Hi Bob!
I'm in a big troulbe. I have shell scripting homework and I'm out of time.
The homework is this: under debian, list packages that installed on the
computer, and write out this packages, and the program also write out the
connection beetwen the program and the packages. like this:
"nparted : libc6, libnewt0, libparted1.4, libuuid1
nano : libc6, libncurses5"
I hope you, or somebody would help me. It's very important for me
Thanks. David
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bash command help
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file]
COPYRIGHT
Bash is 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 conformant 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. If there are arguments after
the string, they are assigned to the positional
parameters, starting with $0.
-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 read and execute the personal initializa
tion file ~/.bashrc if the shell is interactive.
This option is on by default if the shell is
invoked as sh.
-noprofile
Do not read either the system-wide startup file
/etc/profile or any of the personal initializa
these files when it is invoked as a login shell
(see INVOCATION below).
-rcfile file
Execute commands from file instead of the stan
dard personal initialization file ~/.bashrc, if
the shell is interactive (see INVOCATION below).
-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).
This is the default.
-login Make bash act as if it had been invoked as a
login shell.
-nobraceexpansion
Do not perform curly brace expansion (see Brace
Expansion below).
-nolineediting
Do not use the GNU readline library to read com
mand lines if interactive.
-posix Change the behavior of bash where the default
operation differs from the Posix 1003.2 standard
to match the standard
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. Bash's exit
status is the exit status of the last command executed in
the script.
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
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
select then until while { }
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable
assignments followed by blank-separated words and redirec
tions, 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 terminated by
one of ;, &, or <newline>.
Of these list operators, && and || have equal 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,
and the return status is 0. Commands separated by a ; are
executed sequentially; the shell waits for each command to
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
command || command2
command2 is executed if and only if command returns a
non-zero exit status. The return status of AND and OR
lists is the exit status of the last command executed in
the list.
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. The return status is the exit sta
tus of list.
{ list; }
list is simply executed in the current shell envi
ronment. This is known as a group command. The
return status is the exit status of list.
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).
select name [ in word; ] do list ; done
The list of words following in is expanded, gener
ating a list of items. The set of expanded words
is printed on the standard error, each preceded by
a number. If the in word is omitted, the posi
tional parameters are printed (see PARAMETERS
below). The PS3 prompt is then displayed and a
line read from the standard input. If the line
consists of the number corresponding to one of the
displayed words, then the value of name is set to
that word. If the line is empty, the words and
name to be set to null. The line read is saved in
the variable REPLY. The list is executed after
each selection until a break or return command is
executed. The exit status of select is the exit
status of the last command executed in list, 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, using the
same matching rules as for pathname expansion (see
Pathname Expansion below). 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 com
mand 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
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
builtin is enabled, a word beginning with # causes that
word and all remaining characters on that line to be
ignored. An interactive shell without the -o interac
tive-comments option enabled does not allow comments.
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, and the backslash is not quoted, the \<new
line> is treated as a line continuation (that is, it is
effectively ignored).
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.
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
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. Positional parameters may not be assigned to
with assignment statements. 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
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
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. If bash is started with the -c
option, then $0 is set to the first argument after
the string to be executed, if one is present. Oth
erwise, 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, ini
tialized at shell startup.
EUID Expands to the effective user ID of the current
user, initialized at shell startup.
BASH Expands to the full pathname used to invoke this
instance of bash.
BASH_VERSION
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 num
bers 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,
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.
HISTCMD
The history number, or index in the history list,
of the current command. If HISTCMD is unset, it
loses its special properties, even if it is subse
quently 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 next argument to be processed by
the getopts builtin command (see SHELL BUILTIN COM
MANDS below).
HOSTTYPE
Automatically set to a string that uniquely
describes the type of machine on which bash is exe
cuting. The default is system-dependent.
OSTYPE Automatically set to a string that describes the
operating system on which bash is executing. The
default is system-dependent.
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-sepa
rated 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:.''.
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''.
ENV If this parameter is set when bash is executing a
shell, as in .bashrc. The value of ENV is sub
jected to parameter expansion, command substitu
tion, and arithmetic expansion before being inter
preted 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).
MAIL_WARNING
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 and used as
the secondary prompt string. The default is ``>
''.
PS3 The value of this parameter is used as the prompt
for the select command (see SHELL GRAMMAR above).
PS4 The value of this parameter is expanded 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 indirection. The
default is ``+ ''.
HISTSIZE
The number of commands to remember in the command
history (see HISTORY below). The default value is
500.
HISTFILE
The name of the file in which command history is
saved. (See HISTORY below.) The default value is
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. The
default value is 500.
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.
PROMPT_COMMAND
If set, the value is executed as a command prior to
issuing each primary prompt.
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
as the first characters on an input line 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.
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:~''.
