1 This is the Bash FAQ, version 3.20, for Bash version 2.05b.
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 1003.2 standard'?
35 A10) What is the bash `posix mode'?
37 Section B: The latest version
39 B1) What's new in version 2.05b?
40 B2) Are there any user-visible incompatibilities between bash-2.05b 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 `//'?
79 Section F: Things to watch out for on certain Unix versions
81 F1) Why can't I use command line editing in my `cmdtool'?
82 F2) I built bash on Solaris 2. Why do globbing expansions and filename
83 completion chop off the first few characters of each filename?
84 F3) Why does bash dump core after I interrupt username completion or
85 `~user' tilde expansion on a machine running NIS?
86 F4) I'm running SVR4.2. Why is the line erased every time I type `@'?
87 F5) Why does bash report syntax errors when my C News scripts use a
88 redirection before a subshell command?
89 F6) Why can't I use vi-mode editing on Red Hat Linux 6.1?
90 F7) Why do bash-2.05a and bash-2.05b fail to compile `printf.def' on
93 Section G: How can I get bash to do certain common things?
95 G1) How can I get bash to read and display eight-bit characters?
96 G2) How do I write a function `x' to replace builtin command `x', but
97 still invoke the command from within the function?
98 G3) How can I find the value of a shell variable whose name is the value
99 of another shell variable?
100 G4) How can I make the bash `time' reserved word print timing output that
101 looks like the output from my system's /usr/bin/time?
102 G5) How do I get the current directory into my prompt?
103 G6) How can I rename "*.foo" to "*.bar"?
104 G7) How can I translate a filename from uppercase to lowercase?
105 G8) How can I write a filename expansion (globbing) pattern that will match
106 all files in the current directory except "." and ".."?
108 Section H: Where do I go from here?
110 H1) How do I report bugs in bash, and where should I look for fixes and
112 H2) What kind of bash documentation is there?
113 H3) What's coming in future versions?
114 H4) What's on the bash `wish list'?
115 H5) When will the next release appear?
118 Section A: The Basics
122 Bash is a Unix command interpreter (shell). It is an implementation of
123 the Posix 1003.2 shell standard, and resembles the Korn and System V
126 Bash contains a number of enhancements over those shells, both
127 for interactive use and shell programming. Features geared
128 toward interactive use include command line editing, command
129 history, job control, aliases, and prompt expansion. Programming
130 features include additional variable expansions, shell
131 arithmetic, and a number of variables and options to control
134 Bash was originally written by Brian Fox of the Free Software
135 Foundation. The current developer and maintainer is Chet Ramey
136 of Case Western Reserve University.
138 A2) What's the latest version?
140 The latest version is 2.05b, first made available on Wednesday, 17
143 A3) Where can I get it?
145 Bash is the GNU project's shell, and so is available from the
146 master GNU archive site, ftp.gnu.org, and its mirrors. The
147 latest version is also available for FTP from ftp.cwru.edu.
148 The following URLs tell how to get version 2.05b:
150 ftp://ftp.gnu.org/pub/gnu/bash/bash-2.05b.tar.gz
151 ftp://ftp.cwru.edu/pub/bash/bash-2.05b.tar.gz
153 Formatted versions of the documentation are available with the URLs:
155 ftp://ftp.gnu.org/pub/gnu/bash/bash-doc-2.05b.tar.gz
156 ftp://ftp.cwru.edu/pub/bash/bash-doc-2.05b.tar.gz
158 A4) On what machines will bash run?
160 Bash has been ported to nearly every version of UNIX. All you
161 should have to do to build it on a machine for which a port
162 exists is to type `configure' and then `make'. The build process
163 will attempt to discover the version of UNIX you have and tailor
164 itself accordingly, using a script created by GNU autoconf.
166 More information appears in the file `INSTALL' in the distribution.
168 The Bash web page (http://cnswww.cns.cwru.edu/~chet/bash/bashtop.html)
169 explains how to obtain binary versions of bash for most of the major
170 commercial Unix systems.
172 A5) Will bash run on operating systems other than Unix?
174 Configuration specifics for Unix-like systems such as QNX and
175 LynxOS are included in the distribution. Bash-2.05 and later
176 versions should compile and run on Minix 2.0 (patches were
177 contributed), but I don't believe anyone has built bash-2.x on
178 earlier Minix versions yet.
180 Bash has been ported to versions of Windows implementing the Win32
181 programming interface. This includes Windows 95 and Windows NT.
182 The port was done by Cygnus Solutions as part of their CYGWIN
183 project. For more information about the project, look at the URLs
185 http://www.cygwin.com/
186 http://sourceware.cygnus.com/cygwin
188 Cygnus originally ported bash-1.14.7, and that port was part of their
189 early GNU-Win32 (the original name) releases. Cygnus has also done a
190 port of bash-2.05 to the CYGWIN environment, and it is available as
191 part of their current release.
193 Bash-2.05b should require no local Cygnus changes to build and run under
196 The Cygnus port works only on Intel machines. There is a port of bash
197 (I don't know which version) to the alpha/NT environment available from
199 ftp://ftp.gnustep.org//pub/win32/bash-alpha-nt-1.01.tar.gz
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 has begun to work with bash-2.05, but I don't know the status.
217 Ports of bash-1.12 and bash-2.0 are available for OS/2 from
219 ftp://hobbes.nmsu.edu/pub/os2/util/shell/bash_112.zip
220 ftp://hobbes.nmsu.edu/pub/os2/util/shell/bash-2.0(253).zip
222 I haven't looked at either, but the second appears to be a binary-only
223 distribution. Beware.
225 I have received word that Bash (I'm not sure which version, but I
226 believe that it's at least bash-2.02.1) is the standard shell on
229 A6) How can I build bash with gcc?
231 Bash configures to use gcc by default if it is available. Read the
232 file INSTALL in the distribution for more information.
234 A7) How can I make bash my login shell?
236 Some machines let you use `chsh' to change your login shell. Other
237 systems use `passwd -s' or `passwd -e'. If one of these works for
238 you, that's all you need. Note that many systems require the full
239 pathname to a shell to appear in /etc/shells before you can make it
240 your login shell. For this, you may need the assistance of your
241 friendly local system administrator.
243 If you cannot do this, you can still use bash as your login shell, but
244 you need to perform some tricks. The basic idea is to add a command
245 to your login shell's startup file to replace your login shell with
248 For example, if your login shell is csh or tcsh, and you have installed
249 bash in /usr/gnu/bin/bash, add the following line to ~/.login:
251 if ( -f /usr/gnu/bin/bash ) exec /usr/gnu/bin/bash --login
253 (the `--login' tells bash that it is a login shell).
255 It's not a good idea to put this command into ~/.cshrc, because every
256 csh you run without the `-f' option, even ones started to run csh scripts,
257 reads that file. If you must put the command in ~/.cshrc, use something
260 if ( $?prompt ) exec /usr/gnu/bin/bash --login
262 to ensure that bash is exec'd only when the csh is interactive.
264 If your login shell is sh or ksh, you have to do two things.
266 First, create an empty file in your home directory named `.bash_profile'.
267 The existence of this file will prevent the exec'd bash from trying to
268 read ~/.profile, and re-execing itself over and over again. ~/.bash_profile
269 is the first file bash tries to read initialization commands from when
270 it is invoked as a login shell.
272 Next, add a line similar to the above to ~/.profile:
274 [ -f /usr/gnu/bin/bash ] && exec /usr/gnu/bin/bash --login
276 This will cause login shells to replace themselves with bash running as
277 a login shell. Once you have this working, you can copy your initialization
278 code from ~/.profile to ~/.bash_profile.
