash my login shell?
228 Some machines let you use `chsh' to change your login shell. Other
229 systems use `passwd -s' or `passwd -e'. If one of these works for
230 you, that's all you need. Note that many systems require the full
231 pathname to a shell to appear in /etc/shells before you can make it
232 your login shell. For this, you may need the assistance of your
233 friendly local system administrator.
235 If you cannot do this, you can still use bash as your login shell, but
236 you need to perform some tricks. The basic idea is to add a command
237 to your login shell's startup file to replace your login shell with
240 For example, if your login shell is csh or tcsh, and you have installed
241 bash in /usr/gnu/bin/bash, add the following line to ~/.login:
243 if ( -f /usr/gnu/bin/bash ) exec /usr/gnu/bin/bash --login
245 (the `--login' tells bash that it is a login shell).
247 It's not a good idea to put this command into ~/.cshrc, because every
248 csh you run without the `-f' option, even ones started to run csh scripts,
249 reads that file. If you must put the command in ~/.cshrc, use something
252 if ( $?prompt ) exec /usr/gnu/bin/bash --login
254 to ensure that bash is exec'd only when the csh is interactive.
256 If your login shell is sh or ksh, you have to do two things.
258 First, create an empty file in your home directory named `.bash_profile'.
259 The existence of this file will prevent the exec'd bash from trying to
260 read ~/.profile, and re-execing itself over and over again. ~/.bash_profile
261 is the first file bash tries to read initialization commands from when
262 it is invoked as a login shell.
264 Next, add a line similar to the above to ~/.profile:
266 [ -f /usr/gnu/bin/bash ] && [ -x /usr/gnu/bin/bash ] && \
267 exec /usr/gnu/bin/bash --login
269 This will cause login shells to replace themselves with bash running as
270 a login shell. Once you have this working, you can copy your initialization
271 code from ~/.profile to ~/.bash_profile.
273 I have received word that the recipe supplied above is insufficient for
274 machines running CDE. CDE has a maze of twisty little startup files, all
277 If you cannot change your login shell in the password file to bash, you
278 will have to (apparently) live with CDE using the shell in the password
279 file to run its startup scripts. If you have changed your shell to bash,
280 there is code in the CDE startup files (on Solaris, at least) that attempts
281 to do the right thing. It is, however, often broken, and may require that
282 you use the $BASH_ENV trick described below.
284 `dtterm' claims to use $SHELL as the default program to start, so if you
285 can change $SHELL in the CDE startup files, you should be able to use bash
286 in your terminal windows.
288 Setting DTSOURCEPROFILE in ~/.dtprofile will cause the `Xsession' program
289 to read your login shell's startup files. You may be able to use bash for
290 the rest of the CDE programs by setting SHELL to bash in ~/.dtprofile as
291 well, but I have not tried this.
293 You can use the above `exec' recipe to start bash when not logging in with
294 CDE by testing the value of the DT variable:
296 if [ -n "$DT" ]; then
297 [ -f /usr/gnu/bin/bash ] && exec /usr/gnu/bin/bash --login
300 If CDE starts its shells non-interactively during login, the login shell
301 startup files (~/.profile, ~/.bash_profile) will not be sourced at login.
302 To get around this problem, append a line similar to the following to your
305 BASH_ENV=${HOME}/.bash_profile ; export BASH_ENV
307 and add the following line to the beginning of ~/.bash_profile:
311 A8) I just changed my login shell to bash, and now I can't FTP into my
314 You must add the full pathname to bash to the file /etc/shells. As
315 noted in the answer to the previous question, many systems require
316 this before you can make bash your login shell.
318 Most versions of ftpd use this file to prohibit `special' users
319 such as `uucp' and `news' from using FTP.
321 A9) What's the `POSIX Shell and Utilities standard'?
323 POSIX is a name originally coined by Richard Stallman for a
324 family of open system standards based on UNIX. There are a
325 number of aspects of UNIX under consideration for
326 standardization, from the basic system services at the system
327 call and C library level to applications and tools to system
328 administration and management. Each area of standardization is
329 assigned to a working group in the 1003 series.
331 The POSIX Shell and Utilities standard was originally developed by
332 IEEE Working Group 1003.2 (POSIX.2). Today it has been merged with
333 the original 1003.1 Working Group and is maintained by the Austin
334 Group (a joint working group of the IEEE, The Open Group and
335 ISO/IEC SC22/WG15). Today the Shell and Utilities are a volume
336 within the set of documents that make up IEEE Std 1003.1-2001, and
337 thus now the former POSIX.2 (from 1992) is now part of the current
338 POSIX.1 standard (POSIX 1003.1-2001).
340 The Shell and Utilities volume concentrates on the command
341 interpreter interface and utility programs commonly executed from
342 the command line or by other programs. The standard is freely
343 available on the web at http://www.UNIX-systems.org/version3/ .
344 Work continues at the Austin Group on maintenance issues; see
345 http://www.opengroup.org/austin/ to join the discussions.
347 Bash is concerned with the aspects of the shell's behavior defined
348 by the POSIX Shell and Utilities volume. The shell command
349 language has of course been standardized, including the basic flow
350 control and program execution constructs, I/O redirection and
351 pipelining, argument handling, variable expansion, and quoting.
353 The `special' builtins, which must be implemented as part of the
354 shell to provide the desired functionality, are specified as
355 being part of the shell; examples of these are `eval' and
356 `export'. Other utilities appear in the sections of POSIX not
357 devoted to the shell which are commonly (and in some cases must
358 be) implemented as builtin commands, such as `read' and `test'.
359 POSIX also specifies aspects of the shell's interactive
360 behavior as part of the UPE, including job control and command
361 line editing. Only vi-style line editing commands have been
362 standardized; emacs editing commands were left out due to
365 The latest version of the POSIX Shell and Utilities standard is
366 available (now updated to the 2004 Edition) as part of the Single
367 UNIX Specification Version 3 at
369 http://www.UNIX-systems.org/version3/
371 A10) What is the bash `posix mode'?
373 Although bash is an implementation of the POSIX shell
374 specification, there are areas where the bash default behavior
375 differs from that spec. The bash `posix mode' changes the bash
376 behavior in these areas so that it obeys the spec more closely.
378 Posix mode is entered by starting bash with the --posix or
379 '-o posix' option or executing `set -o posix' after bash is running.
381 The specific aspects of bash which change when posix mode is
382 active are listed in the file POSIX in the bash distribution.
383 They are also listed in a section in the Bash Reference Manual
384 (from which that file is generated).
386 Section B: The latest version
388 B1) What's new in version 3.1?
390 Bash-3.1 is the first maintenance release of the third major release of
391 bash. It contains the following significant new features (see the manual
392 page for complete descriptions and the CHANGES and NEWS files in the
393 bash-3.1 distribution).
395 o Bash-3.1 may now be configured and built in a mode that enforces strict
398 o The `+=' assignment operator, which appends to the value of a string or
399 array variable, has been implemented.
401 o It is now possible to ignore case when matching in contexts other than
402 filename generation using the new `nocasematch' shell option.
