-This is the Bash FAQ, version 2.13, for Bash version 2.02.
+This is the Bash FAQ, version 4.13, for Bash version 4.3.
This document contains a set of frequently-asked questions concerning
Bash, the GNU Bourne-Again Shell. Bash is a freely-available command
of FAQ articles periodically posted to comp.unix.shell.
Questions and comments concerning this document should be sent to
-chet@po.cwru.edu.
+chet.ramey@case.edu.
This document is available for anonymous FTP with the URL
ftp://ftp.cwru.edu/pub/bash/FAQ
+The Bash home page is http://cnswww.cns.cwru.edu/~chet/bash/bashtop.html
+
----------
Contents:
Section A: The Basics
-1) What is it?
-2) What's the latest version?
-3) Where can I get it?
-4) On what machines will bash run?
-5) Will bash run on operating systems other than Unix?
-6) How can I build bash with gcc?
-7) How can I make bash my login shell?
-8) I just changed my login shell to bash, and now I can't FTP into my
- machine. Why not?
-9) What's the `POSIX 1003.2 standard'?
-10) What is the bash `posix mode'?
+A1) What is it?
+A2) What's the latest version?
+A3) Where can I get it?
+A4) On what machines will bash run?
+A5) Will bash run on operating systems other than Unix?
+A6) How can I build bash with gcc?
+A7) How can I make bash my login shell?
+A8) I just changed my login shell to bash, and now I can't FTP into my
+ machine. Why not?
+A9) What's the `POSIX Shell and Utilities standard'?
+A10) What is the bash `posix mode'?
Section B: The latest version
-11) What's new in version 2.02?
-12) Are there any user-visible incompatibilities between bash-2.02 and
- bash-1.14.7?
+B1) What's new in version 4.3?
+B2) Are there any user-visible incompatibilities between bash-4.3 and
+ previous bash versions?
Section C: Differences from other Unix shells
-13) How does bash differ from sh, the Bourne shell?
-14) How does bash differ from the Korn shell, version ksh88?
-15) Which new features in ksh-93 are not in bash, and which are?
+C1) How does bash differ from sh, the Bourne shell?
+C2) How does bash differ from the Korn shell, version ksh88?
+C3) Which new features in ksh-93 are not in bash, and which are?
Section D: Why does bash do some things differently than other Unix shells?
-16) Why does bash run a different version of `command' than
+D1) Why does bash run a different version of `command' than
`which command' says it will?
-17) Why doesn't bash treat brace expansions exactly like csh?
-18) Why doesn't bash have csh variable modifiers?
-19) How can I make my csh aliases work when I convert to bash?
-20) How can I pipe standard output and standard error from one command to
+D2) Why doesn't bash treat brace expansions exactly like csh?
+D3) Why doesn't bash have csh variable modifiers?
+D4) How can I make my csh aliases work when I convert to bash?
+D5) How can I pipe standard output and standard error from one command to
another, like csh does with `|&'?
-21) Now that I've converted from ksh to bash, are there equivalents to
+D6) Now that I've converted from ksh to bash, are there equivalents to
ksh features like autoloaded functions and the `whence' command?
-Section E: How can I get bash to do certain things, and why does bash do
- things the way it does?
+Section E: Why does bash do certain things the way it does?
-22) Why is the bash builtin `test' slightly different from /bin/test?
-23) Why does bash sometimes say `Broken pipe'?
-24) How can I get bash to read and display eight-bit characters?
-25) How do I write a function `x' to replace builtin command `x', but
- still invoke the command from within the function?
-26) When I have terminal escape sequences in my prompt, why does bash
+E1) Why is the bash builtin `test' slightly different from /bin/test?
+E2) Why does bash sometimes say `Broken pipe'?
+E3) When I have terminal escape sequences in my prompt, why does bash
wrap lines at the wrong column?
-27) How can I find the value of a shell variable whose name is the value
- of another shell variable?
-28) If I pipe the output of a command into `read variable', why doesn't
+E4) If I pipe the output of a command into `read variable', why doesn't
the output show up in $variable when the read command finishes?
-29) I have a bunch of shell scripts that use backslash-escaped characters
+E5) I have a bunch of shell scripts that use backslash-escaped characters
in arguments to `echo'. Bash doesn't interpret these characters. Why
not, and how can I make it understand them?
-30) Why doesn't a while or for loop get suspended when I type ^Z?
-31) How can I make the bash `time' reserved word print timing output that
- looks like the output from my system's /usr/bin/time?
+E6) Why doesn't a while or for loop get suspended when I type ^Z?
+E7) What about empty for loops in Makefiles?
+E8) Why does the arithmetic evaluation code complain about `08'?
+E9) Why does the pattern matching expression [A-Z]* match files beginning
+ with every letter except `z'?
+E10) Why does `cd //' leave $PWD as `//'?
+E11) If I resize my xterm while another program is running, why doesn't bash
+ notice the change?
+E12) Why don't negative offsets in substring expansion work like I expect?
+E13) Why does filename completion misbehave if a colon appears in the filename?
+E14) Why does quoting the pattern argument to the regular expression matching
+ conditional operator (=~) cause matching to stop working?
+E15) Tell me more about the shell compatibility level.
Section F: Things to watch out for on certain Unix versions
-32) Why can't I use command line editing in my `cmdtool'?
-33) I built bash on Solaris 2. Why do globbing expansions and filename
+F1) Why can't I use command line editing in my `cmdtool'?
+F2) I built bash on Solaris 2. Why do globbing expansions and filename
completion chop off the first few characters of each filename?
-34) Why does bash dump core after I interrupt username completion or
+F3) Why does bash dump core after I interrupt username completion or
`~user' tilde expansion on a machine running NIS?
-35) I'm running SVR4.2. Why is the line erased every time I type `@'?
-36) Why does bash report syntax errors when my C News scripts use a
+F4) I'm running SVR4.2. Why is the line erased every time I type `@'?
+F5) Why does bash report syntax errors when my C News scripts use a
redirection before a subshell command?
+F6) Why can't I use vi-mode editing on Red Hat Linux 6.1?
+F7) Why do bash-2.05a and bash-2.05b fail to compile `printf.def' on
+ HP/UX 11.x?
+
+Section G: How can I get bash to do certain common things?
+
+G1) How can I get bash to read and display eight-bit characters?
+G2) How do I write a function `x' to replace builtin command `x', but
+ still invoke the command from within the function?
+G3) How can I find the value of a shell variable whose name is the value
+ of another shell variable?
+G4) How can I make the bash `time' reserved word print timing output that
+ looks like the output from my system's /usr/bin/time?
+G5) How do I get the current directory into my prompt?
+G6) How can I rename "*.foo" to "*.bar"?
+G7) How can I translate a filename from uppercase to lowercase?
+G8) How can I write a filename expansion (globbing) pattern that will match
+ all files in the current directory except "." and ".."?
-Section G: Where do I go from here?
+Section H: Where do I go from here?
-37) How do I report bugs in bash, and where should I look for fixes and
+H1) How do I report bugs in bash, and where should I look for fixes and
advice?
-38) What kind of bash documentation is there?
-39) What's coming in future versions?
-40) What's on the bash `wish list'?
-41) When will the next release appear?
+H2) What kind of bash documentation is there?
+H3) What's coming in future versions?
+H4) What's on the bash `wish list'?
+H5) When will the next release appear?
----------
Section A: The Basics
-1) What is it?
+A1) What is it?
Bash is a Unix command interpreter (shell). It is an implementation of
the Posix 1003.2 shell standard, and resembles the Korn and System V
Foundation. The current developer and maintainer is Chet Ramey
of Case Western Reserve University.
-2) What's the latest version?
+A2) What's the latest version?
-The latest version is 2.02, first made available on Monday, 20 April, 1998.
+The latest version is 4.3, first made available on xx December, 2013.
-3) Where can I get it?
+A3) Where can I get it?
Bash is the GNU project's shell, and so is available from the
-master GNU archive site, prep.ai.mit.edu, and its mirrors. The
+master GNU archive site, ftp.gnu.org, and its mirrors. The
latest version is also available for FTP from ftp.cwru.edu.
-The following URLs tell how to get version 2.02:
+The following URLs tell how to get version 4.3:
-ftp://prep.ai.mit.edu/pub/gnu/bash-2.02.tar.gz
-ftp://ftp.cwru.edu/pub/bash/bash-2.02.tar.gz
+ftp://ftp.gnu.org/pub/gnu/bash/bash-4.3.tar.gz
+ftp://ftp.cwru.edu/pub/bash/bash-4.3.tar.gz
Formatted versions of the documentation are available with the URLs:
-ftp://prep.ai.mit.edu/pub/gnu/bash-doc-2.02.tar.gz
-ftp://ftp.cwru.edu/pub/bash/bash-doc-2.02.tar.gz
+ftp://ftp.gnu.org/pub/gnu/bash/bash-doc-4.3.tar.gz
+ftp://ftp.cwru.edu/pub/bash/bash-doc-4.3.tar.gz
+
+Any patches for the current version are available with the URL:
-4) On what machines will bash run?
+ftp://ftp.cwru.edu/pub/bash/bash-4.3-patches/
-Bash has been ported to nearly every version of UNIX. All you
+A4) On what machines will bash run?
+
+Bash has been ported to nearly every version of Unix. All you
should have to do to build it on a machine for which a port
exists is to type `configure' and then `make'. The build process
-will attempt to discover the version of UNIX you have and tailor
+will attempt to discover the version of Unix you have and tailor
itself accordingly, using a script created by GNU autoconf.
More information appears in the file `INSTALL' in the distribution.
-5) Will bash run on operating systems other than Unix?
+The Bash web page (http://cnswww.cns.cwru.edu/~chet/bash/bashtop.html)
+explains how to obtain binary versions of bash for most of the major
+commercial Unix systems.
+
+A5) Will bash run on operating systems other than Unix?
Configuration specifics for Unix-like systems such as QNX and
-LynxOS are included in the distribution. Bash-2.02 should
-compile and run on Minix 2.0 (patches were contributed), but I
-don't believe anyone has built bash-2.x on earlier Minix versions
-yet.
+LynxOS are included in the distribution. Bash-2.05 and later
+versions should compile and run on Minix 2.0 (patches were
+contributed), but I don't believe anyone has built bash-2.x on
+earlier Minix versions yet.
Bash has been ported to versions of Windows implementing the Win32
programming interface. This includes Windows 95 and Windows NT.
-The port was done by Cygnus Solutions as part of their GNU-Win32
-project. For more information about the project, look at the URL
-
-http://www.cygnus.com/misc/gnu-win32
-
-Cygnus has ported bash-1.14.7, and their port is part of the current
-gnu-win32 release. Cygnus has also done a port of bash-2.01 to the
-GNU-Win32 environment, and it should be available as part of their next
-release.
+The port was done by Cygnus Solutions (now part of Red Hat) as part
+of their CYGWIN project. For more information about the project, see
+http://www.cygwin.com/.
-Bash-2.02 should require no local Cygnus changes to build and run under
-GNU-WIN32.
+Cygnus originally ported bash-1.14.7, and that port was part of their
+early GNU-Win32 (the original name) releases. Cygnus has also done
+ports of bash-3.2 and bash-4.0 to the CYGWIN environment, and both
+are available as part of their current release.
-The Cygnus port works only on Intel machines. There is a port of bash
-(I don't know which version) to the alpha/NT environment available from
+Bash-2.05b and later versions should require no local Cygnus changes to
+build and run under CYGWIN.
-ftp://ftp.gnustep.org//pub/win32/bash-alpha-nt-1.01.tar.gz
-
-Softway Systems has ported bash-2.01.1 to their Interix (nee OpenNT)
-system, a Unix subsystem for NT that replaces the Microsoft POSIX
-subsystem. Check out http://www.interix.com for more information.