280 I have received word that the recipe supplied above is insufficient for
281 machines running CDE. CDE has a maze of twisty little startup files, all
284 If you cannot change your login shell in the password file to bash, you
285 will have to (apparently) live with CDE using the shell in the password
286 file to run its startup scripts. If you have changed your shell to bash,
287 there is code in the CDE startup files (on Solaris, at least) that attempts
288 to do the right thing. It is, however, often broken, and may require that
289 you use the $BASH_ENV trick described below.
291 `dtterm' claims to use $SHELL as the default program to start, so if you
292 can change $SHELL in the CDE startup files, you should be able to use bash
293 in your terminal windows.
295 Setting DTSOURCEPROFILE in ~/.dtprofile will cause the `Xsession' program
296 to read your login shell's startup files. You may be able to use bash for
297 the rest of the CDE programs by setting SHELL to bash in ~/.dtprofile as
298 well, but I have not tried this.
300 You can use the above `exec' recipe to start bash when not logging in with
301 CDE by testing the value of the DT variable:
303 if [ -n "$DT" ]; then
304 [ -f /usr/gnu/bin/bash ] && exec /usr/gnu/bin/bash --login
307 If CDE starts its shells non-interactively during login, the login shell
308 startup files (~/.profile, ~/.bash_profile) will not be sourced at login.
309 To get around this problem, append a line similar to the following to your
312 BASH_ENV=${HOME}/.bash_profile ; export BASH_ENV
314 and add the following line to the beginning of ~/.bash_profile:
318 A8) I just changed my login shell to bash, and now I can't FTP into my
321 You must add the full pathname to bash to the file /etc/shells. As
322 noted in the answer to the previous question, many systems require
323 this before you can make bash your login shell.
325 Most versions of ftpd use this file to prohibit `special' users
326 such as `uucp' and `news' from using FTP.
328 A9) What's the `POSIX 1003.2 standard'?
330 POSIX is a name originally coined by Richard Stallman for a
331 family of open system standards based on UNIX. There are a
332 number of aspects of UNIX under consideration for
333 standardization, from the basic system services at the system
334 call and C library level to applications and tools to system
335 administration and management. Each area of standardization is
336 assigned to a working group in the 1003 series.
338 The POSIX Shell and Utilities standard has been developed by IEEE
339 Working Group 1003.2 (POSIX.2). It concentrates on the command
340 interpreter interface and utility programs commonly executed from
341 the command line or by other programs. An initial version of the
342 standard has been approved and published by the IEEE, and work is
343 currently underway to update it.
345 Bash is concerned with the aspects of the shell's behavior
346 defined by POSIX.2. The shell command language has of course
347 been standardized, including the basic flow control and program
348 execution constructs, I/O redirection and pipelining, argument
349 handling, variable expansion, and quoting.
351 The `special' builtins, which must be implemented as part of the
352 shell to provide the desired functionality, are specified as
353 being part of the shell; examples of these are `eval' and
354 `export'. Other utilities appear in the sections of POSIX.2 not
355 devoted to the shell which are commonly (and in some cases must
356 be) implemented as builtin commands, such as `read' and `test'.
357 POSIX.2 also specifies aspects of the shell's interactive
358 behavior as part of the UPE, including job control and command
359 line editing. Only vi-style line editing commands have been
360 standardized; emacs editing commands were left out due to
363 The Open Group has made an older version of its Single Unix
364 Specification (version 2), which is very similar to POSIX.2,
365 available on the web at
367 http://www.opengroup.org/onlinepubs/007908799/
369 The Single Unix Specification, version 3, is available on the web at
371 http://www.opengroup.org/onlinepubs/007904975/
373 A10) What is the bash `posix mode'?
375 Although bash is an implementation of the POSIX.2 shell
376 specification, there are areas where the bash default behavior
377 differs from that spec. The bash `posix mode' changes the bash
378 behavior in these areas so that it obeys the spec more closely.
380 Posix mode is entered by starting bash with the --posix or
381 '-o posix' option or executing `set -o posix' after bash is running.
383 The specific aspects of bash which change when posix mode is
384 active are listed in the file POSIX in the bash distribution.
385 They are also listed in a section in the Bash Reference Manual
386 (from which that file is generated).
388 Section B: The latest version
390 B1) What's new in version 2.05b?
392 The raison d'etre for bash-2.05b is to make a second intermediate
393 release containing the first of the new features to be available
394 in bash-3.0 and get feedback on those features before proceeding.
395 The major new feature is multibyte character support in both Bash
398 Bash-2.05b contains the following new features (see the manual page for
399 complete descriptions and the CHANGES and NEWS files in the bash-2.05b
402 o support for multibyte characters has been added to both bash and readline
404 o the DEBUG trap is now run *before* simple commands, ((...)) commands,
405 [[...]] conditional commands, and for ((...)) loops
407 o the shell now performs arithmetic in the largest integer size the machine
410 o there is a new \D{...} prompt expansion; passes the `...' to strftime(3)
411 and inserts the result into the expanded prompt
413 o there is a new `here-string' redirection operator: <<< word
415 o when displaying variables, function attributes and definitions are shown
416 separately, allowing them to be re-used as input (attempting to re-use
417 the old output would result in syntax errors).
419 o `read' has a new `-u fd' option to read from a specified file descriptor
421 o the bash debugger in examples/bashdb has been modified to work with the
422 new DEBUG trap semantics, the command set has been made more gdb-like,
423 and the changes to $LINENO make debugging functions work better
425 o the expansion of $LINENO inside a shell function is only relative to the
426 function start if the shell is interactive -- if the shell is running a
427 script, $LINENO expands to the line number in the script. This is as
431 A short feature history dating from Bash-2.0:
433 Bash-2.05a introduced the following new features:
435 o The `printf' builtin has undergone major work
437 o There is a new read-only `shopt' option: login_shell, which is set by
438 login shells and unset otherwise
440 o New `\A' prompt string escape sequence; expanding to time in 24-hour
443 o New `-A group/-g' option to complete and compgen; goes group name
446 o New [+-]O invocation option to set and unset `shopt' options at startup
448 o ksh-like `ERR' trap
450 o `for' loops now allow empty word lists after the `in' reserved word
452 o new `hard' and `soft' arguments for the `ulimit' builtin
454 o Readline can be configured to place the user at the same point on the line
455 when retrieving commands from the history list
457 o Readline can be configured to skip `hidden' files (filenames with a leading
458 `.' on Unix) when performing completion
460 Bash-2.05 introduced the following new features:
462 o This version has once again reverted to using locales and strcoll(3) when
463 processing pattern matching bracket expressions, as POSIX requires.
464 o Added a new `--init-file' invocation argument as a synonym for `--rcfile',
465 per the new GNU coding standards.
466 o The /dev/tcp and /dev/udp redirections now accept service names as well as
468 o `complete' and `compgen' now take a `-o value' option, which controls some
469 of the aspects of that compspec. Valid values are:
471 default - perform bash default completion if programmable
472 completion produces no matches
473 dirnames - perform directory name completion if programmable
474 completion produces no matches
475 filenames - tell readline that the compspec produces filenames,
476 so it can do things like append slashes to
477 directory names and suppress trailing spaces
478 o A new loadable builtin, realpath, which canonicalizes and expands symlinks
479 in pathname arguments.
480 o When `set' is called without options, it prints function defintions in a
481 way that allows them to be reused as input. This affects `declare' and
482 `declare -p' as well. This only happens when the shell is not in POSIX
483 mode, since POSIX.2 forbids this behavior.