404 A short feature history dating from Bash-2.0:
406 Bash-3.0 contained the following new features:
408 o Features to support the bash debugger have been implemented, and there
409 is a new `extdebug' option to turn the non-default options on
411 o HISTCONTROL is now a colon-separated list of options and has been
412 extended with a new `erasedups' option that will result in only one
413 copy of a command being kept in the history list
415 o Brace expansion has been extended with a new {x..y} form, producing
416 sequences of digits or characters
418 o Timestamps are now kept with history entries, with an option to save
419 and restore them from the history file; there is a new HISTTIMEFORMAT
420 variable describing how to display the timestamps when listing history
423 o The `[[' command can now perform extended regular expression (egrep-like)
424 matching, with matched subexpressions placed in the BASH_REMATCH array
427 o A new `pipefail' option causes a pipeline to return a failure status if
428 any command in it fails
430 o The `jobs', `kill', and `wait' builtins now accept job control notation
431 in their arguments even if job control is not enabled
433 o The `gettext' package and libintl have been integrated, and the shell
434 messages may be translated into other languages
436 Bash-2.05b introduced the following new features:
438 o support for multibyte characters has been added to both bash and readline
440 o the DEBUG trap is now run *before* simple commands, ((...)) commands,
441 [[...]] conditional commands, and for ((...)) loops
443 o the shell now performs arithmetic in the largest integer size the machine
446 o there is a new \D{...} prompt expansion; passes the `...' to strftime(3)
447 and inserts the result into the expanded prompt
449 o there is a new `here-string' redirection operator: <<< word
451 o when displaying variables, function attributes and definitions are shown
452 separately, allowing them to be re-used as input (attempting to re-use
453 the old output would result in syntax errors).
455 o `read' has a new `-u fd' option to read from a specified file descriptor
457 o the bash debugger in examples/bashdb has been modified to work with the
458 new DEBUG trap semantics, the command set has been made more gdb-like,
459 and the changes to $LINENO make debugging functions work better
461 o the expansion of $LINENO inside a shell function is only relative to the
462 function start if the shell is interactive -- if the shell is running a
463 script, $LINENO expands to the line number in the script. This is as
466 Bash-2.05a introduced the following new features:
468 o The `printf' builtin has undergone major work
470 o There is a new read-only `shopt' option: login_shell, which is set by
471 login shells and unset otherwise
473 o New `\A' prompt string escape sequence; expanding to time in 24-hour
476 o New `-A group/-g' option to complete and compgen; goes group name
479 o New [+-]O invocation option to set and unset `shopt' options at startup
481 o ksh-like `ERR' trap
483 o `for' loops now allow empty word lists after the `in' reserved word
485 o new `hard' and `soft' arguments for the `ulimit' builtin
487 o Readline can be configured to place the user at the same point on the line
488 when retrieving commands from the history list
490 o Readline can be configured to skip `hidden' files (filenames with a leading
491 `.' on Unix) when performing completion
493 Bash-2.05 introduced the following new features:
495 o This version has once again reverted to using locales and strcoll(3) when
496 processing pattern matching bracket expressions, as POSIX requires.
497 o Added a new `--init-file' invocation argument as a synonym for `--rcfile',
498 per the new GNU coding standards.
499 o The /dev/tcp and /dev/udp redirections now accept service names as well as
501 o `complete' and `compgen' now take a `-o value' option, which controls some
502 of the aspects of that compspec. Valid values are:
504 default - perform bash default completion if programmable
505 completion produces no matches
506 dirnames - perform directory name completion if programmable
507 completion produces no matches
508 filenames - tell readline that the compspec produces filenames,
509 so it can do things like append slashes to
510 directory names and suppress trailing spaces
511 o A new loadable builtin, realpath, which canonicalizes and expands symlinks
512 in pathname arguments.
513 o When `set' is called without options, it prints function defintions in a
514 way that allows them to be reused as input. This affects `declare' and
515 `declare -p' as well. This only happens when the shell is not in POSIX
516 mode, since POSIX.2 forbids this behavior.
518 Bash-2.04 introduced the following new features:
520 o Programmable word completion with the new `complete' and `compgen' builtins;
521 examples are provided in examples/complete/complete-examples
522 o `history' has a new `-d' option to delete a history entry
523 o `bind' has a new `-x' option to bind key sequences to shell commands
524 o The prompt expansion code has new `\j' and `\l' escape sequences
525 o The `no_empty_cmd_completion' shell option, if enabled, inhibits
526 command completion when TAB is typed on an empty line
527 o `help' has a new `-s' option to print a usage synopsis
528 o New arithmetic operators: var++, var--, ++var, --var, expr1,expr2 (comma)
529 o New ksh93-style arithmetic for command:
530 for ((expr1 ; expr2; expr3 )); do list; done
531 o `read' has new options: `-t', `-n', `-d', `-s'
532 o The redirection code handles several filenames specially: /dev/fd/N,
533 /dev/stdin, /dev/stdout, /dev/stderr
534 o The redirection code now recognizes /dev/tcp/HOST/PORT and
535 /dev/udp/HOST/PORT and tries to open a TCP or UDP socket, respectively,
536 to the specified port on the specified host
537 o The ${!prefix*} expansion has been implemented
538 o A new FUNCNAME variable, which expands to the name of a currently-executing
540 o The GROUPS variable is no longer readonly
541 o A new shopt `xpg_echo' variable, to control the behavior of echo with
542 respect to backslash-escape sequences at runtime
543 o The NON_INTERACTIVE_LOGIN_SHELLS #define has returned
545 The version of Readline released with Bash-2.04, Readline-4.1, had several
546 new features as well:
548 o Parentheses matching is always compiled into readline, and controllable
549 with the new `blink-matching-paren' variable
550 o The history-search-forward and history-search-backward functions now leave
551 point at the end of the line when the search string is empty, like
552 reverse-search-history, and forward-search-history
553 o A new function for applications: rl_on_new_line_with_prompt()
554 o New variables for applications: rl_already_prompted, and rl_gnu_readline_p
557 Bash-2.03 had very few new features, in keeping with the convention
558 that odd-numbered releases provide mainly bug fixes. A number of new
559 features were added to Readline, mostly at the request of the Cygnus
562 A new shopt option, `restricted_shell', so that startup files can test
563 whether or not the shell was started in restricted mode
564 Filename generation is now performed on the words between ( and ) in
565 compound array assignments (this is really a bug fix)