INPUTRC
The filename for the readline startup file, over
riding the default of ~/.inputrc (see READLINE
below).
notify If set, bash reports terminated background jobs
immediately, rather than waiting until before
printing the next primary prompt (see also the -b
option to the set builtin command).
history_control
HISTCONTROL
If set to a value of ignorespace, lines which begin
with a space character are not entered on the his
tory list. If set to a value of ignoredups, lines
matching the last history line are not entered. A
value of ignoreboth combines the two options. If
unset, or if set to any other value than those
command_oriented_history
If set, bash attempts to save all lines of a multi
ple-line command in the same history entry. This
allows easy re-editing of multi-line commands.
glob_dot_filenames
If set, bash includes filenames beginning with a
`.' in the results of pathname expansion.
allow_null_glob_expansion
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 or three characters which control history
expansion and tokenization (see HISTORY EXPANSION
below). The first character is the history expan
sion character, that is, the character which sig
nals the start of a history expansion, normally
`!'. The second character is the quick substitu
tion character, which is used as shorthand for re-
running the previous command entered, substituting
one string for another in the command. The default
is `^'. The optional third character is the char
acter which signifies that the remainder of the
line is a comment, when found as the first charac
ter of a word, normally `#'. The history comment
character causes history substitution to be skipped
for the remaining words on the line. It does not
necessarily cause the shell parser to treat the
rest of the line as a comment.
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 which change the current directory, such as
cd. See also the description of the -P option to
the set builtin ( SHELL BUILTIN COMMANDS below).
hostname_completion_file
HOSTFILE
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. The file may be changed
interactively; the next time hostname completion is
attempted bash adds the contents of the new file to
the already existing database.
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).
auto_resume
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. The name of a stopped job,
in this context, is the command line used to start
it. If set to the value exact, the string supplied
must match the name of a stopped job exactly; if
set to substring, the string supplied needs to
match a substring of the name of a stopped job.
The substring value provides functionality analo
gous to the %? job id (see JOB CONTROL below). If
set to any other value, the supplied string must be
a prefix of a stopped job's name; this provides
functionality analogous to the % job id.
no_exit_on_failed_exec
If this variable exists, a non-interactive shell
will not exit if it cannot execute the file speci
fied in the exec builtin command. An interactive
shell does not exit if exec fails.
cdable_vars
If this is set, an argument to the cd builtin com
mand that is not a directory is assumed to be the
name of a variable whose value is the directory to
change to.
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.
On systems that can support it, there is an additional
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.
A correctly-formed brace expansion must contain unquoted
opening and closing braces, and at least one unquoted
comma. Any incorrectly formed brace expansion is left
unchanged.
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}}
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).
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 replaces the
tilde and `+'. If a `-' follows, the value of OLDPWD is
substituted. If the value following the tilde is a valid
login name, the tilde and login name are replaced with the
home directory associated with that name. If the name is
invalid, or the tilde expansion fails, the word is
unchanged.
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 assignments to PATH, MAILPATH,
and CDPATH, and the shell assigns the expanded value.
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,
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
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
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
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.
There are two formats for arithmetic expansion:
$[expression]
$((expression))
The expression is treated as if it were within double
quotes, but a double quote inside the braces or parenthe
ses is not treated specially. All tokens in the expres
sion 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.
Process Substitution
Process substitution is supported on systems that support
named pipes (FIFOs) or the /dev/fd method of naming open
files. It takes the form of <(list) or >(list). The pro
cess list is run with its input or output connected to a
FIFO or some file in /dev/fd. The name of this file is
passed as an argument to the current command as the result
of the expansion. If the >(list) form is used, writing to
the file will provide input for list. If the <(list) form
is used, the file passed as an argument should be read to
obtain the output of list.
On systems that support it, process substitution is per
formed simultaneously with parameter and variable expan
sion, command substitution, and arithmetic expansion.
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
value other than the default, then sequences of the
whitespace characters space and tab are ignored at the
beginning and end of the word, as long as the whitespace
character is in the value of IFS (an IFS whitespace char
acter). Any character in IFS that is not IFS whitespace,
along with any adjacent IFS whitespace characters, delim
its a field. A sequence of IFS whitespace characters is
also 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 per
formed.
Pathname Expansion
After word splitting, unless the -f option has been set,
bash scans each word for the characters *, ?, and [. 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
pathnames are found, and the shell variable
allow_null_glob_expansion is unset, the word is left
unchanged. If the variable is set, and no matches are
found, the word is removed. 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 glob_dot_filenames
is set. The slash character must always be matched
explicitly. In other cases, the ``.'' character is not
treated specially.
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.
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 precede or appear any
where within a simple command or may follow a command.
Redirections 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).
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.
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.
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
[n]>word
If the redirection operator is >|, then the value of the
-C option to the set builtin command is not tested, and
file creation is attempted. (See also the description of
noclobber under 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:
&>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
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.
If n is not specified, the standard output (file descrip
tor 1) is used. As a special case, if n is omitted, and
word does not expand to one or more digits, the standard
output and standard error are redirected as described pre
viously.
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. If the file does not exist, it is created.
FUNCTIONS
A shell function, defined as described above under SHELL
GRAMMAR, stores a series of commands for later execution.