-
-D. J. Delorie has ported bash-1.14.7 to run under MS-DOS, as part of
-the DJGPP project. For more information on the project, see
+DJ Delorie has a port of bash-2.x which runs under MS-DOS, as part
+of the DJGPP project. For more information on the project, see
http://www.delorie.com/djgpp/
-I picked up a binary of bash-1.14.7 that is purported to work with
-the DJGPP V2 environment from
-
-ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh1147b.zip
-
-The corresponding source is
+I have been told that the original DJGPP port was done by Daisuke Aoyama.
-ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh1147s.zip
+Mark Elbrecht <snowball3@bigfoot.com> has sent me notice that bash-2.04
+is available for DJGPP V2. The files are available as:
-Ports of bash-1.12 and bash-2.0 are available for OS/2 from
+ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh204b.zip binary
+ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh204d.zip documentation
+ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh204s.zip source
-ftp://hobbes.nmsu.edu/pub/os2/util/shell/bash_112.zip
-ftp://hobbes.nmsu.edu/pub/os2/util/shell/bash-2.0(253).zip
+Mark began to work with bash-2.05, but I don't know the current status.
-I haven't looked at either, but the second appears to be a binary-only
-distribution. Beware.
+Bash-3.0 compiles and runs with no modifications under Microsoft's Services
+for Unix (SFU), once known as Interix. I do not anticipate any problems
+with building bash-4.2 and later, but will gladly accept any patches that
+are needed.
-6) How can I build bash with gcc?
+A6) How can I build bash with gcc?
Bash configures to use gcc by default if it is available. Read the
file INSTALL in the distribution for more information.
-7) How can I make bash my login shell?
+A7) How can I make bash my login shell?
Some machines let you use `chsh' to change your login shell. Other
systems use `passwd -s' or `passwd -e'. If one of these works for
Next, add a line similar to the above to ~/.profile:
- [ -f /usr/gnu/bin/bash ] && exec /usr/gnu/bin/bash --login
+ [ -f /usr/gnu/bin/bash ] && [ -x /usr/gnu/bin/bash ] && \
+ exec /usr/gnu/bin/bash --login
This will cause login shells to replace themselves with bash running as
a login shell. Once you have this working, you can copy your initialization
code from ~/.profile to ~/.bash_profile.
-8) I just changed my login shell to bash, and now I can't FTP into my
+I have received word that the recipe supplied above is insufficient for
+machines running CDE. CDE has a maze of twisty little startup files, all
+slightly different.
+
+If you cannot change your login shell in the password file to bash, you
+will have to (apparently) live with CDE using the shell in the password
+file to run its startup scripts. If you have changed your shell to bash,
+there is code in the CDE startup files (on Solaris, at least) that attempts
+to do the right thing. It is, however, often broken, and may require that
+you use the $BASH_ENV trick described below.
+
+`dtterm' claims to use $SHELL as the default program to start, so if you
+can change $SHELL in the CDE startup files, you should be able to use bash
+in your terminal windows.
+
+Setting DTSOURCEPROFILE in ~/.dtprofile will cause the `Xsession' program
+to read your login shell's startup files. You may be able to use bash for
+the rest of the CDE programs by setting SHELL to bash in ~/.dtprofile as
+well, but I have not tried this.
+
+You can use the above `exec' recipe to start bash when not logging in with
+CDE by testing the value of the DT variable:
+
+ if [ -n "$DT" ]; then
+ [ -f /usr/gnu/bin/bash ] && exec /usr/gnu/bin/bash --login
+ fi
+
+If CDE starts its shells non-interactively during login, the login shell
+startup files (~/.profile, ~/.bash_profile) will not be sourced at login.
+To get around this problem, append a line similar to the following to your
+~/.dtprofile:
+
+ BASH_ENV=${HOME}/.bash_profile ; export BASH_ENV
+
+and add the following line to the beginning of ~/.bash_profile:
+
+ unset BASH_ENV
+
+A8) I just changed my login shell to bash, and now I can't FTP into my
machine. Why not?
You must add the full pathname to bash to the file /etc/shells. As
Most versions of ftpd use this file to prohibit `special' users
such as `uucp' and `news' from using FTP.
-9) What's the `POSIX 1003.2 standard'?
+A9) What's the `POSIX Shell and Utilities standard'?
POSIX is a name originally coined by Richard Stallman for a
family of open system standards based on UNIX. There are a
administration and management. Each area of standardization is
assigned to a working group in the 1003 series.
-The POSIX Shell and Utilities standard has been developed by IEEE
-Working Group 1003.2 (POSIX.2). It concentrates on the command
+The POSIX Shell and Utilities standard was originally developed by
+IEEE Working Group 1003.2 (POSIX.2). Today it has been merged with
+the original 1003.1 Working Group and is maintained by the Austin
+Group (a joint working group of the IEEE, The Open Group and
+ISO/IEC SC22/WG15). Today the Shell and Utilities are a volume
+within the set of documents that make up IEEE Std 1003.1-2001, and
+thus now the former POSIX.2 (from 1992) is now part of the current
+POSIX.1 standard (POSIX 1003.1-2001).
+
+The Shell and Utilities volume concentrates on the command
interpreter interface and utility programs commonly executed from
-the command line or by other programs. An initial version of the
-standard has been approved and published by the IEEE, and work is
-currently underway to update it.
+the command line or by other programs. The standard is freely
+available on the web at http://www.UNIX-systems.org/version3/ .
+Work continues at the Austin Group on maintenance issues; see
+http://www.opengroup.org/austin/ to join the discussions.
-Bash is concerned with the aspects of the shell's behavior
-defined by POSIX.2. The shell command language has of course
-been standardized, including the basic flow control and program
-execution constructs, I/O redirection and pipelining, argument
-handling, variable expansion, and quoting.
+Bash is concerned with the aspects of the shell's behavior defined
+by the POSIX Shell and Utilities volume. The shell command
+language has of course been standardized, including the basic flow
+control and program execution constructs, I/O redirection and
+pipelining, argument handling, variable expansion, and quoting.
The `special' builtins, which must be implemented as part of the
shell to provide the desired functionality, are specified as
being part of the shell; examples of these are `eval' and
-`export'. Other utilities appear in the sections of POSIX.2 not
+`export'. Other utilities appear in the sections of POSIX not
devoted to the shell which are commonly (and in some cases must
be) implemented as builtin commands, such as `read' and `test'.
-POSIX.2 also specifies aspects of the shell's interactive
+POSIX also specifies aspects of the shell's interactive
behavior as part of the UPE, including job control and command
line editing. Only vi-style line editing commands have been
standardized; emacs editing commands were left out due to
objections.
-10) What is the bash `posix mode'?
+The latest version of the POSIX Shell and Utilities standard is
+available (now updated to the 2004 Edition) as part of the Single
+UNIX Specification Version 3 at
+
+http://www.UNIX-systems.org/version3/
-Although bash is an implementation of the POSIX.2 shell
+A10) What is the bash `posix mode'?
+
+Although bash is an implementation of the POSIX shell
specification, there are areas where the bash default behavior
differs from that spec. The bash `posix mode' changes the bash
behavior in these areas so that it obeys the spec more closely.
-Posix mode is entered by starting bash with the --posix option or
-executing `set -o posix' after bash is running.
+Posix mode is entered by starting bash with the --posix or
+'-o posix' option or executing `set -o posix' after bash is running.
The specific aspects of bash which change when posix mode is
-active are listed in the file CWRU/POSIX.NOTES in the bash
-distribution. They are also listed in a section in the Bash
-Reference Manual.
+active are listed in the file POSIX in the bash distribution.
+They are also listed in a section in the Bash Reference Manual
+(from which that file is generated).
Section B: The latest version
-11) What's new in version 2.02?
+B1) What's new in version 4.3?
+
+Bash-4.3 is the third revision to the fourth major release of bash.
+
+Bash-4.3 contains the following new features (see the manual page for
+complete descriptions and the CHANGES and NEWS files in the bash-4.3
+distribution):
+
+o The `helptopic' completion action now maps to all the help topics, not just
+ the shell builtins.
+
+o The `help' builtin no longer does prefix substring matching first, so
+ `help read' does not match `readonly', but will do it if exact string
+ matching fails.
+
+o The shell can be compiled to not display a message about processes that
+ terminate due to SIGTERM.
+
+o Non-interactive shells now react to the setting of checkwinsize and set
+ LINES and COLUMNS after a foreground job exits.
+
+o There is a new shell option, `globasciiranges', which, when set to on,
+ forces globbing range comparisons to use character ordering as if they
+ were run in the C locale.
+
+o There is a new shell option, `direxpand', which makes filename completion
+ expand variables in directory names in the way bash-4.1 did.
+
+o In Posix mode, the `command' builtin does not change whether or not a
+ builtin it shadows is treated as an assignment builtin.
+
+o The `return' and `exit' builtins accept negative exit status arguments.
+
+o The word completion code checks whether or not a filename containing a
+ shell variable expands to a directory name and appends `/' to the word
+ as appropriate. The same code expands shell variables in command names
+ when performing command completion.
+
+o In Posix mode, it is now an error to attempt to define a shell function
+ with the same name as a Posix special builtin.
+
+o When compiled for strict Posix conformance, history expansion is disabled
+ by default.
+
+o The history expansion character (!) does not cause history expansion when
+ followed by the closing quote in a double-quoted string.
+
+o `complete' and its siblings compgen/compopt now takes a new `-o noquote'
+ option to inhibit quoting of the completions.
+
+o Setting HISTSIZE to a value less than zero causes the history list to be
+ unlimited (setting it 0 zero disables the history list).
+
+o Setting HISTFILESIZE to a value less than zero causes the history file size
+ to be unlimited (setting it to 0 causes the history file to be truncated
+ to zero size).
+
+o The `read' builtin now skips NUL bytes in the input.
+
+o There is a new `bind -X' option to print all key sequences bound to Unix
+ commands.
+
+o When in Posix mode, `read' is interruptible by a trapped signal. After
+ running the trap handler, read returns 128+signal and throws away any
+ partially-read input.
+
+o The command completion code skips whitespace and assignment statements
+ before looking for the command name word to be completed.
+
+o The build process has a new mechanism for constructing separate help files
+ that better reflects the current set of compilation options.
+
+o The -nt and -ot options to test now work with files with nanosecond
+ timestamp resolution.
+
+o The shell saves the command history in any shell for which history is
+ enabled and HISTFILE is set, not just interactive shells.
+
+o The shell has `nameref' variables and new -n(/+n) options to declare and
+ unset to use them, and a `test -R' option to test for them.
+
+o The shell now allows assigning, referencing, and unsetting elements of
+ indexed arrays using negative subscripts (a[-1]=2, echo ${a[-1]}) which
+ count back from the last element of the array.
+
+o The {x}<word redirection feature now allows words like {array[ind]} and
+ can use variables with special meanings to the shell (e.g., BASH_XTRACEFD).
+
+o There is a new CHILD_MAX special shell variable; its value controls the
+ number of exited child statues the shell remembers.
+
+o There is a new configuration option (--enable-direxpand-default) that
+ causes the `direxpand' shell option to be enabled by default.
+
+o Bash does not do anything special to ensure that the file descriptor
+ assigned to X in {x}<foo remains open after the block containing it
+ completes.
+
+o The `wait' builtin has a new `-n' option to wait for the next child to
+ change status.
+
+o The `printf' %(...)T format specifier now uses the current time if no
+ argument is supplied.
+
+o There is a new variable, BASH_COMPAT, that controls the current shell
+ compatibility level.
+
+o The `popd' builtin now treats additional arguments as errors.
+
+o The brace expansion code now treats a failed sequence expansion as a
+ simple string and will continue to expand brace terms in the remainder
+ of the word.
+
+o Shells started to run process substitutions now run any trap set on EXIT.
+
+o The fc builtin now interprets -0 as the current command line.
+
+o Completing directory names containing shell variables now adds a trailing
+ slash if the expanded result is a directory.
+
+A short feature history dating back to Bash-2.0:
+
+Bash-4.2 contained the following new features:
+
+o `exec -a foo' now sets $0 to `foo' in an executable shell script without a
+ leading #!.
+
+o Subshells begun to execute command substitutions or run shell functions or
+ builtins in subshells do not reset trap strings until a new trap is
+ specified. This allows $(trap) to display the caller's traps and the
+ trap strings to persist until a new trap is set.
