| This is the Bash FAQ, version 2.11, for Bash version 2.02. |
| |
| 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://ftp.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) Will bash run on operating systems other than Unix? |
| 6) How can I build bash with gcc? |
| 7) How can I make bash my login shell? |
| 8) I just changed my login shell to bash, and now I can't FTP into my |
| machine. Why not? |
| 9) What's the `POSIX 1003.2 standard'? |
| 10) What is the bash `posix mode'? |
| |
| Section B: The latest version |
| |
| 11) What's new in version 2.02? |
| 12) Are there any user-visible incompatibilities between bash-2.02 and |
| bash-1.14.7? |
| |
| Section C: Differences from other Unix shells |
| |
| 13) How does bash differ from sh, the Bourne shell? |
| 14) How does bash differ from the Korn shell, version ksh88? |
| 15) Which new features in ksh-93 are not in bash, and which are? |
| |
| Section D: Why does bash do some things differently than other Unix shells? |
| |
| 16) Why does bash run a different version of `command' than |
| `which command' says it will? |
| 17) Why doesn't bash treat brace expansions exactly like csh? |
| 18) Why doesn't bash have csh variable modifiers? |
| 19) How can I make my csh aliases work when I convert to bash? |
| 20) How can I pipe standard output and standard error from one command to |
| another, like csh does with `|&'? |
| 21) 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? |
| |
| 22) Why is the bash builtin `test' slightly different from /bin/test? |
| 23) Why does bash sometimes say `Broken pipe'? |
| 24) How can I get bash to read and display eight-bit characters? |
| 25) How do I write a function `x' to replace builtin command `x', but |
| still invoke the command from within the function? |
| 26) When I have terminal escape sequences in my prompt, why does bash |
| wrap lines at the wrong column? |
| 27) How can I find the value of a shell variable whose name is the value |
| of another shell variable? |
| 28) If I pipe the output of a command into `read variable', why doesn't |
| the output show up in $variable when the read command finishes? |
| 29) I have a bunch of shell scripts that use backslash-escaped characters |
| in arguments to `echo'. Bash doesn't interpret these characters. Why |
| not, and how can I make it understand them? |
| 30) Why doesn't a while or for loop get suspended when I type ^Z? |
| 31) How can I make the bash `time' reserved word print timing output that |
| looks like the output from my system's /usr/bin/time? |
| |
| Section F: Things to watch out for on certain Unix versions |
| |
| 32) Why can't I use command line editing in my `cmdtool'? |
| 33) I built bash on Solaris 2. Why do globbing expansions and filename |
| completion chop off the first few characters of each filename? |
| 34) Why does bash dump core after I interrupt username completion or |
| `~user' tilde expansion on a machine running NIS? |
| 35) I'm running SVR4.2. Why is the line erased every time I type `@'? |
| 36) Why does bash report syntax errors when my C News scripts use a |
| redirection before a subshell command? |
| |
| Section G: Where do I go from here? |
| |
| 37) How do I report bugs in bash, and where should I look for fixes and |
| advice? |
| 38) What kind of bash documentation is there? |
| 39) What's coming in future versions? |
| 40) What's on the bash `wish list'? |
| 41) When will the next release appear? |
| |
| ---------- |
| 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 2.02, first made available on Monday, 20 April, 1998. |
| |
| 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 ftp.cwru.edu. |
| The following URLs tell how to get version 2.02: |
| |
| ftp://prep.ai.mit.edu/pub/gnu/bash-2.02.tar.gz |
| ftp://ftp.cwru.edu/pub/bash/bash-2.02.tar.gz |
| |
| Formatted versions of the documentation are available with the URLs: |
| |
| ftp://prep.ai.mit.edu/pub/gnu/bash-doc-2.02.tar.gz |
| ftp://ftp.cwru.edu/pub/bash/bash-doc-2.02.tar.gz |
| |
| 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) Will bash run on operating systems other than Unix? |
| |
| Configuration specifics for Unix-like systems such as QNX and |
| LynxOS are included in the distribution. Previous versions of |
| bash have been ported to Minix, but I don't believe anyone has |
| built bash-2.x on Minix yet. |
| |
| Bash has been ported to versions of Windows implementing the Win32 |
| programming interface. This includes Windows 95 and Windows NT. |
| The port was done by Cygnus Solutions as part of their GNU-Win32 |
| project. For more information about the project, look at the URL |
| |
| http://www.cygnus.com/misc/gnu-win32 |
| |
