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@@ -0,0 +1,1098 @@ +This is the Bash FAQ, version 2.1, for Bash version 2.0. + +This document contains a set of frequently-asked questions concerning +Bash, the GNU Bourne-Again Shell. Bash is a freely-available command +interpreter with advanced features for both interactive use and shell +programming. + +Another good source of basic information about shells is the collection +of FAQ articles periodically posted to comp.unix.shell. + +Questions and comments concerning this document should be sent to +chet@po.cwru.edu. + +This document is available for anonymous FTP with the URL + +ftp://slc2.ins.cwru.edu/pub/bash/FAQ + +---------- +Contents: + +Section A: The Basics + +1) What is it? +2) What's the latest version? +3) Where can I get it? +4) On what machines will bash run? +5) How can I build bash with gcc? +6) How can I make bash my login shell? +7) I just changed my login shell to bash, and now I can't FTP into my + machine. Why not? +8) What's the `POSIX 1003.2 standard'? +9) What is the bash `posix mode'? + +Section B: The latest version + +10) What's new in version 2.0? +11) Are there any user-visible incompatibilities between bash-2.0 and + bash-1.14.7? + +Section C: Differences from other Unix shells + +12) How does bash differ from sh, the Bourne shell? +13) How does bash differ from the Korn shell, version ksh88? +14) 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? + +15) Why does bash run a different version of `command' than + `which command' says it will? +16) Why doesn't bash treat brace expansions exactly like csh? +17) Why doesn't bash have csh variable modifiers? +18) How can I make my csh aliases work when I convert to bash? +19) How can I pipe standard output and standard error from one command to + another, like csh does with `|&'? +20) Now that I've converted from ksh to bash, are there equivalents to + ksh features like autoloaded functions and the `whence' command? + +Section E: How can I get bash to do certain things, and why does bash do + things the way it does? + +21) Why is the bash builtin `test' slightly different from /bin/test? +22) Why does bash sometimes say `Broken pipe'? +23) How can I get bash to read and display eight-bit characters? +24) How do I write a function `x' to replace builtin command `x', but + still invoke the command from within the function? +25) When I have terminal escape sequences in my prompt, why does bash + wrap lines at the wrong column? +26) How can I find the value of a shell variable whose name is the value + of another shell variable? +27) 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? +28) 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? +29) Why doesn't a while or for loop get suspended when I type ^Z? +30) How can I make the bash `time' reserved word print timing output that + looks like the output from my system's /usr/bin/time? + +Section F: Things to watch out for on certain Unix versions + +31) Why can't I use command line editing in my `cmdtool'? +32) I built bash on Solaris 2. Why do globbing expansions and filename + completion chop off the first few characters of each filename? +33) Why does bash dump core after I interrupt username completion or + `~user' tilde expansion on a machine running NIS? +34) I'm running SVR4.2. Why is the line erased every time I type `@'? +35) Why does bash report syntax errors when my C News scripts use a + redirection before a subshell command? + +Section G: Where do I go from here? + +36) How do I report bugs in bash, and where should I look for fixes and + advice? +37) What kind of bash documentation is there? +38) What's coming in future versions? +39) What's on the bash `wish list'? +40) When will the next release appear? + +---------- +Section A: The Basics + +1) What is it? + +Bash is a Unix command interpreter (shell). It is an implementation of +the Posix 1003.2 shell standard, and resembles the Korn and System V +shells. + +Bash contains a number of enhancements over those shells, both +for interactive use and shell programming. Features geared +toward interactive use include command line editing, command +history, job control, aliases, and prompt expansion. Programming +features include additional variable expansions, shell +arithmetic, and a number of variables and options to control +shell behavior. + +Bash was originally written by Brian Fox of the Free Software +Foundation. The current developer and maintainer is Chet Ramey +of Case Western Reserve University. + +2) What's the latest version? + +The latest version is 2.0, first made available on December 23, 1996. + +3) Where can I get it? + +Bash is the