485 Bash-2.04 introduced the following new features:
487 o Programmable word completion with the new `complete' and `compgen' builtins;
488 examples are provided in examples/complete/complete-examples
489 o `history' has a new `-d' option to delete a history entry
490 o `bind' has a new `-x' option to bind key sequences to shell commands
491 o The prompt expansion code has new `\j' and `\l' escape sequences
492 o The `no_empty_cmd_completion' shell option, if enabled, inhibits
493 command completion when TAB is typed on an empty line
494 o `help' has a new `-s' option to print a usage synopsis
495 o New arithmetic operators: var++, var--, ++var, --var, expr1,expr2 (comma)
496 o New ksh93-style arithmetic for command:
497 for ((expr1 ; expr2; expr3 )); do list; done
498 o `read' has new options: `-t', `-n', `-d', `-s'
499 o The redirection code handles several filenames specially: /dev/fd/N,
500 /dev/stdin, /dev/stdout, /dev/stderr
501 o The redirection code now recognizes /dev/tcp/HOST/PORT and
502 /dev/udp/HOST/PORT and tries to open a TCP or UDP socket, respectively,
503 to the specified port on the specified host
504 o The ${!prefix*} expansion has been implemented
505 o A new FUNCNAME variable, which expands to the name of a currently-executing
507 o The GROUPS variable is no longer readonly
508 o A new shopt `xpg_echo' variable, to control the behavior of echo with
509 respect to backslash-escape sequences at runtime
510 o The NON_INTERACTIVE_LOGIN_SHELLS #define has returned
512 The version of Readline released with Bash-2.04, Readline-4.1, had several
513 new features as well:
515 o Parentheses matching is always compiled into readline, and controllable
516 with the new `blink-matching-paren' variable
517 o The history-search-forward and history-search-backward functions now leave
518 point at the end of the line when the search string is empty, like
519 reverse-search-history, and forward-search-history
520 o A new function for applications: rl_on_new_line_with_prompt()
521 o New variables for applications: rl_already_prompted, and rl_gnu_readline_p
524 Bash-2.03 had very few new features, in keeping with the convention
525 that odd-numbered releases provide mainly bug fixes. A number of new
526 features were added to Readline, mostly at the request of the Cygnus
529 A new shopt option, `restricted_shell', so that startup files can test
530 whether or not the shell was started in restricted mode
531 Filename generation is now performed on the words between ( and ) in
532 compound array assignments (this is really a bug fix)
533 OLDPWD is now auto-exported, as POSIX.2 requires
534 ENV and BASH_ENV are read-only variables in a restricted shell
535 Bash may now be linked against an already-installed Readline library,
536 as long as the Readline library is version 4 or newer
537 All shells begun with the `--login' option will source the login shell
538 startup files, even if the shell is not interactive
540 There were lots of changes to the version of the Readline library released
541 along with Bash-2.03. For a complete list of the changes, read the file
542 CHANGES in the Bash-2.03 distribution.
544 Bash-2.02 contained the following new features:
546 a new version of malloc (based on the old GNU malloc code in previous
547 bash versions) that is more page-oriented, more conservative
548 with memory usage, does not `orphan' large blocks when they
549 are freed, is usable on 64-bit machines, and has allocation
550 checking turned on unconditionally
551 POSIX.2-style globbing character classes ([:alpha:], [:alnum:], etc.)
552 POSIX.2-style globbing equivalence classes
553 POSIX.2-style globbing collating symbols
554 the ksh [[...]] extended conditional command
555 the ksh egrep-style extended pattern matching operators
556 a new `printf' builtin
557 the ksh-like $(<filename) command substitution, which is equivalent to
559 new tilde prefixes that expand to directories from the directory stack
560 new `**' arithmetic operator to do exponentiation
561 case-insensitive globbing (filename expansion)
562 menu completion a la tcsh
563 `magic-space' history expansion function like tcsh
564 the readline inputrc `language' has a new file inclusion directive ($include)
566 Bash-2.01 contained only a few new features:
568 new `GROUPS' builtin array variable containing the user's group list
569 new bindable readline commands: history-and-alias-expand-line and
572 Bash-2.0 contained extensive changes and new features from bash-1.14.7.
575 new `time' reserved word to time pipelines, shell builtins, and
577 one-dimensional arrays with a new compound assignment statement,
578 appropriate expansion constructs and modifications to some
579 of the builtins (read, declare, etc.) to use them
580 new quoting syntaxes for ANSI-C string expansion and locale-specific
582 new expansions to do substring extraction, pattern replacement, and
583 indirect variable expansion
584 new builtins: `disown' and `shopt'
585 new variables: HISTIGNORE, SHELLOPTS, PIPESTATUS, DIRSTACK, GLOBIGNORE,
586 MACHTYPE, BASH_VERSINFO
587 special handling of many unused or redundant variables removed
588 (e.g., $notify, $glob_dot_filenames, $no_exit_on_failed_exec)
589 dynamic loading of new builtin commands; many loadable examples provided
590 new prompt expansions: \a, \e, \n, \H, \T, \@, \v, \V
591 history and aliases available in shell scripts
592 new readline variables: enable-keypad, mark-directories, input-meta,
593 visible-stats, disable-completion, comment-begin
594 new readline commands to manipulate the mark and operate on the region
595 new readline emacs mode commands and bindings for ksh-88 compatibility
596 updated and extended builtins
598 expanded (and now documented) restricted shell mode
600 implementation stuff:
601 autoconf-based configuration
602 nearly all of the bugs reported since version 1.14 have been fixed
603 most builtins converted to use builtin `getopt' for consistency
604 most builtins use -p option to display output in a reusable form
606 grammar tighter and smaller (66 reduce-reduce conflicts gone)
607 lots of code now smaller and faster
608 test suite greatly expanded
610 B2) Are there any user-visible incompatibilities between bash-2.05b and
613 There are a few incompatibilities between version 1.14.7 and version 2.05b.
614 They are detailed in the file COMPAT in the bash distribution. That file
615 is not meant to be all-encompassing; send mail to bash-maintainers@gnu.org
616 if if you find something that's not mentioned there.
618 Section C: Differences from other Unix shells
620 C1) How does bash differ from sh, the Bourne shell?
622 This is a non-comprehensive list of features that differentiate bash
623 from the SVR4.2 shell. The bash manual page explains these more
626 Things bash has that sh does not:
627 long invocation options
628 [+-]O invocation option
630 `!' reserved word to invert pipeline return value
631 `time' reserved word to time pipelines and shell builtins
632 the `function' reserved word
633 the `select' compound command and reserved word
634 arithmetic for command: for ((expr1 ; expr2; expr3 )); do list; done
635 new $'...' and $"..." quoting
636 the $(...) form of command substitution
637 the $(<filename) form of command substitution, equivalent to
639 the ${#param} parameter value length operator
640 the ${!param} indirect parameter expansion operator
641 the ${!param*} prefix expansion operator
642 the ${param:offset[:length]} parameter substring operator
643 the ${param/pat[/string]} parameter pattern substitution operator
644 expansions to perform substring removal (${p%[%]w}, ${p#[#]w})
645 expansion of positional parameters beyond $9 with ${num}
646 variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, REPLY,
647 TIMEFORMAT, PPID, PWD, OLDPWD, SHLVL, RANDOM, SECONDS,
648 LINENO, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE, HOSTNAME,
649 ENV, PS3, PS4, DIRSTACK, PIPESTATUS, HISTSIZE, HISTFILE,
650 HISTFILESIZE, HISTCONTROL, HISTIGNORE, GLOBIGNORE, GROUPS,
651 PROMPT_COMMAND, FCEDIT, FIGNORE, IGNOREEOF, INPUTRC,
652 SHELLOPTS, OPTERR, HOSTFILE, TMOUT, FUNCNAME, histchars,
656 variable arrays with new compound assignment syntax
657 redirections: <>, &>, >|, <<<, [n]<&word-, [n]>&word-
658 prompt string special char translation and variable expansion
659 auto-export of variables in initial environment
660 command search finds functions before builtins
661 bash return builtin will exit a file sourced with `.'