Functions are executed in the context of the current
shell; no new process is created to interpret them (con
trast this with the execution of a shell script). When a
function is executed, the arguments to the function become
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 and definitions may be listed with the -f
option to the declare or typeset builtin commands. Func
tions may be exported so that subshells automatically have
them defined with the -f option to the export builtin.
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 (see
SHELL BUILTIN COMMANDS below). The first word of each
command, if unquoted, 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, with the exception that the alias name may
not contain =. 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, as in csh. If arguments are needed, a shell
function should be used.
Aliases are not expanded when the shell is not interac
tive.
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
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.
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
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 last process in the pipeline associated
with this job is 25647. All of the processes in a single
pipeline are members of the same job. 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, suspends
the process.
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, refers 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 to the
shell's notion of the current job, which is the last job
stopped while it was in the foreground. The previous job
may be referenced using %-. In output pertaining 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 -b option to the set
builtin command is set, bash reports such changes immedi
ately. (See also the description of notify variable under
Shell Variables above.)
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 the wait builtin is interrupt
ible). In all cases, bash ignores SIGQUIT. If job con
Synchronous jobs started by bash have signals set to the
values inherited by the shell from its parent. When job
control is not in effect, background jobs (jobs started
with &) ignore SIGINT and SIGQUIT. Commands run as a
result of command substitution ignore the keyboard-gener
ated job control signals SIGTTIN, SIGTTOU, 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
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.
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.
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.
When bash invokes an external command, the variable _ is
set to the full path name of the command and passed to
that command in its environment.
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.
If a command is not found, the child process created to
execute it returns a status of 127. If a command is found
but is not executable, the return status is 126.
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
a command. Bash allows these prompt strings to be cus
tomized by inserting a number of backslash-escaped special
characters that are decoded as follows:
\t the current time in HH:MM:SS format
\d the date in "Weekday Month Date" format
(e.g., "Tue May 26")
\n newline
\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 the character corresponding to the octal
number nnn
\\ a backslash
\[ begin a sequence of non-printing characters,
which could be used to embed a terminal con
trol sequence into the prompt
\] end a sequence of non-printing characters
The command number and the history number are usually dif
ferent: the history number of a command is its position in
the history list, which may include commands restored from
the history file (see HISTORY below), while the command
number is the position in the sequence of commands exe
cuted during the current shell session. After the string
is decoded, it is expanded via parameter expansion, com
mand substitution, arithmetic expansion, and word split
ting.
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
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. This makes ESC the meta pre
fix. The combination M-C-x means ESC-Control-x, or press
the Escape key then hold the Control key while pressing
the x key.)
file. The value of the shell variable INPUTRC, if set, is
used instead of ~/.inputrc. 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 readline
command universal-argument.
The following symbolic character names are recognized:
RUBOUT, DEL, ESC, LFD, NEWLINE, RET, RETURN, SPC, SPACE,
and TAB. In addition to command names, readline allows
keys to be bound to a string that is inserted when the key
is pressed (a macro).
Readline is customized by putting commands in an initial
ization file. The name of this file is taken from the
value of the INPUTRC variable. If that variable is unset,
the default is ~/.inputrc. When a program which uses the
readline library starts up, the init file is read, and the
key bindings and variables are set. There are only a few
basic constructs allowed in the readline init file. Blank
lines are ignored. Lines beginning with a # are comments.
Lines beginning with a $ indicate conditional constructs.
Other lines denote key bindings and variable settings.
The syntax for controlling key bindings in the ~/.inputrc
file is simple. All that is required is the name of the
command or the text of a macro and a key sequence to which
it should be bound. The name may be specified in one of
two ways: as a symbolic key name, possibly with Meta- or
Control- prefixes, or as a key sequence. When using the
form keyname:function-name or macro, keyname is the name
of a key spelled out in English. For example:
Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: ">&output"
In the above example, C-u is bound to the function univer
sal-argument, M-DEL is bound to the function back
ward-kill-word, and C-o is bound to run the macro
expressed on the right hand side (that is, to insert the
text >&output into the line).
In the second form, "keyseq":function-name or macro, key
seq differs from keyname above in that strings denoting an
entire key sequence may be specified by placing the
sequence within double quotes. Some GNU Emacs style key
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
In this example, C-u is again bound to the function uni
versal-argument. C-x C-r is bound to the function
re-read-init-file, and ESC [ 1 1 ~ is bound to insert the
text Function Key 1. The full set of escape sequences is
\C- control prefix
\M- meta prefix
\e an escape character
\\ backslash
\" literal "
\' literal '
When entering the text of a macro, single or double quotes
should be used to indicate a macro definition. Unquoted
text is assumed to be a function name. Backslash will
quote any character in the macro text, including " and '.
Bash allows the current readline key bindings to be dis
played or modified with the bind builtin command. The
editing mode may be switched during interactive use by
using the -o option to the set builtin command (see SHELL
BUILTIN COMMANDS below).