GNU project's shell, and so is available from the +master GNU archive site, prep.ai.mit.edu, and its mirrors. The +latest version is also available for FTP from slc2.ins.cwru.edu, +the maintainer's machine. The following URLs tell how to get +version 2.0: + +ftp://prep.ai.mit.edu/pub/gnu/bash-2.0.tar.gz +ftp://slc2.ins.cwru.edu/pub/dist/bash-2.0.tar.gz + +Formatted versions of the documentation are available with the URLs: + +ftp://prep.ai.mit.edu/pub/gnu/bash-doc-2.0.tar.gz +ftp://slc2.ins.cwru.edu/pub/dist/bash-doc-2.0.tar.gz + +4) On what machines will bash run? + +Bash has been ported to nearly every version of UNIX. All you +should have to do to build it on a machine for which a port +exists is to type `configure' and then `make'. The build process +will attempt to discover the version of UNIX you have and tailor +itself accordingly, using a script created by GNU autoconf. + +More information appears in the file `INSTALL' in the distribution. + +5) How can I build bash with gcc? + +Bash configures to use gcc by default if it is available. Read the +file INSTALL in the distribution for more information. + +6) How can I make bash my login shell? + +Some machines let you use `chsh' to change your login shell. Other +systems use `passwd -s'. If one of these works for you, that's all +you need. Note that many systems require the full pathname to a shell +to appear in /etc/shells before you can make it your login shell. For +this, you may need the assistance of your friendly local system +administrator. + +If you cannot do this, you can still use bash as your login shell, but +you need to perform some tricks. The basic idea is to add a command +to your login shell's startup file to replace your login shell with +bash. + +For example, if your login shell is csh or tcsh, and you have installed +bash 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 every +csh 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 something +like + + 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, add +a line similar to the above to ~/.profile: + + [ -f /usr/gnu/bin/bash ] && exec /usr/gnu/bin/bash --login + +Next, create an empty file in your home directory named `.bash_profile'. +The existence of this file will prevent the exec'd bash from trying to +read ~/.profile, and re-execing itself over and over again. ~/.bash_profile +is the file bash tries to read initialization commands from when it is +invoked as a login shell. + +7) I just changed my login shell to bash, and now I can't FTP into my + machine. Why not? + +You must add the full pathname to bash to the file /etc/shells. As +noted in the answer to the previous question, many systems require +this before you can make bash your login shell. + +Most versions of ftpd use this file to prohibit `special' users +such as `uucp' and `news' from using FTP. + +8) What's the `POSIX 1003.2 standard'? + +POSIX is a name originally coined by Richard Stallman for a +family of open system standards based on UNIX. There are a +number of aspects of UNIX under consideration for +standardization, from the basic system services at the system +call and C library level to applications and tools to system +administration and management. Each area of standardization is +assigned to a working group in the 1003 series. + +The POSIX Shell and Utilities standard has been developed by IEEE +Working Group 1003.2 (POSIX.2). It concentrates on the command +interpreter interface and utility programs commonly executed from +the command line or by other programs. An initial version of the +standard has been approved and published by the IEEE, and work is +currently underway to update it. + +Bash is concerned with the aspects of the shell's behavior +defined by POSIX.2. The shell command language has of course +been standardized, including the basic flow control and program +execution constructs, I/O redirection and pipelining, argument +handling, variable expansion, and quoting. + +The `special' builtins, which must be implemented as part of the +shell to provide the desired functionality, are specified as +being part of the shell; examples of these are `eval' and +`export'. Other utilities appear in the sections of POSIX.2 not +devoted to the shell which are commonly (and in some cases must +be) implemented as builtin commands, such as `read' and `test'. +POSIX.2 also specifies aspects of the shell's interactive +behavior as part of the UPE, including job control and command +line editing. Only vi-style line editing commands have been +standardized; emacs editing commands were left out due to +objections. + +9) What is the bash `posix mode'? + +Although bash is an implementation