BASH(1) BASH(1)
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file]
COPYRIGHT
Copyright (C) 1989, 1991 by the Free Software Foundation,
Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that
executes commands read from the standard input or from a
file. Bash also incorporates useful features from the
Korn and C shells (ksh and csh).
Bash is ultimately intended to be a faithful implementa-
tion of the IEEE Posix Shell and Tools specification (IEEE
Working Group 1003.2).
OPTIONS
In addition to the single-character shell options docu-
mented in the description of the set builtin command, bash
interprets the following flags when it is invoked:
-c string If the -c flag is present, then commands are
read from string.
-i If the -i flag is present, the shell is interac-
tive.
-s If the -s flag is present, or if no arguments
remain after option processing, then commands
are read from the standard input. This option
allows the positional parameters to be set when
invoking an interactive shell.
- A single - signals the end of options and dis-
ables further option processing. Any arguments
after the - are treated as filenames and argu-
ments. An argument of -- is equivalent to an
argument of -.
Bash also interprets a number of multi-character options.
These options must appear on the command line before the
single-character options to be recognized.
-norc Do not load the personal initialization file
~/.bashrc if the shell is interactive. This is
the default if the shell name is sh.
-noprofile
Do not read either /etc/profile or
~/.bashprofile. By default, bash normally
reads these files when it is invoked as a login
shell.
-rcfile file
Execute commands from file instead of the
GNU 1991 November 24 1
BASH(1) BASH(1)
standard personal initialization file ~/.bashrc,
if the shell is interactive.
-version Show the version number of this instance of bash
when starting.
-quiet Do not be verbose when starting up (do not show
the shell version or any other information).
-login Make bash act as if it had been invoked by
login(1).
-nobraceexpansion
Do not perform curly brace expansion a la csh.
-nolineediting
Do not use the GNU readline library to read com-
mand lines if interactive.
ARGUMENTS
If arguments remain after option processing, and neither
the -c nor the -s option has been supplied, the first
argument is assumed to be the name of a file containing
shell commands. If bash is invoked in this fashion, $0 is
set to the name of the file, and the positional parameters
are set to the remaining arguments. Bash reads and exe-
cutes commands from this file, then exits.
DEFINITIONS
blank A space or tab.
word A sequence of characters considered as a single
unit by the shell. Also known as a token.
name A word consisting only of alphanumeric characters
and underscores, and beginning with an alphabetic
character or an underscore. Also referred to as an
identifier.
metacharacter
A character that, when unquoted, separates words.
One of the following:
| & ; ( ) < > <space> <tab>
control operator
A token that performs a control function. It is
one of the following symbols:
|| & && ; ;; ( ) | <newline>
RESERVED WORDS
Reserved words are words that have a special meaning to
the shell. The following words are recognized as reserved
when unquoted and either the first word of a simple com-
mand (see SHELL GRAMMAR below) or the third word of a case
or for command:
! case do done elif else esac fi for function if in then
until while { }
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable
assignments followed by blank-separated words and
GNU 1991 November 24 2
BASH(1) BASH(1)
redirections, and terminated by a control operator. The
first word specifies the command to be executed. The
remaining words are passed as arguments to the invoked
command.
The return value of a simple command is its exit status,
or 128+n if the command is terminated by signal n.
Pipelines
A pipeline is a sequence of one or more commands separated
by the character |. The format for a pipeline is:
[ ! ] command [ | command2 ... ]
The standard output of command is connected to the stan-
dard input of command2. This connection is performed
before any redirections specified by the command (see
REDIRECTION below).
If the reserved word ! precedes a pipeline, the exit sta-
tus of that pipeline is the logical NOT of the exit status
of the last command. Otherwise, the status of the
pipeline is the exit status of the last command. The
shell waits for all commands in the pipeline to terminate
before returning a value.
Each command in a pipeline is executed as a separate pro-
cess (i.e. in a subshell).
Lists
A list is a sequence of one or more pipelines separated by
one of the operators ;, &, &&, or ||, and optionally ter-
minated by one of ;, &, or <newline>.
Of these list operators, && has highest precedence. ||
has the next highest precedence, followed by ; and &,
which have equal precedence.
If a command is terminated by the control operator &, the
shell executes the command in the background in a sub-
shell. The shell does not wait for the command to finish.
Commands separated by a ; are executed sequentially; the
shell waits for each command to terminate in turn.
The control operators && and || denote AND lists and OR
lists, respectively. An AND list has the form
command && command2
command2 is executed if, and only if, command returns an
exit status of zero.
An OR list has the form
GNU 1991 November 24 3
BASH(1) BASH(1)
command || command2
command2 is executed if and only if command returns a non-
zero exit status.
Compound Commands
A compound command is one of the following:
(list) list is executed in a subshell. Variable assign-
ments and builtin commands that affect the shell's
environment do not remain in effect after the com-
mand completes.
{ list; }
list is simply executed in the current shell envi-
ronment. This is known as a group command.
for name [ in word; ] do list ; done
The list of words following in is expanded, gener-
ating a list of items. The variable name is set to
each element of this list in turn, and list is exe-
cuted each time. If the in word is omitted, the
for command executes list once for each positional
parameter that is set (see PARAMETERS below). The
exit status is the exit status of the last command,
or zero if no commands were executed.
case word in [ pattern [ | pattern ] ... ) list ;; ] ...
esac
A case command first expands word, and tries to
match it against each pattern in turn. When a
match is found, the corresponding list is executed.
After the first match, no subsequent matches are
attempted. The exit status is zero if no patterns
are matches. Otherwise, it is the exit status of
the last command executed in list.
if list then list [ elif list then list ] ... [ else list
] fi
The if list is executed. If its exit status is
zero, the then list is executed. Otherwise, each
elif list is executed in turn, and if its exit sta-
tus is zero, the corresponding then list is exe-
cuted and the command completes. Otherwise, the
else list is executed, if present. The exit status
is the exit status of the last command executed, or
zero if no condition tested true.
while list do list done
until list do list done
The while command continuously executes the do list
as long as the last command in list returns an exit
status of zero. The until command is identical to
GNU 1991 November 24 4
BASH(1) BASH(1)
the while command, except that the test is negated;
the do list is executed as long as the last command
in list returns a non-zero exit status. The exit
status of the while and until commands is the exit
status of the last do list command executed, or
zero if none was executed.
[ function ] name () { list; }
This defines a function named name. The body of
the function is the list of commands between { and
}. This list is executed whenever name is speci-
fied as the name of a simple command. The exit
status of a function is the exit status of the last
command executed in the body. (See FUNCTIONS
below.)
COMMENTS
In a non-interactive shell, a word beginning with # causes
that word and all remaining characters on that line to be
ignored.
QUOTING
Quoting is used to remove the special meaning of certain
characters or words to the shell. Quoting can be used to
disable special treatment for special characters, to pre-
vent reserved words from being recognized as such, and to
prevent parameter expansion.
Each of the metacharacters listed above under DEFINITIONS
has special meaning to the shell and must be quoted if
they are to represent themselves. There are three quoting
mechanisms: the escape character, single quotes, and dou-
ble quotes.
A non-quoted backslash (\) is the escape character. It
preserves the literal value of the next character that
follows, with the exception of <newline>. If a \<newline>
pair appears, it is treated as a line continuation (that
is, it is effectively ignored), if the backslash is non-
quoted.
Enclosing characters in single quotes preserves the lit-
eral value of each character within the quotes. A single
quote may not occur between single quotes, even when pre-
ceded by a backslash.
Enclosing characters in double quotes preserves the lit-
eral value of all characters within the quotes, with the
exception of $, `, and \. The characters $ and ` retain
their special meaning within double quotes. The backslash
retains its special meaning only when followed by one of
the following characters: $, `, ", \, or <newline>. A
double quote may be quoted within double quotes by preced-
ing it with a backslash.
GNU 1991 November 24 5
BASH(1) BASH(1)
The special parameters * and @ have special meaning when
in double quotes (see PARAMETERS below).
PARAMETERS
A parameter is an entity that stores values, somewhat like
a variable in a conventional programming language. It can
be a name, a number, or one of the special characters
listed below under Special Parameters. For the shell's
purposes, a variable is a parameter denoted by a name.
A parameter is set if it has been assigned a value. The
null string is a valid value. Once a variable is set, it
may be unset only by using the unset builtin command (see
SHELL BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null
string. All values undergo tilde expansion, parameter and
variable expansion, command substitution, arithmetic
expansion, and quote removal. If the variable has its -i
attribute set (see declare below in SHELL BUILTIN COM-
MANDS) then value is subject to arithmetic expansion even
if the $[...] syntax does not appear. Word splitting is
not performed, with the exception of "$@" as explained
below under Special Parameters. Pathname expansion is not
performed.
Positional Parameters
A positional parameter is a parameter denoted by one or
more digits, other than the single digit 0. Positional
parameters are assigned from the shell's arguments when it
is invoked, and may be reassigned using the set builtin
command. The positional parameters are temporarily
replaced when a shell function is executed (see FUNCTIONS
below).
When a positional parameter consisting of more than a sin-
gle digit is expanded, it must be enclosed in braces (see
EXPANSION below).
Special Parameters
The shell treats several parameters specially. These
parameters may only be referenced; assignment to them is
not allowed.
* Expands to the positional parameters, starting from
one. When the expansion occurs within double
quotes, it expands to a single word with the value
of each parameter separated by the first character
of the IFS special variable. That is, ``$*'' is
equivalent to ``$1c$2c...'', where c is the first
character of the value of the IFS variable. If IFS
GNU 1991 November 24 6
BASH(1) BASH(1)
is null or unset, the parameters are separated by
spaces.