| Cygnus has ported bash-1.14.7, and their port is part of the current |
| gnu-win32 release. Cygnus has also done a port of bash-2.01 to the |
| GNU-Win32 environment, and it should be available as part of their next |
| release. |
| |
| Bash-2.02 should require no local Cygnus changes to build and run under |
| GNU-WIN32. |
| |
| The Cygnus port works only on Intel machines. There is a port of bash |
| (I don't know which version) to the alpha/NT environment available from |
| |
| ftp://ftp.gnustep.org//pub/win32/bash-alpha-nt-1.01.tar.gz |
| |
| Softway Systems has ported bash-2.01.1 to their OpenNT system, a |
| Unix subsystem for NT that replaces the Microsoft POSIX subsystem. |
| Check out http://www.opennt.com for more information. |
| |
| D. J. Delorie has ported bash-1.14.7 to run under MS-DOS, as part of |
| the DJGPP project. For more information on the project, see |
| |
| http://www.delorie.com/djgpp/ |
| |
| I picked up a binary of bash-1.14.7 that is purported to work with |
| the DJGPP V2 environment from |
| |
| ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh1147b.zip |
| |
| The corresponding source is |
| |
| ftp://ftp.simtel.net/pub/simtelnet/gnu/djgpp/v2gnu/bsh1147s.zip |
| |
| Ports of bash-1.12 and bash-2.0 are available for OS/2 from |
| |
| ftp://hobbes.nmsu.edu/pub/os2/util/shell/bash_112.zip |
| ftp://hobbes.nmsu.edu/pub/os2/util/shell/bash-2.0(253).zip |
| |
| I haven't looked at either, but the second appears to be a binary-only |
| distribution. Beware. |
| |
| 6) How can I build bash with gcc? |
| |
| Bash configures to use gcc by default if it is available. Read the |
| file INSTALL in the distribution for more information. |
| |
| 7) How can I make bash my login shell? |
| |
| Some machines let you use `chsh' to change your login shell. Other |
| systems use `passwd -s' or `passwd -e'. If one of these works for |
| 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, 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 first file bash tries to read initialization commands from when |
| it is invoked as a login shell. |
| |
| Next, add a line similar to the above to ~/.profile: |
| |
| [ -f /usr/gnu/bin/bash ] && exec /usr/gnu/bin/bash --login |
| |
| This will cause login shells to replace themselves with bash running as |
| a login shell. Once you have this working, you can copy your initialization |
| code from ~/.profile to ~/.bash_profile. |
| |
| 8) I just changed my login shell to bash, and now I can't FTP into my |
| 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. |
| |
| 9) 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. |
| |
| 10) 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 |
| |
| 11) What's new in version 2.02? |
| |
| Bash-2.02 has a number of new features. Here's a short list: |
| |
| 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 unconditionally |
| POSIX.2-style globbing character classes ([:alpha:], [:alnum:], etc.) |
| POSIX.2-style globbing equivalence classes |
| POSIX.2-style globbing collating symbols |
| the ksh [[...]] extended conditional command |
| the ksh egrep-style extended pattern matching operators |
| a new `printf' builtin |
| the ksh-like $(<filename) command substitution, which is equivalent to |
| $(cat filename) |
| new tilde prefixes that expand to directories from the directory stack |
| new `**' arithmetic operator to do exponentiation |
| case-insensitive globbing (filename expansion) |
| menu completion a la tcsh |
| `magic-space' history expansion function like tcsh |
| the readline inputrc `language' has a new file inclusion directive ($include) |
| |
| Bash-2.01 contained only a few new features: |
| |
| new `GROUPS' builtin array variable containing the user's group list |
| new 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 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 |
| |
| 12) Are there any user-visible incompatibilities between bash-2.02 and |
| bash-1.14.7? |
| |
| There are a few incompatibilities between version 1.14.7 and version 2.02. |
| They are detailed in the file COMPAT in the bash-2.02 distribution. |
| |
| Section C: Differences from other Unix shells |
| |
| 13) 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' |
| $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, sh goes into an infinite loop.) |
| sh exits a script if any builtin fails; bash exits only if one of |
| the POSIX.2 `special' builtins fails |
| |
| 14) How does bash differ from the Korn shell, version ksh88? |
| |
| 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 |
| 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 |
| |
| 15) Which new features in ksh-93 are not in bash, and which are? |
| |
| New things in ksh-93 not in bash-2.02: |