of the POSIX.2 shell +specification, there are areas where the bash default behavior +differs from that spec. The bash `posix mode' changes the bash +behavior in these areas so that it obeys the spec more closely. + +Posix mode is entered by starting bash with the --posix option or +executing `set -o posix' after bash is running. + +The specific aspects of bash which change when posix mode is +active are listed in the file CWRU/POSIX.NOTES in the bash +distribution. They are also listed in a section in the Bash +Reference Manual. + +Section B: The latest version + +10) What's new in version 2.0? + +This version contains extensive changes and new features. Here's a +short list: + +new `time' reserved word to time pipelines, shell builtins, and + shell functions +one-dimensional arrays with a new compound assignment statement, + appropriate expansion constructs and modifications to some + of the builtins (read, declare, etc.) to use them +new quoting syntaxes for ANSI-C string expansion and locale-specific + string translation +new expansions to do substring extraction, pattern replacement, and + indirect variable expansion +new builtins: `disown' and `shopt' +new variables: HISTIGNORE, SHELLOPTS, PIPESTATUS, DIRSTACK, GLOBIGNORE, + MACHTYPE, BASH_VERSINFO +special 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 provided +new prompt expansions: \a, \e, \n, \H, \T, \@, \v, \V +history and aliases available in shell scripts +new readline variables: enable-keypad, mark-directories, input-meta, + visible-stats, disable-completion, comment-begin +new readline commands to manipulate the mark and operate on the region +new readline emacs mode commands and bindings for ksh-88 compatibility +updated and extended builtins +new DEBUG trap +expanded (and now documented) restricted shell mode + +implementation stuff: +autoconf-based configuration +nearly all of the bugs reported since version 1.14 have been fixed +most builtins converted to use builtin `getopt' for consistency +most 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 faster +test suite greatly expanded + +11) Are there any user-visible incompatibilities between bash-2.0 and + bash-1.14.7? + +There are a few incompatibilities between version 1.14.7 and version 2.0. +They are detailed in the file COMPAT in the bash-2.0 distribution. + +Section C: Differences from other Unix shells + +12) How does bash differ from sh, the Bourne shell? + +This is a non-comprehensive list of features that differentiate bash +from the SVR4.2 shell. The bash manual page explains these more +completely. + +Things bash has that sh does not: + long invocation options + `!' 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 + new $'...' and $"..." quoting + the $(...) form of command substitution + the ${#param} parameter value length operator + the ${!param} indirect parameter expansion operator + the ${param:length[:offset]} parameter substring operator + the ${param/pat[/string]} parameter pattern substitution operator + expansions to perform substring removal (${p%[%]w}, ${p#[#]w}) + expansion of positional parameters beyond $9 with ${num} + variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, REPLY, + TIMEFORMAT, PPID, PWD, OLDPWD, SHLVL, RANDOM, SECONDS, + LINENO, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE, HOSTNAME, + ENV, PS3, PS4, DIRSTACK, PIPESTATUS, HISTSIZE, HISTFILE, + HISTFILESIZE, HISTCONTROL, HISTIGNORE, GLOBIGNORE, + PROMPT_COMMAND, FCEDIT, FIGNORE, IGNOREEOF, INPUTRC, + SHELLOPTS, OPTERR, HOSTFILE, TMOUT, histchars, auto_resume + DEBUG trap + variable arrays with new compound assignment syntax + redirections: <>, &>, >| + prompt string special char translation and variable expansion + auto-export of modified values of variables in initial environment + command search finds functions before builtins + bash return builtin will exit a file sourced with `.' + builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E, hash -p. + export -n/-f/-p/name=value, pwd -L/-P, read -e/-p/-a, + readonly -a/-f/name=value, trap -l, set +o, + set -b/-m/-o option/-h/-p/-B/-C/-H/-P, + unset -f/-v, ulimit -m/-p/-u, + type -a/-p/-t, suspend -f, kill -n, + test -o optname/s1 == s2/s1 < s2/s1 > s2/-nt/-ot/-ef/-O/-G/-S + 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 + process substitution + aliases and alias/unalias builtins + local variables in functions and `local' builtin + readline and command-line editing + command history and history/fc builtins + csh-like history expansion + other new bash builtins: bind, command, builtin, declare/typeset, + dirs, enable, fc, help, history, logout, + popd, pushd, disown, shopt + exported functions + filename generation when using output redirection (command >a*) + variable assignments preceding commands affect only that command, + even for builtins and functions + posix mode + +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' + ulimit attempts to set both soft & hard limits if -S/-H not given + $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) + sh exits a script if any builtin fails; bash exits only if one of + the POSIX.2 `special' builtins fails + +13) How does bash differ from the Korn shell, version ksh88? + +Things bash has or uses that ksh88 does not: + long invocation options + `!' reserved word + posix mode and posix conformance + command hashing + tilde expansion for assignment statements that look like $PATH + process substitution with named pipes if /dev/fd is not available + the ${!param} indirect parameter expansion operator + the ${param:length[:offset]} parameter substring operator + the ${param/pat[/string]} parameter pattern substitution operator + variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, SHLVL, + TIMEFORMAT, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE, + HISTFILESIZE, HISTIGNORE, HISTCONTROL, PROMPT_COMMAND, + IGNOREEOF, FIGNORE, INPUTRC, HOSTFILE, DIRSTACK, + PIPESTATUS, HOSTNAME, OPTERR, SHELLOPTS, GLOBIGNORE, + histchars, auto_resume + prompt expansion with backslash escapes and command substitution + redirection: &> (stdout and stderr) + more extensive and extensible editing and completion + builtins: bind, builtin, command, declare, dirs, echo -e/-E, enable, + exec -l/-c/-a, fc -s, export -n/-f/-p, hash, help, history, + jobs -x/-r/-s, kill -s/-n/-l, local, logout, popd, pushd, + read -e/-p/-a, readonly -a/-n/-f/-p, set -o braceexpand/ + -o histexpand/-o interactive-comments/-o notify/-o physical/ + -o posix/-o hashall/-o onecmd/-h/-B/-C/-b/-H/-P, set +o, + suspend, trap -l, type, typeset -a/-F/-p, ulimit -u, + umask -S, alias -p, shopt, disown + `!' csh-style history expansion + +Things ksh88 has or uses that bash does not: + new version of test: [[...]] + tracked aliases + $(<file) + variables: ERRNO, FPATH, COLUMNS, LINES, EDITOR, VISUAL + extended pattern matching with egrep-style pattern lists + co-processes (|&, >&p, <&p) + weirdly-scoped functions + typeset +f to list all function names without definitions + text of command history kept in a file, not memory + builtins: alias -x, cd old new, fc -e -, newgrp, print, + read -p/-s/-u/var?prompt, set -A/-o gmacs/ + -o bgnice/-o markdirs/-o nolog/-o trackall/-o viraw/-s, + typeset -H/-L/-R/-A/-ft/-fu/-fx/-l/-u/-t, whence + +Implementation differences: + ksh runs last command of a pipeline in parent shell context + ksh ulimit sets hard and soft limits by default + 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 + +14) Which new features in ksh-93 are not in bash, and which are? + +New things in ksh-93 not in bash-2.0: + associative arrays + floating point arithmetic + ++, --, comma arithmetic operators + math library functions + ${!name[sub]} name of subscript for associative array + ${!prefix*} and {!prefix@} variable name prefix expansions + `.' is allowed in variable names to create a hierarchical namespace + more extensive compound assignment syntax + discipline functions + `sleep' and `getconf' builtins (bash has loadable versions) + typeset -n and `nameref' variables + KEYBD trap + variables: .sh.edchar, .sh.edmode, .sh.edcol, .sh.edtext, HISTEDIT, + .sh.version, .sh.name, .sh.subscript, .sh.value + backreferences in pattern matching + print -f and printf (bash has loadable versions) + `fc' has been renamed to `hist' + read -t/-d + `.' can execute shell functions + ENV processed only for interactive shells + +New things in ksh-93 present in bash-2.0: + ?: arithmetic operator + expansions: ${!param}, ${param:len[:offset]}, ${param/pat[/str]} + compound array assignment + the `!' reserved word + loadable builtins -- but ksh uses `builtin' while bash uses `enable' + `command', `builtin', `disown' builtins + new $'...' and $"..." quoting + FIGNORE (but bash uses GLOBIGNORE), HISTCMD + set -o notify/-C + changes to kill builtin + read -A (bash uses read -a) + trap -p + exec -c/-a + `.' restores the positional parameters when it completes + POSIX.2 `test' + umask -S + unalias -a + command and arithmetic substitution performed on PS1, PS4, and ENV + command name completion + +Section D: Why does bash do some things differently than other Unix shells? + +15) Why does bash run a different version of `command' than + `which command' says it will? + +`which' is actually a csh script that assumes you're running csh. +It reads the csh startup files from your home directory