@ Expands to the positional parameters, starting from
one. When the expansion occurs within double
quotes, each parameter expands as a separate word.
That is, `` $@'' is equivalent to ``$1'' ``$2'' ...
When there are no positional parameters, ``$@'' and
$@ expand to nothing (i.e. they are removed).
# Expands to the number of positional parameters in
decimal.
? Expands to the status of the most recently executed
foreground pipeline.
- Expands to the current option flags as specified
upon invocation, by the set builtin command, or
those set by the shell itself (such as the -i
flag).
$ Expands to the process ID of the shell. In a ()
subshell, it expands to the process ID of the cur-
rent shell, not the subshell.
! Expands to the process ID of the most recently exe-
cuted background (asynchronous) command.
0 Expands to the name of the shell or shell script.
This is set at shell initialization. If bash is
invoked with a file of commands, $0 is set to the
name of that file. Otherwise, it is set to the
pathname used to invoke bash, as given by argument
zero.
Expands to the last argument to the previous com-
mand, after expansion. Also set to the full path-
name of each command executed and placed in the
environment exported to that command.
Shell Variables
The following variables are set by the shell:
PPID The process ID of the shell's parent.
PWD The current working directory as set by the cd com-
mand.
OLDPWD The previous working directory as set by the cd
command.
REPLY Set to the line of input read by the read builtin
command when no arguments are supplied.
UID Expands to the user ID of the current user.
EUID Expands to the effective user ID of the current
user.
BASH Expands to the full pathname used to invoke this
instance of bash.
BASHVERSION
Expands to the version number of this instance of
bash.
SHLVL Incremented by one each time an instance of bash is
started.
RANDOM Each time this parameter is referenced, a random
integer is generated. The sequence of random
GNU 1991 November 24 7
BASH(1) BASH(1)
numbers may be initialized by assigning a value to
RANDOM. If RANDOM is unset, it loses its special
properties, even if it is subsequently reset.
SECONDS
Each time this parameter is referenced, the number
of seconds since shell invocation is returned. If
a value is assigned to SECONDS, the value returned
upon subsequent references is the number of seconds
since the assignment plus the value assigned. If
SECONDS is unset, it loses its special properties,
even if it is subsequently reset.
LINENO Each time this parameter is referenced, the shell
substitutes a decimal number representing the cur-
rent sequential line number (starting with 1)
within a script or function. When not in a script
or function, the value substituted is not guaran-
teed to be meaningful. When in a function, the
value is not the number of the source line that the
command appears on (that information has been lost
by the time the function is executed), but is an
approximation of the number of simple commands exe-
cuted in the current function. If LINENO is unset,
it loses its special properties, even if it is sub-
sequently reset.
OPTARG The value of the last option argument processed by
the getopts builtin command (see SHELL BUILTIN COM-
MANDS below).
OPTIND The index of the last option processed by the
getopts builtin command (see SHELL BUILTIN COMMANDS
below).
The following variables are used by the shell. In some
cases, bash assigns a default value to a variable; these
cases are noted below.
IFS The Internal Field Separator that is used for word
splitting after expansion and to split lines into
words with the read builtin command. The default
value is ``<space><tab><newline>''.
PATH The search path for commands. It is a colon-
separated list of directories in which the shell
looks for commands (see COMMAND EXECUTION below).
The default path is system-dependent, and is set by
the administrator who installs bash. A common
value is
``.:/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin:/etc:/usr/etc''.
Note that in some circumstances, however, a leading
`.' in PATH can be a security hazard.
HOME The home directory of the current user; the default
argument for the cd builtin command.
CDPATH The search path for the cd command. This is a
colon-separated list of directories in which the
shell looks for destination directories specified
by the cd command. A sample value is ``.:~:/usr''.
GNU 1991 November 24 8
BASH(1) BASH(1)
ENV If this parameter is set when bash is executing a
shell script, its value is interpreted as a file-
name containing commands to initialize the shell,
as in .bashrc. The value of ENV is subjected to
parameter expansion, command substitution, and
arithmetic expansion before being interpreted as a
pathname. PATH is not used to search for the
resultant pathname.
MAIL If this parameter is set to a filename and the
MAILPATH variable is not set, bash informs the user
of the arrival of mail in the specified file.
MAILCHECK
Specifies how often (in seconds) bash checks for
mail. The default is 60 seconds. When it is time
to check for mail, the shell does so before prompt-
ing. If this variable is unset, the shell disables
mail checking.
MAILPATH
A colon-separated list of pathnames to be checked
for mail. The message to be printed may be speci-
fied by separating the pathname from the message
with a `?'. $_ stands for the name of the current
mailfile. Example:
MAILPATH='/usr/spool/mail/bfox?"You have
mail":~/shell-mail?"$_ has mail!"'
Bash supplies a default value for this variable,
but the location of the user mail files that it
uses is system dependent (e.g.
/usr/spool/mail/$USER).
MAILWARNING
If set, and a file that bash is checking for mail
has been accessed since the last time it was
checked, the message ``The mail in` mailfile has
been read'' is printed.
PS1 The value of this parameter is expanded (see
PROMPTING below) and used as the primary prompt
string. The default value is ``bash\$ ''.
PS2 The value of this parameter is expanded like PS1
and used as the secondary prompt string. The
default is ``> ''.
PS4 The value of this parameter is expanded like PS1
and the value is printed before each command bash
displays during an execution trace. The first
character of PS4 is replicated multiple times, as
necessary, to indicate multiple levels of indirec-
tion. The default is ``+ ''.
NOPROMPTVARS
If set, the decoded prompt string does not undergo
further expansion (see PROMPTING below).
HISTSIZE
The number of commands to remember in the command
history (see HISTORY below).
HISTFILE
The name of the file in which command history is
GNU 1991 November 24 9
BASH(1) BASH(1)
saved. (See HISTORY below.)
HISTFILESIZE
The maximum number of lines contained in the his-
tory file. When this variable is assigned a value,
the history file is truncated, if necessary, to
contain no more than that number of lines.
OPTERR If set to the value 1, bash displays error messages
generated by the getopts builtin command (see SHELL
BUILTIN COMMANDS below). OPTERR is initialized to
1 each time the shell is invoked or a shell script
is executed.
PROMPTCOMMAND
If set, the value is executed as a command prior to
issuing each primary prompt.
IGNOREEOF
ignoreeof
Controls the action of the shell on receipt of an
EOF character as the sole input. If set, the value
is the number of consecutive EOF characters typed
before bash exits. If the variable exists but does
not have a numeric value, or has no value, the
default value is 10. If it does not exist, EOF
signifies the end of input to the shell. This is
only in effect for interactive shells.
HOSTTYPE
Automatically set to a string that uniquely
describes the type of machine on which bash is exe-
cuting. The default is system-dependent.
TMOUT If set to a value greater than zero, the value is
interpreted as the number of seconds to wait for
input after issuing the primary prompt. Bash ter-
minates after waiting for that number of seconds if
input does not arrive.
FCEDIT The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when
performing filename completion (see READLINE
below). A filename whose suffix matches one of the
entries in FIGNORE is excluded from the list of
matched filenames. A sample value is ``.o:~''.
notify If set, bash reports terminated background jobs
immediately, rather than waiting until before
printing the next primary prompt.
historycontrol
If set to a value of ignorespace, it means don't
enter lines which begin with a <space> on the his-
tory list. If set to a value of ignoredups, it
means don't enter lines which match the last
entered line. If unset, or if set to any other
value than those above, all lines read by the
parser are saved on the history list.
commandorientedhistory
If set, bash attempts to save all lines of a multi-
ple-line command in the same history entry. This
GNU 1991 November 24 10
BASH(1) BASH(1)
allows easy re-editing of multi-line commands.
globdotfilenames
If set, bash includes filenames beginning with a
`.' in the results of pathname expansion.
allownullglobexpansion
If set, bash allows pathname patterns which match
no files (see Pathname Expansion below) to expand
to a null string, rather than themselves.
histchars
The two characters which control history expansion
and tokenization. The first character is the his-
tory expansion character, that is, the character
which signals the start of a history expansion,
normally `!'. The second character is the charac-
ter which signifies that the remainder of the line
is a comment, when found as the first character of
a word.
nolinks
If set, the shell does not follow symbolic links
when executing commands that change the current
working directory. It uses the physical directory
structure instead. By default, bash follows the
logical chain of directories when performing com-
mands such as cd.
hostnamecompletionfile
Contains the name of a file in the same format as
/etc/hosts that should be read when the shell needs
to complete a hostname. You can change the file
interactively; the next time you want to complete a
hostname bash adds the contents of the new file to
the already existing database.
noclobber
If set, bash does not overwrite an existing file
with the >, >&, and <> redirection operators. This
variable may be overridden when creating output
files by using the redirection operator >| instead
of > (see also the -C option to the set builtin
command).
autoresume
This variable controls how the shell interacts with
the user and job control. If this variable is set,
single word simple commands without redirections
are treated as candidates for resumption of an
existing stopped job. There is no ambiguity
allowed; if there is more than one job beginning
with the string typed, the job most recently
accessed is selected.
noexitonfailedexec
If this variable exists, the shell does not exit if
it cannot execute the file specified in the exec
command.
cdablevars
If this is set, an argument to the cd builtin com-
mand that is not a directory is assumed to be the
GNU 1991 November 24 11
BASH(1) BASH(1)
name of a variable whose value is the directory to
change to.
pushdsilent
If set, the pushd and popd builtin commands do not
print the current directory stack after successful
execution.