Readline has variables that can be used to further cus
tomize its behavior. A variable may be set in the inputrc
file with a statement of the form
set variable-name value
Except where noted, readline variables can take the values
On or Off. The variables and their default values are:
horizontal-scroll-mode (Off)
When set to On, makes readline use a single line
for display, scrolling the input horizontally on a
single screen line when it becomes longer than the
screen width rather than wrapping to a new line.
editing-mode (emacs)
Controls whether readline begins with a set of key
bindings similar to emacs or vi. editing-mode can
be set to either emacs or vi.
mark-modified-lines (Off)
If set to On, history lines that have been modified
are displayed with a preceding asterisk (*).
Controls what happens when readline wants to ring
the terminal bell. If set to none, readline never
rings the bell. If set to visible, readline uses a
visible bell if one is available. If set to audi
ble, readline attempts to ring the terminal's bell.
comment-begin (``#'')
The string that is inserted in vi mode when the
vi-comment command is executed.
meta-flag (Off)
If set to On, readline will enable eight-bit input
(that is, it will not strip the high bit from the
characters it reads), regardless of what the termi
nal claims it can support.
convert-meta (On)
If set to On, readline will convert characters with
the eighth bit set to an ASCII key sequence by
stripping the eighth bit and prepending an escape
character (in effect, using escape as the meta pre
fix).
output-meta (Off)
If set to On, readline will display characters with
the eighth bit set directly rather than as a meta-
prefixed escape sequence.
completion-query-items (100)
This determines when the user is queried about
viewing the number of possible completions gener
ated by the possible-completions command. It may
be set to any integer value greater than or equal
to zero. If the number of possible completions is
greater than or equal to the value of this vari
able, the user is asked whether or not he wishes to
view them; otherwise they are simply listed on the
terminal.
keymap (emacs)
Set the current readline keymap. The set of legal
keymap names is emacs, emacs-standard, emacs-meta,
emacs-ctlx, vi, vi-move, vi-command, and vi-insert.
vi is equivalent to vi-command; emacs is equivalent
to emacs-standard. The default value is emacs; the
value of editing-mode also affects the default
keymap.
show-all-if-ambiguous (Off)
This alters the default behavior of the completion
functions. If set to on, words which have more
than one possible completion cause the matches to
be listed immediately instead of ringing the bell.
expand-tilde (Off)
If set to on, tilde expansion is performed when
readline attempts word completion.
Readline implements a facility similar in spirit to the
conditional compilation features of the C preprocessor
directives used.
$if The $if construct allows bindings to be made based
on the editing mode, the terminal being used, or
the application using readline. The text of the
test extends to the end of the line; no characters
are required to isolate it.
mode The mode= form of the $if directive is used
to test whether readline is in emacs or vi
mode. This may be used in conjunction with
the set keymap command, for instance, to set
bindings in the emacs-standard and emacs-
ctlx keymaps only if readline is starting
out in emacs mode.
term The term= form may be used to include termi
nal-specific key bindings, perhaps to bind
the key sequences output by the terminal's
function keys. The word on the right side
of the = is tested against the full name of
the terminal and the portion of the terminal
name before the first -. This allows sun to
match both sun and sun-cmd, for instance.
application
The application construct is used to include
application-specific settings. Each program
using the readline library sets the applica
tion name, and an initialization file can
test for a particular value. This could be
used to bind key sequences to functions use
ful for a specific program. For instance,
the following command adds a key sequence
that quotes the current or previous word in
Bash:
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
$endif This command, as you saw in the previous example,
terminates an $if command.
$else Commands in this branch of the $if directive are
executed if the test fails.
Readline commands may be given numeric arguments, which
normally act as a repeat count. Sometimes, however, it is
the sign of the argument that is significant. Passing a
negative argument to a command that acts in the forward
direction (e.g., kill-line) causes that command to act in
When a command is described as killing text, the text
deleted is saved for possible future retrieval (yanking).
The killed text is saved in a kill-ring. Consecutive
kills cause the text to be accumulated into one unit,
which can be yanked all at once. Commands which do not
kill text separate the chunks of text on the kill-ring.
The following is a list of the names of the commands and
the default key sequences to which they are bound.
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. Words
are composed of alphanumeric characters (letters
and digits).
backward-word (M-b)
Move back to the start of this, or the previous,
word. Words are composed of alphanumeric charac
ters (letters and digits).
clear-screen (C-l)
Clear the screen leaving the current line at the
top of the screen. With an argument, refresh the
current line without clearing the screen.
redraw-current-line
Refresh the current line. By default, this is
unbound.
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 HISTCONTROL
variable. If the line is a modified 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.
Move to the end of the input history, i.e., the
line currently being entered.
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.
This is an incremental search.
non-incremental-reverse-search-history (M-p)
Search backward through the history starting at the
current line using a non-incremental search for a
string supplied by the user.
non-incremental-forward-search-history (M-n)
Search forward through the history using a
non-incremental search for a string supplied by the
user.
history-search-forward
Search forward through the history for the string
of characters between the start of the current line
and the current point. This is a non-incremental
search. By default, this command is unbound.
history-search-backward
Search backward through the history for the string
of characters between the start of the current line
and the current point. This is a non-incremental
search. By default, this command is unbound.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command
(usually the second word on the previous line) at
point (the current cursor position). With an argu
ment n, insert the nth word from the previous com
mand (the words in the previous command begin with
word 0). A negative argument inserts the nth word
from the end of the previous command.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command
(the last word on the previous line). With an
argument, behave exactly like yank-nth-arg.