566 OLDPWD is now auto-exported, as POSIX.2 requires
567 ENV and BASH_ENV are read-only variables in a restricted shell
568 Bash may now be linked against an already-installed Readline library,
569 as long as the Readline library is version 4 or newer
570 All shells begun with the `--login' option will source the login shell
571 startup files, even if the shell is not interactive
573 There were lots of changes to the version of the Readline library released
574 along with Bash-2.03. For a complete list of the changes, read the file
575 CHANGES in the Bash-2.03 distribution.
577 Bash-2.02 contained the following new features:
579 a new version of malloc (based on the old GNU malloc code in previous
580 bash versions) that is more page-oriented, more conservative
581 with memory usage, does not `orphan' large blocks when they
582 are freed, is usable on 64-bit machines, and has allocation
583 checking turned on unconditionally
584 POSIX.2-style globbing character classes ([:alpha:], [:alnum:], etc.)
585 POSIX.2-style globbing equivalence classes
586 POSIX.2-style globbing collating symbols
587 the ksh [[...]] extended conditional command
588 the ksh egrep-style extended pattern matching operators
589 a new `printf' builtin
590 the ksh-like $(<filename) command substitution, which is equivalent to
592 new tilde prefixes that expand to directories from the directory stack
593 new `**' arithmetic operator to do exponentiation
594 case-insensitive globbing (filename expansion)
595 menu completion a la tcsh
596 `magic-space' history expansion function like tcsh
597 the readline inputrc `language' has a new file inclusion directive ($include)
599 Bash-2.01 contained only a few new features:
601 new `GROUPS' builtin array variable containing the user's group list
602 new bindable readline commands: history-and-alias-expand-line and
605 Bash-2.0 contained extensive changes and new features from bash-1.14.7.
608 new `time' reserved word to time pipelines, shell builtins, and
610 one-dimensional arrays with a new compound assignment statement,
611 appropriate expansion constructs and modifications to some
612 of the builtins (read, declare, etc.) to use them
613 new quoting syntaxes for ANSI-C string expansion and locale-specific
615 new expansions to do substring extraction, pattern replacement, and
616 indirect variable expansion
617 new builtins: `disown' and `shopt'
618 new variables: HISTIGNORE, SHELLOPTS, PIPESTATUS, DIRSTACK, GLOBIGNORE,
619 MACHTYPE, BASH_VERSINFO
620 special handling of many unused or redundant variables removed
621 (e.g., $notify, $glob_dot_filenames, $no_exit_on_failed_exec)
622 dynamic loading of new builtin commands; many loadable examples provided
623 new prompt expansions: \a, \e, \n, \H, \T, \@, \v, \V
624 history and aliases available in shell scripts
625 new readline variables: enable-keypad, mark-directories, input-meta,
626 visible-stats, disable-completion, comment-begin
627 new readline commands to manipulate the mark and operate on the region
628 new readline emacs mode commands and bindings for ksh-88 compatibility
629 updated and extended builtins
631 expanded (and now documented) restricted shell mode
633 implementation stuff:
634 autoconf-based configuration
635 nearly all of the bugs reported since version 1.14 have been fixed
636 most builtins converted to use builtin `getopt' for consistency
637 most builtins use -p option to display output in a reusable form
639 grammar tighter and smaller (66 reduce-reduce conflicts gone)
640 lots of code now smaller and faster
641 test suite greatly expanded
643 B2) Are there any user-visible incompatibilities between bash-3.1 and
646 There are a few incompatibilities between version 2.05b and version 3.1.
647 They are detailed in the file COMPAT in the bash distribution. That file
648 is not meant to be all-encompassing; send mail to bash-maintainers@gnu.org
649 if if you find something that's not mentioned there.
651 Section C: Differences from other Unix shells
653 C1) How does bash differ from sh, the Bourne shell?
655 This is a non-comprehensive list of features that differentiate bash
656 from the SVR4.2 shell. The bash manual page explains these more
659 Things bash has that sh does not:
660 long invocation options
661 [+-]O invocation option
663 `!' reserved word to invert pipeline return value
664 `time' reserved word to time pipelines and shell builtins
665 the `function' reserved word
666 the `select' compound command and reserved word
667 arithmetic for command: for ((expr1 ; expr2; expr3 )); do list; done
668 new $'...' and $"..." quoting
669 the $(...) form of command substitution
670 the $(<filename) form of command substitution, equivalent to
672 the ${#param} parameter value length operator
673 the ${!param} indirect parameter expansion operator
674 the ${!param*} prefix expansion operator
675 the ${param:offset[:length]} parameter substring operator
676 the ${param/pat[/string]} parameter pattern substitution operator
677 expansions to perform substring removal (${p%[%]w}, ${p#[#]w})
678 expansion of positional parameters beyond $9 with ${num}
679 variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, REPLY,
680 TIMEFORMAT, PPID, PWD, OLDPWD, SHLVL, RANDOM, SECONDS,
681 LINENO, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE, HOSTNAME,
682 ENV, PS3, PS4, DIRSTACK, PIPESTATUS, HISTSIZE, HISTFILE,
683 HISTFILESIZE, HISTCONTROL, HISTIGNORE, GLOBIGNORE, GROUPS,
684 PROMPT_COMMAND, FCEDIT, FIGNORE, IGNOREEOF, INPUTRC,
685 SHELLOPTS, OPTERR, HOSTFILE, TMOUT, FUNCNAME, histchars,
689 variable arrays with new compound assignment syntax
690 redirections: <>, &>, >|, <<<, [n]<&word-, [n]>&word-
691 prompt string special char translation and variable expansion
692 auto-export of variables in initial environment
693 command search finds functions before builtins
694 bash return builtin will exit a file sourced with `.'
695 builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E, hash -d/-l/-p/-t.
696 export -n/-f/-p/name=value, pwd -L/-P,
697 read -e/-p/-a/-t/-n/-d/-s/-u,
698 readonly -a/-f/name=value, trap -l, set +o,
699 set -b/-m/-o option/-h/-p/-B/-C/-H/-P,
700 unset -f/-v, ulimit -m/-p/-u,
701 type -a/-p/-t/-f/-P, suspend -f, kill -n,
702 test -o optname/s1 == s2/s1 < s2/s1 > s2/-nt/-ot/-ef/-O/-G/-S
703 bash reads ~/.bashrc for interactive shells, $ENV for non-interactive
704 bash restricted shell mode is more extensive
705 bash allows functions and variables with the same name
708 arithmetic expansion with $((...)) and `let' builtin
709 the `[[...]]' extended conditional command
711 aliases and alias/unalias builtins
712 local variables in functions and `local' builtin
713 readline and command-line editing with programmable completion
714 command history and history/fc builtins
715 csh-like history expansion
716 other new bash builtins: bind, command, compgen, complete, builtin,
717 declare/typeset, dirs, enable, fc, help,
718 history, logout, popd, pushd, disown, shopt,
721 filename generation when using output redirection (command >a*)
722 POSIX.2-style globbing character classes
723 POSIX.2-style globbing equivalence classes
724 POSIX.2-style globbing collating symbols
725 egrep-like extended pattern matching operators
726 case-insensitive pattern matching and globbing
727 variable assignments preceding commands affect only that command,
728 even for builtins and functions
729 posix mode and strict posix conformance
730 redirection to /dev/fd/N, /dev/stdin, /dev/stdout, /dev/stderr,
731 /dev/tcp/host/port, /dev/udp/host/port
732 debugger support, including `caller' builtin and new variables
734 the `+=' assignment operator
737 Things sh has that bash does not:
738 uses variable SHACCT to do shell accounting
739 includes `stop' builtin (bash can use alias stop='kill -s STOP')
741 turns on job control if called as `jsh'
742 $TIMEOUT (like bash $TMOUT)
743 `^' is a synonym for `|'
744 new SVR4.2 sh builtins: mldmode, priv
746 Implementation differences:
747 redirection to/from compound commands causes sh to create a subshell
748 bash does not allow unbalanced quotes; sh silently inserts them at EOF
749 bash does not mess with signal 11
750 sh sets (euid, egid) to (uid, gid) if -p not supplied and uid < 100
751 bash splits only the results of expansions on IFS, using POSIX.2
752 field splitting rules; sh splits all words on IFS
753 sh does not allow MAILCHECK to be unset (?)
754 sh does not allow traps on SIGALRM or SIGCHLD
755 bash allows multiple option arguments when invoked (e.g. -x -v);
756 sh allows only a single option argument (`sh -x -v' attempts
757 to open a file named `-v', and, on SunOS 4.1.4, dumps core.