662 builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E, hash -d/-l/-p/-t.
663 export -n/-f/-p/name=value, pwd -L/-P,
664 read -e/-p/-a/-t/-n/-d/-s/-u,
665 readonly -a/-f/name=value, trap -l, set +o,
666 set -b/-m/-o option/-h/-p/-B/-C/-H/-P,
667 unset -f/-v, ulimit -m/-p/-u,
668 type -a/-p/-t/-f/-P, suspend -f, kill -n,
669 test -o optname/s1 == s2/s1 < s2/s1 > s2/-nt/-ot/-ef/-O/-G/-S
670 bash reads ~/.bashrc for interactive shells, $ENV for non-interactive
671 bash restricted shell mode is more extensive
672 bash allows functions and variables with the same name
675 arithmetic expansion with $((...)) and `let' builtin
676 the `[[...]]' extended conditional command
678 aliases and alias/unalias builtins
679 local variables in functions and `local' builtin
680 readline and command-line editing with programmable completion
681 command history and history/fc builtins
682 csh-like history expansion
683 other new bash builtins: bind, command, compgen, complete, builtin,
684 declare/typeset, dirs, enable, fc, help,
685 history, logout, popd, pushd, disown, shopt,
688 filename generation when using output redirection (command >a*)
689 POSIX.2-style globbing character classes
690 POSIX.2-style globbing equivalence classes
691 POSIX.2-style globbing collating symbols
692 egrep-like extended pattern matching operators
693 case-insensitive pattern matching and globbing
694 variable assignments preceding commands affect only that command,
695 even for builtins and functions
697 redirection to /dev/fd/N, /dev/stdin, /dev/stdout, /dev/stderr,
698 /dev/tcp/host/port, /dev/udp/host/port
700 Things sh has that bash does not:
701 uses variable SHACCT to do shell accounting
702 includes `stop' builtin (bash can use alias stop='kill -s STOP')
704 turns on job control if called as `jsh'
705 $TIMEOUT (like bash $TMOUT)
706 `^' is a synonym for `|'
707 new SVR4.2 sh builtins: mldmode, priv
709 Implementation differences:
710 redirection to/from compound commands causes sh to create a subshell
711 bash does not allow unbalanced quotes; sh silently inserts them at EOF
712 bash does not mess with signal 11
713 sh sets (euid, egid) to (uid, gid) if -p not supplied and uid < 100
714 bash splits only the results of expansions on IFS, using POSIX.2
715 field splitting rules; sh splits all words on IFS
716 sh does not allow MAILCHECK to be unset (?)
717 sh does not allow traps on SIGALRM or SIGCHLD
718 bash allows multiple option arguments when invoked (e.g. -x -v);
719 sh allows only a single option argument (`sh -x -v' attempts
720 to open a file named `-v', and, on SunOS 4.1.4, dumps core.
721 On Solaris 2.4 and earlier versions, sh goes into an infinite
723 sh exits a script if any builtin fails; bash exits only if one of
724 the POSIX.2 `special' builtins fails
726 C2) How does bash differ from the Korn shell, version ksh88?
728 Things bash has or uses that ksh88 does not:
729 long invocation options
730 [-+]O invocation option
733 arithmetic for command: for ((expr1 ; expr2; expr3 )); do list; done
734 arithmetic in largest machine-supported size (intmax_t)
735 posix mode and posix conformance
737 tilde expansion for assignment statements that look like $PATH
738 process substitution with named pipes if /dev/fd is not available
739 the ${!param} indirect parameter expansion operator
740 the ${!param*} prefix expansion operator
741 the ${param:offset[:length]} parameter substring operator
742 the ${param/pat[/string]} parameter pattern substitution operator
743 variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, SHLVL,
744 TIMEFORMAT, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE,
745 HISTFILESIZE, HISTIGNORE, HISTCONTROL, PROMPT_COMMAND,
746 IGNOREEOF, FIGNORE, INPUTRC, HOSTFILE, DIRSTACK,
747 PIPESTATUS, HOSTNAME, OPTERR, SHELLOPTS, GLOBIGNORE,
748 GROUPS, FUNCNAME, histchars, auto_resume
749 prompt expansion with backslash escapes and command substitution
750 redirection: &> (stdout and stderr), <<<, [n]<&word-, [n]>&word-
751 more extensive and extensible editing and programmable completion
752 builtins: bind, builtin, command, declare, dirs, echo -e/-E, enable,
753 exec -l/-c/-a, fc -s, export -n/-f/-p, hash, help, history,
754 jobs -x/-r/-s, kill -s/-n/-l, local, logout, popd, pushd,
755 read -e/-p/-a/-t/-n/-d/-s, readonly -a/-n/-f/-p,
756 set -o braceexpand/-o histexpand/-o interactive-comments/
757 -o notify/-o physical/-o posix/-o hashall/-o onecmd/
758 -h/-B/-C/-b/-H/-P, set +o, suspend, trap -l, type,
759 typeset -a/-F/-p, ulimit -u, umask -S, alias -p, shopt,
760 disown, printf, complete, compgen
761 `!' csh-style history expansion
762 POSIX.2-style globbing character classes
763 POSIX.2-style globbing equivalence classes
764 POSIX.2-style globbing collating symbols
765 egrep-like extended pattern matching operators
766 case-insensitive pattern matching and globbing
767 `**' arithmetic operator to do exponentiation
768 redirection to /dev/fd/N, /dev/stdin, /dev/stdout, /dev/stderr
769 arrays of unlimited size
770 TMOUT is default timeout for `read' and `select'
772 Things ksh88 has or uses that bash does not:
773 tracked aliases (alias -t)
774 variables: ERRNO, FPATH, EDITOR, VISUAL
775 co-processes (|&, >&p, <&p)
776 weirdly-scoped functions
777 typeset +f to list all function names without definitions
778 text of command history kept in a file, not memory
779 builtins: alias -x, cd old new, fc -e -, newgrp, print,
780 read -p/-s/var?prompt, set -A/-o gmacs/
781 -o bgnice/-o markdirs/-o nolog/-o trackall/-o viraw/-s,
782 typeset -H/-L/-R/-Z/-A/-ft/-fu/-fx/-l/-u/-t, whence
783 using environment to pass attributes of exported variables
784 arithmetic evaluation done on arguments to some builtins
785 reads .profile from $PWD when invoked as login shell
787 Implementation differences:
788 ksh runs last command of a pipeline in parent shell context
789 bash has brace expansion by default (ksh88 compile-time option)