| associative arrays |
| floating point arithmetic |
| ++, --, comma arithmetic operators |
| math library functions |
| ${!name[sub]} name of subscript for associative array |
| ${!prefix*} and {!prefix@} variable name prefix expansions |
| `.' is allowed in variable names to create a hierarchical namespace |
| more extensive compound assignment syntax |
| discipline functions |
| `sleep' and `getconf' builtins (bash has loadable versions) |
| typeset -n and `nameref' variables |
| KEYBD trap |
| variables: .sh.edchar, .sh.edmode, .sh.edcol, .sh.edtext, HISTEDIT, |
| .sh.version, .sh.name, .sh.subscript, .sh.value |
| backreferences in pattern matching |
| print -f (bash has a loadable version) |
| `fc' has been renamed to `hist' |
| read -t/-d |
| `.' can execute shell functions |
| |
| New things in ksh-93 present in bash-2.02: |
| ?: arithmetic operator |
| expansions: ${!param}, ${param:offset[:len]}, ${param/pat[/str]} |
| 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 |
| ENV processed only for interactive shells |
| |
| Section D: Why does bash do some things differently than other Unix shells? |
| |
| 16) 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. |
| |
| 17) 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. |
| |
| 18) 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 |
| |
| |
| 19) 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. |
| |
| 20) 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. |
| |
| 21) 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? |
| |
| 22) 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 $4. |
| Otherwise unspecified |
| 5 or more Args: unspecified. (Historical shells would use their |
| current algorithm). |
| |
| The operators -a and -o are considered binary operators for the purpose |
| of the 3 Arg case. |
| |
| As you can see, the test becomes (not (x or x)), which is false. |
| |
| 23) Why does bash sometimes say `Broken pipe'? |
| |
| If a sequence of commands appears in a pipeline, and one of the |
| reading commands finishes before the writer has finished, the |
| writer receives a SIGPIPE signal. Many other shells special-case |
| SIGPIPE as an exit status in the pipeline and do not report it. |
| For example, in: |
| |
| ps -aux | head |
| |
| `head' can finish before `ps' writes all of its output, and ps |
| will try to write on a pipe without a reader. In that case, bash |
| will print `Broken pipe' to stderr when ps is killed by a |
| SIGPIPE. |
| |
| 24) How can I get bash to read and display eight-bit characters? |
| |
| This is a process requiring several steps. |
| |
| First, you must ensure that the `physical' data path is a full eight |
| bits. For xterms, for example, the `vt100' resources `eightBitInput' |
| and `eightBitOutput' should be set to `true'. |
| |
| Once you have set up an eight-bit path, you must tell the kernel and |
| tty driver to leave the eighth bit of characters alone when processing |
| keyboard input. Use `stty' to do this: |
| |
| stty cs8 -istrip -parenb |
| |
| For old BSD-style systems, you can use |
| |
| stty pass8 |
| |
| You may also need |
| |
| stty even odd |
| |
| Finally, you need to tell readline that you will be inputting and |
| displaying eight-bit characters. You use readline variables to do |
| this. These variables can be set in your .inputrc or using the bash |
| `bind' builtin. Here's an example using `bind': |
| |
| bash$ 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. |
| |
| 25) How do I write a function `x' to replace builtin command `x', but |
| still invoke the command from within the function? |
| |
| This is why the `command' and `builtin' builtins exist. The |
| `command' builtin executes the command supplied as its first |
| argument, skipping over any function defined with that name. The |
| `builtin' builtin executes the builtin command given as its first |
| argument directly. |
| |
| For example, to write a function to replace `cd' that writes the |
| hostname and current directory to an xterm title bar, use |
| something like the following: |
| |
| cd() |
| { |
| builtin cd "$@" && xtitle "$HOST: $PWD" |
| } |
| |
| This could also be written using `command' instead of `builtin'; |
| the version above is marginally more efficient. |
| |
| 26) When I have terminal escape sequences in my prompt, why does bash |
| wrap lines at the wrong column? |
| |
| Readline, the line editing library that bash uses, does not know |
| that the terminal escape sequences do not take up space on the |
| screen. The redisplay code assumes, unless told otherwise, that |
| each character in the prompt is a `printable' character that |
| takes up one character position on the screen. |