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 as well as all of the shell word expansions.
See HISTORY EXPANSION below for a description of
history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See
HISTORY EXPANSION below for a description of his
tory expansion.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
Accept the current line for execution and fetch the
next line relative to the current line from the
history for editing. Any argument is ignored.
Commands for Changing Text
delete-char (C-d)
Delete the character under the cursor. If point is
at the beginning of the line, there are no charac
ters 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. When given
a numeric argument, save the deleted text on the
kill-ring.
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 (C-v 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.
backward-kill-line (C-x C-Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
kill-whole-line
Kill all characters on the current line, no matter
where the cursor is. By default, this is unbound.
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. Word boundaries are the same as those used
by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind the cursor. Word boundaries
are the same as those used by backward-word.
unix-word-rubout (C-w)
Kill the word behind the cursor, using white space
as a word boundary. The word boundaries are dif
ferent from backward-kill-word.
delete-horizontal-space
Delete all spaces and tabs around point. By
default, this is unbound.
yank (C-y)
Yank the top of the kill ring into the buffer at
the cursor.
yank-pop (M-y)
Rotate the kill-ring, and yank the new top. Only
works following yank or yank-pop.
Numeric 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
Each time this is executed, the argument count is
multiplied by four. The argument count is ini
tially one, so executing this function the first
time makes the argument count four. 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 $), username
(if the text begins with ~), hostname (if the text
begins with @), or command (including aliases and
functions) in turn. If none of these produces a
match, filename completion is attempted.
possible-completions (M-?)
List the possible completions of the text before
point.
insert-completions
Insert all completions of the text before point
that would have been generated by possible-comple
Attempt filename completion on the text before
point.
possible-filename-completions (C-x /)
List the possible completions of the text before
point, treating it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treat
ing it as a username.
possible-username-completions (C-x ~)
List the possible completions of the text before
point, treating it as a username.
complete-variable (M-$)
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.
complete-command (M-!)
Attempt completion on the text before point, treat
ing it as a command name. Command completion
attempts to match the text against aliases,
reserved words, shell functions, builtins, and
finally executable filenames, in that order.
possible-command-completions (C-x !)
List the possible completions of the text before
point, treating it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, com
paring the text against lines from the history list
for possible completion matches.
complete-into-braces (M-{)
Perform filename completion and return the list of
possible completions enclosed within braces so the
list is available to the shell (see Brace Expansion
above).
Keyboard Macros
start-kbd-macro (C-x ()
Begin saving the characters typed into the current
keyboard macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current
keyboard macro and save the definition.
call-last-kbd-macro (C-x e)
Re-execute the last keyboard macro defined, by mak
ing the characters in the macro appear as if typed
re-read-init-file (C-x C-r)
Read in the contents of your init file, and incor
porate any bindings or variable assignments found
there.
abort (C-g)
Abort the current editing command and ring the ter
minal's bell (subject to the setting of
bell-style).
do-uppercase-version (M-a, M-b, ...)
Run the command that is bound to the corresponding
uppercase character.
prefix-meta (ESC)
Metafy the next character typed. ESC f is equiva
lent to Meta-f.
undo (C-_, C-x C-u)
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 return the
line to its initial state.
tilde-expand (M-~)
Perform tilde expansion on the current word.
dump-functions
Print all of the functions and their key bindings
to the readline output stream. If a numeric argu
ment is supplied, the output is formatted in such a
way that it can be made part of an inputrc file.
display-shell-version (C-x C-v)
Display version information about the current
instance of bash.
HISTORY
When interactive, the shell provides access to the command
history, the list of commands previously typed. The text
of the last HISTSIZE commands (default 500) is saved in a
history list. The shell stores each command in the his
tory list prior to parameter and variable expansion (see
EXPANSION above) but after history expansion is performed,
subject to the values of the shell variables command_ori
ented_history and HISTCONTROL. On startup, the history is
initialized from the file named by the variable HISTFILE
(default ~/.bash_history). HISTFILE is truncated, if nec
essary, to contain no more than HISTFILESIZE lines. The
builtin command fc (see SHELL BUILTIN COMMANDS below) may
be used to list or edit and re-execute a portion of the
history list. The history builtin can be used to display
the history list and manipulate the history file. When
using the command-line editing, search commands are avail
able in each editing mode that provide access to the his
tory list. When an interactive shell exits, the last
HISTSIZE lines are copied from the history list to HIST
HISTORY EXPANSION
The shell supports a history expansion feature that is
similar to the history expansion in csh. This section
describes what syntax features are available. This fea
ture is enabled by default for interactive shells, and can
be disabled using the +H option to the set builtin command
(see SHELL BUILTIN COMMANDS below). Non-interactive
shells do not perform history expansion.