790 bash has fixed startup file for all interactive shells; ksh reads $ENV
791 bash has exported functions
792 bash command search finds functions before builtins
793 bash waits for all commands in pipeline to exit before returning status
794 emacs-mode editing has some slightly different key bindings
796 C3) Which new features in ksh-93 are not in bash, and which are?
798 New things in ksh-93 not in bash-2.05b:
800 floating point arithmetic and variables
801 math library functions
802 ${!name[sub]} name of subscript for associative array
803 `.' is allowed in variable names to create a hierarchical namespace
804 more extensive compound assignment syntax
806 `sleep' and `getconf' builtins (bash has loadable versions)
807 typeset -n and `nameref' variables
809 variables: .sh.edchar, .sh.edmode, .sh.edcol, .sh.edtext, .sh.version,
810 .sh.name, .sh.subscript, .sh.value, .sh.match, HISTEDIT
811 backreferences in pattern matching (\N)
812 `&' operator in pattern lists for matching
813 print -f (bash uses printf)
814 `fc' has been renamed to `hist'
815 `.' can execute shell functions
816 exit statuses between 0 and 255
818 `+=' variable assignment operator
819 FPATH and PATH mixing
822 DEBUG trap now executed before each simple command, instead of after
823 printf %H, %P, %T, %Z modifiers, output base for %d
825 New things in ksh-93 present in bash-2.05b:
826 [n]<&word- and [n]>&word- redirections (combination dup and close)
827 for (( expr1; expr2; expr3 )) ; do list; done - arithmetic for command
828 ?:, ++, --, `expr1 , expr2' arithmetic operators
829 expansions: ${!param}, ${param:offset[:len]}, ${param/pat[/str]},
831 compound array assignment
832 the `!' reserved word
833 loadable builtins -- but ksh uses `builtin' while bash uses `enable'
834 `command', `builtin', `disown' builtins
835 new $'...' and $"..." quoting
836 FIGNORE (but bash uses GLOBIGNORE), HISTCMD
838 changes to kill builtin
839 read -A (bash uses read -a)
843 `.' restores the positional parameters when it completes
847 command and arithmetic substitution performed on PS1, PS4, and ENV
848 command name completion
849 ENV processed only for interactive shells
851 Section D: Why does bash do some things differently than other Unix shells?
853 D1) Why does bash run a different version of `command' than
854 `which command' says it will?
856 On many systems, `which' is actually a csh script that assumes
857 you're running csh. In tcsh, `which' and its cousin `where'
858 are builtins. On other Unix systems, `which' is a perl script
859 that uses the PATH environment variable.
861 The csh script version reads the csh startup files from your
862 home directory and uses those to determine which `command' will
863 be invoked. Since bash doesn't use any of those startup files,
864 there's a good chance that your bash environment differs from
865 your csh environment. The bash `type' builtin does everything
866 `which' does, and will report correct results for the running
867 shell. If you're really wedded to the name `which', try adding
868 the following function definition to your .bashrc:
875 If you're moving from tcsh and would like to bring `where' along
876 as well, use this function:
883 D2) Why doesn't bash treat brace expansions exactly like csh?
885 The only difference between bash and csh brace expansion is that
886 bash requires a brace expression to contain at least one unquoted
887 comma if it is to be expanded. Any brace-surrounded word not
888 containing an unquoted comma is left unchanged by the brace
889 expansion code. This affords the greatest degree of sh
892 Bash, ksh, zsh, and pd-ksh all implement brace expansion this way.
894 D3) Why doesn't bash have csh variable modifiers?
896 Posix has specified a more powerful, albeit somewhat more cryptic,
897 mechanism cribbed from ksh, and bash implements it.
900 Remove smallest suffix pattern. The WORD is expanded to produce
901 a pattern. It then expands to the value of PARAMETER, with the
902 smallest portion of the suffix matched by the pattern deleted.
910 Remove largest suffix pattern. The WORD is expanded to produce
911 a pattern. It then expands to the value of PARAMETER, with the
912 largest portion of the suffix matched by the pattern deleted.
919 Remove smallest prefix pattern. The WORD is expanded to produce
920 a pattern. It then expands to the value of PARAMETER, with the
921 smallest portion of the prefix matched by the pattern deleted.
928 Remove largest prefix pattern. The WORD is expanded to produce
929 a pattern. It then expands to the value of PARAMETER, with the
930 largest portion of the prefix matched by the pattern deleted.
949 D4) How can I make my csh aliases work when I convert to bash?
951 Bash uses a different syntax to support aliases than csh does.
952 The details can be found in the documentation. We have provided
953 a shell script which does most of the work of conversion for you;
954 this script can be found in ./examples/misc/aliasconv.sh. Here is
957 Start csh in the normal way for you. (e.g., `csh')
959 Pipe the output of `alias' through `aliasconv.sh', saving the
960 results into `bash_aliases':
962 alias | bash aliasconv.sh >bash_aliases
964 Edit `bash_aliases', carefully reading through any created
965 functions. You will need to change the names of some csh specific
966 variables to the bash equivalents. The script converts $cwd to
967 $PWD, $term to $TERM, $home to $HOME, $user to $USER, and $prompt
968 to $PS1. You may also have to add quotes to avoid unwanted
971 For example, the csh alias:
973 alias cd 'cd \!*; echo $cwd'
975 is converted to the bash function:
977 cd () { command cd "$@"; echo $PWD ; }
979 The only thing that needs to be done is to quote $PWD:
981 cd () { command cd "$@"; echo "$PWD" ; }
983 Merge the edited file into your ~/.bashrc.
985 There is an additional, more ambitious, script in
986 examples/misc/cshtobash that attempts to convert your entire csh
987 environment to its bash equivalent. This script can be run as
988 simply `cshtobash' to convert your normal interactive
989 environment, or as `cshtobash ~/.login' to convert your login
992 D5) How can I pipe standard output and standard error from one command to
993 another, like csh does with `|&'?
996 command 2>&1 | command2
998 The key is to remember that piping is performed before redirection, so
999 file descriptor 1 points to the pipe when it is duplicated onto file
1002 D6) Now that I've converted from ksh to bash, are there equivalents to
1003 ksh features like autoloaded functions and the `whence' command?
1005 There are features in ksh-88 and ksh-93 that do not have direct bash
1006 equivalents. Most, however, can be emulated with very little trouble.
1008 ksh-88 feature Bash equivalent
1009 -------------- ---------------
1010 compiled-in aliases set up aliases in .bashrc; some ksh aliases are
1011 bash builtins (hash, history, type)
1012 coprocesses named pipe pairs (one for read, one for write)
1013 typeset +f declare -F
1014 cd, print, whence function substitutes in examples/functions/kshenv
1015 autoloaded functions examples/functions/autoload is the same as typeset -fu
1016 read var?prompt read -p prompt var
1018 ksh-93 feature Bash equivalent
1019 -------------- ---------------
1020 sleep, getconf Bash has loadable versions in examples/loadables
1021 ${.sh.version} $BASH_VERSION
1026 Section E: How can I get bash to do certain things, and why does bash do
1027 things the way it does?
1029 E1) Why is the bash builtin `test' slightly different from /bin/test?
1031 The specific example used here is [ ! x -o x ], which is false.
1033 Bash's builtin `test' implements the Posix.2 spec, which can be
1034 summarized as follows (the wording is due to David Korn):
1036 Here is the set of rules for processing test arguments.
1039 1 Arg: True iff argument is not null.
1040 2 Args: If first arg is !, True iff second argument is null.
1041 If first argument is unary, then true if unary test is true
1043 3 Args: If second argument is a binary operator, do binary test of $1 $3
1044 If first argument is !, negate two argument test of $2 $3
1045 If first argument is `(' and third argument is `)', do the
1046 one-argument test of the second argument.
1048 4 Args: If first argument is !, negate three argument test of $2 $3 $4.
