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This is the Bash FAQ, version 4.15, for Bash version 5.1. [This document is no longer maintained.] This document contains a set of frequently-asked questions concerningBash, the GNU Bourne-Again Shell. Bash is a freely-available commandinterpreter with advanced features for both interactive use and shellprogramming. Another good source of basic information about shells is the collectionof FAQ articles periodically posted to comp.unix.shell. Questions and comments concerning this document should be sent tochet.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 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 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 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? D1) Why does bash run a different version of `command' than `which command' says it will?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 `|&'?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: Why does bash do certain things the way it does? 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?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?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?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 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?F3) Why does bash dump core after I interrupt username completion or `~user' tilde expansion on a machine running NIS?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 H: Where do I go from here? H1) How do I report bugs in bash, and where should I look for fixes and advice?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 A1) What is it? Bash is a Unix command interpreter (shell). It is an implementation ofthe Posix 1003.2 shell standard, and resembles the Korn and System Vshells. Bash contains a number of enhancements over those shells, bothfor interactive use and shell programming. Features gearedtoward interactive use include command line editing, commandhistory, job control, aliases, and prompt expansion. Programmingfeatures include additional variable expansions, shellarithmetic, and a number of variables and options to controlshell behavior. Bash was originally written by Brian Fox of the Free SoftwareFoundation. The current developer and maintainer is Chet Rameyof Case Western Reserve University. A2) What's the latest version? The latest version is 4.3, first made available on 26 February, 2014. A3) Where can I get it? Bash is the GNU project's shell, and so is available from themaster GNU archive site, ftp.gnu.org, and its mirrors. Thelatest version is also available for FTP from ftp.cwru.edu.The following URLs tell how to get version 4.3: ftp://ftp.gnu.org/pub/gnu/bash/bash-4.3.tar.gzftp://ftp.cwru.edu/pub/bash/bash-4.3.tar.gz Formatted versions of the documentation are available with the URLs: ftp://ftp.gnu.org/pub/gnu/bash/bash-doc-4.3.tar.gzftp://ftp.cwru.edu/pub/bash/bash-doc-4.3.tar.gz Any patches for the current version are available with the URL: ftp://ftp.cwru.edu/pub/bash/bash-4.3-patches/ A4) On what machines will bash run? Bash has been ported to nearly every version of Unix. All youshould have to do to build it on a machine for which a portexists is to type `configure' and then `make'. The build processwill attempt to discover the version of Unix you have and tailoritself accordingly, using a script created by GNU autoconf. More information appears in the file `INSTALL' in the distribution. 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 majorcommercial Unix systems. A5) Will bash run on operating systems other than Unix? Configuration specifics for Unix-like systems such as QNX andLynxOS are included in the distribution. Bash-2.05 and laterversions should compile and run on Minix 2.0 (patches werecontributed), but I don't believe anyone has built bash-2.x onearlier Minix versions yet. Bash has been ported to versions of Windows implementing the Win32programming interface. This includes Windows 95 and Windows NT.The port was done by Cygnus Solutions (now part of Red Hat) as partof their CYGWIN project. For more information about the project, seehttp://www.cygwin.com/. Cygnus originally ported bash-1.14.7, and that port was part of theirearly GNU-Win32 (the original name) releases. Cygnus has also doneports of bash-3.2 and bash-4.0 to the CYGWIN environment, and bothare available as part of their current release. Bash-2.05b and later versions should require no local Cygnus changes tobuild and run under CYGWIN. DJ Delorie has a port of bash-2.x which runs under MS-DOS, as partof the DJGPP project. For more information on the project, see http://www.delorie.com/djgpp/ I have been told that the original DJGPP port was done by Daisuke Aoyama. Mark Elbrecht <snowball3@bigfoot.com> has sent me notice that bash-2.04is available for DJGPP V2. The files are available as: ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh204b.zip binaryftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh204d.zip documentationftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh204s.zip source Mark began to work with bash-2.05, but I don't know the current status. Bash-3.0 compiles and runs with no modifications under Microsoft's Servicesfor Unix (SFU), once known as Interix. I do not anticipate any problemswith building bash-4.2 and later, but will gladly accept any patches thatare needed. A6) How can I build bash with gcc? Bash configures to use gcc by default if it is available. Read thefile INSTALL in the distribution for more information. A7) How can I make bash my login shell? Some machines let you use `chsh' to change your login shell. Othersystems use `passwd -s' or `passwd -e'. If one of these works foryou, that's all you need. Note that many systems require the fullpathname to a shell to appear in /etc/shells before you can make ityour login shell. For this, you may need the assistance of yourfriendly local system administrator. If you cannot do this, you can still use bash as your login shell, butyou need to perform some tricks. The basic idea is to add a commandto your login shell's startup file to replace your login shell withbash. For example, if your login shell is csh or tcsh, and you have installedbash in /usr/gnu/bin/bash, add the following line to ~/.login: if ( -f /usr/gnu/bin/bash ) exec /usr/gnu/bin/bash --login (the `--login' tells bash