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 to determine
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 metacharacter-separated words surrounded
by quotes are considered as one word. Only backslash (\)
and single quotes can quote the history escape character,
which is ! by default.
The shell allows control of the various characters used by
the history expansion mechanism (see the description of
histchars above under Shell Variables).
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 blank, newline, = or (.
!! Refer to the previous command. This is a synonym
for `!-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.
^string1^string2^
Quick substitution. Repeat the last command,
replacing string1 with string2. Equivalent to
``!!:s/string1/string2/'' (see Modifiers below).
!# The entire command line typed so far.
Word Designators
A : separates the event specification from the word desig
nator. It can be omitted if the word designator begins
by a 0 (zero).
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-$'. It is not an error to use * if there is
just one word in the event; the empty string is
returned in that case.
x* Abbreviates x-$.
x- Abbreviates x-$ like x*, but omits the last word.
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, leaving
the basename.
e Remove all but the trailing suffix.
t Remove all leading pathname components, leaving the
tail.
p Print the new command but do not execute it.
q Quote the substituted words, escaping further sub
stitutions.
x Quote the substituted words as with q, but break
into words at blanks and newlines.
s/old/new/
Substitute new for the first occurrence of old in
the event line. Any delimiter can be used in place
of /. The final delimiter is optional if it is the
last character of the event line. The delimiter
may be quoted in old and new with a single back
slash. If & appears in new, it is replaced by old.
A single backslash will quote the &.
& Repeat the previous substitution.
g Cause changes to be applied over the entire event
line. This is used in conjunction with `:s' (e.g.,
`:gs/old/new/') or `:&'. If used with `:s', any
delimiter can be used in place of /, and the final
delimiter is optional if it is the last character
of the event line.
ARITHMETIC EVALUATION
and Arithmetic Expansion). Evaluation is done in long
integers with no check for overflow, though division by 0
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 and plus
! ~ logical and bitwise negation
* / % multiplication, division, remainder
+ - addition, subtraction
<< >> left and right bitwise shifts
<= >= < >
comparison
== != equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
= *= /= %= += -= <<= >>= &= ^= |=
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.
Constants with a leading 0 are interpreted as octal num
bers. A leading 0x or 0X denotes hexadecimal. Otherwise,
numbers take the form [base#]n, where base is a decimal
number between 2 and 36 representing the arithmetic base,
and n is a number in that base. If base is omitted, then
base 10 is used.
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 [arguments]
source filename [arguments]
Read and execute commands from filename in the cur
rent shell environment and return the exit status
of the last command executed from filename. If
filename does not contain a slash, pathnames in
be executable. The current directory is searched
if no file is found in PATH. If any arguments are
supplied, they become the positional parameters
when file is executed. Otherwise the positional
parameters are unchanged. The return status is the
status of the last command exited within the script
(0 if no commands are executed), and false if file
name 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. For
each name in the argument list for which no value
is supplied, the name and value of the alias is
printed. Alias returns true unless a name is given
for which no alias has been defined.
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. bg job
spec returns 0 unless run when job control is dis
abled or, when run with job control enabled, if
jobspec was not found or started without job con
trol.
bind [-m keymap] [-lvd] [-q name]
bind [-m keymap] -f filename
bind [-m keymap] 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
following meanings:
-m keymap
Use keymap as the keymap to be affected by
the subsequent bindings. Acceptable keymap
names are emacs, emacs-standard, emacs-meta,
emacs-ctlx, vi, vi-move, vi-command, and vi-
insert. vi is equivalent to vi-command;
emacs is equivalent to emacs-standard.
-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
Query about which keys invoke the named
function
The return value is 0 unless an unrecognized option
is given or an error occurred.
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. The return status is false if shell-builtin
is not a shell builtin command.
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.
command [-pVv] 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. If either the -V or
-v option is supplied, a description of command is
printed. The -v option causes a single word indi
cating the command or pathname used to invoke com
mand to be printed; the -V option produces a more
verbose description. An argument of -- disables
option checking for the rest of the arguments. If
the -V or -v option is supplied, the exit status is
0 if command was found, and 1 if not. If neither
option is supplied and an error occurred or command
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. The options can be used to
restrict output to variables with the specified
attribute.
-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. The return value
is 0 unless an illegal option is encountered, an
attempt is made to define a function using "-f
foo=bar", one of the names is not a legal shell
variable name, an attempt is made to turn off read
only status for a readonly variable, or an attempt
is made to display a non-existant function with -f.
dirs [-l] [+/-n]
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.
+n displays the nth entry counting from the
left of the list shown by dirs when invoked
without options, starting with zero.
-n displays the nth entry counting from the
right of the list shown by dirs when invoked
without options, starting with zero.
-l produces a longer listing; the default list
ing format uses a tilde to denote the home
supplied or n indexes beyond the end of the direc
tory stack.
echo [-neE] [arg ...]
Output the args, separated by spaces. The return
status is always 0. If -n is specified, the trail
ing newline is suppressed. If the -e option is
given, interpretation of the following backslash-
escaped characters is enabled. The -E option dis
ables the interpretation of these escape charac
ters, even on systems where they are interpreted by
default.