758 On Solaris 2.4 and earlier versions, sh goes into an infinite
760 sh exits a script if any builtin fails; bash exits only if one of
761 the POSIX.2 `special' builtins fails
763 C2) How does bash differ from the Korn shell, version ksh88?
765 Things bash has or uses that ksh88 does not:
766 long invocation options
767 [-+]O invocation option
770 arithmetic for command: for ((expr1 ; expr2; expr3 )); do list; done
771 arithmetic in largest machine-supported size (intmax_t)
772 posix mode and posix conformance
774 tilde expansion for assignment statements that look like $PATH
775 process substitution with named pipes if /dev/fd is not available
776 the ${!param} indirect parameter expansion operator
777 the ${!param*} prefix expansion operator
778 the ${param:offset[:length]} parameter substring operator
779 the ${param/pat[/string]} parameter pattern substitution operator
780 variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, SHLVL,
781 TIMEFORMAT, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE,
782 HISTFILESIZE, HISTIGNORE, HISTCONTROL, PROMPT_COMMAND,
783 IGNOREEOF, FIGNORE, INPUTRC, HOSTFILE, DIRSTACK,
784 PIPESTATUS, HOSTNAME, OPTERR, SHELLOPTS, GLOBIGNORE,
785 GROUPS, FUNCNAME, histchars, auto_resume
786 prompt expansion with backslash escapes and command substitution
787 redirection: &> (stdout and stderr), <<<, [n]<&word-, [n]>&word-
788 more extensive and extensible editing and programmable completion
789 builtins: bind, builtin, command, declare, dirs, echo -e/-E, enable,
790 exec -l/-c/-a, fc -s, export -n/-f/-p, hash, help, history,
791 jobs -x/-r/-s, kill -s/-n/-l, local, logout, popd, pushd,
792 read -e/-p/-a/-t/-n/-d/-s, readonly -a/-n/-f/-p,
793 set -o braceexpand/-o histexpand/-o interactive-comments/
794 -o notify/-o physical/-o posix/-o hashall/-o onecmd/
795 -h/-B/-C/-b/-H/-P, set +o, suspend, trap -l, type,
796 typeset -a/-F/-p, ulimit -u, umask -S, alias -p, shopt,
797 disown, printf, complete, compgen
798 `!' csh-style history expansion
799 POSIX.2-style globbing character classes
800 POSIX.2-style globbing equivalence classes
801 POSIX.2-style globbing collating symbols
802 egrep-like extended pattern matching operators
803 case-insensitive pattern matching and globbing
804 `**' arithmetic operator to do exponentiation
805 redirection to /dev/fd/N, /dev/stdin, /dev/stdout, /dev/stderr
806 arrays of unlimited size
807 TMOUT is default timeout for `read' and `select'
808 debugger support, including the `caller' builtin
810 Timestamps in history entries
811 {x..y} brace expansion
812 The `+=' assignment operator
814 Things ksh88 has or uses that bash does not:
815 tracked aliases (alias -t)
816 variables: ERRNO, FPATH, EDITOR, VISUAL
817 co-processes (|&, >&p, <&p)
818 weirdly-scoped functions
819 typeset +f to list all function names without definitions
820 text of command history kept in a file, not memory
821 builtins: alias -x, cd old new, newgrp, print,
822 read -p/-s/var?prompt, set -A/-o gmacs/
823 -o bgnice/-o markdirs/-o trackall/-o viraw/-s,
824 typeset -H/-L/-R/-Z/-A/-ft/-fu/-fx/-l/-u/-t, whence
825 using environment to pass attributes of exported variables
826 arithmetic evaluation done on arguments to some builtins
827 reads .profile from $PWD when invoked as login shell
829 Implementation differences:
830 ksh runs last command of a pipeline in parent shell context
831 bash has brace expansion by default (ksh88 compile-time option)
832 bash has fixed startup file for all interactive shells; ksh reads $ENV
833 bash has exported functions
834 bash command search finds functions before builtins
835 bash waits for all commands in pipeline to exit before returning status
836 emacs-mode editing has some slightly different key bindings
838 C3) Which new features in ksh-93 are not in bash, and which are?
840 New things in ksh-93 not in bash-3.0:
842 floating point arithmetic and variables
843 math library functions
844 ${!name[sub]} name of subscript for associative array
845 `.' is allowed in variable names to create a hierarchical namespace
846 more extensive compound assignment syntax
848 `sleep' and `getconf' builtins (bash has loadable versions)
849 typeset -n and `nameref' variables
851 variables: .sh.edchar, .sh.edmode, .sh.edcol, .sh.edtext, .sh.version,
852 .sh.name, .sh.subscript, .sh.value, .sh.match, HISTEDIT
853 backreferences in pattern matching (\N)
854 `&' operator in pattern lists for matching
855 print -f (bash uses printf)
856 `fc' has been renamed to `hist'
857 `.' can execute shell functions
858 exit statuses between 0 and 255
859 FPATH and PATH mixing
862 printf %H, %P, %T, %Z modifiers, output base for %d
863 lexical scoping for local variables in `ksh' functions
864 no scoping for local variables in `POSIX' functions
866 New things in ksh-93 present in bash-3.0:
867 [n]<&word- and [n]>&word- redirections (combination dup and close)
868 for (( expr1; expr2; expr3 )) ; do list; done - arithmetic for command
869 ?:, ++, --, `expr1 , expr2' arithmetic operators
870 expansions: ${!param}, ${param:offset[:len]}, ${param/pat[/str]},
872 compound array assignment
873 the `!' reserved word
874 loadable builtins -- but ksh uses `builtin' while bash uses `enable'
875 `command', `builtin', `disown' builtins
876 new $'...' and $"..." quoting
877 FIGNORE (but bash uses GLOBIGNORE), HISTCMD
879 changes to kill builtin
880 read -A (bash uses read -a)
884 `.' restores the positional parameters when it completes
888 command and arithmetic substitution performed on PS1, PS4, and ENV
889 command name completion
890 ENV processed only for interactive shells
892 The `+=' assignment operator
894 Section D: Why does bash do some things differently than other Unix shells?
896 D1) Why does bash run a different version of `command' than
897 `which command' says it will?
899 On many systems, `which' is actually a csh script that assumes
900 you're running csh. In tcsh, `which' and its cousin `where'
901 are builtins. On other Unix systems, `which' is a perl script
902 that uses the PATH environment variable.
904 The csh script version reads the csh startup files from your
905 home directory and uses those to determine which `command' will
906 be invoked. Since bash doesn't use any of those startup files,
907 there's a good chance that your bash environment differs from
908 your csh environment. The bash `type' builtin does everything
909 `which' does, and will report correct results for the running
910 shell. If you're really wedded to the name `which', try adding
911 the following function definition to your .bashrc:
918 If you're moving from tcsh and would like to bring `where' along
919 as well, use this function:
926 D2) Why doesn't bash treat brace expansions exactly like csh?
928 The only difference between bash and csh brace expansion is that
929 bash requires a brace expression to contain at least one unquoted
930 comma if it is to be expanded. Any brace-surrounded word not
931 containing an unquoted comma is left unchanged by the brace
932 expansion code. This affords the greatest degree of sh
935 Bash, ksh, zsh, and pd-ksh all implement brace expansion this way.
937 D3) Why doesn't bash have csh variable modifiers?
939 Posix has specified a more powerful, albeit somewhat more cryptic,
940 mechanism cribbed from ksh, and bash implements it.