EXPANSION
Expansion is performed on the command line after it has
been split into words. There are seven kinds of expansion
performed: brace expansion, tilde expansion, parameter and
variable expansion, command substitution, arithmetic
expansion, word splitting, and pathname expansion.
The order of expansions is: brace expansion, tilde expan-
sion, parameter, variable, command, and arithmetic substi-
tution (done in a left-to-right fashion), word splitting,
and pathname expansion.
Only brace expansion, word splitting, and pathname expan-
sion can change the number of words of the expansion;
other expansions expand a single word to a single word.
The single exception to this is the expansion of ``$@'' as
explained above (see PARAMETERS).
Brace Expansion
Brace expansion is a mechanism by which arbitrary strings
may be generated. This mechanism is similar to pathname
expansion, but the filenames generated need not exist.
Patterns to be brace expanded take the form of an optional
preamble, followed by a series of comma-separated strings
between a pair of braces, followed by an optional postam-
ble. The preamble is prepended to each string contained
within the braces, and the postamble is then appended to
each resulting string, expanding left to right.
Brace expansions may be nested. The results of each
expanded string are not sorted; left to right order is
preserved. For example, a{d,c,b}e expands into `ade ace
abe'.
Brace expansion is performed before any other expansions,
and any characters special to other expansions are pre-
served in the result. It is strictly textual. Bash does
not apply any syntactic interpretation to the context of
the expansion or the text between the braces.
This construct is typically used as shorthand when the
common prefix of the strings to be generated is longer
than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
GNU 1991 November 24 12
BASH(1) BASH(1)
Brace expansion introduces a slight incompatibility with
traditional versions of sh, the Bourne shell. sh does not
treat opening or closing braces specially when they appear
as part of a word, and preserves them in the output. Bash
removes braces from words as a consequence of brace expan-
sion. For example, a word entered to sh as file{1,2}
appears identically in the output. The same word is out-
put as file1 file2 after expansion by bash. If strict
compatibility with sh is desired, start bash with the
-nobraceexpansion flag (see OPTIONS above) or disable
brace expansion with the +o braceexpand option to the set
command (see SHELL BUILTIN COMMANDS below).
Tilde Expansion
If a word begins with a tilde character (`~'), all of the
characters preceding the first slash (or all characters,
if there is no slash) are treated as a possible login
name. If this login name is the null string, the tilde is
replaced with the value of the parameter HOME. If HOME is
unset, the home directory of the user executing the shell
is substituted instead.
If a `+' follows the tilde, the value of PWD is substi-
tuted. If a `-' follows, the value of OLDPWD is used.
Each variable assignment is checked for unquoted instances
of tildes following a : or =. In these cases, tilde sub-
stitution is also performed. Consequently, one may use
pathnames with tildes in PATH, MAILPATH, and CDPATH, and
the shell will export the expanded variables.
Parameter Expansion
The `$' character introduces parameter expansion, command
substitution, or arithmetic expansion. The parameter name
or symbol to be expanded may be enclosed in braces, which
are optional but serve to protect the variable to be
expanded from characters immediately following it which
could be interpreted as part of the name.
${parameter}
The value of parameter is substituted. The braces
are required when parameter is a positional parame-
ter with more than one digit, or when parameter is
followed by a character which is not to be inter-
preted as part of its name.
In each of the cases below, word is subject to tilde
expansion, parameter expansion, command substitution, and
arithmetic expansion. Bash tests for a parameter that is
unset or null; omitting the colon results in a test only
for a parameter that is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null,
GNU 1991 November 24 13
BASH(1) BASH(1)
the expansion of word is substituted. Otherwise,
the value of parameter is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or
null, the expansion of word is assigned to parame-
ter. The value of parameter is then substituted.
Positional parameters and special parameters may
not be assigned to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is
null or unset, the expansion of word (or a message
to that effect if word is not present) is written
to the standard error and the shell, if it is not
interactive, exits. Otherwise, the value of param-
eter is substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or
unset, nothing is substituted, otherwise the expan-
sion of word is substituted.
${#parameter}
The length in characters of the value of parameter
is substituted. If parameter is * or @, the length
substituted is the length of * expanded within dou-
ble quotes.
${parameter#word}
${parameter##word}
The word is expanded to produce a pattern just as
in pathname expansion. If the pattern matches the
beginning of the value of parameter, then the
expansion is the value of parameter with the short-
est matching pattern deleted (the ``#'' case) or
the longest matching pattern deleted (the ``##''
case).
${parameter%word}
${parameter%%word}
The word is expanded to produce a pattern just as
in pathname expansion. If the pattern matches a
trailing portion of the value of parameter, then
the expansion is the value of parameter with the
shortest matching pattern deleted (the ``%'' case)
or the longest matching pattern deleted (the ``%%''
case).
Command Substitution
Command substitution allows the output of a command to
replace the command name. There are two forms:
$(command)
or
`command`
Bash performs the expansion by executing command and
GNU 1991 November 24 14
BASH(1) BASH(1)
replacing the command substitution with the standard out-
put of the command, with any trailing newlines deleted.
When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed
by $, `, or \. When using the $(command) form, all char-
acters between the parentheses make up the command; none
are treated specially.
Command substitutions may be nested. To nest when using
the old form, escape the inner backquotes with back-
slashes.
If the substitution appears within double quotes, word
splitting and pathname expansion are not performed on the
results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arith-
metic expression and the substitution of the result. The
format for arithmetic expansion is:
$[expression]
The expression is treated as if it were within double
quotes, but a double quote inside the braces is not
treated specially. All tokens in the expression undergo
parameter expansion, command substitution, and quote
removal. Arithmetic substitutions may be nested.
The evaluation is performed according to the rules listed
below under ARITHMETIC EVALUATION. If expression is
invalid, bash prints a message indicating failure and no
substitution occurs.
Word Splitting
The shell scans the results of parameter expansion, com-
mand substitution, and arithmetic expansion that did not
occur within double quotes for word splitting.
The shell treats each character of IFS as a delimiter, and
splits the results of the other expansions into words on
these characters. If the value of IFS is exactly
<space><tab><newline>, the default, then any sequence of
IFS characters serves to delimit words; otherwise each
occurrence of an IFS character is treated as a delimiter.
If the value of IFS is null, no word splitting occurs.
IFS cannot be unset.
Explicit null arguments ("" or '') are retained. Implicit
null arguments, resulting from the expansion of parameters
that have no values, are removed.
Note that if no expansion occurs, no splitting is
GNU 1991 November 24 15
BASH(1) BASH(1)
performed.
Pathname Expansion
After word splitting, bash scans each word for the charac-
ters *, ?, and [, unless the -f flag has been set. If one
of these characters appears, then the word is regarded as
a pattern, and replaced with an alphabetically sorted list
of pathnames matching the pattern. If no matching path-
names are found, and the shell variable
allownullglobexpansion is unset, the word is left
unchanged. If the variable is set, the word is removed if
no matches are found. When a pattern is used for pathname
generation, the character ``.'' at the start of a name or
immediately following a slash must be matched explicitly,
unless the shell variable globdotfilenames is set. The
slash character must always be matched explicitly. In
other cases, the ``.'' character is not treated spe-
cially.
The special pattern characters have the following mean-
ings:
* Matches any string, including the null string.
? Matches any single character.
[...] Matches any one of the enclosed characters. A pair
of characters separated by a minus sign denotes a
range; any character lexically between those two
characters, inclusive, is matched. If the first
character following the [ is a ! or a ^ then any
character not enclosed is matched. A - or ] may be
matched by including it as the first or last char-
acter in the set.
Quote Removal
After the preceding expansions, all unquoted occurrences
of the characters \, `, and " are removed.
REDIRECTION
Before a command is executed, its input and output may be
redirected using a special notation interpreted by the
shell. Redirection may also be used to open and close
files for the current shell execution environment. The
following redirection operators may appear anywhere in a
simple command or may precede or follow a command. Redi-
rections are processed in the order they appear, from left
to right.
In the following descriptions, if the file descriptor num-
ber is omitted, and the first character of the redirection
operator is <, the redirection refers to the standard
input (file descriptor 0). If the first character of the
redirection operator is >, the redirection refers to the
standard output (file descriptor 1).
GNU 1991 November 24 16
BASH(1) BASH(1)
The word that follows the redirection operator in the fol-
lowing descriptions is subjected to brace expansion, tilde
expansion, parameter expansion, command substitution,
arithmetic expansion, quote removal, and pathname expan-
sion. If it expands to more than one word, bash reports
an error.
Redirecting Input
Redirection of input causes the file whose name results
from the expansion of word to be opened for reading on
file descriptor n, or the standard input (file descriptor
0) if n is not specified.
The general format for redirecting input is:
[n]<word
Redirecting Output
Redirection of output causes the file whose name results
from the expansion of word to be opened for writing on
file descriptor n, or the standard output (file descriptor
1) if n is not specified. If the file does not exist it
is created; if it does exist it is truncated to zero size.
The general format for redirecting output is:
[n]>word
If the redirection operator is >|, then the variable
noclobber is not consulted, and the file is created
regardless of the value of noclobber (see Shell Variables
above).
Appending Redirected Output
Redirection of output in this fashion causes the file
whose name results from the expansion of word to be opened
for appending on file descriptor n, or the standard output
(file descriptor 1) if n is not specified. If the file
does not exist it is created.
The general format for appending output is:
[n]>>word
Redirecting Standard Output and Standard Error
Bash allows both the standard output (file descriptor 1)
and the standard error output (file descriptor 2) to be
redirected to the file whose name is the expansion of word
with this construct.