+
+o `trap -p' will now show signals ignored at shell startup, though their
+ disposition still cannot be modified.
+
+o $'...', echo, and printf understand \uXXXX and \UXXXXXXXX escape sequences.
+
+o declare/typeset has a new `-g' option, which creates variables in the
+ global scope even when run in a shell function.
+
+o test/[/[[ have a new -v variable unary operator, which returns success if
+ `variable' has been set.
+
+o Posix parsing changes to allow `! time command' and multiple consecutive
+ instances of `!' (which toggle) and `time' (which have no cumulative
+ effect).
+
+o Posix change to allow `time' as a command by itself to print the elapsed
+ user, system, and real times for the shell and its children.
+
+o $((...)) is always parsed as an arithmetic expansion first, instead of as
+ a potential nested command substitution, as Posix requires.
+
+o A new FUNCNEST variable to allow the user to control the maximum shell
+ function nesting (recursive execution) level.
+
+o The mapfile builtin now supplies a third argument to the callback command:
+ the line about to be assigned to the supplied array index.
+
+o The printf builtin has as new %(fmt)T specifier, which allows time values
+ to use strftime-like formatting.
+
+o There is a new `compat41' shell option.
+
+o The cd builtin has a new Posix-mandated `-e' option.
+
+o Negative subscripts to indexed arrays, previously errors, now are treated
+ as offsets from the maximum assigned index + 1.
+
+o Negative length specifications in the ${var:offset:length} expansion,
+ previously errors, are now treated as offsets from the end of the variable.
+
+o Parsing change to allow `time -p --'.
+
+o Posix-mode parsing change to not recognize `time' as a keyword if the
+ following token begins with a `-'. This means no more Posix-mode
+ `time -p'. Posix interpretation 267.
+
+o There is a new `lastpipe' shell option that runs the last command of a
+ pipeline in the current shell context. The lastpipe option has no
+ effect if job control is enabled.
+
+o History expansion no longer expands the `$!' variable expansion.
+
+o Posix mode shells no longer exit if a variable assignment error occurs
+ with an assignment preceding a command that is not a special builtin.
+
+o Non-interactive mode shells exit if -u is enabled an an attempt is made
+ to use an unset variable with the % or # expansions, the `//', `^', or
+ `,' expansions, or the parameter length expansion.
+
+o Posix-mode shells use the argument passed to `.' as-is if a $PATH search
+ fails, effectively searching the current directory. Posix-2008 change.
+
+A short feature history dating back to Bash-2.0:
+
+Bash-4.1 contained the following new features:
+
+o Here-documents within $(...) command substitutions may once more be
+ delimited by the closing right paren, instead of requiring a newline.
+
+o Bash's file status checks (executable, readable, etc.) now take file
+ system ACLs into account on file systems that support them.
+
+o Bash now passes environment variables with names that are not valid
+ shell variable names through into the environment passed to child
+ processes.
+
+o The `execute-unix-command' readline function now attempts to clear and
+ reuse the current line rather than move to a new one after the command
+ executes.
+
+o `printf -v' can now assign values to array indices.
+
+o New `complete -E' and `compopt -E' options that work on the "empty"
+ completion: completion attempted on an empty command line.
+
+o New complete/compgen/compopt -D option to define a `default' completion:
+ a completion to be invoked on command for which no completion has been
+ defined. If this function returns 124, programmable completion is
+ attempted again, allowing a user to dynamically build a set of completions
+ as completion is attempted by having the default completion function
+ install individual completion functions each time it is invoked.
+
+o When displaying associative arrays, subscripts are now quoted.
+
+o Changes to dabbrev-expand to make it more `emacs-like': no space appended
+ after matches, completions are not sorted, and most recent history entries
+ are presented first.
+
+o The [[ and (( commands are now subject to the setting of `set -e' and the
+ ERR trap.
+
+o The source/. builtin now removes NUL bytes from the file before attempting
+ to parse commands.
+
+o There is a new configuration option (in config-top.h) that forces bash to
+ forward all history entries to syslog.
+
+o A new variable $BASHOPTS to export shell options settable using `shopt' to
+ child processes.
+
+o There is a new confgure option that forces the extglob option to be
+ enabled by default.
+
+o New variable $BASH_XTRACEFD; when set to an integer bash will write xtrace
+ output to that file descriptor.
+
+o If the optional left-hand-side of a redirection is of the form {var}, the
+ shell assigns the file descriptor used to $var or uses $var as the file
+ descriptor to move or close, depending on the redirection operator.
+
+o The < and > operators to the [[ conditional command now do string
+ comparison according to the current locale.
+
+o Programmable completion now uses the completion for `b' instead of `a'
+ when completion is attempted on a line like: a $(b c.
+
+o Force extglob on temporarily when parsing the pattern argument to
+ the == and != operators to the [[ command, for compatibility.
+
+o Changed the behavior of interrupting the wait builtin when a SIGCHLD is
+ received and a trap on SIGCHLD is set to be Posix-mode only.
+
+o The read builtin has a new `-N nchars' option, which reads exactly NCHARS
+ characters, ignoring delimiters like newline.
+
+o The mapfile/readarray builtin no longer stores the commands it invokes via
+ callbacks in the history list.
+
+o There is a new `compat40' shopt option.
+
+o The < and > operators to [[ do string comparisons using the current locale
+ only if the compatibility level is greater than 40 (set to 41 by default).
+
+o New bindable readline function: menu-complete-backward.
+
+o In the readline vi-mode insertion keymap, C-n is now bound to menu-complete
+ by default, and C-p to menu-complete-backward.
+
+o When in readline vi command mode, repeatedly hitting ESC now does nothing,
+ even when ESC introduces a bound key sequence. This is closer to how
+ historical vi behaves.
+
+o New bindable readline function: skip-csi-sequence. Can be used as a
+ default to consume key sequences generated by keys like Home and End
+ without having to bind all keys.
+
+o New bindable readline variable: skip-completed-text, active when
+ completing in the middle of a word. If enabled, it means that characters
+ in the completion that match characters in the remainder of the word are
+ "skipped" rather than inserted into the line.
+
+o The pre-readline-6.0 version of menu completion is available as
+ "old-menu-complete" for users who do not like the readline-6.0 version.
-Bash-2.02 has a number of new features. Here's a short list:
+o New bindable readline variable: echo-control-characters. If enabled, and
+ the tty ECHOCTL bit is set, controls the echoing of characters
+ corresponding to keyboard-generated signals.
+
+o New bindable readline variable: enable-meta-key. Controls whether or not
+ readline sends the smm/rmm sequences if the terminal indicates it has a
+ meta key that enables eight-bit characters.
+
+Bash-4.0 contained the following new features:
+
+o When using substring expansion on the positional parameters, a starting
+ index of 0 now causes $0 to be prefixed to the list.
+
+o There is a new variable, $BASHPID, which always returns the process id of
+ the current shell.
+
+o There is a new `autocd' option that, when enabled, causes bash to attempt
+ to `cd' to a directory name that is supplied as the first word of a
+ simple command.
+
+o There is a new `checkjobs' option that causes the shell to check for and
+ report any running or stopped jobs at exit.
+
+o The programmable completion code exports a new COMP_TYPE variable, set to
+ a character describing the type of completion being attempted.
+
+o The programmable completion code exports a new COMP_KEY variable, set to
+ the character that caused the completion to be invoked (e.g., TAB).
+
+o The programmable completion code now uses the same set of characters as
+ readline when breaking the command line into a list of words.
+
+o The block multiplier for the ulimit -c and -f options is now 512 when in
+ Posix mode, as Posix specifies.
+
+o Changed the behavior of the read builtin to save any partial input received
+ in the specified variable when the read builtin times out. This also
+ results in variables specified as arguments to read to be set to the empty
+ string when there is no input available. When the read builtin times out,
+ it returns an exit status greater than 128.
+
+o The shell now has the notion of a `compatibility level', controlled by
+ new variables settable by `shopt'. Setting this variable currently
+ restores the bash-3.1 behavior when processing quoted strings on the rhs
+ of the `=~' operator to the `[[' command.
+
+o The `ulimit' builtin now has new -b (socket buffer size) and -T (number
+ of threads) options.
+
+o There is a new `compopt' builtin that allows completion functions to modify
+ completion options for existing completions or the completion currently
+ being executed.
+
+o The `read' builtin has a new -i option which inserts text into the reply
+ buffer when using readline.
+
+o A new `-E' option to the complete builtin allows control of the default
+ behavior for completion on an empty line.
+
+o There is now limited support for completing command name words containing
+ globbing characters.
+
+o The `help' builtin now has a new -d option, to display a short description,
+ and a -m option, to print help information in a man page-like format.
+
+o There is a new `mapfile' builtin to populate an array with lines from a
+ given file.
+
+o If a command is not found, the shell attempts to execute a shell function
+ named `command_not_found_handle', supplying the command words as the
+ function arguments.
+
+o There is a new shell option: `globstar'. When enabled, the globbing code
+ treats `**' specially -- it matches all directories (and files within
+ them, when appropriate) recursively.
+
+o There is a new shell option: `dirspell'. When enabled, the filename
+ completion code performs spelling correction on directory names during
+ completion.
+
+o The `-t' option to the `read' builtin now supports fractional timeout
+ values.
+
+o Brace expansion now allows zero-padding of expanded numeric values and
+ will add the proper number of zeroes to make sure all values contain the
+ same number of digits.
+
+o There is a new bash-specific bindable readline function: `dabbrev-expand'.
+ It uses menu completion on a set of words taken from the history list.
+
+o The command assigned to a key sequence with `bind -x' now sets two new
+ variables in the environment of the executed command: READLINE_LINE_BUFFER
+ and READLINE_POINT. The command can change the current readline line
+ and cursor position by modifying READLINE_LINE_BUFFER and READLINE_POINT,
+ respectively.
+
+o There is a new >>& redirection operator, which appends the standard output
+ and standard error to the named file.
+
+o The parser now understands `|&' as a synonym for `2>&1 |', which redirects
+ the standard error for a command through a pipe.
+
+o The new `;&' case statement action list terminator causes execution to
+ continue with the action associated with the next pattern in the
+ statement rather than terminating the command.
+
+o The new `;;&' case statement action list terminator causes the shell to
+ test the next set of patterns after completing execution of the current
+ action, rather than terminating the command.
+
+o The shell understands a new variable: PROMPT_DIRTRIM. When set to an
+ integer value greater than zero, prompt expansion of \w and \W will
+ retain only that number of trailing pathname components and replace
+ the intervening characters with `...'.
+
+o There are new case-modifying word expansions: uppercase (^[^]) and
+ lowercase (,[,]). They can work on either the first character or
+ array element, or globally. They accept an optional shell pattern
+ that determines which characters to modify. There is an optionally-
+ configured feature to include capitalization operators.
+
+o The shell provides associative array variables, with the appropriate
+ support to create, delete, assign values to, and expand them.
+
+o The `declare' builtin now has new -l (convert value to lowercase upon
+ assignment) and -u (convert value to uppercase upon assignment) options.
+ There is an optionally-configurable -c option to capitalize a value at
+ assignment.
+
+o There is a new `coproc' reserved word that specifies a coprocess: an
+ asynchronous command run with two pipes connected to the creating shell.
+ Coprocs can be named. The input and output file descriptors and the
+ PID of the coprocess are available to the calling shell in variables
+ with coproc-specific names.
+
+o A value of 0 for the -t option to `read' now returns success if there is
+ input available to be read from the specified file descriptor.
+
+o CDPATH and GLOBIGNORE are ignored when the shell is running in privileged
+ mode.
+
+o New bindable readline functions shell-forward-word and shell-backward-word,
+ which move forward and backward words delimited by shell metacharacters
+ and honor shell quoting.
+
+o New bindable readline functions shell-backward-kill-word and shell-kill-word
+ which kill words backward and forward, but use the same word boundaries
+ as shell-forward-word and shell-backward-word.