and uses +those to determine which `command' will be invoked. Since bash +doesn't use any of those startup files, there's a good chance +that your bash environment differs from your csh environment. + +16) Why doesn't bash treat brace expansions exactly like csh? + +The only difference between bash and csh brace expansion is that +bash requires a brace expression to contain at least one unquoted +comma if it is to be expanded. Any brace-surrounded word not +containing an unquoted comma is left unchanged by the brace +expansion code. This affords the greatest degree of sh +compatibility. + +Bash, ksh, zsh, and pd-ksh all implement brace expansion this way. + +17) Why doesn't bash have csh variable modifiers? + +Posix has specified a more powerful, albeit somewhat more cryptic, +mechanism cribbed from ksh, and bash implements it. + +${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 + + +18) How can I make my csh aliases work when I convert to bash? + +Bash uses a different syntax to support aliases than csh does. +The details can be found in the documentation. We have provided +a shell script which does most of the work of conversion for you; +this script can be found in ./examples/misc/alias-conv.sh. Here is +how you use it: + +Start csh in the normal way for you. (e.g., `csh') + +Pipe the output of `alias' through `alias-conv.sh', saving the +results into `bash_aliases': + + alias | alias-conv.sh >bash_aliases + +Edit `bash_aliases', carefully reading through any created +functions. You will need to change the names of some csh specific +variables to the bash equivalents. The script converts $cwd to +$PWD, $term to $TERM, $home to $HOME, $user to $USER, and $prompt +to $PS1. You may also have to add quotes to avoid unwanted +expansion. + +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 "$PWD" ; } + +Merge the edited file into your ~/.bashrc. + +There is an additional, more ambitious, script in +examples/misc/cshtobash that attempts to convert your entire csh +environment to its bash equivalent. This script can be run as +simply `cshtobash' to convert your normal interactive +environment, or as `cshtobash ~/.login' to convert your login +environment. + +19) How can I pipe standard output and standard error from one command to + another, like csh does with `|&'? + +Use + command 2>&1 | command2 + +The key is to remember that piping is performed before redirection, so +file descriptor 1 points to the pipe when it is duplicated onto file +descriptor 2. + +20) Now that I've converted from ksh to bash, are there equivalents to + ksh features like autoloaded functions and the `whence' command? + +There are features in ksh-88 that do not have direct bash equivalents. +Most, however, can be emulated with very little trouble. + +ksh-88 feature Bash equivalent +-------------- --------------- +[[...]] can usually use [...]; minor differences (no + pattern matching, for one) +compiled-in aliases set up aliases in .bashrc; some ksh aliases are + bash builtins (hash, history, type) +$(<file) $(cat file) +extended patterns no good substitute +coprocesses named pipe pairs (one for read, one for write) +typeset +f declare -F +cd, print, whence function substitutes in examples/functions/kshenv +autoloaded functions examples/functions/autoload is the same as typeset -fu +read var?prompt read -p prompt var + +Section E: How can I get bash to do certain things, and why does bash do + things the way it does? + +21) 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 be +summarized 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 $4. + Otherwise unspecified + 5 or more Args: unspecified. (Historical shells would use their + current algorithm). + +The operators -a and -o are considered binary operators for the purpose +of the 3 Arg case. + +As you can see, the test becomes (not (x or x)), which is false. + +22) Why does bash sometimes say `Broken pipe'? + +If a sequence of commands appears in a pipeline, and one of the +reading commands finishes before the writer has finished, the +writer receives a SIGPIPE signal. Many other shells special-case +SIGPIPE as an exit status in the pipeline and do not report it. +For example, in: + + ps -aux | head + +`head' can finish before `ps' writes all of its output, and ps +will try to write on a pipe without a reader. In that case, bash +will print `Broken pipe' to stderr when ps is killed by a +SIGPIPE. + +23) How can I get bash to read and display eight-bit characters? + +This is a process requiring several steps. + +First, you must ensure that the `physical' data path is a full eight +bits. For xterms, for example, the `vt100' resources `eightBitInput' +and `eightBitOutput' should be set to `true'. + +Once you have set up an eight-bit path, you must tell the kernel and +tty driver to leave the eighth bit of characters alone when processing +keyboard input. Use `stty' to do this: + + stty cs8 -istrip -parenb + +For old BSD-style systems, you can use + + stty pass8 + +You may also need + + stty even odd + +Finally, you need to tell readline that you will be inputting and +displaying eight-bit characters. You use readline variables to do +this. These variables can be set in your .inputrc or using the bash +`bind' builtin. Here's an example using `bind': + + bash$ bind 'set convert-meta off' + bash$ bind 'set meta-flag on' + bash$ bind 'set output-meta on' + +The `set' commands between the single quotes may also be placed +in ~/.inputrc. + +24) How do I write a function `x' to replace builtin command `x', but + still invoke the command from within the function? + +This is why the `command' and `builtin' builtins exist. The +`command' builtin executes the command supplied as its first +argument, skipping over any function defined with that name. The +`builtin' builtin executes the builtin command given as its first +argument directly. + +For example, to write a function to replace `cd' that writes the +hostname and current directory to an xterm title bar, use +something like the following: + + cd() + { + builtin cd "$@" && xtitle "$HOST: $PWD" + } + +This could also be written using `command' instead of `builtin'; +the version above is marginally more efficient. + +25) When I have terminal escape sequences in my prompt, why does bash + wrap lines at the wrong column? + +Readline, the line editing library that bash uses, does not know +that the terminal escape sequences do not take up space on the +screen. The redisplay code assumes, unless told otherwise, that +each character in the prompt is a `printable' character that +takes up one character position on the screen. + +You can use the bash prompt expansion facility (see the PROMPTING +section in the manual page) to tell readline that sequences of +characters in the prompt strings take up no screen space. + +Use the \[ escape to begin a sequence of non-printing characters, +and the \] escape to signal the end of such a sequence. + +26) How can I find the value of a shell variable whose name is the value + of another shell variable? + +Bash-2.0 supports this directly. You can use + + ${!var} + +For example, the following sequence of commands will echo `z': + + var1=var2 + var2=z + echo ${!var1} + +For sh compatibility, use the `eval' builtin. The important +thing to remember is that `eval' expands the arguments you give +it again, so you need to quote the parts of the arguments that +you want `eval' to act on. + +For example, this expression prints the value of the last positional +parameter: + + eval echo \"\$\{$#\}\" + +The expansion of the quoted portions of this expression will be +deferred until `eval' runs, while the `$#' will be expanded +before `eval' is executed. In bash-2.0, + + echo ${!#} + +does the same thing. + +27) If I pipe the output of a command into `read variable', why doesn't + the output show up in $variable when the read command finishes? + +This has to do with the parent-child relationship between Unix +processes. + +Each element of a pipeline runs in a separate process, a child of +the shell running the pipeline. A subprocess cannot affect its +parent's environment. When the `read' command sets the variable +to the input, that variable is set only in the subshell, not the +parent shell. When the subshell exits, the value of the variable +is lost. + +Many pipelines that end with `read variable' can be converted +into command substitutions, which will capture the output of +a specified command. The output can then be assigned to a +variable: + + grep ^gnu /usr/lib/news/active | wc -l | read ngroup + +can be converted into + + ngroup=$(grep ^gnu /usr/lib/news/active | wc -l) + +This does not, unfortunately, work to split the text among +multiple variables, as read does when given multiple variable +arguments. If you need to do this, you can either use the +command substitution above to read the output into a variable +and chop up the variable using the bash pattern removal +expansion operators or use some variant of the following +approach. + +Say /usr/local/bin/ipaddr is the following shell script: + +#! /bin/sh +host `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 positional +parameters. If you need them, you should save them before doing +this. + +This is the general approach -- in most cases you will not need to +set $IFS to a different value. + +28) I have a bunch of