\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] [-all] [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 binary found via the PATH instead
of the shell builtin version, type ``enable -n
test''. If no arguments are given, a list of all
enabled shell builtins is printed. If only -n is
supplied, a list of all disabled builtins is
printed. If only -all is supplied, the list
printed includes all builtins, with an indication
of whether or not each is enabled. enable accepts
-a as a synonym for -all. The return value is 0
unless a name is not a shell builtin.
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 -,
cannot be executed for some reason, a non-interac
tive shell exits, unless the shell variable
no_exit_on_failed_exec exists, in which case it
returns failure. An interactive shell returns
failure if the file cannot be executed. If command
is not specified, any redirections take effect in
the current shell, and the return status is 0.
exit [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 [-nf] [name[=word]] ...
export -p
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
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 0 unless an illegal
option is encountered, one of the names is not a
legal shell variable name, or -f is supplied with a
name that is not a function.
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
edited commands are echoed and executed.
In the second form, command is re-executed after
each instance of pat is replaced by rep. A useful
alias to use with this is ``r=fc -s'', so that typ
ing ``r cc'' runs the last command beginning with
``cc'' and typing ``r'' re-executes the last com
mand.
If the first form is used, the return value is 0
unless an illegal option is encountered or first or
last specify history lines out of range. If the -e
option is supplied, the return value is the value
of the last command executed or failure if an error
occurs with the temporary file of commands. If the
second form is used, the return status is that of
the command re-executed, unless cmd does not spec
ify a valid history line, in which case fc returns
failure.
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. The
return value is that of the command placed into the
foreground, or failure if run when job control is
disabled or, when run with job control enabled, if
jobspec does not specify a valid job or jobspec
specifies a job that was started without job con
trol.
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
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
options or missing option arguments are encoun
tered. If the variable OPTERR is set to 0, no
error message will be displayed, even if the first
character of optstring 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. The return status is 0
unless no command matches pattern.
history [n]
history -rwan [filename]
With no options, display the command history list
with line numbers. Lines listed with a * have been
modified. An argument of n lists only the last n
lines. If a non-option argument is supplied, it is
used as the name of the history file; if not, the
value of HISTFILE is used. Options, if supplied,
have the following meanings:
-a Append the ``new'' history lines (history
-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.
The return value is 0 unless an illegal option is
encountered or an error occurs while reading or
writing the history file.
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 notified. If jobspec is given, output
is restricted to information about that job. The
return status is 0 unless an illegal option is
encountered or an illegal jobspec is supplied.
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, returning its exit status.
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, and the
return status is 0. An argument of -- disables
option checking for the rest of the arguments.
kill returns true if at least one signal was suc
cessfully sent, or false if an error occurs or an
illegal option is encountered.
let arg [arg ...]
Each arg is an arithmetic expression to be evalu
ated (see ARITHMETIC EVALUATION). If the last arg
local [name[=value] ...]
For each argument, 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 func
tion. The return status is 0 unless local is used
outside a function, or an illegal name is supplied.
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 popd command is successful, a dirs is per
formed as well, and the return status is 0. popd
returns false if an illegal option is encountered,
the directory stack is empty, a non-existent direc
tory stack entry is specified, or the directory
change fails.
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 and
returns 0, unless the directory stack is empty.
+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 pushd command is successful, a dirs is per
formed as well. If the first form is used, pushd
stack is empty, a non-existant directory stack ele
ment is specified, or the directory change to the
specified new current directory fails.
pwd Print the absolute pathname of the current working
directory. The path printed contains no symbolic
links if the -P option to the set builtin command
is set. See also the description of nolinks under
Shell Variables above). The return status is 0
unless an error occurs while reading the pathname
of the current directory.
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. If no names are supplied, the line read is
assigned to the variable REPLY. The return code is
zero, unless end-of-file is encountered. If the -r
option is given, a backslash-newline pair is not
ignored, and the backslash is considered to be part
of the line.
readonly [-f] [name ...]
readonly -p
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. The return status is 0
unless an illegal option is encountered, one of the
names is not a legal shell variable name, or -f is
supplied with a name that is not a function.
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. If used outside a function and
not during execution of a script by ., the return
status is false.
-a Automatically mark variables which are mod
ified or created for export to the environ
ment of subsequent commands.
-b Cause the status of terminated background
jobs to be reported immediately, rather
than before the next primary prompt. (Also
see notify under Shell Variables above).
-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.
-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 brace expansion
(see Brace Expansion above). This
is on by default.
emacs Use an emacs-style command line
editing interface. This is enabled
by default when the shell is inter
active, unless the shell is started
with the -nolineediting option.
errexit Same as -e.
histexpand
Same as -H.
ignoreeof
The effect is as if the shell com
above).
interactive-comments
Allow a word beginning with # to
cause that word and all remaining
characters on that line to be
ignored in an interactive shell
(see COMMENTS above).