that it is a login shell). It's not a good idea to put this command into ~/.cshrc, because everycsh you run without the `-f' option, even ones started to run csh scripts,reads that file. If you must put the command in ~/.cshrc, use somethinglike if ( $?prompt ) exec /usr/gnu/bin/bash --login to ensure that bash is exec'd only when the csh is interactive. If your login shell is sh or ksh, you have to do two things. First, create an empty file in your home directory named `.bash_profile'.The existence of this file will prevent the exec'd bash from trying toread ~/.profile, and re-execing itself over and over again. ~/.bash_profileis the first file bash tries to read initialization commands from whenit is invoked as a login shell. Next, add a line similar to the above to ~/.profile: [ -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 asa login shell. Once you have this working, you can copy your initializationcode from ~/.profile to ~/.bash_profile. I have received word that the recipe supplied above is insufficient formachines running CDE. CDE has a maze of twisty little startup files, allslightly different. If you cannot change your login shell in the password file to bash, youwill have to (apparently) live with CDE using the shell in the passwordfile 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 attemptsto do the right thing. It is, however, often broken, and may require thatyou use the $BASH_ENV trick described below. `dtterm' claims to use $SHELL as the default program to start, so if youcan change $SHELL in the CDE startup files, you should be able to use bashin your terminal windows. Setting DTSOURCEPROFILE in ~/.dtprofile will cause the `Xsession' programto read your login shell's startup files. You may be able to use bash forthe rest of the CDE programs by setting SHELL to bash in ~/.dtprofile aswell, but I have not tried this. You can use the above `exec' recipe to start bash when not logging in withCDE by testing the value of the DT variable: if [ -n "$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 shellstartup 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. Asnoted in the answer to the previous question, many systems requirethis before you can make bash your login shell. Most versions of ftpd use this file to prohibit `special' userssuch as `uucp' and `news' from using FTP. A9) What's the `POSIX Shell and Utilities standard'? POSIX is a name originally coined by Richard Stallman for afamily of open system standards based on UNIX. There are anumber of aspects of UNIX under consideration forstandardization, from the basic system services at the systemcall and C library level to applications and tools to systemadministration and management. Each area of standardization isassigned to a working group in the 1003 series. The POSIX Shell and Utilities standard was originally developed byIEEE Working Group 1003.2 (POSIX.2). Today it has been merged withthe original 1003.1 Working Group and is maintained by the AustinGroup (a joint working group of the IEEE, The Open Group andISO/IEC SC22/WG15). Today the Shell and Utilities are a volumewithin the set of documents that make up IEEE Std 1003.1-2001, andthus now the former POSIX.2 (from 1992) is now part of the currentPOSIX.1 standard (POSIX 1003.1-2001). The Shell and Utilities volume concentrates on the commandinterpreter interface and utility programs commonly executed fromthe command line or by other programs. The standard is freelyavailable on the web at http://www.UNIX-systems.org/version3/ . Work continues at the Austin Group on maintenance issues; seehttp://www.opengroup.org/austin/ to join the discussions. Bash is concerned with the aspects of the shell's behavior definedby the POSIX Shell and Utilities volume. The shell commandlanguage has of course been standardized, including the basic flowcontrol and program execution constructs, I/O redirection andpipelining, argument handling, variable expansion, and quoting. The `special' builtins, which must be implemented as part of theshell to provide the desired functionality, are specified asbeing part of the shell; examples of these are `eval' and`export'. Other utilities appear in the sections of POSIX notdevoted to the shell which are commonly (and in some cases mustbe) implemented as builtin commands, such as `read' and `test'. POSIX also specifies aspects of the shell's interactivebehavior as part of the UPE, including job control and commandline editing. Only vi-style line editing commands have beenstandardized; emacs editing commands were left out due toobjections. The latest version of the POSIX Shell and Utilities standard isavailable (now updated to the 2004 Edition) as part of the SingleUNIX Specification Version 3 at http://www.UNIX-systems.org/version3/ A10) What is the bash `posix mode'? Although bash is an implementation of the POSIX shellspecification, there are areas where the bash default behaviordiffers from that spec. The bash `posix mode' changes the bashbehavior in these areas so that it obeys the spec more closely. 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 isactive 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 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 forcomplete descriptions and the CHANGES and NEWS files in the bash-4.3distribution): 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 configure 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. 