| |
| You can use the bash prompt expansion facility (see the PROMPTING |
| section in the manual page) to tell readline that sequences of |
| characters in the prompt strings take up no screen space. |
| |
| Use the \[ escape to begin a sequence of non-printing characters, |
| and the \] escape to signal the end of such a sequence. |
| |
| 27) How can I find the value of a shell variable whose name is the value |
| of another shell variable? |
| |
| Bash-2.02 supports this directly. You can use |
| |
| ${!var} |
| |
| For example, the following sequence of commands will echo `z': |
| |
| var1=var2 |
| var2=z |
| echo ${!var1} |
| |
| For sh compatibility, use the `eval' builtin. The important |
| thing to remember is that `eval' expands the arguments you give |
| it again, so you need to quote the parts of the arguments that |
| you want `eval' to act on. |
| |
| For example, this expression prints the value of the last positional |
| parameter: |
| |
| eval echo \"\$\{$#\}\" |
| |
| The expansion of the quoted portions of this expression will be |
| deferred until `eval' runs, while the `$#' will be expanded |
| before `eval' is executed. In bash-2.02, |
| |
| echo ${!#} |
| |
| does the same thing. |
| |
| 28) If I pipe the output of a command into `read variable', why doesn't |
| the output show up in $variable when the read command finishes? |
| |
| This has to do with the parent-child relationship between Unix |
| processes. |
| |
| Each element of a pipeline runs in a separate process, a child of |
| the shell running the pipeline. A subprocess cannot affect its |
| parent's environment. When the `read' command sets the variable |
| to the input, that variable is set only in the subshell, not the |
| parent shell. When the subshell exits, the value of the variable |
| is lost. |
| |
| Many pipelines that end with `read variable' can be converted |
| into command substitutions, which will capture the output of |
| a specified command. The output can then be assigned to a |
| variable: |
| |
| grep ^gnu /usr/lib/news/active | wc -l | read ngroup |
| |
| can be converted into |
| |
| ngroup=$(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. |
| |
| 29) 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. |
| |
| 30) 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. |
| |
| 31) How can I make the bash `time' reserved word print timing output that |
| looks like the output from my system's /usr/bin/time? |
| |
| 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 |
| |
| 32) 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. |
| |
| 33) 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. |
| |
| 34) 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. |
| |
| 35) 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. |
| |
| 36) 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-2.02 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? |
| |
| 37) 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. |
| |
| 38) 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 |
| ftp.cwru.edu in the `pub/bash' directory. |
| |
| Cameron Newham and Bill Rosenblatt have written a book on bash, published |
| by O'Reilly and Associates. The book is based on Bill Rosenblatt's Korn |
| Shell book. The title is ``Learning the Bash Shell'', and the ISBN number |
| is 1-56592-147-X. Look for it in fine bookstores near you. This book |
| covers bash-1.14, but has an appendix describing some of the new features |
| in bash-2.0. |
| |
| A second edition of this book is available, just published in January, 1998. |
| The ISBN number is 1-56592-347-2. Look for it in the same fine bookstores |
| or on the web. |
| |
| 39) What's coming in future versions? |
| |
| These are features I plan to include in a future version of bash. |
| |
| a bash debugger (a minimally-tested version is included with bash-2.02) |
| Programmable completion a la zsh |
| |
| 40) What's on the bash `wish list' for future versions? |
| |
| These are features that may or may not appear in a future version of bash. |
| |
| associative arrays (not really all that hard) |
| breaking some of the shell functionality into embeddable libraries |
| better internationalization using GNU `gettext' |
| an option to use external files for the long `help' text |
| timeouts for the `read' builtin |
| the ksh-93 ${!prefix*} and ${!prefix@} operators |
| arithmetic ++ and -- prefix and postfix operators |
| date-stamped command history |
| |
| 41) When will the next release appear? |
| |
| The next version will appear sometime in 1998. Never make |
| predictions. |
| |
| |
| This document is Copyright 1995, 1996, 1998 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. |