943 Remove smallest suffix pattern. The WORD is expanded to produce
944 a pattern. It then expands to the value of PARAMETER, with the
945 smallest portion of the suffix matched by the pattern deleted.
953 Remove largest suffix pattern. The WORD is expanded to produce
954 a pattern. It then expands to the value of PARAMETER, with the
955 largest portion of the suffix matched by the pattern deleted.
962 Remove smallest prefix pattern. The WORD is expanded to produce
963 a pattern. It then expands to the value of PARAMETER, with the
964 smallest portion of the prefix matched by the pattern deleted.
971 Remove largest prefix pattern. The WORD is expanded to produce
972 a pattern. It then expands to the value of PARAMETER, with the
973 largest portion of the prefix matched by the pattern deleted.
992 D4) How can I make my csh aliases work when I convert to bash?
994 Bash uses a different syntax to support aliases than csh does.
995 The details can be found in the documentation. We have provided
996 a shell script which does most of the work of conversion for you;
997 this script can be found in ./examples/misc/aliasconv.sh. Here is
1000 Start csh in the normal way for you. (e.g., `csh')
1002 Pipe the output of `alias' through `aliasconv.sh', saving the
1003 results into `bash_aliases':
1005 alias | bash aliasconv.sh >bash_aliases
1007 Edit `bash_aliases', carefully reading through any created
1008 functions. You will need to change the names of some csh specific
1009 variables to the bash equivalents. The script converts $cwd to
1010 $PWD, $term to $TERM, $home to $HOME, $user to $USER, and $prompt
1011 to $PS1. You may also have to add quotes to avoid unwanted
1014 For example, the csh alias:
1016 alias cd 'cd \!*; echo $cwd'
1018 is converted to the bash function:
1020 cd () { command cd "$@"; echo $PWD ; }
1022 The only thing that needs to be done is to quote $PWD:
1024 cd () { command cd "$@"; echo "$PWD" ; }
1026 Merge the edited file into your ~/.bashrc.
1028 There is an additional, more ambitious, script in
1029 examples/misc/cshtobash that attempts to convert your entire csh
1030 environment to its bash equivalent. This script can be run as
1031 simply `cshtobash' to convert your normal interactive
1032 environment, or as `cshtobash ~/.login' to convert your login
1035 D5) How can I pipe standard output and standard error from one command to
1036 another, like csh does with `|&'?
1039 command 2>&1 | command2
1041 The key is to remember that piping is performed before redirection, so
1042 file descriptor 1 points to the pipe when it is duplicated onto file
1045 D6) Now that I've converted from ksh to bash, are there equivalents to
1046 ksh features like autoloaded functions and the `whence' command?
1048 There are features in ksh-88 and ksh-93 that do not have direct bash
1049 equivalents. Most, however, can be emulated with very little trouble.
1051 ksh-88 feature Bash equivalent
1052 -------------- ---------------
1053 compiled-in aliases set up aliases in .bashrc; some ksh aliases are
1054 bash builtins (hash, history, type)
1055 coprocesses named pipe pairs (one for read, one for write)
1056 typeset +f declare -F
1057 cd, print, whence function substitutes in examples/functions/kshenv
1058 autoloaded functions examples/functions/autoload is the same as typeset -fu
1059 read var?prompt read -p prompt var
1061 ksh-93 feature Bash equivalent
1062 -------------- ---------------
1063 sleep, getconf Bash has loadable versions in examples/loadables
1064 ${.sh.version} $BASH_VERSION
1069 Section E: How can I get bash to do certain things, and why does bash do
1070 things the way it does?
1072 E1) Why is the bash builtin `test' slightly different from /bin/test?
1074 The specific example used here is [ ! x -o x ], which is false.
1076 Bash's builtin `test' implements the Posix.2 spec, which can be
1077 summarized as follows (the wording is due to David Korn):
1079 Here is the set of rules for processing test arguments.
1082 1 Arg: True iff argument is not null.
1083 2 Args: If first arg is !, True iff second argument is null.
1084 If first argument is unary, then true if unary test is true
1086 3 Args: If second argument is a binary operator, do binary test of $1 $3
1087 If first argument is !, negate two argument test of $2 $3
1088 If first argument is `(' and third argument is `)', do the
1089 one-argument test of the second argument.
1091 4 Args: If first argument is !, negate three argument test of $2 $3 $4.
1092 Otherwise unspecified
1093 5 or more Args: unspecified. (Historical shells would use their
1096 The operators -a and -o are considered binary operators for the purpose
1099 As you can see, the test becomes (not (x or x)), which is false.
1101 E2) Why does bash sometimes say `Broken pipe'?
1103 If a sequence of commands appears in a pipeline, and one of the
1104 reading commands finishes before the writer has finished, the
1105 writer receives a SIGPIPE signal. Many other shells special-case
1106 SIGPIPE as an exit status in the pipeline and do not report it.
1111 `head' can finish before `ps' writes all of its output, and ps
1112 will try to write on a pipe without a reader. In that case, bash
1113 will print `Broken pipe' to stderr when ps is killed by a
1116 As of bash-3.1, bash will not report SIGPIPE errors by default. You
1117 can build a version of bash that will report such errors.
1119 E3) When I have terminal escape sequences in my prompt, why does bash
1120 wrap lines at the wrong column?
1122 Readline, the line editing library that bash uses, does not know
1123 that the terminal escape sequences do not take up space on the
1124 screen. The redisplay code assumes, unless told otherwise, that
1125 each character in the prompt is a `printable' character that
1126 takes up one character position on the screen.
1128 You can use the bash prompt expansion facility (see the PROMPTING
1129 section in the manual page) to tell readline that sequences of
1130 characters in the prompt strings take up no screen space.
1132 Use the \[ escape to begin a sequence of non-printing characters,
1133 and the \] escape to signal the end of such a sequence.
1135 E4) If I pipe the output of a command into `read variable', why doesn't
1136 the output show up in $variable when the read command finishes?
1138 This has to do with the parent-child relationship between Unix
1139 processes. It affects all commands run in pipelines, not just
1140 simple calls to `read'. For example, piping a command's output
1141 into a `while' loop that repeatedly calls `read' will result in
1144 Each element of a pipeline, even a builtin or shell function,
1145 runs in a separate process, a child of the shell running the
1146 pipeline. A subprocess cannot affect its parent's environment.
1147 When the `read' command sets the variable to the input, that
1148 variable is set only in the subshell, not the parent shell. When
1149 the subshell exits, the value of the variable is lost.
1151 Many pipelines that end with `read variable' can be converted
1152 into command substitutions, which will capture the output of
1153 a specified command. The output can then be assigned to a
1156 grep ^gnu /usr/lib/news/active | wc -l | read ngroup
1158 can be converted into
1160 ngroup=$(grep ^gnu /usr/lib/news/active | wc -l)
1162 This does not, unfortunately, work to split the text among
1163 multiple variables, as read does when given multiple variable
1164 arguments. If you need to do this, you can either use the
1165 command substitution above to read the output into a variable
1166 and chop up the variable using the bash pattern removal
1167 expansion operators or use some variant of the following
1170 Say /usr/local/bin/ipaddr is the following shell script:
1173 host `hostname` | awk '/address/ {print $NF}'
1177 /usr/local/bin/ipaddr | read A B C D
1179 to break the local machine's IP address into separate octets, use
1183 set -- $(/usr/local/bin/ipaddr)
1185 A="$1" B="$2" C="$3" D="$4"
1187 Beware, however, that this will change the shell's positional
1188 parameters. If you need them, you should save them before doing
1191 This is the general approach -- in most cases you will not need to
1192 set $IFS to a different value.