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 spaceso A new loadable builtin, realpath, which canonicalizes and expands symlinks in pathname arguments.o When `set' is called without options, it prints function definitions 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-exampleso `history' has a new `-d' option to delete a history entryo `bind' has a new `-x' option to bind key sequences to shell commandso The prompt expansion code has new `\j' and `\l' escape sequenceso The `no_empty_cmd_completion' shell option, if enabled, inhibits command completion when TAB is typed on an empty lineo `help' has a new `-s' option to print a usage synopsiso New arithmetic operators: var++, var--, ++var, --var, expr1,expr2 (comma)o New ksh93-style arithmetic for command: for ((expr1 ; expr2; expr3 )); do list; doneo `read' has new options: `-t', `-n', `-d', `-s'o The redirection code handles several filenames specially: /dev/fd/N, /dev/stdin, /dev/stdout, /dev/stderro 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 hosto The ${!prefix*} expansion has been implementedo A new FUNCNAME variable, which expands to the name of a currently-executing functiono The GROUPS variable is no longer readonlyo A new shopt `xpg_echo' variable, to control the behavior of echo with respect to backslash-escape sequences at runtimeo The NON_INTERACTIVE_LOGIN_SHELLS #define has returned The version of Readline released with Bash-2.04, Readline-4.1, had severalnew features as well: o Parentheses matching is always compiled into readline, and controllable with the new `blink-matching-paren' variableo 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-historyo 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 conventionthat odd-numbered releases provide mainly bug fixes. A number of newfeatures were added to Readline, mostly at the request of the Cygnusfolks. A new shopt option, `restricted_shell', so that startup files can test whether or not the shell was started in restricted modeFilename 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 requiresENV and BASH_ENV are read-only variables in a restricted shellBash may now be linked against an already-installed Readline library, as long as the Readline library is version 4 or newerAll 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 releasedalong with Bash-2.03. For a complete list of the changes, read the fileCHANGES 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 with memory usage, does not `orphan' large blocks when they are freed, is usable on 64-bit machines, and has allocation checking turned on unconditionallyPOSIX.2-style globbing character classes ([:alpha:], [:alnum:], etc.)POSIX.2-style globbing equivalence classesPOSIX.2-style globbing collating symbolsthe ksh [[...]] extended conditional commandthe ksh egrep-style extended pattern matching operatorsa new `printf' builtinthe ksh-like $(<filename) command substitution, which is equivalent to $(cat filename)new tilde prefixes that expand to directories from the directory stacknew `**' arithmetic operator to do exponentiationcase-insensitive globbing (filename expansion)menu completion a la tcsh`magic-space' history expansion function like tcshthe readline inputrc `language' has a new file inclusion directive ($include) Bash-2.01 contained only a few new features: new `GROUPS' builtin array variable containing the user's group listnew bindable readline commands: history-and-alias-expand-line and alias-expand-line Bash-2.0 contained extensive changes and new features from bash-1.14.7.Here's a short list: new `time' reserved word to time pipelines, shell builtins, and shell functionsone-dimensional arrays with a new compound assignment statement, appropriate expansion constructs and modifications to some of the builtins (read, declare, etc.) to use themnew quoting syntaxes for ANSI-C string expansion and locale-specific string translationnew expansions to do substring extraction, pattern replacement, and indirect variable expansionnew builtins: `disown' and `shopt'new variables: HISTIGNORE, SHELLOPTS, PIPESTATUS, DIRSTACK, GLOBIGNORE, MACHTYPE, BASH_VERSINFOspecial handling of many unused or redundant variables removed (e.g., $notify, $glob_dot_filenames, $no_exit_on_failed_exec)dynamic loading of new builtin commands; many loadable examples providednew prompt expansions: \a, \e, \n, \H, \T, \@, \v, \Vhistory and aliases available in shell scriptsnew readline variables: enable-keypad, mark-directories, input-meta, visible-stats, disable-completion, comment-beginnew readline commands to manipulate the mark and operate on the regionnew readline emacs mode commands and bindings for ksh-88 compatibilityupdated and extended builtinsnew DEBUG trapexpanded (and now documented) restricted shell mode implementation stuff: autoconf-based configurationnearly all of the bugs reported since version 1.14 have been fixedmost builtins converted to use builtin `getopt' for consistencymost builtins use -p option to display output in a reusable form (for consistency)grammar tighter and smaller (66 reduce-reduce conflicts gone)lots of code now smaller and fastertest suite greatly expanded B2) Are there any user-visible incompatibilities between bash-4.3 and previous bash versions? There are a few incompatibilities between version 4.3 and previousversions. They are detailed in the file COMPAT in the bash distribution.That file is not meant to be all-encompassing; send mail tobash-maintainers@gnu.org (or bug-bash@gnu.org if you would likecommunity discussion) if you find something that's not mentioned there. Section C: Differences from other Unix shells C1) How does bash differ from sh, the Bourne shell? This is a non-comprehensive list of features that differentiate bashfrom the SVR4.2 shell. The bash manual page explains these morecompletely. 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 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*} 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, 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, FUNCNAME, histchars, auto_resume, PROMPT_DIRTRIM, BASHOPTS, BASH_XTRACEFD DEBUG trap ERR trap variable arrays with new compound assignment syntax redirections: <>, &>, >|, <<<, [n]<&word-, [n]>&word-, >>& prompt string special char translation and variable expansion 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 -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/-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 brace expansion tilde expansion arithmetic expansion with $((...)) and `let' builtin the `[[...]]' extended conditional command process substitution aliases and alias/unalias builtins local variables in functions and `local' builtin readline and command-line editing with programmable completion command history and history/fc builtins csh-like history expansion 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 POSIX.2-style globbing equivalence classes POSIX.2-style globbing collating symbols egrep-like extended pattern matching operators case-insensitive pattern matching and globbing variable assignments preceding commands affect only that command, even for builtins and functions 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 includes `stop' builtin (bash can use alias stop='kill -s STOP') `newgrp' builtin turns on