shell scripts that use backslash-escaped characters + in arguments to `echo'. Bash doesn't interpret these characters. Why + not, and how can I make it understand them? + +This is the behavior of echo on most Unix System V machines. + +The bash builtin `echo' is modelled after the 9th Edition +Research Unix version of `echo'. It does not interpret +backslash-escaped characters in its argument strings by default; +it requires the use of the -e option to enable the +interpretation. The System V echo provides no way to disable the +special characters; the bash echo has a -E option to disable +them. + +There is a configuration option that will make bash behave like +the System V echo and interpret things like `\t' by default. Run +configure with the --enable-usg-echo-default option to turn this +on. Be aware that this will cause some of the tests run when you +type `make tests' to fail. + +29) Why doesn't a while or for loop get suspended when I type ^Z? + +This is a consequence of how job control works on Unix. The only +thing that can be suspended is the process group. This is a single +command or pipeline of commands that the shell forks and executes. + +When you run a while or for loop, the only thing that the shell forks +and executes are any commands in the while loop test and commands in +the loop bodies. These, therefore, are the only things that can be +suspended when you type ^Z. + +If you want to be able to stop the entire loop, you need to put it +within parentheses, which will force the loop into a subshell that +may be stopped (and subsequently restarted) as a single unit. + +30) How can I make the bash `time' reserved word print timing output that + looks like the output from my system's /usr/bin/time? + +The bash command timing code looks for a variable `TIMEFORMAT' and +uses its value as a format string to decide how to display the +timing statistics. + +The value of TIMEFORMAT is a string with `%' escapes expanded in a +fashion similar in spirit to printf(3). The manual page explains +the meanings of the escape sequences in the format string. + +If TIMEFORMAT is not set, bash acts as if the following assignment had +been performed: + + TIMEFORMAT=$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS' + +The POSIX.2 default time format (used by `time -p command') is + + TIMEFORMAT=$'real %2R\nuser %2U\nsys %2S' + +The BSD /usr/bin/time format can be emulated with: + + TIMEFORMAT=$'\t%1R real\t%1U user\t%1S sys' + +The System V /usr/bin/time format can be emulated with: + + TIMEFORMAT=$'\nreal\t%1R\nuser\t%1U\nsys\t%1S' + +The ksh format can be emulated with: + + TIMEFORMAT=$'\nreal\t%2lR\nuser\t%2lU\nsys\t%2lS' + +Section F: Things to watch out for on certain Unix versions + +31) Why can't I use command line editing in my `cmdtool'? + +The problem is `cmdtool' and bash fighting over the input. When +scrolling is enabled in a cmdtool window, cmdtool puts the tty in +`raw mode' to permit command-line editing using the mouse for +applications that cannot do it themselves. As a result, bash and +cmdtool each try to read keyboard input immediately, with neither +getting enough of it to be useful. + +This mode also causes cmdtool to not implement many of the +terminal functions and control sequences appearing in the +`sun-cmd' termcap entry. For a more complete explanation, see +that file examples/suncmd.termcap in the bash distribution. + +`xterm' is a better choice, and gets along with bash much more +smoothly. + +If you must use cmdtool, you can use the termcap description in +examples/suncmd.termcap. Set the TERMCAP variable to the terminal +description 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 new +cmdtool. If this works, you can put the assignment to TERMCAP +in your bashrc file. + +32) I built bash on Solaris 2. Why do globbing expansions and filename + completion chop off the first few characters of each filename? + +This is the consequence of building bash on SunOS 5 and linking +with the libraries in /usr/ucblib, but using the definitions +and 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 $PATH +when configuring and building bash. This will ensure that you +use /usr/ccs/bin/cc or acc instead of /usr/ucb/cc and that you +link with libc before libucb. + +If you have installed the Sun C compiler, you may also need to +put /usr/ccs/bin and /opt/SUNWspro/bin into your $PATH before +/usr/ucb. + +33) 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 data +returned 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 (the +exact function is interpretwithsave()), and returns NULL, the +pointer is freed without being reset to NULL, and the function +returns. The