1194 Some other user-supplied alternatives include:
1196 read A B C D << HERE
1197 $(IFS=.; echo $(/usr/local/bin/ipaddr))
1200 and, where process substitution is available,
1202 read A B C D < <(IFS=.; echo $(/usr/local/bin/ipaddr))
1204 E5) I have a bunch of shell scripts that use backslash-escaped characters
1205 in arguments to `echo'. Bash doesn't interpret these characters. Why
1206 not, and how can I make it understand them?
1208 This is the behavior of echo on most Unix System V machines.
1210 The bash builtin `echo' is modeled after the 9th Edition
1211 Research Unix version of `echo'. It does not interpret
1212 backslash-escaped characters in its argument strings by default;
1213 it requires the use of the -e option to enable the
1214 interpretation. The System V echo provides no way to disable the
1215 special characters; the bash echo has a -E option to disable
1218 There is a configuration option that will make bash behave like
1219 the System V echo and interpret things like `\t' by default. Run
1220 configure with the --enable-xpg-echo-default option to turn this
1221 on. Be aware that this will cause some of the tests run when you
1222 type `make tests' to fail.
1224 There is a shell option, `xpg_echo', settable with `shopt', that will
1225 change the behavior of echo at runtime. Enabling this option turns
1226 on expansion of backslash-escape sequences.
1228 E6) Why doesn't a while or for loop get suspended when I type ^Z?
1230 This is a consequence of how job control works on Unix. The only
1231 thing that can be suspended is the process group. This is a single
1232 command or pipeline of commands that the shell forks and executes.
1234 When you run a while or for loop, the only thing that the shell forks
1235 and executes are any commands in the while loop test and commands in
1236 the loop bodies. These, therefore, are the only things that can be
1237 suspended when you type ^Z.
1239 If you want to be able to stop the entire loop, you need to put it
1240 within parentheses, which will force the loop into a subshell that
1241 may be stopped (and subsequently restarted) as a single unit.
1243 E7) What about empty for loops in Makefiles?
1245 It's fairly common to see constructs like this in automatically-generated
1253 for d in ${SUBDIRS}; do \
1254 ( cd $$d && ${MAKE} ${MFLAGS} clean ) \
1257 When SUBDIRS is empty, this results in a command like this being passed to
1261 ( cd $d && ${MAKE} ${MFLAGS} clean )
1264 In versions of bash before bash-2.05a, this was a syntax error. If the
1265 reserved word `in' was present, a word must follow it before the semicolon
1266 or newline. The language in the manual page referring to the list of words
1267 being empty referred to the list after it is expanded. These versions of
1268 bash required that there be at least one word following the `in' when the
1269 construct was parsed.
1271 The idiomatic Makefile solution is something like:
1276 subdirs=$SUBDIRS ; for d in $$subdirs; do \
1277 ( cd $$d && ${MAKE} ${MFLAGS} clean ) \
1280 The latest updated POSIX standard has changed this: the word list
1281 is no longer required. Bash versions 2.05a and later accept the
1284 E8) Why does the arithmetic evaluation code complain about `08'?
1286 The bash arithmetic evaluation code (used for `let', $(()), (()), and in
1287 other places), interprets a leading `0' in numeric constants as denoting
1288 an octal number, and a leading `0x' as denoting hexadecimal. This is
1289 in accordance with the POSIX.2 spec, section 2.9.2.1, which states that
1290 arithmetic constants should be handled as signed long integers as defined
1291 by the ANSI/ISO C standard.
1293 The POSIX.2 interpretation committee has confirmed this:
1295 http://www.pasc.org/interps/unofficial/db/p1003.2/pasc-1003.2-173.html
1297 E9) Why does the pattern matching expression [A-Z]* match files beginning
1298 with every letter except `z'?
1300 Bash-2.03, Bash-2.05 and later versions honor the current locale setting
1301 when processing ranges within pattern matching bracket expressions ([A-Z]).
1302 This is what POSIX.2 and SUSv3/XPG6 specify.
1304 The behavior of the matcher in bash-2.05 and later versions depends on the
1305 current LC_COLLATE setting. Setting this variable to `C' or `POSIX' will
1306 result in the traditional behavior ([A-Z] matches all uppercase ASCII
1307 characters). Many other locales, including the en_US locale (the default
1308 on many US versions of Linux) collate the upper and lower case letters like
1313 which means that [A-Z] matches every letter except `z'. Others collate like
1317 which means that [A-Z] matches every letter except `a'.
1319 The portable way to specify upper case letters is [:upper:] instead of
1320 A-Z; lower case may be specified as [:lower:] instead of a-z.
1322 Look at the manual pages for setlocale(3), strcoll(3), and, if it is
1323 present, locale(1). If you have locale(1), you can use it to find
1324 your current locale information even if you do not have any of the
1331 into /etc/profile and inspect any shell scripts run from cron for
1332 constructs like [A-Z]. This will prevent things like
1336 from removing every file in the current directory except those beginning
1337 with `z' and still allow individual users to change the collation order.
1338 Users may put the above command into their own profiles as well, of course.
1340 E10) Why does `cd //' leave $PWD as `//'?
1342 POSIX.2, in its description of `cd', says that *three* or more leading
1343 slashes may be replaced with a single slash when canonicalizing the
1344 current working directory.
1346 This is, I presume, for historical compatibility. Certain versions of
1347 Unix, and early network file systems, used paths of the form
1348 //hostname/path to access `path' on server `hostname'.
1350 E11) If I resize my xterm while another program is running, why doesn't bash
1353 This is another issue that deals with job control.
1355 The kernel maintains a notion of a current terminal process group. Members
1356 of this process group (processes whose process group ID is equal to the
1357 current terminal process group ID) receive terminal-generated signals like
1358 SIGWINCH. (For more details, see the JOB CONTROL section of the bash
1361 If a terminal is resized, the kernel sends SIGWINCH to each member of
1362 the terminal's current process group (the `foreground' process group).
1364 When bash is running with job control enabled, each pipeline (which may be
1365 a single command) is run in its own process group, different from bash's
1366 process group. This foreground process group receives the SIGWINCH; bash
1367 does not. Bash has no way of knowing that the terminal has been resized.
1369 There is a `checkwinsize' option, settable with the `shopt' builtin, that
1370 will cause bash to check the window size and adjust its idea of the
1371 terminal's dimensions each time a process stops or exits and returns control
1372 of the terminal to bash. Enable it with `shopt -s checkwinsize'.
1374 E12) Why don't negative offsets in substring expansion work like I expect?
1376 When substring expansion of the form ${param:offset[:length} is used,
1377 an `offset' that evaluates to a number less than zero counts back from
1378 the end of the expanded value of $param.
1380 When a negative `offset' begins with a minus sign, however, unexpected things
1381 can happen. Consider
1386 intending to print the last four characters of $a. The problem is that
1387 ${param:-word} already has a well-defined meaning: expand to word if the
1388 expanded value of param is unset or null, and $param otherwise.