+
+Bash-3.2 contained the following new features:
+
+o Bash-3.2 now checks shell scripts for NUL characters rather than non-printing
+ characters when deciding whether or not a script is a binary file.
+
+o Quoting the string argument to the [[ command's =~ (regexp) operator now
+ forces string matching, as with the other pattern-matching operators.
+
+Bash-3.1 contained the following new features:
+
+o Bash-3.1 may now be configured and built in a mode that enforces strict
+ POSIX compliance.
+
+o The `+=' assignment operator, which appends to the value of a string or
+ array variable, has been implemented.
+
+o It is now possible to ignore case when matching in contexts other than
+ filename generation using the new `nocasematch' shell option.
+
+Bash-3.0 contained the following new features:
+
+o Features to support the bash debugger have been implemented, and there
+ is a new `extdebug' option to turn the non-default options on
+
+o HISTCONTROL is now a colon-separated list of options and has been
+ extended with a new `erasedups' option that will result in only one
+ copy of a command being kept in the history list
+
+o Brace expansion has been extended with a new {x..y} form, producing
+ sequences of digits or characters
+
+o Timestamps are now kept with history entries, with an option to save
+ and restore them from the history file; there is a new HISTTIMEFORMAT
+ variable describing how to display the timestamps when listing history
+ entries
+
+o The `[[' command can now perform extended regular expression (egrep-like)
+ matching, with matched subexpressions placed in the BASH_REMATCH array
+ variable
+
+o A new `pipefail' option causes a pipeline to return a failure status if
+ any command in it fails
+
+o The `jobs', `kill', and `wait' builtins now accept job control notation
+ in their arguments even if job control is not enabled
+
+o The `gettext' package and libintl have been integrated, and the shell
+ messages may be translated into other languages
+
+Bash-2.05b introduced the following new features:
+
+o support for multibyte characters has been added to both bash and readline
+
+o the DEBUG trap is now run *before* simple commands, ((...)) commands,
+ [[...]] conditional commands, and for ((...)) loops
+
+o the shell now performs arithmetic in the largest integer size the machine
+ supports (intmax_t)
+
+o there is a new \D{...} prompt expansion; passes the `...' to strftime(3)
+ and inserts the result into the expanded prompt
+
+o there is a new `here-string' redirection operator: <<< word
+
+o when displaying variables, function attributes and definitions are shown
+ separately, allowing them to be re-used as input (attempting to re-use
+ the old output would result in syntax errors).
+
+o `read' has a new `-u fd' option to read from a specified file descriptor
+
+o the bash debugger in examples/bashdb has been modified to work with the
+ new DEBUG trap semantics, the command set has been made more gdb-like,
+ and the changes to $LINENO make debugging functions work better
+
+o the expansion of $LINENO inside a shell function is only relative to the
+ function start if the shell is interactive -- if the shell is running a
+ script, $LINENO expands to the line number in the script. This is as
+ POSIX-2001 requires
+
+Bash-2.05a introduced the following new features:
+
+o The `printf' builtin has undergone major work
+
+o There is a new read-only `shopt' option: login_shell, which is set by
+ login shells and unset otherwise
+
+o New `\A' prompt string escape sequence; expanding to time in 24-hour
+ HH:MM format
+
+o New `-A group/-g' option to complete and compgen; goes group name
+ completion
+
+o New [+-]O invocation option to set and unset `shopt' options at startup
+
+o ksh-like `ERR' trap
+
+o `for' loops now allow empty word lists after the `in' reserved word
+
+o new `hard' and `soft' arguments for the `ulimit' builtin
+
+o Readline can be configured to place the user at the same point on the line
+ when retrieving commands from the history list
+
+o Readline can be configured to skip `hidden' files (filenames with a leading
+ `.' on Unix) when performing completion
+
+Bash-2.05 introduced the following new features:
+
+o This version has once again reverted to using locales and strcoll(3) when
+ processing pattern matching bracket expressions, as POSIX requires.
+o Added a new `--init-file' invocation argument as a synonym for `--rcfile',
+ per the new GNU coding standards.
+o The /dev/tcp and /dev/udp redirections now accept service names as well as
+ port numbers.
+o `complete' and `compgen' now take a `-o value' option, which controls some
+ of the aspects of that compspec. Valid values are:
+
+ default - perform bash default completion if programmable
+ completion produces no matches
+ dirnames - perform directory name completion if programmable
+ completion produces no matches
+ filenames - tell readline that the compspec produces filenames,
+ so it can do things like append slashes to
+ directory names and suppress trailing spaces
+o A new loadable builtin, realpath, which canonicalizes and expands symlinks
+ in pathname arguments.
+o When `set' is called without options, it prints function defintions in a
+ way that allows them to be reused as input. This affects `declare' and
+ `declare -p' as well. This only happens when the shell is not in POSIX
+ mode, since POSIX.2 forbids this behavior.
+
+Bash-2.04 introduced the following new features:
+
+o Programmable word completion with the new `complete' and `compgen' builtins;
+ examples are provided in examples/complete/complete-examples
+o `history' has a new `-d' option to delete a history entry
+o `bind' has a new `-x' option to bind key sequences to shell commands
+o The prompt expansion code has new `\j' and `\l' escape sequences
+o The `no_empty_cmd_completion' shell option, if enabled, inhibits
+ command completion when TAB is typed on an empty line
+o `help' has a new `-s' option to print a usage synopsis
+o New arithmetic operators: var++, var--, ++var, --var, expr1,expr2 (comma)
+o New ksh93-style arithmetic for command:
+ for ((expr1 ; expr2; expr3 )); do list; done
+o `read' has new options: `-t', `-n', `-d', `-s'
+o The redirection code handles several filenames specially: /dev/fd/N,
+ /dev/stdin, /dev/stdout, /dev/stderr
+o The redirection code now recognizes /dev/tcp/HOST/PORT and
+ /dev/udp/HOST/PORT and tries to open a TCP or UDP socket, respectively,
+ to the specified port on the specified host
+o The ${!prefix*} expansion has been implemented
+o A new FUNCNAME variable, which expands to the name of a currently-executing
+ function
+o The GROUPS variable is no longer readonly
+o A new shopt `xpg_echo' variable, to control the behavior of echo with
+ respect to backslash-escape sequences at runtime
+o The NON_INTERACTIVE_LOGIN_SHELLS #define has returned
+
+The version of Readline released with Bash-2.04, Readline-4.1, had several
+new features as well:
+
+o Parentheses matching is always compiled into readline, and controllable
+ with the new `blink-matching-paren' variable
+o The history-search-forward and history-search-backward functions now leave
+ point at the end of the line when the search string is empty, like
+ reverse-search-history, and forward-search-history
+o A new function for applications: rl_on_new_line_with_prompt()
+o New variables for applications: rl_already_prompted, and rl_gnu_readline_p
+
+
+Bash-2.03 had very few new features, in keeping with the convention
+that odd-numbered releases provide mainly bug fixes. A number of new
+features were added to Readline, mostly at the request of the Cygnus
+folks.
+
+A new shopt option, `restricted_shell', so that startup files can test
+ whether or not the shell was started in restricted mode
+Filename generation is now performed on the words between ( and ) in
+ compound array assignments (this is really a bug fix)
+OLDPWD is now auto-exported, as POSIX.2 requires
+ENV and BASH_ENV are read-only variables in a restricted shell
+Bash may now be linked against an already-installed Readline library,
+ as long as the Readline library is version 4 or newer
+All shells begun with the `--login' option will source the login shell
+ startup files, even if the shell is not interactive
+
+There were lots of changes to the version of the Readline library released
+along with Bash-2.03. For a complete list of the changes, read the file
+CHANGES in the Bash-2.03 distribution.
+
+Bash-2.02 contained the following new features:
a new version of malloc (based on the old GNU malloc code in previous
bash versions) that is more page-oriented, more conservative
lots of code now smaller and faster
test suite greatly expanded
-12) Are there any user-visible incompatibilities between bash-2.02 and
- bash-1.14.7?
+B2) Are there any user-visible incompatibilities between bash-4.3 and
+ previous bash versions?
-There are a few incompatibilities between version 1.14.7 and version 2.02.
-They are detailed in the file COMPAT in the bash-2.02 distribution.
+There are a few incompatibilities between version 4.3 and previous
+versions. They are detailed in the file COMPAT in the bash distribution.
+That file is not meant to be all-encompassing; send mail to
+bash-maintainers@gnu.org (or bug-bash@gnu.org if you would like
+community discussion) if you find something that's not mentioned there.
Section C: Differences from other Unix shells
-13) How does bash differ from sh, the Bourne shell?
+C1) How does bash differ from sh, the Bourne shell?
This is a non-comprehensive list of features that differentiate bash
from the SVR4.2 shell. The bash manual page explains these more
Things bash has that sh does not:
long invocation options
+ [+-]O invocation option
+ -l invocation option
`!' reserved word to invert pipeline return value
`time' reserved word to time pipelines and shell builtins
the `function' reserved word
- the select compound command and reserved word
+ the `select' compound command and reserved word
+ arithmetic for command: for ((expr1 ; expr2; expr3 )); do list; done
new $'...' and $"..." quoting
the $(...) form of command substitution
the $(<filename) form of command substitution, equivalent to
$(cat filename)
the ${#param} parameter value length operator
the ${!param} indirect parameter expansion operator
- the ${param:length[:offset]} parameter substring operator
+ the ${!param*} prefix expansion operator
+ the ${param:offset[:length]} parameter substring operator
the ${param/pat[/string]} parameter pattern substitution operator
expansions to perform substring removal (${p%[%]w}, ${p#[#]w})
expansion of positional parameters beyond $9 with ${num}
- variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, REPLY,
+ variables: BASH, BASHPID, BASH_VERSION, BASH_VERSINFO, UID, EUID, REPLY,
TIMEFORMAT, PPID, PWD, OLDPWD, SHLVL, RANDOM, SECONDS,
LINENO, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE, HOSTNAME,
ENV, PS3, PS4, DIRSTACK, PIPESTATUS, HISTSIZE, HISTFILE,
HISTFILESIZE, HISTCONTROL, HISTIGNORE, GLOBIGNORE, GROUPS,
PROMPT_COMMAND, FCEDIT, FIGNORE, IGNOREEOF, INPUTRC,
- SHELLOPTS, OPTERR, HOSTFILE, TMOUT, histchars, auto_resume
+ SHELLOPTS, OPTERR, HOSTFILE, TMOUT, FUNCNAME, histchars,
+ auto_resume, PROMPT_DIRTRIM, BASHOPTS, BASH_XTRACEFD
DEBUG trap
+ ERR trap
variable arrays with new compound assignment syntax
- redirections: <>, &>, >|
+ redirections: <>, &>, >|, <<<, [n]<&word-, [n]>&word-, >>&
prompt string special char translation and variable expansion
- auto-export of modified values of variables in initial environment
+ auto-export of variables in initial environment
command search finds functions before builtins
bash return builtin will exit a file sourced with `.'
- builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E, hash -p.
- export -n/-f/-p/name=value, pwd -L/-P, read -e/-p/-a,
+ builtins: cd -/-L/-P/-@, exec -l/-c/-a, echo -e/-E, hash -d/-l/-p/-t.