job control if called as `jsh' $TIMEOUT (like bash $TMOUT) `^' is a synonym for `|' new SVR4.2 sh builtins: mldmode, priv Implementation differences: redirection to/from compound commands causes sh to create a subshell bash does not allow unbalanced quotes; sh silently inserts them at EOF bash does not mess with signal 11 sh sets (euid, egid) to (uid, gid) if -p not supplied and uid < 100 bash splits only the results of expansions on IFS, using POSIX.2 field splitting rules; sh splits all words on IFS sh does not allow MAILCHECK to be unset (?) sh does not allow traps on SIGALRM or SIGCHLD 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 2.4 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 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*} prefix expansion operator the ${param:offset[:length]} parameter substring operator the ${param/pat[/string]} parameter pattern substitution operator 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, FUNCNAME, histchars, auto_resume, PROMPT_DIRTRIM prompt expansion with backslash escapes and command substitution 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/-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 POSIX.2-style globbing collating symbols 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 (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, 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 brace expansion by default (ksh88 compile-time option) 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 C3) Which new features in ksh-93 are not in bash, and which are? 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 `.' is allowed in variable names to create a hierarchical namespace more extensive compound assignment syntax discipline functions KEYBD trap 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' `.' can execute shell functions 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' new $'...' and $"..." quoting FIGNORE (but bash uses GLOBIGNORE), HISTCMD 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 `.' 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, 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 ${param:offset:length} redirection operators preceded with {varname} to store fd number in varname DEBUG can force skipping following command [[ -v var ]] operator (checks whether or not var is set) typeset -n and `nameref' variables process substitutions work without /dev/fd Section D: Why does bash do some things differently than other Unix shells? D1) Why does bash run a different version of `command' than `which command' says it will? On many systems, `which' is actually a csh script that assumesyou're running csh. In tcsh, `which' and its cousin `where'are builtins. On other Unix systems, `which' is a perl scriptthat uses the PATH environment variable. Many Linux distributionsuse GNU `which', which is a C program that can understand shellaliases. The csh script version reads the csh startup files from yourhome directory and uses those to determine which `command' willbe invoked. Since bash doesn't use any of those startup files,there's a good chance that your bash environment differs fromyour csh environment. The bash `type' builtin does everything`which' does, and will report correct results for the runningshell. If you're really wedded to the name `which', try addingthe following function definition to your .bashrc: which() { builtin type "$@" } If you're moving from tcsh and would like to bring `where' alongas well, use this function: where() { builtin type -a "$@" } D2) Why doesn't bash treat brace expansions exactly like csh? The only difference between bash and csh brace expansion is thatbash requires a brace expression to contain at least one unquotedcomma if it is to be expanded. Any brace-surrounded word notcontaining an unquoted comma is left unchanged by the braceexpansion code. This affords the greatest degree of shcompatibility. Bash, ksh, zsh, and pd-ksh all implement brace expansion this way. 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. ${parameter%word} Remove smallest suffix pattern. The WORD is expanded to produce a pattern. It then expands to the value of PARAMETER, with the smallest portion of the suffix matched by the pattern deleted. x=file.c echo ${x%.c}.o -->file.o ${parameter%%word} Remove largest suffix pattern. The WORD is expanded to produce a pattern. It then expands to the value of PARAMETER, with the largest portion of the suffix matched by the pattern deleted. x=posix/src/std echo ${x%%/*} -->posix ${parameter#word} Remove smallest prefix pattern. The WORD is expanded to produce a pattern. It then expands to the value of PARAMETER, with the smallest portion of the prefix matched by the pattern deleted. x=$HOME/src/cmd echo ${x#$HOME} -->/src/cmd ${parameter##word} Remove largest prefix pattern. The WORD is expanded to produce a pattern. It then expands to the value of PARAMETER, with the largest portion of the prefix matched by the pattern deleted. x=/one/two/three echo ${x##*/} -->three Given a=/a/b/c/d b=b.xxx csh bash result --- ---- ------ $a:h ${a%/*} /a/b/c $a:t ${a##*/} d $b:r ${b%.*} b $b:e ${b##*.} xxx 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 provideda shell script which does most of the work of conversion for you;this script can be found in ./examples/misc/aliasconv.sh. Here ishow you use it: Start csh in the normal way for you. (e.g., `csh') Pipe the output of `alias' through `aliasconv.sh', saving theresults into `bash_aliases': alias | bash aliasconv.sh >bash_aliases Edit `bash_aliases', carefully reading through any createdfunctions. You will need to change the names of some csh specificvariables to the bash equivalents. The script converts $cwd to$PWD, $term to $TERM, $home to $HOME, $user to $USER, and $promptto $PS1. You may also have to add quotes to avoid unwantedexpansion. For example, the csh alias: alias cd 'cd \!