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f.
nohash Same as -d.
notify Same as -b.
nounset Same as -u.
physical
Same as -P.
posix Change the behavior of bash where
the default operation differs from
the Posix 1003.2 standard to match
the standard.
privileged
Same as -p.
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.
-p Turn on privileged mode. In this mode, the
$ENV file is not processed, and shell func
tions are not inherited from the environ
ment. This is enabled automatically on
startup if the effective user (group) id is
not equal to the real user (group) id.
Turning this option off causes the effec
tive user and group ids to be set to the
real user and group ids.
-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
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
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 default when the shell is
interactive.
-P If set, do not follow symbolic links when
performing commands such as cd which change
the current directory. The physical direc
tory is used instead.
-- 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.
The flags are off by default unless otherwise
noted. 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 n args are
treated as values for the positional parameters and
are assigned, in order, to $1, $2, ... $n. 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 .... Parameters represented by the numbers
$# down to $#-n+1 are unset. If n is 0, no parame
ters are changed. If n is not given, it is assumed
to be 1. n must be a non-negative number less than
or equal to $#. If n is greater than $#, the posi
tional parameters are not changed. The return sta
tus is greater than 0 if n is greater than $# or
less than 0; 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
control is not enabled.
test expr
[ expr ]
Return a status of 0 (true) or 1 (false) depending
on the evaluation of the conditional expression
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. Each operator and
operand must be a separate argument. If file is of
the form /dev/fd/n, then file descriptor n is
checked.
-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.
-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
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
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. The
return status is 0.
trap [-l] [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 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 defined
in <signal.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 arguments. Signals ignored upon entry
to the shell cannot be trapped or reset. Trapped
tus is false if either the trap name or number is
invalid; otherwise trap returns true.
type [-all] [-type | -path] name [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
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 [-SHacdfmstpnuv [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
not be set)
-n the maximum number of open file descriptors
(most systems do not allow this value to be
set, only displayed)
-u the maximum number of processes available to
a single user
-v The maximum amount of virtual memory avail
able to the shell
An argument of -- disables option checking for the
rest of the arguments. If limit is given, it is
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, -p,
which is in units of 512-byte blocks, and -n and
-u, which are unscaled values. The return status
is 0 unless an illegal option is encountered, a
non-numeric argument other than unlimited is sup
plied as limit, or an error occurs while setting a
new limit.
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. The return
status is 0 if the mode was successfully changed or
if no mode argument was supplied, and false other
wise.
unalias [-a] [name ...]
Remove names from the list of defined aliases. If
-a is supplied, all alias definitions are removed.
The return value is true unless a supplied 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, LINENO, or HISTCMD are unset, they lose
their special properties, even if they are subse
quently reset. The exit status is true unless a
Wait for the specified process and return 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 status is zero. If
n specifies a non-existant process or job, the
return status is 127. Otherwise, the return status
is the exit status of the last process or job
waited for.
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 option. PS1 is set
and $- includes i if bash is interactive, allowing a shell
script or a startup file to test this state.
Login shells:
On login (subject to the -noprofile option):
if /etc/profile exists, source it.
if ~/.bash_profile exists, source it,
else if ~/.bash_login exists, source it,
else if ~/.profile exists, source it.
On exit:
if ~/.bash_logout exists, source it.
Non-login interactive shells:
On startup (subject to the -norc and -rcfile options):
if ~/.bashrc exists, source it.
Non-interactive shells:
On startup:
if the environment variable ENV is non-null, expand
it and source the file it names, as if the command
if [ "$ENV" ]; then . $ENV; fi
had been executed, but do not use PATH to search
for the pathname. When not started in Posix mode, bash
looks for BASH_ENV before ENV.
If Bash is invoked as sh, it tries to mimic the behavior
of sh as closely as possible. For a login shell, it
attempts to source only /etc/profile and ~/.profile, in
that order. The -noprofile option may still be used to
disable this behavior. A shell invoked as sh does not
attempt to source any other startup files.
command line option, it follows the Posix standard for
startup files. In this mode, the ENV variable is expanded
and that file sourced; no other startup files are read.
SEE ALSO
Bash Features, Brian Fox and Chet Ramey
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
A System V Compatible Implementation of 4.2BSD Job
Control, David Lennert
Portable Operating System Interface (POSIX) Part 2:
Shell and Utilities, IEEE
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)
FILES
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for
login shells
~/.bash_profile
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.
Once you have determined that a bug actually exists, use
the bashbug command to submit a bug report. If you have a
fix, you are welcome to mail that as well! Suggestions
and `philosophical' bug reports may be mailed to bug-
bash@prep.ai.MIT.Edu or posted to the Usenet newsgroup
gnu.bash.bug.
ALL bug reports should include:
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
bashbug inserts the first three items automatically into
the template it provides for filing a bug report.
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.
Comments - most recent first
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I welcome your comments. However... I am puzzled by many people
who say "Please send me the Linux tutorial." This website *is* your Linux Tutorial! Read everything here, learn
all you can, ask questions if you like. But don't ask me to send what you already have. :-)
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