+ export -n/-f/-p/name=value, pwd -L/-P,
+ read -e/-p/-a/-t/-n/-d/-s/-u/-i/-N,
readonly -a/-f/name=value, trap -l, set +o,
set -b/-m/-o option/-h/-p/-B/-C/-H/-P,
- unset -f/-v, ulimit -m/-p/-u,
- type -a/-p/-t, suspend -f, kill -n,
- test -o optname/s1 == s2/s1 < s2/s1 > s2/-nt/-ot/-ef/-O/-G/-S
+ unset -f/-n/-v, ulimit -i/-m/-p/-q/-u/-x,
+ type -a/-p/-t/-f/-P, suspend -f, kill -n,
+ test -o optname/s1 == s2/s1 < s2/s1 > s2/-nt/-ot/-ef/-O/-G/-S/-R
bash reads ~/.bashrc for interactive shells, $ENV for non-interactive
bash restricted shell mode is more extensive
bash allows functions and variables with the same name
process substitution
aliases and alias/unalias builtins
local variables in functions and `local' builtin
- readline and command-line editing
+ readline and command-line editing with programmable completion
command history and history/fc builtins
csh-like history expansion
- other new bash builtins: bind, command, builtin, declare/typeset,
- dirs, enable, fc, help, history, logout,
- popd, pushd, disown, shopt, printf
+ other new bash builtins: bind, command, compgen, complete, builtin,
+ declare/typeset, dirs, enable, fc, help,
+ history, logout, popd, pushd, disown, shopt,
+ printf, compopt, mapfile
exported functions
filename generation when using output redirection (command >a*)
POSIX.2-style globbing character classes
case-insensitive pattern matching and globbing
variable assignments preceding commands affect only that command,
even for builtins and functions
- posix mode
+ posix mode and strict posix conformance
+ redirection to /dev/fd/N, /dev/stdin, /dev/stdout, /dev/stderr,
+ /dev/tcp/host/port, /dev/udp/host/port
+ debugger support, including `caller' builtin and new variables
+ RETURN trap
+ the `+=' assignment operator
+ autocd shell option and behavior
+ command-not-found hook with command_not_found_handle shell function
+ globstar shell option and `**' globbing behavior
+ |& synonym for `2>&1 |'
+ ;& and ;;& case action list terminators
+ case-modifying word expansions and variable attributes
+ associative arrays
+ coprocesses using the `coproc' reserved word and variables
+ shell assignment of a file descriptor used in a redirection to a variable
Things sh has that bash does not:
uses variable SHACCT to do shell accounting
bash allows multiple option arguments when invoked (e.g. -x -v);
sh allows only a single option argument (`sh -x -v' attempts
to open a file named `-v', and, on SunOS 4.1.4, dumps core.
- On Solaris 2, sh goes into an infinite loop.)
+ On Solaris 2.4 and earlier versions, sh goes into an infinite
+ loop.)
sh exits a script if any builtin fails; bash exits only if one of
the POSIX.2 `special' builtins fails
-14) How does bash differ from the Korn shell, version ksh88?
+C2) How does bash differ from the Korn shell, version ksh88?
Things bash has or uses that ksh88 does not:
long invocation options
+ [-+]O invocation option
+ -l invocation option
`!' reserved word
+ arithmetic for command: for ((expr1 ; expr2; expr3 )); do list; done
+ arithmetic in largest machine-supported size (intmax_t)
posix mode and posix conformance
command hashing
tilde expansion for assignment statements that look like $PATH
process substitution with named pipes if /dev/fd is not available
the ${!param} indirect parameter expansion operator
- the ${param:length[:offset]} parameter substring operator
+ the ${!param*} prefix expansion operator
+ the ${param:offset[:length]} parameter substring operator
the ${param/pat[/string]} parameter pattern substitution operator
- variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, SHLVL,
+ variables: BASH, BASH_VERSION, BASH_VERSINFO, BASHPID, UID, EUID, SHLVL,
TIMEFORMAT, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE,
HISTFILESIZE, HISTIGNORE, HISTCONTROL, PROMPT_COMMAND,
IGNOREEOF, FIGNORE, INPUTRC, HOSTFILE, DIRSTACK,
PIPESTATUS, HOSTNAME, OPTERR, SHELLOPTS, GLOBIGNORE,
- GROUPS, histchars, auto_resume
+ GROUPS, FUNCNAME, histchars, auto_resume, PROMPT_DIRTRIM
prompt expansion with backslash escapes and command substitution
- redirection: &> (stdout and stderr)
- more extensive and extensible editing and completion
+ redirection: &> (stdout and stderr), <<<, [n]<&word-, [n]>&word-, >>&
+ more extensive and extensible editing and programmable completion
builtins: bind, builtin, command, declare, dirs, echo -e/-E, enable,
exec -l/-c/-a, fc -s, export -n/-f/-p, hash, help, history,
jobs -x/-r/-s, kill -s/-n/-l, local, logout, popd, pushd,
- read -e/-p/-a, readonly -a/-n/-f/-p, set -o braceexpand/
- -o histexpand/-o interactive-comments/-o notify/-o physical/
- -o posix/-o hashall/-o onecmd/-h/-B/-C/-b/-H/-P, set +o,
- suspend, trap -l, type, typeset -a/-F/-p, ulimit -u,
- umask -S, alias -p, shopt, disown, printf
+ read -e/-p/-a/-t/-n/-d/-s/-N, readonly -a/-n/-f/-p,
+ set -o braceexpand/-o histexpand/-o interactive-comments/
+ -o notify/-o physical/-o posix/-o hashall/-o onecmd/
+ -h/-B/-C/-b/-H/-P, set +o, suspend, trap -l, type,
+ typeset -a/-F/-p, ulimit -i/-q/-u/-x, umask -S, alias -p,
+ shopt, disown, printf, complete, compgen, compopt, mapfile
`!' csh-style history expansion
POSIX.2-style globbing character classes
POSIX.2-style globbing equivalence classes
egrep-like extended pattern matching operators
case-insensitive pattern matching and globbing
`**' arithmetic operator to do exponentiation
+ redirection to /dev/fd/N, /dev/stdin, /dev/stdout, /dev/stderr
+ arrays of unlimited size
+ TMOUT is default timeout for `read' and `select'
+ debugger support, including the `caller' builtin
+ RETURN trap
+ Timestamps in history entries
+ {x..y} brace expansion
+ The `+=' assignment operator
+ autocd shell option and behavior
+ command-not-found hook with command_not_found_handle shell function
+ globstar shell option and `**' globbing behavior
+ |& synonym for `2>&1 |'
+ ;& and ;;& case action list terminators
+ case-modifying word expansions and variable attributes
+ associative arrays
+ coprocesses using the `coproc' reserved word and variables
+ shell assignment of a file descriptor used in a redirection to a variable
Things ksh88 has or uses that bash does not:
- tracked aliases
- variables: ERRNO, FPATH, COLUMNS, LINES, EDITOR, VISUAL
- co-processes (|&, >&p, <&p)
+ tracked aliases (alias -t)
+ variables: ERRNO, FPATH, EDITOR, VISUAL
+ co-processes (bash uses different syntax)
weirdly-scoped functions
typeset +f to list all function names without definitions
text of command history kept in a file, not memory
- builtins: alias -x, cd old new, fc -e -, newgrp, print,
- read -p/-s/-u/var?prompt, set -A/-o gmacs/
- -o bgnice/-o markdirs/-o nolog/-o trackall/-o viraw/-s,
- typeset -H/-L/-R/-A/-ft/-fu/-fx/-l/-u/-t, whence
+ builtins: alias -x, cd old new, newgrp, print,
+ read -p/-s/var?prompt, set -A/-o gmacs/
+ -o bgnice/-o markdirs/-o trackall/-o viraw/-s,
+ typeset -H/-L/-R/-Z/-A/-ft/-fu/-fx/-t, whence
+ using environment to pass attributes of exported variables
+ arithmetic evaluation done on arguments to some builtins
+ reads .profile from $PWD when invoked as login shell
Implementation differences:
ksh runs last command of a pipeline in parent shell context
bash has fixed startup file for all interactive shells; ksh reads $ENV
bash has exported functions
bash command search finds functions before builtins
+ bash waits for all commands in pipeline to exit before returning status
+ emacs-mode editing has some slightly different key bindings
-15) Which new features in ksh-93 are not in bash, and which are?
+C3) Which new features in ksh-93 are not in bash, and which are?
-New things in ksh-93 not in bash-2.02:
- associative arrays
- floating point arithmetic
- ++, --, comma arithmetic operators
- math library functions
+This list is current through ksh93v (10/08/2013)
+
+New things in ksh-93 not in bash-4.3:
+ floating point arithmetic, variables, and constants
+ math library functions, including user-defined math functions
${!name[sub]} name of subscript for associative array
- ${!prefix*} and {!prefix@} variable name prefix expansions
`.' is allowed in variable names to create a hierarchical namespace
more extensive compound assignment syntax
discipline functions
- `sleep' and `getconf' builtins (bash has loadable versions)
- typeset -n and `nameref' variables
KEYBD trap
- variables: .sh.edchar, .sh.edmode, .sh.edcol, .sh.edtext, HISTEDIT,
- .sh.version, .sh.name, .sh.subscript, .sh.value
- backreferences in pattern matching
- print -f (bash has a loadable version of print and the printf builtin)
+ variables: .sh.edchar, .sh.edmode, .sh.edcol, .sh.edtext, .sh.version,
+ .sh.name, .sh.subscript, .sh.value, .sh.match, HISTEDIT,
+ .sh.sig, .sh.stats, .sh.siginfo, .sh.pwdfd, .sh.op_astbin,
+ .sh.pool
+ backreferences in pattern matching (\N)
+ `&' operator in pattern lists for matching (match all instead of any)
+ exit statuses between 0 and 255
+ FPATH and PATH mixing
+ lexical scoping for local variables in `ksh' functions
+ no scoping for local variables in `POSIX' functions
+ $'' \C[.collating-element.] escape sequence
+ -C/-I invocation options
+ print -f (bash uses printf) and rest of print builtin options
+ printf %(type)q, %#q
`fc' has been renamed to `hist'
- read -t/-d
`.' can execute shell functions
-
-New things in ksh-93 present in bash-2.02:
- ?: arithmetic operator
- expansions: ${!param}, ${param:offset[:len]}, ${param/pat[/str]}
+ getopts -a
+ printf %B, %H, %P, %R, %Z modifiers, output base for %d, `=' flag
+ read -n/-N differ/-v/-S
+ set -o showme/-o multiline (bash default)
+ set -K
+ kill -Q/-q/-L
+ trap -a
+ `sleep' and `getconf' builtins (bash has loadable versions)
+ [[ -R name ]] (checks whether or not name is a nameref)
+ typeset -C/-S/-T/-X/-h/-s/-c/-M
+ experimental `type' definitions (a la typedef) using typeset
+ array expansions ${array[sub1..sub2]} and ${!array[sub1..sub2]}
+ associative array assignments using `;' as element separator
+ command substitution $(n<#) expands to current byte offset for fd N
+ new '${ ' form of command substitution, executed in current shell
+ new >;/<>;/<#pat/<##pat/<#/># redirections
+ brace expansion printf-like formats
+ CHLD trap triggered by SIGSTOP and SIGCONT
+ ~{fd} expansion, which replaces fd with the corresponding path name
+ $"string" expanded when referenced rather than when first parsed
+ job "pools", which allow a collection of jobs to be managed as a unit
+
+New things in ksh-93 present in bash-4.3:
+ associative arrays
+ [n]<&word- and [n]>&word- redirections (combination dup and close)
+ for (( expr1; expr2; expr3 )) ; do list; done - arithmetic for command
+ ?:, ++, --, `expr1 , expr2' arithmetic operators
+ expansions: ${!param}, ${param:offset[:len]}, ${param/pat[/str]},
+ ${!param*}
compound array assignment
+ negative subscripts for indexed array variables
the `!' reserved word
loadable builtins -- but ksh uses `builtin' while bash uses `enable'
- `command', `builtin', `disown' builtins
new $'...' and $"..." quoting
FIGNORE (but bash uses GLOBIGNORE), HISTCMD
- set -o notify/-C
+ brace expansion and set -B
changes to kill builtin
+ `command', `builtin', `disown' builtins
+ echo -e
+ exec -c/-a
+ printf %T modifier
read -A (bash uses read -a)
+ read -t/-d
trap -p
- exec -c/-a
`.' restores the positional parameters when it completes
+ set -o notify/-C
+ set -o pipefail
+ set -G (-o globstar) and **
POSIX.2 `test'
umask -S
unalias -a
command and arithmetic substitution performed on PS1, PS4, and ENV
- command name completion
+ command name completion, TAB displaying possible completions
ENV processed only for interactive shells
+ The `+=' assignment operator
+ the `;&' case statement "fallthrough" pattern list terminator
+ csh-style history expansion and set -H
+ negative offsets in ${param:offset:length}
+ redirection operators preceded with {varname} to store fd number in varname
+ DEBUG can force skipping following command
+ [[ -v var ]] operator (checks whether or not var is set)
+ typeset -n and `nameref' variables
+ process substitutions work without /dev/fd
Section D: Why does bash do some things differently than other Unix shells?