1049 Otherwise unspecified
1050 5 or more Args: unspecified. (Historical shells would use their
1053 The operators -a and -o are considered binary operators for the purpose
1056 As you can see, the test becomes (not (x or x)), which is false.
1058 E2) Why does bash sometimes say `Broken pipe'?
1060 If a sequence of commands appears in a pipeline, and one of the
1061 reading commands finishes before the writer has finished, the
1062 writer receives a SIGPIPE signal. Many other shells special-case
1063 SIGPIPE as an exit status in the pipeline and do not report it.
1068 `head' can finish before `ps' writes all of its output, and ps
1069 will try to write on a pipe without a reader. In that case, bash
1070 will print `Broken pipe' to stderr when ps is killed by a
1073 You can build a version of bash that will not report SIGPIPE errors
1074 by uncommenting the definition of DONT_REPORT_SIGPIPE in the file
1077 E3) When I have terminal escape sequences in my prompt, why does bash
1078 wrap lines at the wrong column?
1080 Readline, the line editing library that bash uses, does not know
1081 that the terminal escape sequences do not take up space on the
1082 screen. The redisplay code assumes, unless told otherwise, that
1083 each character in the prompt is a `printable' character that
1084 takes up one character position on the screen.
1086 You can use the bash prompt expansion facility (see the PROMPTING
1087 section in the manual page) to tell readline that sequences of
1088 characters in the prompt strings take up no screen space.
1090 Use the \[ escape to begin a sequence of non-printing characters,
1091 and the \] escape to signal the end of such a sequence.
1093 E4) If I pipe the output of a command into `read variable', why doesn't
1094 the output show up in $variable when the read command finishes?
1096 This has to do with the parent-child relationship between Unix
1097 processes. It affects all commands run in pipelines, not just
1098 simple calls to `read'. For example, piping a command's output
1099 into a `while' loop that repeatedly calls `read' will result in
1102 Each element of a pipeline runs in a separate process, a child of
1103 the shell running the pipeline. A subprocess cannot affect its
1104 parent's environment. When the `read' command sets the variable
1105 to the input, that variable is set only in the subshell, not the
1106 parent shell. When the subshell exits, the value of the variable
1109 Many pipelines that end with `read variable' can be converted
1110 into command substitutions, which will capture the output of
1111 a specified command. The output can then be assigned to a
1114 grep ^gnu /usr/lib/news/active | wc -l | read ngroup
1116 can be converted into
1118 ngroup=$(grep ^gnu /usr/lib/news/active | wc -l)
1120 This does not, unfortunately, work to split the text among
1121 multiple variables, as read does when given multiple variable
1122 arguments. If you need to do this, you can either use the
1123 command substitution above to read the output into a variable
1124 and chop up the variable using the bash pattern removal
1125 expansion operators or use some variant of the following
1128 Say /usr/local/bin/ipaddr is the following shell script:
1131 host `hostname` | awk '/address/ {print $NF}'
1135 /usr/local/bin/ipaddr | read A B C D
1137 to break the local machine's IP address into separate octets, use
1141 set -- $(/usr/local/bin/ipaddr)
1143 A="$1" B="$2" C="$3" D="$4"
1145 Beware, however, that this will change the shell's positional
1146 parameters. If you need them, you should save them before doing
1149 This is the general approach -- in most cases you will not need to
1150 set $IFS to a different value.
1152 Some other user-supplied alternatives include:
1154 read A B C D << HERE
1155 $(IFS=.; echo $(/usr/local/bin/ipaddr))
1158 and, where process substitution is available,
1160 read A B C D < <(IFS=.; echo $(/usr/local/bin/ipaddr))
1162 E5) I have a bunch of shell scripts that use backslash-escaped characters
1163 in arguments to `echo'. Bash doesn't interpret these characters. Why
1164 not, and how can I make it understand them?
1166 This is the behavior of echo on most Unix System V machines.
1168 The bash builtin `echo' is modeled after the 9th Edition
1169 Research Unix version of `echo'. It does not interpret
1170 backslash-escaped characters in its argument strings by default;
1171 it requires the use of the -e option to enable the
1172 interpretation. The System V echo provides no way to disable the
1173 special characters; the bash echo has a -E option to disable
1176 There is a configuration option that will make bash behave like
1177 the System V echo and interpret things like `\t' by default. Run
1178 configure with the --enable-xpg-echo-default option to turn this
1179 on. Be aware that this will cause some of the tests run when you
1180 type `make tests' to fail.
1182 There is a shell option, `xpg_echo', settable with `shopt', that will
1183 change the behavior of echo at runtime. Enabling this option turns
1184 on expansion of backslash-escape sequences.
1186 E6) Why doesn't a while or for loop get suspended when I type ^Z?
1188 This is a consequence of how job control works on Unix. The only
1189 thing that can be suspended is the process group. This is a single
1190 command or pipeline of commands that the shell forks and executes.
1192 When you run a while or for loop, the only thing that the shell forks
1193 and executes are any commands in the while loop test and commands in
1194 the loop bodies. These, therefore, are the only things that can be
1195 suspended when you type ^Z.
1197 If you want to be able to stop the entire loop, you need to put it
1198 within parentheses, which will force the loop into a subshell that
1199 may be stopped (and subsequently restarted) as a single unit.
1201 E7) What about empty for loops in Makefiles?
1203 It's fairly common to see constructs like this in automatically-generated
1211 for d in ${SUBDIRS}; do \
1212 ( cd $$d && ${MAKE} ${MFLAGS} clean ) \
1215 When SUBDIRS is empty, this results in a command like this being passed to
1219 ( cd $d && ${MAKE} ${MFLAGS} clean )
1222 In versions of bash before bash-2.05a, this was a syntax error. If the
1223 reserved word `in' was present, a word must follow it before the semicolon
1224 or newline. The language in the manual page referring to the list of words
1225 being empty referred to the list after it is expanded. These versions of
1226 bash required that there be at least one word following the `in' when the
1227 construct was parsed.
1229 The idiomatic Makefile solution is something like:
1234 subdirs=$SUBDIRS ; for d in $$subdirs; do \
1235 ( cd $$d && ${MAKE} ${MFLAGS} clean ) \
1238 The latest drafts of the updated POSIX standard have changed this: the
1239 word list is no longer required. Bash versions 2.05a and later accept
1242 E8) Why does the arithmetic evaluation code complain about `08'?
1244 The bash arithmetic evaluation code (used for `let', $(()), (()), and in
1245 other places), interprets a leading `0' in numeric constants as denoting
1246 an octal number, and a leading `0x' as denoting hexadecimal. This is
1247 in accordance with the POSIX.2 spec, section 2.9.2.1, which states that
1248 arithmetic constants should be handled as signed long integers as defined
1249 by the ANSI/ISO C standard.
1251 The POSIX.2 interpretation committee has confirmed this:
1253 http://www.pasc.org/interps/unofficial/db/p1003.2/pasc-1003.2-173.html
1255 E9) Why does the pattern matching expression [A-Z]* match files beginning
1256 with every letter except `z'?
1258 Bash-2.03, Bash-2.05 and later versions honor the current locale setting
1259 when processing ranges within pattern matching bracket expressions ([A-Z]).
1260 This is what POSIX.2 and SUSv3/XPG6 specify.
1262 The behavior of the matcher in bash-2.05 and later versions depends on the
1263 current LC_COLLATE setting. Setting this variable to `C' or `POSIX' will
1264 result in the traditional behavior ([A-Z] matches all uppercase ASCII
1265 characters). Many other locales, including the en_US locale (the default
1266 on many US versions of Linux) collate the upper and lower case letters like
1271 which means that [A-Z] matches every letter except `z'. Others collate like
1275 which means that [A-Z] matches every letter except `a'.