*; echo $cwd' is converted to the bash function: cd () { command cd "$@"; echo $PWD ; } The only thing that needs to be done is to quote $PWD: cd () { command cd "$@"; echo "$PWD" ; } Merge the edited file into your ~/.bashrc. There is an additional, more ambitious, script inexamples/misc/cshtobash that attempts to convert your entire cshenvironment to its bash equivalent. This script can be run assimply `cshtobash' to convert your normal interactiveenvironment, or as `cshtobash ~/.login' to convert your loginenvironment. D5) How can I pipe standard output and standard error from one command to another, like csh does with `|&'? Use command 2>&1 | command2 The key is to remember that piping is performed before redirection, sofile descriptor 1 points to the pipe when it is duplicated onto filedescriptor 2. D6) Now that I've converted from ksh to bash, are there equivalents to ksh features like autoloaded functions and the `whence' command? There are features in ksh-88 and ksh-93 that do not have direct bashequivalents. Most, however, can be emulated with very little trouble. ksh-88 feature Bash equivalent-------------- ---------------compiled-in aliases set up aliases in .bashrc; some ksh aliases are bash builtins (hash, history, type)coprocesses named pipe pairs (one for read, one for write)typeset +f declare -Fcd, print, whence function substitutes in examples/functions/kshenvautoloaded functions examples/functions/autoload is the same as typeset -furead var?prompt read -p prompt var ksh-93 feature Bash equivalent-------------- ---------------sleep, getconf Bash has loadable versions in examples/loadables${.sh.version} $BASH_VERSIONprint -f printfhist alias hist=fc$HISTEDIT $FCEDIT Section E: How can I get bash to do certain things, and why does bash do things the way it does? E1) Why is the bash builtin `test' slightly different from /bin/test? The specific example used here is [ ! x -o x ], which is false. Bash's builtin `test' implements the Posix.2 spec, which can besummarized as follows (the wording is due to David Korn): Here is the set of rules for processing test arguments. 0 Args: False 1 Arg: True iff argument is not null. 2 Args: If first arg is !, True iff second argument is null. If first argument is unary, then true if unary test is true Otherwise error. 3 Args: If second argument is a binary operator, do binary test of $1 $3 If first argument is !, negate two argument test of $2 $3 If first argument is `(' and third argument is `)', do the one-argument test of the second argument. Otherwise error. 4 Args: If first argument is !, negate three argument test of $2 $3 $4. Otherwise unspecified 5 or more Args: unspecified. (Historical shells would use their current algorithm). The operators -a and -o are considered binary operators for the purposeof the 3 Arg case. As you can see, the test becomes (not (x or x)), which is false. E2) Why does bash sometimes say `Broken pipe'? If a sequence of commands appears in a pipeline, and one of thereading commands finishes before the writer has finished, thewriter receives a SIGPIPE signal. Many other shells special-caseSIGPIPE as an exit status in the pipeline and do not report it. For example, in: ps -aux | head `head' can finish before `ps' writes all of its output, and pswill try to write on a pipe without a reader. In that case, bashwill print `Broken pipe' to stderr when ps is killed by aSIGPIPE. As of bash-3.1, bash does not report SIGPIPE errors by default. Youcan build a version of bash that will report such errors. E3) 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 knowthat the terminal escape sequences do not take up space on thescreen. The redisplay code assumes, unless told otherwise, thateach character in the prompt is a `printable' character thattakes up one character position on the screen. You can use the bash prompt expansion facility (see the PROMPTINGsection in the manual page) to tell readline that sequences ofcharacters 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. E4) If I pipe the output of a command into `read variable', why doesn't the output show up in $variable when the read command finishes? This has to do with the parent-child relationship between Unixprocesses. It affects all commands run in pipelines, not justsimple calls to `read'. For example, piping a command's outputinto a `while' loop that repeatedly calls `read' will result inthe same behavior. Each element of a pipeline, even a builtin or shell function,runs in a separate process, a child of the shell running thepipeline. A subprocess cannot affect its parent's environment. When the `read' command sets the variable to the input, thatvariable is set only in the subshell, not the parent shell. Whenthe subshell exits, the value of the variable is lost. Many pipelines that end with `read variable' can be convertedinto command substitutions, which will capture the output ofa specified command. The output can then be assigned to avariable: grep ^gnu /usr/lib/news/active | wc -l | read ngroup can be converted into ngroup=$(grep ^gnu /usr/lib/news/active | wc -l) This does not, unfortunately, work to split the text amongmultiple variables, as read does when given multiple variablearguments. If you need to do this, you can either use thecommand substitution above to read the output into a variableand chop up the variable using the bash pattern removalexpansion operators or use some variant of the followingapproach. Say /usr/local/bin/ipaddr is the following shell script: #! /bin/shhost `hostname` | awk '/address/ {print $NF}' Instead of using /usr/local/bin/ipaddr | read A B C D to break the local machine's IP address into separate octets, use OIFS="$IFS" IFS=. set -- $(/usr/local/bin/ipaddr) IFS="$OIFS" A="$1" B="$2" C="$3" D="$4" Beware, however, that this will change the shell's positionalparameters. If you need them, you should save them before doingthis. This is the general approach -- in most cases you will not need toset $IFS to a different value. 