-16) Why does bash run a different version of `command' than
+D1) Why does bash run a different version of `command' than
`which command' says it will?
-`which' is actually a csh script that assumes you're running csh.
-It reads the csh startup files from your home directory and uses
-those to determine which `command' will be invoked. Since bash
-doesn't use any of those startup files, there's a good chance
-that your bash environment differs from your csh environment.
+On many systems, `which' is actually a csh script that assumes
+you're running csh. In tcsh, `which' and its cousin `where'
+are builtins. On other Unix systems, `which' is a perl script
+that uses the PATH environment variable. Many Linux distributions
+use GNU `which', which is a C program that can understand shell
+aliases.
+
+The csh script version reads the csh startup files from your
+home directory and uses those to determine which `command' will
+be invoked. Since bash doesn't use any of those startup files,
+there's a good chance that your bash environment differs from
+your csh environment. The bash `type' builtin does everything
+`which' does, and will report correct results for the running
+shell. If you're really wedded to the name `which', try adding
+the following function definition to your .bashrc:
+
+ which()
+ {
+ builtin type "$@"
+ }
+
+If you're moving from tcsh and would like to bring `where' along
+as well, use this function:
+
+ where()
+ {
+ builtin type -a "$@"
+ }
-17) Why doesn't bash treat brace expansions exactly like csh?
+D2) Why doesn't bash treat brace expansions exactly like csh?
The only difference between bash and csh brace expansion is that
bash requires a brace expression to contain at least one unquoted
Bash, ksh, zsh, and pd-ksh all implement brace expansion this way.
-18) Why doesn't bash have csh variable modifiers?
+D3) Why doesn't bash have csh variable modifiers?
Posix has specified a more powerful, albeit somewhat more cryptic,
mechanism cribbed from ksh, and bash implements it.
$b:e ${b##*.} xxx
-19) How can I make my csh aliases work when I convert to bash?
+D4) How can I make my csh aliases work when I convert to bash?
Bash uses a different syntax to support aliases than csh does.
The details can be found in the documentation. We have provided
a shell script which does most of the work of conversion for you;
-this script can be found in ./examples/misc/alias-conv.sh. Here is
+this script can be found in ./examples/misc/aliasconv.sh. Here is
how you use it:
Start csh in the normal way for you. (e.g., `csh')
-Pipe the output of `alias' through `alias-conv.sh', saving the
+Pipe the output of `alias' through `aliasconv.sh', saving the
results into `bash_aliases':
- alias | alias-conv.sh >bash_aliases
+ alias | bash aliasconv.sh >bash_aliases
Edit `bash_aliases', carefully reading through any created
functions. You will need to change the names of some csh specific
environment, or as `cshtobash ~/.login' to convert your login
environment.
-20) How can I pipe standard output and standard error from one command to
+D5) How can I pipe standard output and standard error from one command to
another, like csh does with `|&'?
Use
file descriptor 1 points to the pipe when it is duplicated onto file
descriptor 2.
-21) Now that I've converted from ksh to bash, are there equivalents to
+D6) Now that I've converted from ksh to bash, are there equivalents to
ksh features like autoloaded functions and the `whence' command?
-There are features in ksh-88 that do not have direct bash equivalents.
-Most, however, can be emulated with very little trouble.
+There are features in ksh-88 and ksh-93 that do not have direct bash
+equivalents. Most, however, can be emulated with very little trouble.
ksh-88 feature Bash equivalent
-------------- ---------------
autoloaded functions examples/functions/autoload is the same as typeset -fu
read var?prompt read -p prompt var
+ksh-93 feature Bash equivalent
+-------------- ---------------
+sleep, getconf Bash has loadable versions in examples/loadables
+${.sh.version} $BASH_VERSION
+print -f printf
+hist alias hist=fc
+$HISTEDIT $FCEDIT
+
Section E: How can I get bash to do certain things, and why does bash do
things the way it does?
-22) Why is the bash builtin `test' slightly different from /bin/test?
+E1) Why is the bash builtin `test' slightly different from /bin/test?
The specific example used here is [ ! x -o x ], which is false.
As you can see, the test becomes (not (x or x)), which is false.
-23) Why does bash sometimes say `Broken pipe'?
+E2) Why does bash sometimes say `Broken pipe'?
If a sequence of commands appears in a pipeline, and one of the
reading commands finishes before the writer has finished, the
will print `Broken pipe' to stderr when ps is killed by a
SIGPIPE.
-24) How can I get bash to read and display eight-bit characters?
-
-This is a process requiring several steps.
+As of bash-3.1, bash does not report SIGPIPE errors by default. You
+can build a version of bash that will report such errors.
-First, you must ensure that the `physical' data path is a full eight
-bits. For xterms, for example, the `vt100' resources `eightBitInput'
-and `eightBitOutput' should be set to `true'.
+E3) When I have terminal escape sequences in my prompt, why does bash
+ wrap lines at the wrong column?
-Once you have set up an eight-bit path, you must tell the kernel and
-tty driver to leave the eighth bit of characters alone when processing
-keyboard input. Use `stty' to do this:
+Readline, the line editing library that bash uses, does not know
+that the terminal escape sequences do not take up space on the
+screen. The redisplay code assumes, unless told otherwise, that
+each character in the prompt is a `printable' character that
+takes up one character position on the screen.
- stty cs8 -istrip -parenb
+You can use the bash prompt expansion facility (see the PROMPTING
+section in the manual page) to tell readline that sequences of
+characters in the prompt strings take up no screen space.
-For old BSD-style systems, you can use
+Use the \[ escape to begin a sequence of non-printing characters,
+and the \] escape to signal the end of such a sequence.
- stty pass8
+E4) If I pipe the output of a command into `read variable', why doesn't
+ the output show up in $variable when the read command finishes?
-You may also need
-
- stty even odd
-
-Finally, you need to tell readline that you will be inputting and
-displaying eight-bit characters. You use readline variables to do
-this. These variables can be set in your .inputrc or using the bash
-`bind' builtin. Here's an example using `bind':
-
- bash$ bind 'set convert-meta off'
- bash$ bind 'set meta-flag on'
- bash$ bind 'set output-meta on'
-
-The `set' commands between the single quotes may also be placed
-in ~/.inputrc.
-
-25) How do I write a function `x' to replace builtin command `x', but
- still invoke the command from within the function?
-
-This is why the `command' and `builtin' builtins exist. The
-`command' builtin executes the command supplied as its first
-argument, skipping over any function defined with that name. The
-`builtin' builtin executes the builtin command given as its first
-argument directly.
-
-For example, to write a function to replace `cd' that writes the
-hostname and current directory to an xterm title bar, use
-something like the following:
-
- cd()
- {
- builtin cd "$@" && xtitle "$HOST: $PWD"
- }
-
-This could also be written using `command' instead of `builtin';
-the version above is marginally more efficient.
-
-26) When I have terminal escape sequences in my prompt, why does bash
- wrap lines at the wrong column?
-
-Readline, the line editing library that bash uses, does not know
-that the terminal escape sequences do not take up space on the
-screen. The redisplay code assumes, unless told otherwise, that
-each character in the prompt is a `printable' character that
-takes up one character position on the screen.
-
-You can use the bash prompt expansion facility (see the PROMPTING
-section in the manual page) to tell readline that sequences of
-characters in the prompt strings take up no screen space.
-
-Use the \[ escape to begin a sequence of non-printing characters,
-and the \] escape to signal the end of such a sequence.
-
-27) How can I find the value of a shell variable whose name is the value
- of another shell variable?
-
-Bash-2.02 supports this directly. You can use
-
- ${!var}
-
-For example, the following sequence of commands will echo `z':
-
- var1=var2
- var2=z
- echo ${!var1}
-
-For sh compatibility, use the `eval' builtin. The important
-thing to remember is that `eval' expands the arguments you give
-it again, so you need to quote the parts of the arguments that
-you want `eval' to act on.
-
-For example, this expression prints the value of the last positional
-parameter:
-
- eval echo \"\$\{$#\}\"
-
-The expansion of the quoted portions of this expression will be
-deferred until `eval' runs, while the `$#' will be expanded
-before `eval' is executed. In bash-2.02,
-
- echo ${!#}
-
-does the same thing.
-
-28) If I pipe the output of a command into `read variable', why doesn't
- the output show up in $variable when the read command finishes?
-
-This has to do with the parent-child relationship between Unix
-processes.
-
-Each element of a pipeline runs in a separate process, a child of
-the shell running the pipeline. A subprocess cannot affect its
-parent's environment. When the `read' command sets the variable
-to the input, that variable is set only in the subshell, not the
-parent shell. When the subshell exits, the value of the variable
-is lost.
+This has to do with the parent-child relationship between Unix
+processes. It affects all commands run in pipelines, not just
+simple calls to `read'. For example, piping a command's output
+into a `while' loop that repeatedly calls `read' will result in
+the same behavior.
+
+Each element of a pipeline, even a builtin or shell function,
+runs in a separate process, a child of the shell running the
+pipeline. A subprocess cannot affect its parent's environment.
+When the `read' command sets the variable to the input, that
+variable is set only in the subshell, not the parent shell. When
+the subshell exits, the value of the variable is lost.
Many pipelines that end with `read variable' can be converted
into command substitutions, which will capture the output of
This is the general approach -- in most cases you will not need to
set $IFS to a different value.
-29) I have a bunch of shell scripts that use backslash-escaped characters
+Some other user-supplied alternatives include:
+
+read A B C D << HERE
+ $(IFS=.; echo $(/usr/local/bin/ipaddr))
+HERE
+
+and, where process substitution is available,
+
+read A B C D < <(IFS=.; echo $(/usr/local/bin/ipaddr))
+
+E5) I have a bunch of shell scripts that use backslash-escaped characters
in arguments to `echo'. Bash doesn't interpret these characters. Why
not, and how can I make it understand them?
This is the behavior of echo on most Unix System V machines.
-The bash builtin `echo' is modelled after the 9th Edition
+The bash builtin `echo' is modeled after the 9th Edition
Research Unix version of `echo'. It does not interpret
backslash-escaped characters in its argument strings by default;
it requires the use of the -e option to enable the
There is a configuration option that will make bash behave like
the System V echo and interpret things like `\t' by default. Run
-configure with the --enable-usg-echo-default option to turn this
+configure with the --enable-xpg-echo-default option to turn this
on. Be aware that this will cause some of the tests run when you
type `make tests' to fail.
-30) Why doesn't a while or for loop get suspended when I type ^Z?
+There is a shell option, `xpg_echo', settable with `shopt', that will
+change the behavior of echo at runtime. Enabling this option turns
+on expansion of backslash-escape sequences.
+
+E6) Why doesn't a while or for loop get suspended when I type ^Z?
This is a consequence of how job control works on Unix. The only
thing that can be suspended is the process group. This is a single
within parentheses, which will force the loop into a subshell that
may be stopped (and subsequently restarted) as a single unit.
-31) How can I make the bash `time' reserved word print timing output that
- looks like the output from my system's /usr/bin/time?
+E7) What about empty for loops in Makefiles?
-The bash command timing code looks for a variable `TIMEFORMAT' and
-uses its value as a format string to decide how to display the
-timing statistics.
+It's fairly common to see constructs like this in automatically-generated
+Makefiles:
-The value of TIMEFORMAT is a string with `%' escapes expanded in a
-fashion similar in spirit to printf(3). The manual page explains
-the meanings of the escape sequences in the format string.
+SUBDIRS = @SUBDIRS@
-If TIMEFORMAT is not set, bash acts as if the following assignment had
-been performed:
+ ...