There are two formats for redirecting standard output and
standard error:
GNU 1991 November 24 17
BASH(1) BASH(1)
&>word
and
>&word
Of the two forms, the first is preferred. This is seman-
tically equivalent to
>word 2>&1
Here Documents
This type of redirection instructs the shell to read input
from the current source until a line containing only word
(with no trailing blanks) is seen. All of the lines read
up to that point are then used as the standard input for a
command.
The format of here-documents is as follows:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, pathname
expansion, or arithmetic expansion is performed on word.
If any characters in word are quoted, the delimiter is the
result of quote removal on word, and the lines in the
here-document are not expanded. Otherwise, all lines of
the here-document are subjected to parameter expansion,
command substitution, and arithmetic expansion. In the
latter case, the pair \<newline> is ignored, and \ must be
used to quote the characters \, $, and `.
If the redirection operator is <<-, then all leading tab
characters are stripped from input lines and the line con-
taining delimiter. This allows here-documents within
shell scripts to be indented in a natural fashion.
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word
expands to one or more digits, the file descriptor denoted
by n is made to be a copy of that file descriptor. If
word evaluates to -, file descriptor n is closed. If n is
not specified, the standard input (file descriptor 0) is
used.
The operator
[n]>&word
is used similarly to duplicate output file descriptors.
GNU 1991 November 24 18
BASH(1) BASH(1)
If n is not specified, the standard output (file descrip-
tor 1) is used.
Opening File Descriptors for Reading and Writing
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be
opened for both reading and writing on file descriptor n,
or as the standard input and standard output if n is not
specified.
Note that the order of redirections is significant. For
example, the command
ls > dirlist 2>&1
directs both standard output and standard error to the
file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because
the standard error was duplicated as standard output
before the standard output was redirected to dirlist.
FUNCTIONS
A shell function, defined as described above under SHELL
GRAMMAR, stores a series of commands for later execution.
However, functions are executed in the context of the cur-
rent shell; no new process is created to interpret them
(contrast this with the execution of a shell script).
When a function is executed, the arguments to the function
become the positional parameters during its execution.
The special parameter # is updated to reflect the change.
Positional parameter 0 is unchanged.
Variables local to the function may be declared with the
local builtin command. Ordinarily, variables and their
values are shared between the function and its caller.
If the builtin command return is executed in a function,
the function completes and execution resumes with the next
command after the function call. When a function com-
pletes, the values of the positional parameters and the
special parameter # are restored to the values they had
prior to function execution.
Function names may be listed with the -f option to the
declare or typeset builtin commands. Functions may be
exported so that subshells automatically have them defined
with the -f option to the export builtin.
GNU 1991 November 24 19
BASH(1) BASH(1)
Functions may be recursive. No limit is imposed on the
number of recursive calls.
ALIASES
The shell maintains a list of aliases that may be set and
unset with the alias and unalias builtin commands. The
first word of each command is checked to see if it has an
alias. If so, that word is replaced by the text of the
alias. The alias name and the replacement text may con-
tain any valid shell input, including the metacharacters
listed above. The first word of the replacement text is
tested for aliases, but a word that is identical to an
alias being expanded is not expanded a second time. This
means that one may alias ls to ls -F, for instance, and
bash does not try to recursively expand the replacement
text. If the last character of the alias value is a
blank, then the next command word following the alias is
also checked for alias expansion.
Aliases are created and listed with the alias command, and
removed with the unalias command.
There is no mechanism for using arguments in the replace-
ment text, a la csh. If arguments are needed, a shell
function should be used.
The rules concerning the definition and use of aliases are
somewhat confusing. Bash always reads at least one com-
plete line of input before executing any of the commands
on that line. Aliases are expanded when a command is
read, not when it is executed. Therefore, an alias defi-
nition appearing on the same line as another command does
not take effect until the next line of input is read.
This means that the commands following the alias defini-
tion on that line are not affected by the new alias. This
behavior is also an issue when functions are executed.
Aliases are expanded when the function definition is read,
not when the function is executed, because a function def-
inition is itself a compound command. As a consequence,
aliases defined in a function are not available until
after that function is executed. To be safe, always put
alias definitions on a separate line, and do not use alias
in compound commands.
Aliases are not expanded when the shell is not interac-
tive.
Note that for almost every purpose, aliases are superseded
by shell functions.
JOB CONTROL
Job control refers to the ability to selectively stop
(suspend) the execution of processes and continue (resume)
their execution at a later point. A user typically
GNU 1991 November 24 20
BASH(1) BASH(1)
employs this facility via an interactive interface sup-
plied jointly by the system's terminal driver and bash.
The shell associates a job with each pipeline. It keeps a
table of currently executing jobs, which may be listed
with the jobs command. When bash starts a job asyn-
chronously (in the background), it prints a line that
looks like:
[1] 25647
indicating that this job is job number 1 and that the pro-
cess ID of the single process in the job is 25647. Bash
uses the job abstraction as the basis for job control.
To facilitate the implementation of the user interface to
job control, the system maintains the notion of a current
terminal process group ID. Members of this process group
(processes whose process group ID is equal to the current
terminal process group ID) receive keyboard-generated sig-
nals such as SIGINT. These processes are said to be in
the foreground. Background processes are those whose pro-
cess group ID differs from the terminal's; such processes
are immune to keyboard-generated signals. Only foreground
processes are allowed to read from or write to the termi-
nal. Background processes which attempt to read from
(write to) the terminal are sent a SIGTTIN (SIGTTOU) sig-
nal by the terminal driver, which, unless caught, causes
the process to stop.
If the operating system on which bash is running supports
job control, bash allows you to use it. Typing the sus-
pend character (typically ^Z, Control-Z) while a process
is running causes that process to be stopped and returns
you to bash. Typing the delayed suspend character (typi-
cally ^Y, Control-Y) causes the process to be stopped when
it attempts to read input from the terminal, and control
to be returned to bash. You may then manipulate the state
of this job, using the bg command to continue it in the
background, the fg command to continue it in the fore-
ground, or the kill command to kill it. A ^Z takes effect
immediately, and has the additional side effect of causing
pending output and typeahead to be discarded.
There are a number of ways to refer to a job in the shell.
The character % introduces a job name. Job number n may
be referred to as %n. A job may also be referred to using
a prefix of the name used to start it, or using a sub-
string that appears in its command line. For example, %ce
refers to a stopped ce job. If a prefix matches more than
one job, bash reports an error. Using %?ce, on the other
hand, would refer to any job containing the string ce in
its command line. If the substring matches more than one
job, bash reports an error. The symbols %% and %+ refer
GNU 1991 November 24 21
BASH(1) BASH(1)
to the shell's notion of the current job, which is the
last job stopped while it was in the foreground. The pre-
vious job may be referenced using %-. In output pertain-
ing to jobs (e.g. the output of the jobs command), the
current job is always flagged with a +, and the previous
job with a -.
Simply naming a job can be used to bring it into the fore-
ground: %1 is a synonym for ``fg %1'', bringing job 1 from
the background into the foreground. Similarly, ``%1 &''
resumes job 1 in the background, equivalent to ``bg %1''.
The shell learns immediately whenever a job changes state.
Normally, bash waits until it is about to print a prompt
before reporting changes in a job's status so as to not
interrupt any other output. If the variable notify is
set, bash reports such changes immediately. (See also the
-o notify option to the set builtin command.)
If you attempt to exit bash while jobs are stopped, the
shell prints a message warning you. You may then use the
jobs command to inspect their status. If you do this, or
try to exit again immediately, you are not warned again,
and the stopped jobs are terminated.
SIGNALS
When bash is interactive, it ignores SIGTERM (so that kill
0 does not kill an interactive shell), and SIGINT is
caught and handled (so that wait is interruptible). In
all cases, bash ignores SIGQUIT. If job control is in
effect, bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.
Synchronous jobs started by bash have signals set to the
values inherited by the shell from its parent. Background
jobs (jobs started with &) ignore SIGINT and SIGQUIT.
Commands run as a result of command substitution ignore
the keyboard-generated job control signals SIGTTIN, SIGT-
TOU, and SIGTSTP.
COMMAND EXECUTION
After a command has been split into words, if it results
in a simple command and an optional list of arguments, the
following actions are taken.
If the command name contains no slashes, the shell
attempts to locate it. If there exists a shell function
by that name, that function is invoked as described above
in FUNCTIONS. If the name does not match a function, the
shell searches for it in the list of shell builtins. If a
match is found, that builtin is invoked.
If the name is neither a shell function nor a builtin, and
contains no slashes, bash searches each element of the
PATH for a directory containing an executable file by that
GNU 1991 November 24 22
BASH(1) BASH(1)
name. If the search is unsuccessful, the shell prints an
error message and returns a nonzero exit status.
If the search is successful, or if the command name con-
tains one or more slashes, the shell executes the named
program. Argument 0 is set to the name given, and the
remaining arguments to the command are set to the argu-
ments given, if any.
If this execution fails because the file is not in exe-
cutable format, and the file is not a directory, it is
assumed to be a shell script, a file containing shell com-
mands. A subshell is spawned to execute it. This sub-
shell reinitializes itself, so that the effect is as if a
new shell had been invoked to handle the script, with the
exception that the locations of commands remembered by the
parent (see hash below under SHELL BUILTIN COMMANDS) are
retained by the child.
If the program is a file beginning with #!, the remainder
of the first line specifies an interpreter for the pro-
gram. The shell executes the specified interpreter on
operating systems that do not handle this executable for-
mat themselves. The arguments to the interpreter consist
of a single optional argument following the interpreter
name on the first line of the program, followed by the
name of the program, followed by the command arguments, if
any.