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 modeled after the 9th EditionResearch Unix version of `echo'. It does not interpretbackslash-escaped characters in its argument strings by default;it requires the use of the -e option to enable theinterpretation. The System V echo provides no way to disable thespecial characters; the bash echo has a -E option to disablethem. There is a configuration option that will make bash behave likethe System V echo and interpret things like `\t' by default. Runconfigure with the --enable-xpg-echo-default option to turn thison. Be aware that this will cause some of the tests run when youtype `make tests' to fail. There is a shell option, `xpg_echo', settable with `shopt', that willchange the behavior of echo at runtime. Enabling this option turnson 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 onlything that can be suspended is the process group. This is a singlecommand or pipeline of commands that the shell forks and executes. When you run a while or for loop, the only thing that the shell forksand executes are any commands in the while loop test and commands inthe loop bodies. These, therefore, are the only things that can besuspended when you type ^Z. If you want to be able to stop the entire loop, you need to put itwithin parentheses, which will force the loop into a subshell thatmay be stopped (and subsequently restarted) as a single unit. E7) What about empty for loops in Makefiles? It's fairly common to see constructs like this in automatically-generatedMakefiles: SUBDIRS = @SUBDIRS@ ... subdirs-clean: for d in ${SUBDIRS}; do \ ( cd $$d && ${MAKE} ${MFLAGS} clean ) \ done When SUBDIRS is empty, this results in a command like this being passed tobash: for d in ; do ( cd $d && ${MAKE} ${MFLAGS} clean ) done In versions of bash before bash-2.05a, this was a syntax error. If thereserved word `in' was present, a word must follow it before the semicolonor newline. The language in the manual page referring to the list of wordsbeing empty referred to the list after it is expanded. These versions ofbash required that there be at least one word following the `in' when theconstruct was parsed. The idiomatic Makefile solution is something like: SUBDIRS = @SUBDIRS@ subdirs-clean: subdirs=$SUBDIRS ; for d in $$subdirs; do \ ( cd $$d && ${MAKE} ${MFLAGS} clean ) \ done The latest updated POSIX standard has changed this: the word listis no longer required. Bash versions 2.05a and later accept thenew syntax. E8) Why does the arithmetic evaluation code complain about `08'? The bash arithmetic evaluation code (used for `let', $(()), (()), and inother places), interprets a leading `0' in numeric constants as denotingan octal number, and a leading `0x' as denoting hexadecimal. This isin accordance with the POSIX.2 spec, section 2.9.2.1, which states thatarithmetic constants should be handled as signed long integers as definedby 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 settingwhen 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 thecurrent LC_COLLATE setting. Setting this variable to `C' or `POSIX' willresult in the traditional behavior ([A-Z] matches all uppercase ASCIIcharacters). Many other locales, including the en_US locale (the defaulton many US versions of Linux) collate the upper and lower case letters likethis: 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 ofA-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 ispresent, locale(1). If you have locale(1), you can use it to findyour current locale information even if you do not have any of theLC_ variables set. My advice is to put export LC_COLLATE=C into /etc/profile and inspect any shell scripts run from cron forconstructs like [A-Z]. This will prevent things like rm [A-Z]* from removing every file in the current directory except those beginningwith `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 leadingslashes may be replaced with a single slash when canonicalizing thecurrent working directory. This is, I presume, for historical compatibility. Certain versions ofUnix, 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. Membersof this process group (processes whose process group ID is equal to thecurrent terminal process group ID) receive terminal-generated signals likeSIGWINCH. (For more details, see the JOB CONTROL section of the bashman page.) If a terminal is resized, the kernel sends SIGWINCH to each member ofthe terminal's current process group (the `foreground' process group). When bash is running with job control enabled, each pipeline (which may bea single command) is run in its own process group, different from bash'sprocess group. This foreground process group receives the SIGWINCH; bashdoes not. Bash has no way of knowing that the terminal has been resized. There is a `checkwinsize' option, settable with the `shopt' builtin, thatwill cause bash to check the window size and adjust its idea of theterminal's dimensions each time a process stops or exits and returns controlof 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 fromthe end of the expanded value of $param. When a negative `offset' begins with a minus sign, however, unexpected thingscan 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 theexpanded value of param is unset or null, and $param otherwise. To use negative offsets that begin with a minus sign, separate theminus 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 behaveimproperly 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 thecharacters that breaks words for the completer. Readline uses these charactersin sort of the same way that bash uses $IFS: they break or separate the wordsthe completion code hands to the application-specific or default wordcompletion functions. The original intent was to make it easy to editcolon-separated lists (such as $PATH in bash) in various applications usingreadline for input. This is complicated by the fact that some versions of the popular`bash-completion' programmable completion package have problems with thedefault completion behavior in the presence of colons. The current set of completion word break characters is available in bash asthe value of the COMP_WORDBREAKS variable. Removing `:' from that value isenough to make the colon not special to completion: COMP_WORDBREAKS=${COMP_WORDBREAKS//:} You can also quote the colon with a backslash to achieve the same resulttemporarily. 