1390 To use negative offsets that begin with a minus sign, separate the
1391 minus sign and the colon with a space.
1393 Section F: Things to watch out for on certain Unix versions
1395 F1) Why can't I use command line editing in my `cmdtool'?
1397 The problem is `cmdtool' and bash fighting over the input. When
1398 scrolling is enabled in a cmdtool window, cmdtool puts the tty in
1399 `raw mode' to permit command-line editing using the mouse for
1400 applications that cannot do it themselves. As a result, bash and
1401 cmdtool each try to read keyboard input immediately, with neither
1402 getting enough of it to be useful.
1404 This mode also causes cmdtool to not implement many of the
1405 terminal functions and control sequences appearing in the
1406 `sun-cmd' termcap entry. For a more complete explanation, see
1407 that file examples/suncmd.termcap in the bash distribution.
1409 `xterm' is a better choice, and gets along with bash much more
1412 If you must use cmdtool, you can use the termcap description in
1413 examples/suncmd.termcap. Set the TERMCAP variable to the terminal
1414 description contained in that file, i.e.
1416 TERMCAP='Mu|sun-cmd:am:bs:km:pt:li#34:co#80:cl=^L:ce=\E[K:cd=\E[J:rs=\E[s:'
1418 Then export TERMCAP and start a new cmdtool window from that shell.
1419 The bash command-line editing should behave better in the new
1420 cmdtool. If this works, you can put the assignment to TERMCAP
1421 in your bashrc file.
1423 F2) I built bash on Solaris 2. Why do globbing expansions and filename
1424 completion chop off the first few characters of each filename?
1426 This is the consequence of building bash on SunOS 5 and linking
1427 with the libraries in /usr/ucblib, but using the definitions
1428 and structures from files in /usr/include.
1430 The actual conflict is between the dirent structure in
1431 /usr/include/dirent.h and the struct returned by the version of
1432 `readdir' in libucb.a (a 4.3-BSD style `struct direct').
1434 Make sure you've got /usr/ccs/bin ahead of /usr/ucb in your $PATH
1435 when configuring and building bash. This will ensure that you
1436 use /usr/ccs/bin/cc or acc instead of /usr/ucb/cc and that you
1437 link with libc before libucb.
1439 If you have installed the Sun C compiler, you may also need to
1440 put /usr/ccs/bin and /opt/SUNWspro/bin into your $PATH before
1443 F3) Why does bash dump core after I interrupt username completion or
1444 `~user' tilde expansion on a machine running NIS?
1446 This is a famous and long-standing bug in the SunOS YP (sorry, NIS)
1447 client library, which is part of libc.
1449 The YP library code keeps static state -- a pointer into the data
1450 returned from the server. When YP initializes itself (setpwent),
1451 it looks at this pointer and calls free on it if it's non-null.
1454 If one of the YP functions is interrupted during getpwent (the
1455 exact function is interpretwithsave()), and returns NULL, the
1456 pointer is freed without being reset to NULL, and the function
1457 returns. The next time getpwent is called, it sees that this
1458 pointer is non-null, calls free, and the bash free() blows up
1459 because it's being asked to free freed memory.
1461 The traditional Unix mallocs allow memory to be freed multiple
1462 times; that's probably why this has never been fixed. You can
1463 run configure with the `--without-gnu-malloc' option to use
1464 the C library malloc and avoid the problem.
1466 F4) I'm running SVR4.2. Why is the line erased every time I type `@'?
1468 The `@' character is the default `line kill' character in most
1469 versions of System V, including SVR4.2. You can change this
1470 character to whatever you want using `stty'. For example, to
1471 change the line kill character to control-u, type
1475 where the `^' and `U' can be two separate characters.
1477 F5) Why does bash report syntax errors when my C News scripts use a
1478 redirection before a subshell command?
1480 The actual command in question is something like
1484 According to the grammar given in the POSIX.2 standard, this construct
1485 is, in fact, a syntax error. Redirections may only precede `simple
1486 commands'. A subshell construct such as the above is one of the shell's
1487 `compound commands'. A redirection may only follow a compound command.
1489 This affects the mechanical transformation of commands that use `cat'
1490 to pipe a file into a command (a favorite Useless-Use-Of-Cat topic on
1491 comp.unix.shell). While most commands of the form
1495 can be converted to `< file command', shell control structures such as
1496 loops and subshells require `command < file'.
1498 The file CWRU/sh-redir-hack in the bash distribution is an
1499 (unofficial) patch to parse.y that will modify the grammar to
1500 support this construct. It will not apply with `patch'; you must
1501 modify parse.y by hand. Note that if you apply this, you must
1502 recompile with -DREDIRECTION_HACK. This introduces a large
1503 number of reduce/reduce conflicts into the shell grammar.
1505 F6) Why can't I use vi-mode editing on Red Hat Linux 6.1?
1507 The short answer is that Red Hat screwed up.
1509 The long answer is that they shipped an /etc/inputrc that only works
1510 for emacs mode editing, and then screwed all the vi users by setting
1511 INPUTRC to /etc/inputrc in /etc/profile.
1513 The short fix is to do one of the following: remove or rename
1514 /etc/inputrc, set INPUTRC=~/.inputrc in ~/.bashrc (or .bash_profile,
1515 but make sure you export it if you do), remove the assignment to
1516 INPUTRC from /etc/profile, add
1520 to the beginning of /etc/inputrc, or bracket the key bindings in
1521 /etc/inputrc with these lines
1527 F7) Why do bash-2.05a and bash-2.05b fail to compile `printf.def' on
1530 HP/UX's support for long double is imperfect at best.
1532 GCC will support it without problems, but the HP C library functions
1533 like strtold(3) and printf(3) don't actually work with long doubles.
1534 HP implemented a `long_double' type as a 4-element array of 32-bit
1535 ints, and that is what the library functions use. The ANSI C
1536 `long double' type is a 128-bit floating point scalar.
1538 The easiest fix, until HP fixes things up, is to edit the generated
1539 config.h and #undef the HAVE_LONG_DOUBLE line. After doing that,
1540 the compilation should complete successfully.
1542 Section G: How can I get bash to do certain common things?
1544 G1) How can I get bash to read and display eight-bit characters?
1546 This is a process requiring several steps.
1548 First, you must ensure that the `physical' data path is a full eight
1549 bits. For xterms, for example, the `vt100' resources `eightBitInput'
1550 and `eightBitOutput' should be set to `true'.
1552 Once you have set up an eight-bit path, you must tell the kernel and
1553 tty driver to leave the eighth bit of characters alone when processing
1554 keyboard input. Use `stty' to do this:
1556 stty cs8 -istrip -parenb
1558 For old BSD-style systems, you can use
1566 Finally, you need to tell readline that you will be inputting and
1567 displaying eight-bit characters. You use readline variables to do
1568 this. These variables can be set in your .inputrc or using the bash
1569 `bind' builtin. Here's an example using `bind':
1571 bash$ bind 'set convert-meta off'
1572 bash$ bind 'set meta-flag on'
1573 bash$ bind 'set output-meta on'
1575 The `set' commands between the single quotes may also be placed
1578 G2) How do I write a function `x' to replace builtin command `x', but
1579 still invoke the command from within the function?