next time getpwent is called, it sees that this +pointer is non-null, calls free, and the bash free() blows up +because it's being asked to free freed memory. + +The traditional Unix mallocs allow memory to be freed multiple +times; that's probably why this has never been fixed. You can +run configure with the `--without-gnu-malloc' option to use +the C library malloc and avoid the problem. + +34) I'm running SVR4.2. Why is the line erased every time I type `@'? + +The `@' character is the default `line kill' character in most +versions of System V, including SVR4.2. You can change this +character to whatever you want using `stty'. For example, to +change the line kill character to control-u, type + + stty kill ^U + +where the `^' and `U' can be two separate characters. + +35) 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 construct +is, in fact, a syntax error. Redirections may only precede `simple +commands'. A subshell construct such as the above is one of the shell's +`compound commands'. A redirection may only follow a compound command. + +The file CWRU/sh-redir-hack in the bash-2.0 distribution is an +(unofficial) patch to parse.y that will modify the grammar to +support this construct. It will not apply with `patch'; you must +modify parse.y by hand. Note that if you apply this, you must +recompile with -DREDIRECTION_HACK. This introduces a large +number of reduce/reduce conflicts into the shell grammar. + +Section G: Where do I go from here? + +36) How do I report bugs in bash, and where should I look for fixes and + advice? + +Use the `bashbug' script to report bugs. It is built and +installed at the same time as bash. It provides a standard +template for reporting a problem and automatically includes +information about your configuration and build environment. + +`bashbug' sends its reports to bug-bash@prep.ai.mit.edu, which +is a large mailing list gatewayed to the usenet newsgroup gnu.bash.bug. + +Bug fixes, answers to questions, and announcements of new releases +are all posted to gnu.bash.bug. Discussions concerning bash features +and problems also take place there. + +To reach the bash maintainers directly, send mail to +bash-maintainers@prep.ai.mit.edu. + +37) What kind of bash documentation is there? + +First, look in the doc directory in the bash distribution. It should +contain at least the following files: + +bash.1 an extensive, thorough Unix-style manual page +builtins.1 a manual page covering just bash builtin commands +bashref.texi a reference manual in GNU info format +bash.html an HTML version of the manual page +bashref.html an HTML version of the reference manual +FAQ this file +article.ms text of an article written for The Linux Journal +readline.3 a man page describing readline + +Postscript files created from the above source are available in +the documentation distribution. + +There is additional documentation available for anonymous FTP from host +slc2.ins.cwru.edu in the `pub/bash' directory. + +Cameron Newham and Bill Rosenblatt have written a book on bash, published +by O'Reilly and Associates. The book is based on Bill Rosenblatt's Korn +Shell book. The title is ``Learning the Bash Shell''. The ISBN number is +1-56592-147-X. Look for it in fine bookstores near you. This book covers +bash-1.14, but has an appendix describing some of the new features in +bash-2.0. + +38) What's coming in future versions? + +These are features I plan to include in a future version of bash. + +POSIX.2-style globbing character classes ([:alpha:], [:alnum:], etc.) +a bash debugger (an incomplete, untested version is included with bash-2.0) + +39) 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. + +Programmable completion a la zsh +menu completion a la tcsh +the ksh [[...]] extended test command +the ksh egrep-style extended pattern matching operators +associative arrays (not really all that hard) +breaking some of the shell functionality into embeddable libraries +better internationalization using GNU `gettext' +an option to use external files for the long `help' text +timeouts for the `read' builtin +the ksh-93 ${!prefix*} and ${!prefix@} operators +arithmetic ++ and -- prefix and postfix operators + +40) When will the next release appear? + +The next version will appear sometime in 1997. Never make predictions. + + +This document is Copyright 1995, 1996 by Chester Ramey. + +Permission is hereby granted, without written agreement and +without license or royalty fees, to use, copy, and distribute +this document for any purpose, provided that the above copyright +notice appears in all copies of this document and that the +contents of this document remain unaltered. |