ENVIRONMENT
When a program is invoked it is given an array of strings
called the environment. This is a list of name-value
pairs, of the form name=value.
The shell allows you to manipulate the environment in sev-
eral ways. On invocation, the shell scans its own envi-
ronment and creates a parameter for each name found, auto-
matically marking it for export to child processes. Exe-
cuted commands inherit the environment. The export and
declare -x commands allow parameters and functions to be
added to and deleted from the environment. If the value
of a parameter in the environment is modified, the new
value becomes part of the environment, replacing the old.
The environment inherited by any executed command consists
of the shell's initial environment, whose values may be
modified in the shell, less any pairs removed by the unset
command, plus any additions via the export and declare -x
commands.
The environment for any simple command or function may be
augmented temporarily by prefixing it with parameter
assignments, as described above in PARAMETERS. These
assignment statements affect only the environment seen by
that command.
GNU 1991 November 24 23
BASH(1) BASH(1)
If the -k flag is set (see the set builtin command below),
then all parameter assignments are placed in the environ-
ment for a command, not just those that precede the com-
mand name.
EXIT STATUS
For the purposes of the shell, a command which exits with
a zero exit status has succeeded. An exit status of zero
indicates success. A non-zero exit status indicates fail-
ure. When a command terminates on a fatal signal, bash
uses the value of 128+signal as the exit status.
Bash itself returns the exit status of the last command
executed, unless a syntax error occurs, in which case it
exits with a non-zero value. See also the exit builtin
command below.
PROMPTING
When executing interactively, bash displays the primary
prompt PS1 when it is ready to read a command, and the
secondary prompt PS2 when it needs more input to complete
a command. Bash allows the prompt to be customized by
inserting a number of backslash-escaped special characters
that are decoded as follows:
\t the time
\d the date
\n CRLF
\s the name of the shell, the basename of $0
(the portion following the final slash)
\w the current working directory
\W the basename of the current working direc-
tory
\u the username of the current user
\h the hostname
\# the command number of this command
\! the history number of this command
\$ if the effective UID is 0, a #, otherwise a
$
\nnn character code in octal
\\ a backslash
After the string is decoded, if the variable
NOPROMPTVARS is not set, it is expanded via parameter
expansion, command substitution, arithmetic expansion, and
word splitting.
READLINE
This is the library that handles reading input when using
an interactive shell, unless the -nolineediting option is
given. By default, the line editing commands are similar
to those of emacs. A vi-style line editing interface is
also available.
In this section, the emacs-style notation is used to
GNU 1991 November 24 24
BASH(1) BASH(1)
denote keystrokes. Control keys are denoted by C-key,
e.g. C-n means Control-N. Similarly, meta keys are
denoted by M-key, so M-x means Meta-X. (On keyboards
without a meta key, M-x means ESC x, i.e. press the Escape
key then the x key. The combination M-C-x means
ESC-Control-x, or press the Escape key then hold the Con-
trol key while pressing the x key.)
The default key-bindings may be changed with an ~/.inputrc
file. Other programs that use this library may add their
own commands and bindings.
For example, placing
M-Control-u: universal-argument
or
C-Meta-u: universal-argument
into the ~/.inputrc would make M-C-u execute the command
universal-argument.
The following symbolic character names are recognized:
RUBOUT, DEL, ESC, NEWLINE, SPACE, RETURN, LFD, TAB.
Placing
set editing-mode vi
into a ~/.inputrc file causes bash to start with a vi-like
editing mode. The editing mode may be switched during
interactive use by using the -o option to the set builtin
command (see SHELL BUILTIN COMMANDS below).
You can have readline use a single line for display,
scrolling the input between the two borders by placing
set horizontal-scroll-mode On
into a ~/.inputrc file.
The following is a list of the names of the commands and
the default key-strokes to get them.
Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word.
GNU 1991 November 24 25
BASH(1) BASH(1)
backward-word (M-b)
Move back to the start of this, or the previous,
word.
clear-screen (C-l)
Clear the screen leaving the current line at the
top of the screen.
Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is.
If this line is non-empty, add it to the history
list according to the state of the historycontrol
variable. If this line was a history line, then
restore the history line to its original state.
previous-history (C-p)
Fetch the previous command from the history list,
moving back in the list.
next-history (C-n)
Fetch the next command from the history list, mov-
ing forward in the list.
beginning-of-history (M-<)
Move to the first line in the history, the first
line entered.
end-of-history (M->)
Move to the end of the input history, i.e., the
line you are entering.
reverse-search-history (C-r)
Search backward starting at the current line and
moving `up' through the history as necessary. This
is an incremental search.
forward-search-history (C-s)
Search forward starting at the current line and
moving `down' through the history as necessary.
shell-expand-line (M-C-e)
Expand the line the way the shell does when it
reads it. This performs alias and history expan-
sion. See HISTORY below.
insert-last-argument (M-., M-)
Insert the last argument to the previous command
(the last word on the previous line).
operate-and-get-next (C-O)
Accept the current line for execution and fetch the
next line relative to the current line from the
history file for editing.
Commands for Changing Text
delete-char (C-d)
Delete the character under the cursor. If the cur-
sor is at the beginning of the line, and there are
no characters in the line, and the last character
typed was not C-d, then return EOF.
backward-delete-char (Rubout)
Delete the character behind the cursor. A numeric
arg says to kill the characters instead of deleting
GNU 1991 November 24 26
BASH(1) BASH(1)
them.
quoted-insert (C-q, C-v)
Add the next character that you type to the line
verbatim. This is how to insert characters like
C-q, for example.
tab-insert (M-TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert the character typed.
transpose-chars (C-t)
Drag the character before point forward over the
character at point. Point moves forward as well.
If point is at the end of the line, then transpose
the two characters before point. Negative argu-
ments don't work.
transpose-words (M-t)
Drag the word behind the cursor past the word in
front of the cursor moving the cursor over that
word as well.
upcase-word (M-u)
Uppercase the current (or following) word. With a
negative argument, do the previous word, but do not
move point.
downcase-word (M-l)
Lowercase the current (or following) word. With a
negative argument, do the previous word, but do not
move point.
capitalize-word (M-c)
Capitalize the current (or following) word. With a
negative argument, do the previous word, but do not
move point.
Killing and Yanking
kill-line (C-k)
Kill the text from the current cursor position to
the end of the line. This saves the killed text on
the kill-ring. (see below)
backward-kill-line
Kill backward to the beginning of the line. This
is normally unbound, in favor of unix-line-discard,
which emulates the behavior of the standard Unix
terminal driver.
kill-word (M-d)
Kill from the cursor to the end of the current
word, or if between words, to the end of the next
word.
backward-kill-word (M-Rubout)
Kill the word behind the cursor.
unix-line-discard (C-u)
Do what C-u used to do in Unix line input. We save
the killed text on the kill-ring, though.
unix-word-rubout (C-w)
Do what C-w used to do in Unix line input. The
killed text is saved on the kill-ring. This is
GNU 1991 November 24 27
BASH(1) BASH(1)
different than backward-kill-word because the word
boundaries differ.
yank (C-y)
Yank the top of the kill ring into the buffer at
point.
yank-pop (M-y)
Rotate the kill-ring, and yank the new top. Only
works following `yank' or `yank-pop'.
Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulat-
ing, or start a new argument. M-- starts a nega-
tive argument.
universal-argument
Do what C-u does in emacs. By default, this is not
bound to a key.
Completing
complete (TAB)
Attempt to perform completion on the text before
point. Bash attempts completion treating the text
as a variable (if the text begins with $), username
(if the text begins with ~), hostname (if the text
begins with @), or command (including aliases and
functions) in turn. If none of these produces a
match, filename completion is attempted.
possible-completions (M-?)
List the possible completions of the text before
point.
complete-filename (M-/)
Attempt filename completion on the text before
point.
possible-filename-completions (C-x /)
List the possible completions of the text before
point, treating it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treat-
ing it as a username.
possible-username-completions (C-x ~)
List the possible completions of the text before
point, treating it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treat-
ing it as a shell variable.
possible-variable-completions (C-x $)
List the possible completions of the text before
point, treating it as a shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treat-
ing it as a hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before
point, treating it as a hostname.
GNU 1991 November 24 28
BASH(1) BASH(1)
Miscellaneous
abort (C-g)
Abort the current editing command and ring the ter-
minal's bell.
do-uppercase-version (M-a, M-b, ...)
Run the command that is bound to the uppercased
key.
prefix-meta (ESC)
Metafy the next character typed. This is for peo-
ple without a meta key. ESC f is equivalent to
Meta-f.
undo (C-)
Incremental undo, separately remembered for each
line.
revert-line (M-r)
Undo all changes made to this line. This is like
typing the `undo' command enough times to get back
to the beginning.
display-shell-version (C-x C-v)
Display version information about the current
instance of bash.
emacs-editing-mode (C-e)
When in vi editing mode, this causes a switch to
emacs editing mode.
vi-editing-mode (M-C-j or M-C-m)
When in emacs editing mode, this causes a switch to
vi editing mode.
HISTORY
The shell supports a history expansion feature that is
similar to the history expansion in csh. This section
describes what syntax features are available.
History expansion is performed immediately after a com-
plete line is read, before the shell breaks it into words.
It takes place in two parts. The first is determining
which line from the previous history to use during substi-
tution. The second is to select portions of that line for
inclusion into the current one. The line selected from
the previous history is the event, and the portions of
that line that are acted upon are words. The line is bro-
ken into words in the same fashion as when reading input,
so that several English, or Unix, words surrounded by
quotes are considered as one word. Only backslash (\) can
quote the history escape character, which is ! by default.