1581 This is why the `command' and `builtin' builtins exist. The
1582 `command' builtin executes the command supplied as its first
1583 argument, skipping over any function defined with that name. The
1584 `builtin' builtin executes the builtin command given as its first
1587 For example, to write a function to replace `cd' that writes the
1588 hostname and current directory to an xterm title bar, use
1589 something like the following:
1593 builtin cd "$@" && xtitle "$HOST: $PWD"
1596 This could also be written using `command' instead of `builtin';
1597 the version above is marginally more efficient.
1599 G3) How can I find the value of a shell variable whose name is the value
1600 of another shell variable?
1602 Versions of Bash newer than Bash-2.0 support this directly. You can use
1606 For example, the following sequence of commands will echo `z':
1612 For sh compatibility, use the `eval' builtin. The important
1613 thing to remember is that `eval' expands the arguments you give
1614 it again, so you need to quote the parts of the arguments that
1615 you want `eval' to act on.
1617 For example, this expression prints the value of the last positional
1620 eval echo \"\$\{$#\}\"
1622 The expansion of the quoted portions of this expression will be
1623 deferred until `eval' runs, while the `$#' will be expanded
1624 before `eval' is executed. In versions of bash later than bash-2.0,
1628 does the same thing.
1630 This is not the same thing as ksh93 `nameref' variables, though the syntax
1631 is similar. I may add namerefs in a future bash version.
1633 G4) How can I make the bash `time' reserved word print timing output that
1634 looks like the output from my system's /usr/bin/time?
1636 The bash command timing code looks for a variable `TIMEFORMAT' and
1637 uses its value as a format string to decide how to display the
1640 The value of TIMEFORMAT is a string with `%' escapes expanded in a
1641 fashion similar in spirit to printf(3). The manual page explains
1642 the meanings of the escape sequences in the format string.
1644 If TIMEFORMAT is not set, bash acts as if the following assignment had
1647 TIMEFORMAT=$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
1649 The POSIX.2 default time format (used by `time -p command') is
1651 TIMEFORMAT=$'real %2R\nuser %2U\nsys %2S'
1653 The BSD /usr/bin/time format can be emulated with:
1655 TIMEFORMAT=$'\t%1R real\t%1U user\t%1S sys'
1657 The System V /usr/bin/time format can be emulated with:
1659 TIMEFORMAT=$'\nreal\t%1R\nuser\t%1U\nsys\t%1S'
1661 The ksh format can be emulated with:
1663 TIMEFORMAT=$'\nreal\t%2lR\nuser\t%2lU\nsys\t%2lS'
1665 G5) How do I get the current directory into my prompt?
1667 Bash provides a number of backslash-escape sequences which are expanded
1668 when the prompt string (PS1 or PS2) is displayed. The full list is in
1671 The \w expansion gives the full pathname of the current directory, with
1672 a tilde (`~') substituted for the current value of $HOME. The \W
1673 expansion gives the basename of the current directory. To put the full
1674 pathname of the current directory into the path without any tilde
1675 subsitution, use $PWD. Here are some examples:
1677 PS1='\w$ ' # current directory with tilde
1678 PS1='\W$ ' # basename of current directory
1679 PS1='$PWD$ ' # full pathname of current directory
1681 The single quotes are important in the final example to prevent $PWD from
1682 being expanded when the assignment to PS1 is performed.
1684 G6) How can I rename "*.foo" to "*.bar"?
1686 Use the pattern removal functionality described in D3. The following `for'
1687 loop will do the trick:
1693 G7) How can I translate a filename from uppercase to lowercase?
1695 The script examples/functions/lowercase, originally written by John DuBois,
1696 will do the trick. The converse is left as an exercise.
1698 G8) How can I write a filename expansion (globbing) pattern that will match
1699 all files in the current directory except "." and ".."?
1701 You must have set the `extglob' shell option using `shopt -s extglob' to use
1706 A solution that works without extended globbing is given in the Unix Shell
1707 FAQ, posted periodically to comp.unix.shell.
1709 Section H: Where do I go from here?
1711 H1) How do I report bugs in bash, and where should I look for fixes and
1714 Use the `bashbug' script to report bugs. It is built and
1715 installed at the same time as bash. It provides a standard
1716 template for reporting a problem and automatically includes
1717 information about your configuration and build environment.
1719 `bashbug' sends its reports to bug-bash@gnu.org, which
1720 is a large mailing list gatewayed to the usenet newsgroup gnu.bash.bug.
1722 Bug fixes, answers to questions, and announcements of new releases
1723 are all posted to gnu.bash.bug. Discussions concerning bash features
1724 and problems also take place there.
1726 To reach the bash maintainers directly, send mail to
1727 bash-maintainers@gnu.org.
1729 H2) What kind of bash documentation is there?
1731 First, look in the doc directory in the bash distribution. It should
1732 contain at least the following files:
1734 bash.1 an extensive, thorough Unix-style manual page
1735 builtins.1 a manual page covering just bash builtin commands
1736 bashref.texi a reference manual in GNU tex`info format
1737 bashref.info an info version of the reference manual
1739 article.ms text of an article written for The Linux Journal
1740 readline.3 a man page describing readline
1742 Postscript, HTML, and ASCII files created from the above source are
1743 available in the documentation distribution.
1745 There is additional documentation available for anonymous FTP from host
1746 ftp.cwru.edu in the `pub/bash' directory.
1748 Cameron Newham and Bill Rosenblatt have written a book on bash, published
1749 by O'Reilly and Associates. The book is based on Bill Rosenblatt's Korn
1750 Shell book. The title is ``Learning the Bash Shell'', and the ISBN number
1751 is 1-56592-147-X. Look for it in fine bookstores near you. This book
1752 covers bash-1.14, but has an appendix describing some of the new features
1755 A second edition of this book is available, published in January, 1998.
1756 The ISBN number is 1-56592-347-2. Look for it in the same fine bookstores
1759 The GNU Bash Reference Manual has been published as a printed book by
1760 Network Theory Ltd (Paperback, ISBN: 0-9541617-7-7, Feb 2003). It covers
1761 bash-2.0 and is available from most online bookstores (see
1762 http://www.network-theory.co.uk/bash/manual/ for details). The publisher
1763 will donate $1 to the Free Software Foundation for each copy sold.
1765 H3) What's coming in future versions?
1767 These are features I hope to include in a future version of bash.
1769 Rocky Bernstein's bash debugger (support is included with bash-3.0)
1771 co-processes, but with a new-style syntax that looks like function declaration
1773 H4) What's on the bash `wish list' for future versions?
1775 These are features that may or may not appear in a future version of bash.
1777 breaking some of the shell functionality into embeddable libraries
1778 a module system like zsh's, using dynamic loading like builtins
1779 a bash programmer's guide with a chapter on creating loadable builtins
1780 a better loadable interface to perl with access to the shell builtins and
1781 variables (contributions gratefully accepted)
1782 ksh93-like `nameref' variables
1783 ksh93-like `xx.yy' variables (including some of the .sh.* variables) and
1784 associated disipline functions
1785 Some of the new ksh93 pattern matching operators, like backreferencing
1787 H5) When will the next release appear?
1789 The next version will appear sometime in 2006. Never make predictions.
1791 This document is Copyright 1995-2005 by Chester Ramey.
1793 Permission is hereby granted, without written agreement and
1794 without license or royalty fees, to use, copy, and distribute
1795 this document for any purpose, provided that the above copyright
1796 notice appears in all copies of this document and that the
1797 contents of this document remain unaltered.