path: root/doc/FAQ

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.