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 regularexpression argument to the [[ command's =~ operator was not specified.The practical effect was that double-quoting the pattern argument requiredbackslashes to quote special pattern characters, which interfered with thebackslash processing performed by double-quoted word expansion and wasinconsistent with how the == shell pattern matching operator treatedquoted 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 thespecial meaning of the characters special to regular expression processing(`.', `[', `\', `(', `), `*', `+', `?', `{', `|', `^', and `$') and forcesthem to be matched literally. This is consistent with how the `==' patternmatching operator treats quoted portions of its pattern argument. Since the treatment of quoted string arguments was changed, several issueshave arisen, chief among them the problem of white space in pattern argumentsand 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 alloperands of the [[ command, this allows users to quote patterns as they wishwhen assigning the variable, then expand the values to a single string thatmay contain whitespace. The first problem may be solved by using backslashesor any other quoting mechanism to escape the white space in the patterns. Bash-4.0 introduces the concept of a `compatibility level', controlled byseveral 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 ofthe =~ operator. E15) Tell me more about the shell compatibility level. Bash-4.0 introduced the concept of a `shell compatibility level', specifiedas a set of options to the shopt builtin (compat31, compat32, compat40 atthis writing). There is only one current compatibility level -- eachoption is mutually exclusive. This list does not mention behavior that isstandard for a particular version (e.g., setting compat32 means that quotingthe rhs of the regexp matching operator quotes special regexp characters inthe 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 F1) Why can't I use command line editing in my `cmdtool'? The problem is `cmdtool' and bash fighting over the input. Whenscrolling is enabled in a cmdtool window, cmdtool puts the tty in`raw mode' to permit command-line editing using the mouse forapplications that cannot do it themselves. As a result, bash andcmdtool each try to read keyboard input immediately, with neithergetting enough of it to be useful. This mode also causes cmdtool to not implement many of theterminal functions and control sequences appearing in the`sun-cmd' termcap entry. For a more complete explanation, seethat file examples/suncmd.termcap in the bash distribution. `xterm' is a better choice, and gets along with bash much moresmoothly. If you must use cmdtool, you can use the termcap description inexamples/suncmd.termcap. Set the TERMCAP variable to the terminaldescription contained in that file, i.e. TERMCAP='Mu|sun-cmd:am:bs:km:pt:li#34:co#80:cl=^L:ce=\E[K:cd=\E[J:rs=\E[s:' Then export TERMCAP and start a new cmdtool window from that shell.The bash command-line editing should behave better in the newcmdtool. If this works, you can put the assignment to TERMCAPin your bashrc file. 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 linkingwith the libraries in /usr/ucblib, but using the definitionsand structures from files in /usr/include. The actual conflict is between the dirent structure in/usr/include/dirent.h and the struct returned by the version of`readdir' in libucb.a (a 4.3-BSD style `struct direct'). Make sure you've got /usr/ccs/bin ahead of /usr/ucb in your $PATHwhen configuring and building bash. This will ensure that youuse /usr/ccs/bin/cc or acc instead of /usr/ucb/cc and that youlink with libc before libucb. If you have installed the Sun C compiler, you may also need toput /usr/ccs/bin and /opt/SUNWspro/bin into your $PATH before/usr/ucb. 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)client library, which is part of libc. The YP library code keeps static state -- a pointer into the datareturned from the server. When YP initializes itself (setpwent),it looks at this pointer and calls free on it if it's non-null. So far, so good. If one of the YP functions is interrupted during getpwent (theexact function is interpretwithsave()), and returns NULL, thepointer is freed without being reset to NULL, and the functionreturns. The next time getpwent is called, it sees that thispointer is non-null, calls free, and the bash free() blows upbecause it's being asked to free freed memory. The traditional Unix mallocs allow memory to be freed multipletimes; that's probably why this has never been fixed. You canrun configure with the `--without-gnu-malloc' option to usethe C library malloc and avoid the problem. F4) I'm running SVR4.2. Why is the line erased every time I type `@'? The `@' character is the default `line kill' character in mostversions of System V, including SVR4.2. You can change thischaracter to whatever you want using `stty'. For example, tochange the line kill character to control-u, type stty kill ^U where the `^' and `U' can be two separate characters. 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 < file ( command ) According to the grammar given in the POSIX.2 standard, this constructis, in fact, a syntax error. Redirections may only precede `simplecommands'. A subshell construct such as the above is one of the shell's`compound commands'. A redirection may only follow a compound command. This affects the mechanical transformation of commands that use `cat'to pipe a file into a command (a favorite Useless-Use-Of-Cat topic oncomp.unix.shell). While most commands of the form cat file | command can be converted to `< file command', shell control structures such asloops 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 tosupport this construct. It will not apply with `patch'; you mustmodify parse.y by hand. Note that if you apply this, you mustrecompile with -DREDIRECTION_HACK. This introduces a largenumber of reduce/reduce conflicts into the shell grammar. 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 worksfor emacs mode editing, and then screwed all the vi users by settingINPUTRC to /etc/inputrc in /etc/profile. 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 toINPUTRC 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 [...] $endif F7) Why do bash-2.05a and bash-2.05b fail to compile `printf.def' on HP/UX 11.x? HP/UX's support for long double is imperfect at best. GCC will support it without problems, but the HP C library functionslike strtold(3) and printf(3) don't actually work with long doubles.HP implemented a `long_double' type as a 4-element array of 32-bitints, and that is what the library functions use. The ANSI C`long double' type is a 128-bit floating point scalar. The easiest fix, until HP fixes things up, is to edit the generatedconfig.h and #undef the HAVE_LONG_DOUBLE line. After doing that,the compilation should complete successfully. Section G: How can I get bash to do certain common things? G1) How can I get bash to read and display eight-bit characters? This is a process requiring several steps. First, you must ensure that the `physical' data path is