1277 The portable way to specify upper case letters is [:upper:] instead of
1278 A-Z; lower case may be specified as [:lower:] instead of a-z.
1280 Look at the manual pages for setlocale(3), strcoll(3), and, if it is
1281 present, locale(1). If you have locale(1), you can use it to find
1282 your current locale information even if you do not have any of the
1289 into /etc/profile and inspect any shell scripts run from cron for
1290 constructs like [A-Z]. This will prevent things like
1294 from removing every file in the current directory except those beginning
1295 with `z' and still allow individual users to change the collation order.
1296 Users may put the above command into their own profiles as well, of course.
1298 E10) Why does `cd //' leave $PWD as `//'?
1300 POSIX.2, in its description of `cd', says that *three* or more leading
1301 slashes may be replaced with a single slash when canonicalizing the
1302 current working directory.
1304 This is, I presume, for historical compatibility. Certain versions of
1305 Unix, and early network file systems, used paths of the form
1306 //hostname/path to access `path' on server `hostname'.
1308 Section F: Things to watch out for on certain Unix versions
1310 F1) Why can't I use command line editing in my `cmdtool'?
1312 The problem is `cmdtool' and bash fighting over the input. When
1313 scrolling is enabled in a cmdtool window, cmdtool puts the tty in
1314 `raw mode' to permit command-line editing using the mouse for
1315 applications that cannot do it themselves. As a result, bash and
1316 cmdtool each try to read keyboard input immediately, with neither
1317 getting enough of it to be useful.
1319 This mode also causes cmdtool to not implement many of the
1320 terminal functions and control sequences appearing in the
1321 `sun-cmd' termcap entry. For a more complete explanation, see
1322 that file examples/suncmd.termcap in the bash distribution.
1324 `xterm' is a better choice, and gets along with bash much more
1327 If you must use cmdtool, you can use the termcap description in
1328 examples/suncmd.termcap. Set the TERMCAP variable to the terminal
1329 description contained in that file, i.e.
1331 TERMCAP='Mu|sun-cmd:am:bs:km:pt:li#34:co#80:cl=^L:ce=\E[K:cd=\E[J:rs=\E[s:'
1333 Then export TERMCAP and start a new cmdtool window from that shell.
1334 The bash command-line editing should behave better in the new
1335 cmdtool. If this works, you can put the assignment to TERMCAP
1336 in your bashrc file.
1338 F2) I built bash on Solaris 2. Why do globbing expansions and filename
1339 completion chop off the first few characters of each filename?
1341 This is the consequence of building bash on SunOS 5 and linking
1342 with the libraries in /usr/ucblib, but using the definitions
1343 and structures from files in /usr/include.
1345 The actual conflict is between the dirent structure in
1346 /usr/include/dirent.h and the struct returned by the version of
1347 `readdir' in libucb.a (a 4.3-BSD style `struct direct').
1349 Make sure you've got /usr/ccs/bin ahead of /usr/ucb in your $PATH
1350 when configuring and building bash. This will ensure that you
1351 use /usr/ccs/bin/cc or acc instead of /usr/ucb/cc and that you
1352 link with libc before libucb.
1354 If you have installed the Sun C compiler, you may also need to
1355 put /usr/ccs/bin and /opt/SUNWspro/bin into your $PATH before
1358 F3) Why does bash dump core after I interrupt username completion or
1359 `~user' tilde expansion on a machine running NIS?
1361 This is a famous and long-standing bug in the SunOS YP (sorry, NIS)
1362 client library, which is part of libc.
1364 The YP library code keeps static state -- a pointer into the data
1365 returned from the server. When YP initializes itself (setpwent),
1366 it looks at this pointer and calls free on it if it's non-null.
1369 If one of the YP functions is interrupted during getpwent (the
1370 exact function is interpretwithsave()), and returns NULL, the
1371 pointer is freed without being reset to NULL, and the function
1372 returns. The next time getpwent is called, it sees that this
1373 pointer is non-null, calls free, and the bash free() blows up
1374 because it's being asked to free freed memory.
1376 The traditional Unix mallocs allow memory to be freed multiple
1377 times; that's probably why this has never been fixed. You can
1378 run configure with the `--without-gnu-malloc' option to use
1379 the C library malloc and avoid the problem.
1381 F4) I'm running SVR4.2. Why is the line erased every time I type `@'?
1383 The `@' character is the default `line kill' character in most
1384 versions of System V, including SVR4.2. You can change this
1385 character to whatever you want using `stty'. For example, to
1386 change the line kill character to control-u, type
1390 where the `^' and `U' can be two separate characters.
1392 F5) Why does bash report syntax errors when my C News scripts use a
1393 redirection before a subshell command?
1395 The actual command in question is something like
1399 According to the grammar given in the POSIX.2 standard, this construct
1400 is, in fact, a syntax error. Redirections may only precede `simple
1401 commands'. A subshell construct such as the above is one of the shell's
1402 `compound commands'. A redirection may only follow a compound command.
1404 This affects the mechanical transformation of commands that use `cat'
1405 to pipe a file into a command (a favorite Useless-Use-Of-Cat topic on
1406 comp.unix.shell). While most commands of the form
1410 can be converted to `< file command', shell control structures such as
1411 loops and subshells require `command < file'.
1413 The file CWRU/sh-redir-hack in the bash-2.05a distribution is an
1414 (unofficial) patch to parse.y that will modify the grammar to
1415 support this construct. It will not apply with `patch'; you must
1416 modify parse.y by hand. Note that if you apply this, you must
1417 recompile with -DREDIRECTION_HACK. This introduces a large
1418 number of reduce/reduce conflicts into the shell grammar.
1420 F6) Why can't I use vi-mode editing on Red Hat Linux 6.1?
1422 The short answer is that Red Hat screwed up.
1424 The long answer is that they shipped an /etc/inputrc that only works
1425 for emacs mode editing, and then screwed all the vi users by setting
1426 INPUTRC to /etc/inputrc in /etc/profile.
1428 The short fix is to do one of the following: remove or rename
1429 /etc/inputrc, set INPUTRC=~/.inputrc in ~/.bashrc (or .bash_profile,
1430 but make sure you export it if you do), remove the assignment to
1431 INPUTRC from /etc/profile, add
1435 to the beginning of /etc/inputrc, or bracket the key bindings in
1436 /etc/inputrc with these lines
1442 F7) Why do bash-2.05a and bash-2.05b fail to compile `printf.def' on
1445 HP/UX's support for long double is imperfect at best.
1447 GCC will support it without problems, but the HP C library functions
1448 like strtold(3) and printf(3) don't actually work with long doubles.
1449 HP implemented a `long_double' type as a 4-element array of 32-bit
1450 ints, and that is what the library functions use. The ANSI C
1451 `long double' type is a 128-bit floating point scalar.
1453 The easiest fix, until HP fixes things up, is to edit the generated
1454 config.h and #undef the HAVE_LONG_DOUBLE line. After doing that,
1455 the compilation should complete successfully.
1457 Section G: How can I get bash to do certain common things?
1459 G1) How can I get bash to read and display eight-bit characters?
1461 This is a process requiring several steps.
1463 First, you must ensure that the `physical' data path is a full eight
1464 bits. For xterms, for example, the `vt100' resources `eightBitInput'
1465 and `eightBitOutput' should be set to `true'.
1467 Once you have set up an eight-bit path, you must tell the kernel and
1468 tty driver to leave the eighth bit of characters alone when processing
1469 keyboard input. Use `stty' to do this:
1471 stty cs8 -istrip -parenb
1473 For old BSD-style systems, you can use
1481 Finally, you need to tell readline that you will be inputting and
1482 displaying eight-bit characters. You use readline variables to do
1483 this. These variables can be set in your .inputrc or using the bash
1484 `bind' builtin. Here's an example using `bind':
1486 bash$ bind 'set convert-meta off'
1487 bash$ bind 'set meta-flag on'
1488 bash$ bind 'set output-meta on'
1490 The `set' commands between the single quotes may also be placed
1493 G2) How do I write a function `x' to replace builtin command `x', but
1494 still invoke the command from within the function?