Event Designators
An event designator is a reference to a command line entry
in the history list.
! Start a history substitution, except when followed
by a <space>, <tab>, <newline>, = or (.
!! Refer to the previous command. This is a synonym
for `!-1'.
GNU 1991 November 24 29
BASH(1) BASH(1)
!n Refer to command line n.
!-n Refer to the current command line minus n.
!string
Refer to the most recent command starting with
string.
!?string[?]
Refer to the most recent command containing string.
Word Designators
A : separates the event specification from the word desig-
nator. It can be omitted if the word designator begins
with a ^, $, *, or %. Words are numbered from the begin-
ning of the line, with the first word being denoted by a 0
(zero).
# The entire command line typed so far. This means
the current command, not the previous command, so
it really isn't a word designator, and doesn't
belong in this section.
0 (zero)
The zeroth word. For the shell, this is the com-
mand word.
n The nth word.
^ The first argument. That is, word 1.
$ The last argument.
% The word matched by the most recent `?string?'
search.
x-y A range of words; `-y' abbreviates `0-y'.
* All of the words but the zeroth. This is a synonym
for `1-$'. It is not an error to use * if there is
just one word in the event; the empty string is
returned in that case.
Modifiers
After the optional word designator, you can add a sequence
of one or more of the following modifiers, each preceded
by a `:'.
h Remove a trailing pathname component, leaving only
the head.
r Remove a trailing suffix of the form ".xxx", leav-
ing the basename.
e Remove all but the suffix.
t Remove all leading pathname components, leaving the
tail.
p Print the new command but do not execute it. This
takes effect immediately, so it should be the last
specifier on the line.
ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated,
under certain circumstances (see the let builtin command
and Arithmetic Expansion). Evaluation is done in long
integers with no check for overflow, though division by 0
GNU 1991 November 24 30
BASH(1) BASH(1)
is trapped and flagged as an error. The following list of
operators is grouped into levels of equal-precedence oper-
ators. The levels are listed in order of decreasing
precedence.
- unary minus
! logical NOT
* / % multiplication, division, remainder
+ - addition, subtraction
<= >= < >
comparison
== != equality and inequality
= assignment
Shell variables are allowed as operands; parameter expan-
sion is performed before the expression is evaluated. The
value of a parameter is coerced to a long integer within
an expression. A shell variable need not have its integer
attribute turned on to be used in an expression.
Operators are evaluated in order of precedence. Sub-
expressions in parentheses are evaluated first and may
override the precedence rules above.
SHELL BUILTIN COMMANDS
: [arguments]
No effect; the command does nothing beyond expand-
ing arguments and performing any specified redirec-
tions. A zero exit code is returned.
. filename
source filename
Read and execute commands from filename in the cur-
rent shell environment and return the exit status
of the last command executed from filename. Path-
names in PATH are used to find the directory con-
taining filename, if filename does not contain a
slash. The file searched for in PATH need not be
executable. The current directory is searched if
no file is found in PATH. The return status is the
status of the last command exited within the script
(true if no commands are executed), and false if
filename is not found.
alias [name[=value] ...]
Alias with no arguments prints the list of aliases
in the form name=value on standard output. When
arguments are supplied, an alias is defined for
each name whose value is given. A trailing space
in value causes the next word to be checked for
alias substitution when the alias is expanded.
Alias returns true unless a name is given for which
no alias has been defined.
GNU 1991 November 24 31
BASH(1) BASH(1)
bg [jobspec]
Place jobspec in the background, as if it had been
started with &. If jobspec is not present, the
shell's notion of the current job is used.
bind [-lvd] [-q name]
bind -f filename
bind keyseq:function-name
Display current readline key and function bindings,
or bind a key sequence to a readline function or
macro. The binding syntax accepted is identical to
that of .inputrc, but each binding must be passed
as a separate argument; e.g. '"\C-x\C-r": re-read-
init-file'. Options, if supplied, have the follow-
ing meanings:
-l List the names of all readline functions
-v List current function names and bindings
-d Dump function names and bindings in such a
way that they can be re-read
-f filename
Read key bindings from filename
-q function
Query about which keys invoke the named
function
break [n]
Exit from within a for, while, or until loop. If n
is specified, break n levels. n must be >= 1. If
n is greater than the number of enclosing loops,
all enclosing loops are exited. The return value
is 0 unless the shell is not executing a loop when
break is executed.
builtin [shell-builtin [arguments]]
Execute the specified shell builtin, passing it
arguments, and return its exit status. This is
useful when you wish to define a function whose
name is the same as a shell builtin, but need the
functionality of the builtin within the function
itself. The cd builtin is commonly redefined this
way.
cd [dir]
Change the current directory to dir. The variable
HOME is the default dir. The variable CDPATH
defines the search path for the directory contain-
ing dir. Alternative directory names are separated
by a colon (:). A null directory name in CDPATH is
the same as the current directory, i.e. ``.''. If
dir begins with a slash (/), then CDPATH is not
used. An argument of - is equivalent to $OLDPWD.
The return value is true if the directory was suc-
cessfully changed; false otherwise.
GNU 1991 November 24 32
BASH(1) BASH(1)
command [-p] [command [arg ...]]
Run command with args suppressing the normal shell
function lookup. Only builtin commands or commands
found in the PATH are executed. If the -p option
is given, the search for command is performed using
a default value for PATH that is guaranteed to find
all of the standard utilities. An argument of --
disables option checking for the rest of the argu-
ments. If an error occurred or command cannot be
found, the exit status is 127. Otherwise, the exit
status of the command builtin is the exit status of
command.
continue [n]
Resume the next iteration of the enclosing for,
while, or until loop. If n is specified, resume at
the nth enclosing loop. n must be >= 1. If n is
greater than the number of enclosing loops, the
last enclosing loop (the `top-level' loop) is
resumed. The return value is 0 unless the shell is
not executing a loop when continue is executed.
declare [-frxi] [name[=value]]
typeset [-frxi] [name[=value]]
Declare variables and/or give them attributes. If
no names are given, then display the values of
variables instead.
-f Use function names only
-r Make names readonly. These names cannot
then be assigned values by subsequent
assignment statements.
-x Mark names for export to subsequent commands
via the environment.
-i The variable is treated as an integer;
arithmetic evaluation (see ARITHMETIC EVALU-
ATION ) is performed when the variable is
assigned a value.
Using `+' instead of `-' turns off the attribute
instead. When used in a function, makes names
local, as with the local command.
dirs [-l]
Display the list of currently remembered directo-
ries. Directories are added to the list with the
pushd command; the popd command moves back up
through the list. The -l option produces a longer
listing; the default listing format uses a tilde to
denote the home directory.
echo [-ne] [arg ...]
Output the args, separated by spaces. If -n is
specified, the trailing newline is suppressed. If
the -e option is given, interpretation of the
GNU 1991 November 24 33
BASH(1) BASH(1)
following backslash-escaped characters is enabled:
\a alert (bell)
\b backspace
\c suppress trailing newline
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\nnn the character whose ASCII code is nnn
(octal)
enable [-n] [name ...]
Enable and disable builtin shell commands. This
allows the execution of a disk command which has
the same name as a shell builtin without specifying
a full pathname. If -n is used, each name is dis-
abled; otherwise, names are enabled. For example,
to use the test found in PATH instead of the shell
builtin version, type ``enable -n test''.
eval [arg ...]
The args are read and concatenated together into a
single command. This command is then read and exe-
cuted by the shell, and its exit status is returned
as the value of the eval command. If there are no
args, or only null arguments, eval returns true.
exec [[-] command [arguments]]
If command is specified, it replaces the shell. No
new process is created. The arguments become the
arguments to command. If the first argument is -,
the shell places a dash in the zeroth arg passed to
command. This is what login does. If the file
cannot be executed for some reason, the shell
exits, unless the shell variable
noexitonfailedexec exists. If command is not
specified, any redirections take effect in the cur-
rent shell.
exit [n]
bye [n]
Cause the shell to exit with a status of n. If n
is omitted, the exit status is that of the last
command executed. A trap on EXIT is executed
before the shell terminates.
export [-npf] [name[=word]] ...
The supplied names are marked for automatic export
to the environment of subsequently executed com-
mands. If the -f option is given, the names refer
to functions. If no names are given, or if the -p
option is supplied, a list of all names that are
GNU 1991 November 24 34
BASH(1) BASH(1)
exported in this shell is printed. The -n option
causes the export property to be removed from the
named variables. An argument of -- disables option
checking for the rest of the arguments. export
returns an exit status of true unless an illegal
option is encountered.
fc [-e ename] [-nlr] [first] [last]
fc -s [pat=rep] [cmd]
Fix Command. In the first form, a range of com-
mands from first to last is selected from the his-
tory list. First and last may be specified as a
string (to locate the last command beginning with
that string) or as a number (an index into the his-
tory list, where a negative number is used as an
offset from the current command number). If last
is not specified it is set to the current command
for listing (so that fc -l -10 prints the last 10
commands) and to first otherwise. If first is not
specified it is set to the previous command for
editing and -16 for listing.
The -n flag suppresses the command numbers when
listing. The -r flag reverses the order of the
commands. If the -l flag is given, the commands
are listed on standard output. Otherwise, the edi-
tor given by ename is invoked on a file containing
those commands. If ename is not given, the value
of the FCEDIT variable is used, and the value of
EDITOR if FCEDIT is not set. If neither variable
is set, vi is used. When editing is complete, the
edited commands are echoed and executed.