a full eightbits. For xterms, for example, the `vt100' resources `eightBitInput'and `eightBitOutput' should be set to `true'. Once you have set up an eight-bit path, you must tell the kernel andtty driver to leave the eighth bit of characters alone when processingkeyboard input. Use `stty' to do this: stty cs8 -istrip -parenb For old BSD-style systems, you can use stty pass8 You may also need stty even odd Finally, you need to tell readline that you will be inputting anddisplaying eight-bit characters. You use readline variables to dothis. These variables can be set in your .inputrc or using the bash`bind' builtin. Here's an example using `bind': bash$ 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 placedin ~/.inputrc. The script examples/scripts.noah/meta.bash encapsulates the bindcommands 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 firstargument, skipping over any function defined with that name. The`builtin' builtin executes the builtin command given as its firstargument directly. For example, to write a function to replace `cd' that writes thehostname and current directory to an xterm title bar, usesomething 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 ${!var1} For sh compatibility, use the `eval' builtin. The importantthing to remember is that `eval' expands the arguments you giveit again, so you need to quote the parts of the arguments thatyou want `eval' to act on. For example, this expression prints the value of the last positionalparameter: eval echo \"\$\{$#\}\" The expansion of the quoted portions of this expression will bedeferred until `eval' runs, while the `$#' will be expandedbefore `eval' is executed. In versions of bash later than bash-2.0, echo ${!#} does the same thing. This is not the same thing as ksh93 `nameref' variables, though the syntaxis similar. Namerefs are available bash version 4.3, and work as in ksh93. G4) How can I make the bash `time' reserved word print timing output that looks like the output from my system's /usr/bin/time? The bash command timing code looks for a variable `TIMEFORMAT' anduses its value as a format string to decide how to display thetiming statistics. The value of TIMEFORMAT is a string with `%' escapes expanded in afashion similar in spirit to printf(3). The manual page explainsthe meanings of the escape sequences in the format string. If TIMEFORMAT is not set, bash acts as if the following assignment hadbeen performed: TIMEFORMAT=$'\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 expandedwhen the prompt string (PS1 or PS2) is displayed. The full list is inthe manual page. The \w expansion gives the full pathname of the current directory, witha tilde (`~') substituted for the current value of $HOME. The \Wexpansion gives the basename of the current directory. To put the fullpathname of the current directory into the path without any tildesubstitution, 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 frombeing 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 usethis: echo .!(.|) * A solution that works without extended globbing is given in the Unix ShellFAQ, posted periodically to comp.unix.shell. It's a variant of echo .[!.]* ..?* * (The ..?* catches files with names of three or more characters beginningwith `..') 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 andinstalled at the same time as bash. It provides a standardtemplate for reporting a problem and automatically includesinformation about your configuration and build environment. `bashbug' sends its reports to bug-bash@gnu.org, whichis a large mailing list gatewayed to the usenet newsgroup gnu.bash.bug. Bug fixes, answers to questions, and announcements of new releasesare all posted to gnu.bash.bug. Discussions concerning bash featuresand problems also take place there. To reach the bash maintainers directly, send mail tobash-maintainers@gnu.org. H2) What kind of bash documentation is there? First, look in the doc directory in the bash distribution. It shouldcontain at least the following files: bash.1 an extensive, thorough Unix-style manual pagebuiltins.1 a manual page covering just bash builtin commandsbashref.texi a reference manual in GNU tex`info formatbashref.info an info version of the reference manualFAQ this filearticle.ms text of an article written for The Linux Journalreadline.3 a man page describing readline Postscript, HTML, and ASCII files created from the above source areavailable in the documentation distribution. There is additional documentation available for anonymous FTP from hostftp.cwru.edu in the `pub/bash' directory. Cameron Newham and Bill Rosenblatt have written a book on bash, publishedby O'Reilly and Associates. The book is based on Bill Rosenblatt's KornShell book. The title is ``Learning the Bash Shell'', and the ISBN numberof the third edition, published in March, 2005, is 0-596-00965-8. Look forit in fine bookstores near you. This edition of the book has been updatedto cover bash-3.0. The GNU Bash Reference Manual has been published as a printed book byNetwork Theory Ltd (Paperback, ISBN: 0-9541617-7-7, Nov. 2006). It coversbash-3.2 and is available from most online bookstores (seehttp://www.network-theory.co.uk/bash/manual/ for details). The publisherwill donate $1 to the Free Software Foundation for each copy sold. 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. Chris F. A. Johnson, a frequent contributor to comp.unix.shell andgnu.bash.bug, has written ``Shell Scripting Recipes: A Problem-SolutionApproach,'' a new book on shell scripting, concentrating on features ofthe POSIX standard helpful to shell script writers. The first edition fromApress, with ISBN number 1-59059-471-1, was published in May, 2005. H3) What's coming in future versions? These are features I hope to include in a future version of bash. 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. breaking some of the shell functionality into embeddable librariesa module system like zsh's, using dynamic loading like builtinsa bash programmer's guide with a chapter on creating loadable builtinsa 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 discipline functionsSome of the new ksh93 pattern matching operators, like backreferencing H5) When will the next release appear? The next version will appear sometime in 2015. Never make predictions. 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