1496 This is why the `command' and `builtin' builtins exist. The
1497 `command' builtin executes the command supplied as its first
1498 argument, skipping over any function defined with that name. The
1499 `builtin' builtin executes the builtin command given as its first
1502 For example, to write a function to replace `cd' that writes the
1503 hostname and current directory to an xterm title bar, use
1504 something like the following:
1508 builtin cd "$@" && xtitle "$HOST: $PWD"
1511 This could also be written using `command' instead of `builtin';
1512 the version above is marginally more efficient.
1514 G3) How can I find the value of a shell variable whose name is the value
1515 of another shell variable?
1517 Versions of Bash newer than Bash-2.0 support this directly. You can use
1521 For example, the following sequence of commands will echo `z':
1527 For sh compatibility, use the `eval' builtin. The important
1528 thing to remember is that `eval' expands the arguments you give
1529 it again, so you need to quote the parts of the arguments that
1530 you want `eval' to act on.
1532 For example, this expression prints the value of the last positional
1535 eval echo \"\$\{$#\}\"
1537 The expansion of the quoted portions of this expression will be
1538 deferred until `eval' runs, while the `$#' will be expanded
1539 before `eval' is executed. In versions of bash later than bash-2.0,
1543 does the same thing.
1545 This is not the same thing as ksh93 `nameref' variables, though the syntax
1546 is similar. I may add namerefs in a future bash version.
1548 G4) How can I make the bash `time' reserved word print timing output that
1549 looks like the output from my system's /usr/bin/time?
1551 The bash command timing code looks for a variable `TIMEFORMAT' and
1552 uses its value as a format string to decide how to display the
1555 The value of TIMEFORMAT is a string with `%' escapes expanded in a
1556 fashion similar in spirit to printf(3). The manual page explains
1557 the meanings of the escape sequences in the format string.
1559 If TIMEFORMAT is not set, bash acts as if the following assignment had
1562 TIMEFORMAT=$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
1564 The POSIX.2 default time format (used by `time -p command') is
1566 TIMEFORMAT=$'real %2R\nuser %2U\nsys %2S'
1568 The BSD /usr/bin/time format can be emulated with:
1570 TIMEFORMAT=$'\t%1R real\t%1U user\t%1S sys'
1572 The System V /usr/bin/time format can be emulated with:
1574 TIMEFORMAT=$'\nreal\t%1R\nuser\t%1U\nsys\t%1S'
1576 The ksh format can be emulated with:
1578 TIMEFORMAT=$'\nreal\t%2lR\nuser\t%2lU\nsys\t%2lS'
1580 G5) How do I get the current directory into my prompt?
1582 Bash provides a number of backslash-escape sequences which are expanded
1583 when the prompt string (PS1 or PS2) is displayed. The full list is in
1586 The \w expansion gives the full pathname of the current directory, with
1587 a tilde (`~') substituted for the current value of $HOME. The \W
1588 expansion gives the basename of the current directory. To put the full
1589 pathname of the current directory into the path without any tilde
1590 subsitution, use $PWD. Here are some examples:
1592 PS1='\w$ ' # current directory with tilde
1593 PS1='\W$ ' # basename of current directory
1594 PS1='$PWD$ ' # full pathname of current directory
1596 The single quotes are important in the final example to prevent $PWD from
1597 being expanded when the assignment to PS1 is performed.
1599 G6) How can I rename "*.foo" to "*.bar"?
1601 Use the pattern removal functionality described in D3. The following `for'
1602 loop will do the trick:
1608 G7) How can I translate a filename from uppercase to lowercase?
1610 The script examples/functions/lowercase, originally written by John DuBois,
1611 will do the trick. The converse is left as an exercise.
1613 G8) How can I write a filename expansion (globbing) pattern that will match
1614 all files in the current directory except "." and ".."?
1616 You must have set the `extglob' shell option using `shopt -s extglob' to use
1621 A solution that works without extended globbing is given in the Unix Shell
1622 FAQ, posted periodically to comp.unix.shell.
1624 Section H: Where do I go from here?
1626 H1) How do I report bugs in bash, and where should I look for fixes and
1629 Use the `bashbug' script to report bugs. It is built and
1630 installed at the same time as bash. It provides a standard
1631 template for reporting a problem and automatically includes
1632 information about your configuration and build environment.
1634 `bashbug' sends its reports to bug-bash@gnu.org, which
1635 is a large mailing list gatewayed to the usenet newsgroup gnu.bash.bug.
1637 Bug fixes, answers to questions, and announcements of new releases
1638 are all posted to gnu.bash.bug. Discussions concerning bash features
1639 and problems also take place there.
1641 To reach the bash maintainers directly, send mail to
1642 bash-maintainers@gnu.org.
1644 H2) What kind of bash documentation is there?
1646 First, look in the doc directory in the bash distribution. It should
1647 contain at least the following files:
1649 bash.1 an extensive, thorough Unix-style manual page
1650 builtins.1 a manual page covering just bash builtin commands
1651 bashref.texi a reference manual in GNU tex`info format
1652 bashref.info an info version of the reference manual
1654 article.ms text of an article written for The Linux Journal
1655 readline.3 a man page describing readline
1657 Postscript, HTML, and ASCII files created from the above source are
1658 available in the documentation distribution.
1660 There is additional documentation available for anonymous FTP from host
1661 ftp.cwru.edu in the `pub/bash' directory.
1663 Cameron Newham and Bill Rosenblatt have written a book on bash, published
1664 by O'Reilly and Associates. The book is based on Bill Rosenblatt's Korn
1665 Shell book. The title is ``Learning the Bash Shell'', and the ISBN number
1666 is 1-56592-147-X. Look for it in fine bookstores near you. This book
1667 covers bash-1.14, but has an appendix describing some of the new features
1670 A second edition of this book is available, published in January, 1998.
1671 The ISBN number is 1-56592-347-2. Look for it in the same fine bookstores
1674 H3) What's coming in future versions?
1676 These are features I hope to include in a future version of bash.
1678 a better bash debugger (a minimally-tested version is included with bash-2.05b)
1680 co-processes, but with a new-style syntax that looks like function declaration
1682 H4) What's on the bash `wish list' for future versions?
1684 These are features that may or may not appear in a future version of bash.
1686 breaking some of the shell functionality into embeddable libraries
1687 a module system like zsh's, using dynamic loading like builtins
1688 better internationalization using GNU `gettext'
1689 date-stamped command history
1690 a bash programmer's guide with a chapter on creating loadable builtins
1691 a better loadable interface to perl with access to the shell builtins and
1692 variables (contributions gratefully accepted)
1693 ksh93-like `nameref' variables
1694 ksh93-like `+=' variable assignment operator
1695 ksh93-like `xx.yy' variables (including some of the .sh.* variables) and
1696 associated disipline functions
1697 Some of the new ksh93 pattern matching operators, like backreferencing
1699 H5) When will the next release appear?
1701 The next version will appear sometime in 2002. Never make predictions.
1704 This document is Copyright 1995-2002 by Chester Ramey.
1706 Permission is hereby granted, without written agreement and
1707 without license or royalty fees, to use, copy, and distribute
1708 this document for any purpose, provided that the above copyright
1709 notice appears in all copies of this document and that the
1710 contents of this document remain unaltered.