- TIMEFORMAT=$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
+subdirs-clean:
+ for d in ${SUBDIRS}; do \
+ ( cd $$d && ${MAKE} ${MFLAGS} clean ) \
+ done
-The POSIX.2 default time format (used by `time -p command') is
+When SUBDIRS is empty, this results in a command like this being passed to
+bash:
- TIMEFORMAT=$'real %2R\nuser %2U\nsys %2S'
+ for d in ; do
+ ( cd $d && ${MAKE} ${MFLAGS} clean )
+ done
-The BSD /usr/bin/time format can be emulated with:
+In versions of bash before bash-2.05a, this was a syntax error. If the
+reserved word `in' was present, a word must follow it before the semicolon
+or newline. The language in the manual page referring to the list of words
+being empty referred to the list after it is expanded. These versions of
+bash required that there be at least one word following the `in' when the
+construct was parsed.
- TIMEFORMAT=$'\t%1R real\t%1U user\t%1S sys'
+The idiomatic Makefile solution is something like:
-The System V /usr/bin/time format can be emulated with:
+SUBDIRS = @SUBDIRS@
- TIMEFORMAT=$'\nreal\t%1R\nuser\t%1U\nsys\t%1S'
+subdirs-clean:
+ subdirs=$SUBDIRS ; for d in $$subdirs; do \
+ ( cd $$d && ${MAKE} ${MFLAGS} clean ) \
+ done
-The ksh format can be emulated with:
+The latest updated POSIX standard has changed this: the word list
+is no longer required. Bash versions 2.05a and later accept the
+new syntax.
- TIMEFORMAT=$'\nreal\t%2lR\nuser\t%2lU\nsys\t%2lS'
+E8) Why does the arithmetic evaluation code complain about `08'?
+
+The bash arithmetic evaluation code (used for `let', $(()), (()), and in
+other places), interprets a leading `0' in numeric constants as denoting
+an octal number, and a leading `0x' as denoting hexadecimal. This is
+in accordance with the POSIX.2 spec, section 2.9.2.1, which states that
+arithmetic constants should be handled as signed long integers as defined
+by the ANSI/ISO C standard.
+
+The POSIX.2 interpretation committee has confirmed this:
+
+http://www.pasc.org/interps/unofficial/db/p1003.2/pasc-1003.2-173.html
+
+E9) Why does the pattern matching expression [A-Z]* match files beginning
+ with every letter except `z'?
+
+Bash-2.03, Bash-2.05 and later versions honor the current locale setting
+when processing ranges within pattern matching bracket expressions ([A-Z]).
+This is what POSIX.2 and SUSv3/XPG6 specify.
+
+The behavior of the matcher in bash-2.05 and later versions depends on the
+current LC_COLLATE setting. Setting this variable to `C' or `POSIX' will
+result in the traditional behavior ([A-Z] matches all uppercase ASCII
+characters). Many other locales, including the en_US locale (the default
+on many US versions of Linux) collate the upper and lower case letters like
+this:
+
+ AaBb...Zz
+
+which means that [A-Z] matches every letter except `z'. Others collate like
+
+ aAbBcC...zZ
+
+which means that [A-Z] matches every letter except `a'.
+
+The portable way to specify upper case letters is [:upper:] instead of
+A-Z; lower case may be specified as [:lower:] instead of a-z.
+
+Look at the manual pages for setlocale(3), strcoll(3), and, if it is
+present, locale(1). If you have locale(1), you can use it to find
+your current locale information even if you do not have any of the
+LC_ variables set.
+
+My advice is to put
+
+ export LC_COLLATE=C
+
+into /etc/profile and inspect any shell scripts run from cron for
+constructs like [A-Z]. This will prevent things like
+
+ rm [A-Z]*
+
+from removing every file in the current directory except those beginning
+with `z' and still allow individual users to change the collation order.
+Users may put the above command into their own profiles as well, of course.
+
+E10) Why does `cd //' leave $PWD as `//'?
+
+POSIX.2, in its description of `cd', says that *three* or more leading
+slashes may be replaced with a single slash when canonicalizing the
+current working directory.
+
+This is, I presume, for historical compatibility. Certain versions of
+Unix, and early network file systems, used paths of the form
+//hostname/path to access `path' on server `hostname'.
+
+E11) If I resize my xterm while another program is running, why doesn't bash
+ notice the change?
+
+This is another issue that deals with job control.
+
+The kernel maintains a notion of a current terminal process group. Members
+of this process group (processes whose process group ID is equal to the
+current terminal process group ID) receive terminal-generated signals like
+SIGWINCH. (For more details, see the JOB CONTROL section of the bash
+man page.)
+
+If a terminal is resized, the kernel sends SIGWINCH to each member of
+the terminal's current process group (the `foreground' process group).
+
+When bash is running with job control enabled, each pipeline (which may be
+a single command) is run in its own process group, different from bash's
+process group. This foreground process group receives the SIGWINCH; bash
+does not. Bash has no way of knowing that the terminal has been resized.
+
+There is a `checkwinsize' option, settable with the `shopt' builtin, that
+will cause bash to check the window size and adjust its idea of the
+terminal's dimensions each time a process stops or exits and returns control
+of the terminal to bash. Enable it with `shopt -s checkwinsize'.
+
+E12) Why don't negative offsets in substring expansion work like I expect?
+
+When substring expansion of the form ${param:offset[:length} is used,
+an `offset' that evaluates to a number less than zero counts back from
+the end of the expanded value of $param.
+
+When a negative `offset' begins with a minus sign, however, unexpected things
+can happen. Consider
+
+ a=12345678
+ echo ${a:-4}
+
+intending to print the last four characters of $a. The problem is that
+${param:-word} already has a well-defined meaning: expand to word if the
+expanded value of param is unset or null, and $param otherwise.
+
+To use negative offsets that begin with a minus sign, separate the
+minus sign and the colon with a space.
+
+E13) Why does filename completion misbehave if a colon appears in the filename?
+
+Filename completion (and word completion in general) may appear to behave
+improperly if there is a colon in the word to be completed.
+
+The colon is special to readline's word completion code: it is one of the
+characters that breaks words for the completer. Readline uses these characters
+in sort of the same way that bash uses $IFS: they break or separate the words
+the completion code hands to the application-specific or default word
+completion functions. The original intent was to make it easy to edit
+colon-separated lists (such as $PATH in bash) in various applications using
+readline for input.
+
+This is complicated by the fact that some versions of the popular
+`bash-completion' programmable completion package have problems with the
+default completion behavior in the presence of colons.
+
+The current set of completion word break characters is available in bash as
+the value of the COMP_WORDBREAKS variable. Removing `:' from that value is
+enough to make the colon not special to completion:
+
+COMP_WORDBREAKS=${COMP_WORDBREAKS//:}
+
+You can also quote the colon with a backslash to achieve the same result
+temporarily.
+
+E14) Why does quoting the pattern argument to the regular expression matching
+ conditional operator (=~) cause regexp matching to stop working?
+
+In versions of bash prior to bash-3.2, the effect of quoting the regular
+expression argument to the [[ command's =~ operator was not specified.
+The practical effect was that double-quoting the pattern argument required
+backslashes to quote special pattern characters, which interfered with the
+backslash processing performed by double-quoted word expansion and was
+inconsistent with how the == shell pattern matching operator treated
+quoted characters.
+
+In bash-3.2, the shell was changed to internally quote characters in single-
+and double-quoted string arguments to the =~ operator, which suppresses the
+special meaning of the characters special to regular expression processing
+(`.', `[', `\', `(', `), `*', `+', `?', `{', `|', `^', and `$') and forces
+them to be matched literally. This is consistent with how the `==' pattern
+matching operator treats quoted portions of its pattern argument.
+
+Since the treatment of quoted string arguments was changed, several issues
+have arisen, chief among them the problem of white space in pattern arguments
+and the differing treatment of quoted strings between bash-3.1 and bash-3.2.
+Both problems may be solved by using a shell variable to hold the pattern.
+Since word splitting is not performed when expanding shell variables in all
+operands of the [[ command, this allows users to quote patterns as they wish
+when assigning the variable, then expand the values to a single string that
+may contain whitespace. The first problem may be solved by using backslashes
+or any other quoting mechanism to escape the white space in the patterns.
+
+Bash-4.0 introduces the concept of a `compatibility level', controlled by
+several options to the `shopt' builtin. If the `compat31' option is enabled,
+bash reverts to the bash-3.1 behavior with respect to quoting the rhs of
+the =~ operator.
+
+E15) Tell me more about the shell compatibility level.
+
+Bash-4.0 introduced the concept of a `shell compatibility level', specified
+as a set of options to the shopt builtin (compat31, compat32, compat40 at
+this writing). There is only one current compatibility level -- each
+option is mutually exclusive. This list does not mention behavior that is
+standard for a particular version (e.g., setting compat32 means that quoting
+the rhs of the regexp matching operator quotes special regexp characters in
+the word, which is default behavior in bash-3.2 and above).
+
+compat31 set
+ - the < and > operators to the [[ command do not consider the current
+ locale when comparing strings
+ - quoting the rhs of the regexp matching operator (=~) has no
+ special effect
+
+compat32 set
+ - the < and > operators to the [[ command do not consider the current
+ locale when comparing strings
+
+compat40 set
+ - the < and > operators to the [[ command do not consider the current
+ locale when comparing strings
+ - interrupting a command list such as "a ; b ; c" causes the execution
+ of the entire list to be aborted (in versions before bash-4.0,
+ interrupting one command in a list caused the next to be executed)
+
+compat41 set
+ - interrupting a command list such as "a ; b ; c" causes the execution
+ of the entire list to be aborted (in versions before bash-4.1,
+ interrupting one command in a list caused the next to be executed)
+ - when in posix mode, single quotes in the `word' portion of a
+ double-quoted parameter expansion define a new quoting context and
+ are treated specially
+
+compat42 set
+ - the replacement string in double-quoted pattern substitution is not
+ run through quote removal, as in previous versions
Section F: Things to watch out for on certain Unix versions
-32) Why can't I use command line editing in my `cmdtool'?
+F1) Why can't I use command line editing in my `cmdtool'?
The problem is `cmdtool' and bash fighting over the input. When
scrolling is enabled in a cmdtool window, cmdtool puts the tty in
cmdtool. If this works, you can put the assignment to TERMCAP
in your bashrc file.
-33) I built bash on Solaris 2. Why do globbing expansions and filename
+F2) I built bash on Solaris 2. Why do globbing expansions and filename
completion chop off the first few characters of each filename?
This is the consequence of building bash on SunOS 5 and linking
put /usr/ccs/bin and /opt/SUNWspro/bin into your $PATH before
/usr/ucb.
-34) Why does bash dump core after I interrupt username completion or
+F3) Why does bash dump core after I interrupt username completion or
`~user' tilde expansion on a machine running NIS?
This is a famous and long-standing bug in the SunOS YP (sorry, NIS)
run configure with the `--without-gnu-malloc' option to use
the C library malloc and avoid the problem.
-35) I'm running SVR4.2. Why is the line erased every time I type `@'?
+F4) I'm running SVR4.2. Why is the line erased every time I type `@'?
The `@' character is the default `line kill' character in most
versions of System V, including SVR4.2. You can change this
where the `^' and `U' can be two separate characters.
-36) Why does bash report syntax errors when my C News scripts use a
+F5) Why does bash report syntax errors when my C News scripts use a
redirection before a subshell command?
The actual command in question is something like
commands'. A subshell construct such as the above is one of the shell's
`compound commands'. A redirection may only follow a compound command.
-The file CWRU/sh-redir-hack in the bash-2.02 distribution is an
+This affects the mechanical transformation of commands that use `cat'
+to pipe a file into a command (a favorite Useless-Use-Of-Cat topic on
+comp.unix.shell). While most commands of the form
+
+ cat file | command
+
+can be converted to `< file command', shell control structures such as
+loops and subshells require `command < file'.