In the second form, the command is re-executed
after the substitution old=new is performed. A
useful alias to use with this is ``r=fc -s'', so
that typing ``r cc'' runs the last command begin-
ning with ``cc'' and typing ``r'' re-executes the
last command.
fg [jobspec]
Place jobspec in the foreground, and make it the
current job. If jobspec is not present, the
shell's notion of the current job is used.
getopts optstring name [args]
getopts is used by shell procedures to parse posi-
tional parameters. optstring contains the option
letters to be recognized; if a letter is followed
by a colon, the option is expected to have an argu-
ment, which should be separated from it by white
space. Each time it is invoked, getopts places the
next option in the shell variable name, initializ-
ing name if it does not exist, and the index of the
GNU 1991 November 24 35
BASH(1) BASH(1)
next argument to be processed into the variable
OPTIND. OPTIND is initialized to 1 each time the
shell or a shell script is invoked. When an option
requires an argument, getopts places that argument
into the variable OPTARG. The shell does not reset
OPTIND automatically; it must be manually reset
between multiple calls to getopts within the same
shell invocation if a new set of parameters is to
be used.
getopts can report errors in two ways. If the
first character of optstring is a colon, silent
error reporting is used. In normal operation diag-
nostic messages are printed when illegal options or
missing option arguments are encountered. If the
variable OPTERR is set to 0, no error message will
be displayed, even if the first character of opt-
string is not a colon.
If an illegal option is seen, getopts places ? into
name and, if not silent, prints an error message
and unsets OPTARG. If getopts is silent, the
option character found is placed in OPTARG and no
diagnostic message is printed.
If a required argument is not found, and getopts is
not silent, a question mark (?) is placed in name,
OPTARG is unset, and a diagnostic message is
printed. If getopts is silent, then a colon (:) is
placed in name and OPTARG is set to the option
character found.
getopts normally parses the positional parameters,
but if more arguments are given in args, getopts
parses those instead. getopts returns true if an
option, specified or unspecified, is found. It
returns false if the end of options is encountered
or an error occurs.
hash [-r] [name]
For each name, the full pathname of the command is
determined and remembered. The -r option causes
the shell to forget all remembered locations. If
no arguments are given, information about remem-
bered commands is printed. An argument of -- dis-
ables option checking for the rest of the argu-
ments. The return status is true unless a name is
not found or an illegal option is supplied.
help [pattern]
Display helpful information about builtin commands.
If pattern is specified, help gives detailed help
on all commands matching pattern; otherwise a list
of the builtins is printed.
GNU 1991 November 24 36
BASH(1) BASH(1)
history [n]
history -rwan [filename]
With no options, display the command history list
with line numbers. Lines listed with with a * have
been modified. An argument of n lists only the
last n lines. If a non-option argument is sup-
plied, it is used as the name of the history file;
if not, the value of HISTFILE (default
~/.bashhistory) is used. Options, if supplied,
have the following meanings:
-a Append the ``new'' history lines (history
lines entered since the beginning of the
current bash session) to the history file
-n Read the history lines not already read from
the history file into the current history
list. These are lines appended to the his-
tory file since the beginning of the current
bash session.
-r read the contents of the history file and
use them as the current history
-w write the current history to the history
file, overwriting the history file's con-
tents.
jobs [-lnp] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The -l
option lists process IDs in addition to the normal
information; the -p option lists only the process
ID of the job's process group leader. The -n
option displays only jobs that have changed status
since last notfied. If jobspec is given, output is
restricted to information about that job.
If the -x option is supplied, jobs replaces any
jobspec found in command or args with the corre-
sponding process group ID, and executes command
passing it args.
kill [-s sigspec | -sigspec] [pid | jobspec] ...
kill -l [signum]
Send the signal named by sigspec to the processes
named by pid or jobspec. sigspec is either a sig-
nal name such as SIGKILL or a signal number. If
sigspec is a signal name, the name is case insensi-
tive and may be given with or without the SIG pre-
fix. If sigspec is not present, then SIGTERM is
assumed. An argument of -l lists the signal names.
If any arguments are supplied when -l is given, the
names of the specified signals are listed. An
argument of -- disables option checking for the
rest of the arguments. kill returns true if at
least one signal was successfully sent, or false if
an error occurs.
GNU 1991 November 24 37
BASH(1) BASH(1)
let arg [arg ...]
Each arg is an arithmetic expression to be evalu-
ated (see ARITHMETIC EVALUATION). If the last arg
evaluates to 0, let returns 1; 0 is returned other-
wise.
local [name[=value]]
Create a local variable named name, and assign it
value. When local is used within a function, it
causes the variable name to have a visible scope
restricted to that function and its children. With
no operands, local writes a list of local variables
to the standard output. It is an error to use
local when not within a function.
logout Exit a login shell.
popd [+/-n]
Removes entries from the directory stack. With no
arguments, removes the top directory from the
stack, and performs a cd to the new top directory.
+n removes the nth entry counting from the left
of the list shown by dirs, starting with
zero. For example: ``popd +0'' removes the
first directory, ``popd +1'' the second.
-n removes the nth entry counting from the
right of the list shown by dirs, starting
with zero. For example: ``popd -0'' removes
the last directory, ``popd -1'' the next to
last.
If the variable pushdsilent is unset and the popd
command is successful, a dirs is performed as well.
pushd dir
pushd +/-n
Adds a directory to the top of the directory stack,
or rotates the stack, making the new top of the
stack the current working directory. With no argu-
ments, exchanges the top two directories.
+n Rotates the stack so that the nth directory
(counting from the left of the list shown by
dirs) is at the top.
-n Rotates the stack so that the nth directory
(counting from the right) is at the top.
dir adds dir to the directory stack at the top,
making it the new current working directory.
If the variable pushdsilent is not set and the
pushd command is successful, a dirs is performed as
well.
pwd Print the absolute pathname of the current working
directory. The path printed contains no symbolic
GNU 1991 November 24 38
BASH(1) BASH(1)
links (but see the description of nolinks under
Shell Variables above).
read [-r] [name ...]
One line is read from the standard input, and the
first word is assigned to the first name, the sec-
ond word to the second name, and so on, with left-
over words assigned to the last name. Only the
characters in IFS are recognized as word delim-
iters. The return code is zero, unless end-of-file
is encountered. If the -r option is given, a back-
slash-newline pair is not ignored, and the back-
slash is considered to be part of the line.
readonly [-pf] [name ...]
The given names are marked readonly and the values
of these names may not be changed by subsequent
assignment. If the -f option is supplied, the
functions corresponding to the names are so marked.
If no arguments are given, or if the -p option is
supplied, a list of all readonly names is printed.
An argument of -- disables option checking for the
rest of the arguments.
return [n]
Causes a function to exit with the return value
specified by n. If n is omitted, the return status
is that of the last command executed in the func-
tion body. If used outside a function, but during
execution of a script by the . (source) command,
it causes the shell to stop executing that script
and return either n or the exit status of the last
command executed within the script as the exit sta-
tus of the script.
set [-aefhknotuvxldCH] [arg ...]
-a Automatically mark variables which are mod-
ified or created for export to the environ-
ment of subsequent commands.
-e Exit immediately if a simple-command (see
SHELL GRAMMAR above) exits with a non-zero
status. The shell does not exit if the
command that fails is part of an until or
while loop, part of an if statement, part
of a && or || list, or if the command's
return value is being inverted via !.
-f Disable pathname expansion.
-h Locate and remember function commands as
functions are defined. Function commands
are normally looked up when the function is
executed.
-k All keyword arguments are placed in the
environment for a command, not just those
that precede the command name.
GNU 1991 November 24 39
BASH(1) BASH(1)
-m Monitor mode. Job control is enabled.
This flag is on by default for interactive
shells on systems that support it (see JOB
CONTROL above). Background processes run
in a separate process group and a line con-
taining their exit status is printed upon
their completion.
-n Read commands but do not execute them.
This may be used to check a shell script
for syntax errors. This is ignored for
interactive shells.
-o option-name
The option-name can be one of the follow-
ing:
allexport
Same as -a.
braceexpand
The shell performs curly brace
expansion (see Brace Expansion
above). This is on by default.
emacs Use an emacs-style command line
editing interface.
errexit Same as -e.
histexpand
Same as -H.
ignoreeof
The effect is as if the shell com-
mand `IGNOREEOF=10' had been exe-
cuted (see Shell Variables above).
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f.
nohash Same as -d.
notify The effect is as if the shell com-
mand `notify=' had been executed
(see Shell Variables above).
nounset Same as -u.
verbose Same as -v.
vi Use a vi-style command line editing
interface.
xtrace Same as -x.
If no option-name is supplied, the values
of the current options are printed.
-t Exit after reading and executing one com-
mand.
-u Treat unset variables as an error when per-
forming parameter expansion. If expansion
is attempted on an unset variable, the
shell prints an error message, and, if not
interactive, exits with a non-zero status.
-v Print shell input lines as they are read.
-x After expanding each simple-command, bash
GNU 1991 November 24 40
BASH(1) BASH(1)
displays the expanded value of PS4, fol-
lowed by the command and its expanded argu-
ments.
-l Save and restore the binding of name in a
for name [in word] command (see SHELL GRAM-
MAR above).
-d Disable the hashing of commands that are
looked up for execution. Normally, com-
mands are remembered in a hash table, and
once found, do not have to be looked up
again.
-C The effect is as if the shell command
`noclobber=' had been executed (see Shell
Variables above).
-H Enable ! style history substitution. This
flag is on by by default.
-- If no arguments follow this flag, then the
positional parameters are unset. Other-
wise, the positional parameters are set to
the args, even if some of them begin with a
-.