+
+The file CWRU/sh-redir-hack in the bash distribution is an
(unofficial) patch to parse.y that will modify the grammar to
support this construct. It will not apply with `patch'; you must
modify parse.y by hand. Note that if you apply this, you must
recompile with -DREDIRECTION_HACK. This introduces a large
number of reduce/reduce conflicts into the shell grammar.
-Section G: Where do I go from here?
+F6) Why can't I use vi-mode editing on Red Hat Linux 6.1?
+
+The short answer is that Red Hat screwed up.
+
+The long answer is that they shipped an /etc/inputrc that only works
+for emacs mode editing, and then screwed all the vi users by setting
+INPUTRC to /etc/inputrc in /etc/profile.
-37) How do I report bugs in bash, and where should I look for fixes and
+The short fix is to do one of the following: remove or rename
+/etc/inputrc, set INPUTRC=~/.inputrc in ~/.bashrc (or .bash_profile,
+but make sure you export it if you do), remove the assignment to
+INPUTRC from /etc/profile, add
+
+ set keymap emacs
+
+to the beginning of /etc/inputrc, or bracket the key bindings in
+/etc/inputrc with these lines
+
+ $if mode=emacs
+ [...]
+ $endif
+
+F7) Why do bash-2.05a and bash-2.05b fail to compile `printf.def' on
+ HP/UX 11.x?
+
+HP/UX's support for long double is imperfect at best.
+
+GCC will support it without problems, but the HP C library functions
+like strtold(3) and printf(3) don't actually work with long doubles.
+HP implemented a `long_double' type as a 4-element array of 32-bit
+ints, and that is what the library functions use. The ANSI C
+`long double' type is a 128-bit floating point scalar.
+
+The easiest fix, until HP fixes things up, is to edit the generated
+config.h and #undef the HAVE_LONG_DOUBLE line. After doing that,
+the compilation should complete successfully.
+
+Section G: How can I get bash to do certain common things?
+
+G1) How can I get bash to read and display eight-bit characters?
+
+This is a process requiring several steps.
+
+First, you must ensure that the `physical' data path is a full eight
+bits. For xterms, for example, the `vt100' resources `eightBitInput'
+and `eightBitOutput' should be set to `true'.
+
+Once you have set up an eight-bit path, you must tell the kernel and
+tty driver to leave the eighth bit of characters alone when processing
+keyboard input. Use `stty' to do this:
+
+ stty cs8 -istrip -parenb
+
+For old BSD-style systems, you can use
+
+ stty pass8
+
+You may also need
+
+ stty even odd
+
+Finally, you need to tell readline that you will be inputting and
+displaying eight-bit characters. You use readline variables to do
+this. These variables can be set in your .inputrc or using the bash
+`bind' builtin. Here's an example using `bind':
+
+ bash$ bind 'set convert-meta off'
+ bash$ bind 'set meta-flag on'
+ bash$ bind 'set output-meta on'
+
+The `set' commands between the single quotes may also be placed
+in ~/.inputrc.
+
+The script examples/scripts.noah/meta.bash encapsulates the bind
+commands in a shell function.
+
+G2) How do I write a function `x' to replace builtin command `x', but
+ still invoke the command from within the function?
+
+This is why the `command' and `builtin' builtins exist. The
+`command' builtin executes the command supplied as its first
+argument, skipping over any function defined with that name. The
+`builtin' builtin executes the builtin command given as its first
+argument directly.
+
+For example, to write a function to replace `cd' that writes the
+hostname and current directory to an xterm title bar, use
+something like the following:
+
+ cd()
+ {
+ builtin cd "$@" && xtitle "$HOST: $PWD"
+ }
+
+This could also be written using `command' instead of `builtin';
+the version above is marginally more efficient.
+
+G3) How can I find the value of a shell variable whose name is the value
+ of another shell variable?
+
+Versions of Bash newer than Bash-2.0 support this directly. You can use
+
+ ${!var}
+
+For example, the following sequence of commands will echo `z':
+
+ var1=var2
+ var2=z
+ echo ${!var1}
+
+For sh compatibility, use the `eval' builtin. The important
+thing to remember is that `eval' expands the arguments you give
+it again, so you need to quote the parts of the arguments that
+you want `eval' to act on.
+
+For example, this expression prints the value of the last positional
+parameter:
+
+ eval echo \"\$\{$#\}\"
+
+The expansion of the quoted portions of this expression will be
+deferred until `eval' runs, while the `$#' will be expanded
+before `eval' is executed. In versions of bash later than bash-2.0,
+
+ echo ${!#}
+
+does the same thing.
+
+This is not the same thing as ksh93 `nameref' variables, though the syntax
+is similar. Namerefs are available bash version 4.3, and work as in ksh93.
+
+G4) How can I make the bash `time' reserved word print timing output that
+ looks like the output from my system's /usr/bin/time?
+
+The bash command timing code looks for a variable `TIMEFORMAT' and
+uses its value as a format string to decide how to display the
+timing statistics.
+
+The value of TIMEFORMAT is a string with `%' escapes expanded in a
+fashion similar in spirit to printf(3). The manual page explains
+the meanings of the escape sequences in the format string.
+
+If TIMEFORMAT is not set, bash acts as if the following assignment had
+been performed:
+
+ TIMEFORMAT=$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
+
+The POSIX.2 default time format (used by `time -p command') is
+
+ TIMEFORMAT=$'real %2R\nuser %2U\nsys %2S'
+
+The BSD /usr/bin/time format can be emulated with:
+
+ TIMEFORMAT=$'\t%1R real\t%1U user\t%1S sys'
+
+The System V /usr/bin/time format can be emulated with:
+
+ TIMEFORMAT=$'\nreal\t%1R\nuser\t%1U\nsys\t%1S'
+
+The ksh format can be emulated with:
+
+ TIMEFORMAT=$'\nreal\t%2lR\nuser\t%2lU\nsys\t%2lS'
+
+G5) How do I get the current directory into my prompt?
+
+Bash provides a number of backslash-escape sequences which are expanded
+when the prompt string (PS1 or PS2) is displayed. The full list is in
+the manual page.
+
+The \w expansion gives the full pathname of the current directory, with
+a tilde (`~') substituted for the current value of $HOME. The \W
+expansion gives the basename of the current directory. To put the full
+pathname of the current directory into the path without any tilde
+subsitution, use $PWD. Here are some examples:
+
+ PS1='\w$ ' # current directory with tilde
+ PS1='\W$ ' # basename of current directory
+ PS1='$PWD$ ' # full pathname of current directory
+
+The single quotes are important in the final example to prevent $PWD from
+being expanded when the assignment to PS1 is performed.
+
+G6) How can I rename "*.foo" to "*.bar"?
+
+Use the pattern removal functionality described in D3. The following `for'
+loop will do the trick:
+
+ for f in *.foo; do
+ mv $f ${f%foo}bar
+ done
+
+G7) How can I translate a filename from uppercase to lowercase?
+
+The script examples/functions/lowercase, originally written by John DuBois,
+will do the trick. The converse is left as an exercise.
+
+G8) How can I write a filename expansion (globbing) pattern that will match
+ all files in the current directory except "." and ".."?
+
+You must have set the `extglob' shell option using `shopt -s extglob' to use
+this:
+
+ echo .!(.|) *
+
+A solution that works without extended globbing is given in the Unix Shell
+FAQ, posted periodically to comp.unix.shell. It's a variant of
+
+ echo .[!.]* ..?* *
+
+(The ..?* catches files with names of three or more characters beginning
+with `..')
+
+Section H: Where do I go from here?
+
+H1) How do I report bugs in bash, and where should I look for fixes and
advice?
Use the `bashbug' script to report bugs. It is built and
template for reporting a problem and automatically includes
information about your configuration and build environment.
-`bashbug' sends its reports to bug-bash@prep.ai.mit.edu, which
+`bashbug' sends its reports to bug-bash@gnu.org, which
is a large mailing list gatewayed to the usenet newsgroup gnu.bash.bug.
Bug fixes, answers to questions, and announcements of new releases
and problems also take place there.
To reach the bash maintainers directly, send mail to
-bash-maintainers@prep.ai.mit.edu.
+bash-maintainers@gnu.org.
-38) What kind of bash documentation is there?
+H2) What kind of bash documentation is there?
First, look in the doc directory in the bash distribution. It should
contain at least the following files:
bash.1 an extensive, thorough Unix-style manual page
builtins.1 a manual page covering just bash builtin commands
-bashref.texi a reference manual in GNU info format
-bash.html an HTML version of the manual page
-bashref.html an HTML version of the reference manual
+bashref.texi a reference manual in GNU tex`info format
+bashref.info an info version of the reference manual
FAQ this file
article.ms text of an article written for The Linux Journal
readline.3 a man page describing readline
-Postscript files created from the above source are available in
-the documentation distribution.
+Postscript, HTML, and ASCII files created from the above source are
+available in the documentation distribution.
There is additional documentation available for anonymous FTP from host
ftp.cwru.edu in the `pub/bash' directory.
Cameron Newham and Bill Rosenblatt have written a book on bash, published
by O'Reilly and Associates. The book is based on Bill Rosenblatt's Korn
Shell book. The title is ``Learning the Bash Shell'', and the ISBN number
-is 1-56592-147-X. Look for it in fine bookstores near you. This book
-covers bash-1.14, but has an appendix describing some of the new features
-in bash-2.0.
+of the third edition, published in March, 2005, is 0-596-00965-8. Look for
+it in fine bookstores near you. This edition of the book has been updated
+to cover bash-3.0.
+
+The GNU Bash Reference Manual has been published as a printed book by
+Network Theory Ltd (Paperback, ISBN: 0-9541617-7-7, Nov. 2006). It covers
+bash-3.2 and is available from most online bookstores (see
+http://www.network-theory.co.uk/bash/manual/ for details). The publisher
+will donate $1 to the Free Software Foundation for each copy sold.
-A second edition of this book is available, published in January, 1998.
-The ISBN number is 1-56592-347-2. Look for it in the same fine bookstores
-or on the web.
+Arnold Robbins and Nelson Beebe have written ``Classic Shell Scripting'',
+published by O'Reilly. The first edition, with ISBN number 0-596-00595-4,
+was published in May, 2005.
-39) What's coming in future versions?
+Chris F. A. Johnson, a frequent contributor to comp.unix.shell and
+gnu.bash.bug, has written ``Shell Scripting Recipes: A Problem-Solution
+Approach,'' a new book on shell scripting, concentrating on features of
+the POSIX standard helpful to shell script writers. The first edition from
+Apress, with ISBN number 1-59059-471-1, was published in May, 2005.
-These are features I plan to include in a future version of bash.
+H3) What's coming in future versions?
-a bash debugger (a minimally-tested version is included with bash-2.02)
-Programmable completion a la zsh
+These are features I hope to include in a future version of bash.
-40) What's on the bash `wish list' for future versions?
+Rocky Bernstein's bash debugger (support is included with bash-4.0)
+
+H4) What's on the bash `wish list' for future versions?
These are features that may or may not appear in a future version of bash.
-associative arrays (not really all that hard)
breaking some of the shell functionality into embeddable libraries
-better internationalization using GNU `gettext'
-an option to use external files for the long `help' text
-timeouts for the `read' builtin
-the ksh-93 ${!prefix*} and ${!prefix@} operators
-arithmetic ++ and -- prefix and postfix operators
-date-stamped command history
-
-41) When will the next release appear?
+a module system like zsh's, using dynamic loading like builtins
+a bash programmer's guide with a chapter on creating loadable builtins
+a better loadable interface to perl with access to the shell builtins and
+ variables (contributions gratefully accepted)
+ksh93-like `xx.yy' variables (including some of the .sh.* variables) and
+ associated disipline functions
+Some of the new ksh93 pattern matching operators, like backreferencing
-The next version will appear sometime in 1998. Never make
-predictions.
+H5) When will the next release appear?
+The next version will appear sometime in 2015. Never make predictions.
-This document is Copyright 1995, 1996, 1998 by Chester Ramey.
+This document is Copyright 1995-2014 by Chester Ramey.
Permission is hereby granted, without written agreement and
without license or royalty fees, to use, copy, and distribute
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