- Signal the end of options, cause all
remaining args to be assigned to the posi-
tional parameters. The -x and -v options
are turned off. If there are no args, the
positional parameters remain unchanged.
Using + rather than - causes these flags to be
turned off. The flags can also be specified as
options to an invocation of the shell. The current
set of flags may be found in $-. After the option
arguments are processed, the remaining args are
treated as values for the positional parameters and
are assigned, in order, to $1, $2, ... $9. If no
options or args are supplied, all shell variables
are printed. The return status is always true
unless an illegal option is encountered.
shift [n]
The positional parameters from n+1 ... are renamed
to $1 .... If n is not given, it is assumed to be
1. The exit status is 1 if n is greater than $#;
otherwise 0.
suspend [-f]
Suspend the execution of this shell until it
receives a SIGCONT signal. The -f option says not
to complain if this is a login shell; just suspend
anyway.
test expr
[ expr ]
Return a status of 0 (true) or 1 (false) depending
on the evaluation of the conditional expression
GNU 1991 November 24 41
BASH(1) BASH(1)
expr. Expressions may be unary or binary. Unary
expressions are often used to examine the status of
a file. There are string operators and numeric
comparison operators as well.
-b file
True if file exists and is block special.
-c file
True if file exists and is character spe-
cial.
-d file
True if file exists and is a directory.
-e file
True if file exists
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-k file
True if file has its ``sticky'' bit set.
-L file
True if file exists and is a symbolic link.
-p file
True if file exists and is a named pipe.
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater
than zero.
-S file
True if file exists and is a socket.
-t [fd]
True if fd is opened on a terminal. If fd
is omitted, it defaults to 1 (standard out-
put).
-u file
True if file exists and its set-user-id bit
is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-O file
True if file exists and is owned by the
effective user id.
-G file
True if file exists and is owned by the
effective group id.
file1 -nt file2
True if file1 is newer (according to modifi-
cation date) than file2.
file1 -ot file2
True if file1 is older than file2.
file1 -ef file
True if file1 and file2 have the same device
GNU 1991 November 24 42
BASH(1) BASH(1)
and inode numbers.
-z string
True if the length of string is zero.
-n string
string True if the length of string is non-zero.
string1 = string2
True if the strings are equal.
string1 != string2
True if the strings are not equal.
! expr True if expr is false.
expr1 -a expr2
True if both expr1 AND expr2 are true.
expr1 -o expr2
True if either expr1 OR expr2 is true.
arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or
-ge. These arithmetic binary operators
return true if arg1 is equal, not-equal,
less-than, less-than-or-equal, greater-than,
or greater-than-or-equal than arg2, respec-
tively. Arg1 and arg2 may be positive inte-
gers, negative integers, or the special
expression -l string, which evaluates to the
length of string.
times Print the accumulated user and system times for the
shell and for processes run from the shell.
trap [arg] [sigspec]
The command arg is to be read and executed when the
shell receives signal(s) sigspec. If arg is absent
or -, all specified signals are are reset to their
original values (the values they had upon entrance
to the shell). If arg is the null string this sig-
nal is ignored by the shell and by the commands it
invokes. sigspec is either a signal name in <sig-
nal.h>, or a signal number. If sigspec is EXIT (0)
the command arg is executed on exit from the shell.
With no arguments, trap prints the list of commands
associated with each signal number. The -l option
causes the shell to print a list of signal names
and their corresponding numbers. An argument of --
disables option checking for the rest of the argu-
ments. Signals ignored upon entry to the shell
cannot be trapped or reset. Trapped signals are
reset to their original values in a child process
when it is created. The return status is false if
either then trap name or number is invalid; other-
wise trap returns true.
type [-all] [-type | -path] [name ...]
With no options, indicate how each name would be
interpreted if used as a command name. If the
-type flag is used, type prints a phrase which is
GNU 1991 November 24 43
BASH(1) BASH(1)
one of alias, keyword, function, builtin, or file
if name is an alias, shell reserved word, function,
builtin, or disk file, respectively. If the name is
not found, then nothing is printed, and an exit
status of false is returned. If the -path flag is
used, type either returns the name of the disk file
that would be executed if name were specified as a
command name, or nothing if -type would not return
file. If a command is hashed, -path prints the
hashed value, not necessarily the file that appears
first in PATH. If the -all flag is used, type
prints all of the places that contain an executable
named name. This includes aliases and functions,
if and only if the -path flag is not also used.
The table of hashed commands is not consulted when
using -all. type accepts -a, -t, and -p in place
of -all, -type, and -path, respectively. An argu-
ment of -- disables option checking for the rest of
the arguments. type returns true if any of the
arguments are found, false if none are found.
ulimit [-SHacdfmstpn [limit]]
Ulimit provides control over the resources avail-
able to the shell and to processes started by it,
on systems that allow such control. The value of
limit can be a number in the unit specified for the
resource, or the value unlimited. The H and S
options specify that the hard or soft limit is set
for the given resource. A hard limit cannot be
increased once it is set; a soft limit may be
increased up to the value of the hard limit. If
neither H nor S is specified, the command applies
to the soft limit. If limit is omitted, the cur-
rent value of the soft limit of the resource is
printed, unless the H option is given. When more
than one resource is specified, the limit name and
unit is printed before the value. Other options
are interpreted as follows:
-a all current limits are reported
-c the maximum size of core files created
-d the maximum size of a process's data segment
-f the maximum size of files created by the
shell
-m the maximum resident set size
-s the maximum stack size
-t the maximum amount of cpu time in seconds
-p the pipe size in 512-byte blocks (this may
not be set)
-n the maximum number of open file descriptors
(most systems do not allow this value to be
set, only displayed)
An argument of -- disables option checking for the
rest of the arguments. If limit is given, it is
GNU 1991 November 24 44
BASH(1) BASH(1)
the new value of the specified resource (the -a
option is display only). If no option is given,
then -f is assumed. Values are in 1024-byte incre-
ments, except for -t, which is in seconds, and -p,
which is in units of 512-byte blocks.
umask [-S] [mode]
The user file-creation mask is set to mode. If
mode begins with a digit, it is interpreted as an
octal number; otherwise it is interpreted as a sym-
bolic mode mask similar to that accepted by
chmod(1). If mode is omitted, or if the -S option
is supplied, the current value of the mask is
printed. The -S option causes the mask to be
printed in symbolic form; the default output is an
octal number. An argument of -- disables option
checking for the rest of the arguments.
unalias [name ...]
Remove names from the list of defined aliases. The
return value is true unless name is not a defined
alias.
unset [-fv] [name ...]
For each name, remove the corresponding variable
or, given the -f option, function. An argument of
-- disables option checking for the rest of the
arguments. Note that PATH, IFS, PPID, PS1, PS2,
UID, and EUID cannot be unset. If any of RANDOM,
SECONDS, or LINENO are unset, they lose their spe-
cial properties, even if they are subsequently
reset. The exit status is true unless the variable
name does not exist or is non-unsettable.
wait [n]
Wait for the specified process and report its ter-
mination status. n may be a process ID or a job
specification; if a job spec is given, all pro-
cesses in that job's pipeline are waited for. If n
is not given, all currently active child processes
are waited for, and the return code is zero.
INVOCATION
A login shell is one whose first character of argument
zero is a - , or one started with the -login flag.
An interactive shell is one whose standard input and out-
put are both connected to terminals (as determined by
isatty(3)), or one started with the -i flag. PS1 is set
and $- includes i if bash is interactive, allowing a way
to test this state from a shell script or a startup file.
Login shells:
On login:
GNU 1991 November 24 45
BASH(1) BASH(1)
if /etc/profile exists, source it.
if ~/.bashprofile exists, source it,
else if ~/.bashlogin exists, source it,
else if ~/.profile exists, source it.
On logout:
if ~/.bashlogout exists, source it.
Non-login interactive shells:
On startup:
if ~/.bashrc exists, source it.
Non-interactive shells:
On startup:
if the environment variable ENV is non-null, expand
it and source the file it names.
SEE ALSO
The Gnu Readline Library, Brian Fox
The Gnu History Library, Brian Fox
A System V Compatible Implementation of 4.2BSD Job
Control, David Lennert
How to wear weird pants for fun and profit, Brian Fox
sh(1), ksh(1), csh(1)
FILES
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for
login shells
~/.bashprofile
The personal initialization file, executed for
login shells
~/.bashrc
The individual per-interactive-shell startup file
~/.inputrc
Individual Readline initialization file
AUTHORS
Brian Fox, Free Software Foundation (primary
author)
bfox@ai.MIT.Edu
Chet Ramey, Case Western Reserve University
chet@ins.CWRU.Edu
BUG REPORTS
If you find a bug in bash, you should report it. But
first, you should make sure that it really is a bug, and
that it appears in the latest version of bash that you
have.
GNU 1991 November 24 46
BASH(1) BASH(1)
Once you have determined that a bug actually exists, mail
a bug report to bash-maintainers@ai.MIT.Edu. If you have
a fix, you are welcome to mail that as well! Suggestions
and `philosophical' bug reports may be mailed to bug-
bash@ai.MIT.Edu or posted to the Usenet newsgroup
gnu.bash.bug.
ALL bug reports should include:
The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behaviour
A short script or `recipe' which exercises the bug
Comments and bug reports concerning this manual page
should be directed to chet@ins.CWRU.Edu.
BUGS
It's too big and too slow.
There are some subtle differences between bash and tradi-
tional versions of sh, mostly because of the POSIX speci-
fication.
Aliases are confusing in some uses.
GNU 1991 November 24 47