为什么80%的码农都做不了架构师？>>> 
NAME
bash, :, ., [, alias, bg, bind, break, builtin, caller, cd, command,
compgen, complete, compopt, continue, declare, dirs, disown, echo,
enable, eval, exec, exit, export, false, fc, fg, getopts, hash, help,
history, jobs, kill, let, local, logout, mapfile, popd, printf, pushd,
pwd, read, readonly, return, set, shift, shopt, source, suspend, test,
times, trap, true, type, typeset, ulimit, umask, unalias, unset, wait -
bash built-in commands, see bash(1)
BASH BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in this section
as accepting options preceded by - accepts -- to signify the end of the
options. The :, true, false, and test builtins do not accept options
and do not treat -- specially. The exit, logout, break, continue, let,
and shift builtins accept and process arguments beginning with - with-
out requiring --. Other builtins that accept arguments but are not
specified as accepting options interpret arguments beginning with - as
invalid options and require -- to prevent this interpretation.
: [arguments]
No effect; the command does nothing beyond expanding arguments
and performing any specified redirections. A zero exit code is
returned.
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current shell
environment and return the exit status of the last command exe-
cuted from filename. If filename does not contain a slash, file
names in PATH are used to find the directory containing file-
name. The file searched for in PATH need not be executable.
When bash is not in posix mode, the current directory is
searched if no file is found in PATH. If the sourcepath option
to the shopt builtin command is turned off, the PATH is not
searched. If any arguments are supplied, they become the posi-
tional parameters when filename 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 filename is not found or
cannot be read.
alias [-p] [name[=value] ...]
Alias with no arguments or with the -p option prints the list of
aliases in the form alias 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 sup-
plied, 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 ...]
Resume each suspended job 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 jobspec returns 0 unless
run when job control is disabled or, when run with job control
enabled, any specified jobspec was not found or was started
without job control.
bind [-m keymap] [-lpsvPSV]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq:shell-command
bind [-m keymap] keyseq:function-name
bind readline-command
Display current readline key and function bindings, bind a key
sequence to a readline function or macro, or set a readline
variable. Each non-option argument is a command as it would
appear in .inputrc, but each binding or command 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-stan-
dard, 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.
-p Display readline function names and bindings in such a
way that they can be re-read.
-P List current readline function names and bindings.
-s Display readline key sequences bound to macros and the
strings they output in such a way that they can be re-
read.
-S Display readline key sequences bound to macros and the
strings they output.
-v Display readline variable names and values in such a way
that they can be re-read.
-V List current readline variable names and values.
-f filename
Read key bindings from filename.
-q function
Query about which keys invoke the named function.
-u function
Unbind all keys bound to the named function.
-r keyseq
Remove any current binding for keyseq.
-x keyseq:shell-command
Cause shell-command to be executed whenever keyseq is
entered. When shell-command is executed, the shell sets
the READLINE_LINE variable to the contents of the read-
line line buffer and the READLINE_POINT variable to the
current location of the insertion point. If the executed
command changes the value of READLINE_LINE or READ-
LINE_POINT, those new values will be reflected in the
editing state.
The return value is 0 unless an unrecognized option is given or
an error occurred.
break [n]
Exit from within a for, while, until, or select 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 non-zero when n is ≤ 0; Otherwise, break
returns 0 value.
builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it arguments, and
return its exit status. This is useful when defining a function
whose name is the same as a shell builtin, retaining the func-
tionality of the builtin within the function. The cd builtin is
commonly redefined this way. The return status is false if
shell-builtin is not a shell builtin command.
caller [expr]
Returns the context of any active subroutine call (a shell func-
tion or a script executed with the . or source builtins. With-
out expr, caller displays the line number and source filename of
the current subroutine call. If a non-negative integer is sup-
plied as expr, caller displays the line number, subroutine name,
and source file corresponding to that position in the current
execution call stack. This extra information may be used, for
example, to print a stack trace. The current frame is frame 0.
The return value is 0 unless the shell is not executing a sub-
routine call or expr does not correspond to a valid position in
the call stack.
cd [-L|-P] [dir]
Change the current directory to dir. The variable HOME is the
default dir. The variable CDPATH defines the search path for
the directory containing dir. Alternative directory names in
CDPATH 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. The -P
option says to use the physical directory structure instead of
following symbolic links (see also the -P option to the set
builtin command); the -L option forces symbolic links to be fol-
lowed. An argument of - is equivalent to $OLDPWD. If a non-
empty directory name from CDPATH is used, or if - is the first
argument, and the directory change is successful, the absolute
pathname of the new working directory is written to the standard
output. The return value is true if the directory was success-
fully 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 indicating the command or file name
used to invoke command to be displayed; the -V option produces a
more verbose description. 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 can-
not be found, the exit status is 127. Otherwise, the exit sta-
tus of the command builtin is the exit status of command.
compgen [option] [word]
Generate possible completion matches for word according to the
options, which may be any option accepted by the complete
builtin with the exception of -p and -r, and write the matches
to the standard output. When using the -F or -C options, the
various shell variables set by the programmable completion
facilities, while available, will not have useful values.
The matches will be generated in the same way as if the pro-
grammable completion code had generated them directly from a
completion specification with the same flags. If word is speci-
fied, only those completions matching word will be displayed.
The return value is true unless an invalid option is supplied,
or no matches were generated.
complete [-abcdefgjksuv] [-o comp-option] [-DE] [-A action] [-G glob-
pat] [-W wordlist] [-F function] [-C command]
[-X filterpat] [-P prefix] [-S suffix] name [name ...]
complete -pr [-DE] [name ...]
Specify how arguments to each name should be completed. If the
-p option is supplied, or if no options are supplied, existing
completion specifications are printed in a way that allows them
to be reused as input. The -r option removes a completion spec-
ification for each name, or, if no names are supplied, all com-
pletion specifications. The -D option indicates that the
remaining options and actions should apply to the ‘‘default’’
command completion; that is, completion attempted on a command
for which no completion has previously been defined. The -E
option indicates that the remaining options and actions should
apply to ‘‘empty’’ command completion; that is, completion
attempted on a blank line.
The process of applying these completion specifications when
word completion is attempted is described above under Pro-
grammable Completion.
Other options, if specified, have the following meanings. The
arguments to the -G, -W, and -X options (and, if necessary, the
-P and -S options) should be quoted to protect them from expan-
sion before the complete builtin is invoked.
-o comp-option
The comp-option controls several aspects of the comp-
spec’s behavior beyond the simple generation of comple-
tions. comp-option may be one of:
bashdefault
Perform the rest of the default bash completions
if the compspec generates no matches.
default Use readline’s default filename completion if
the compspec generates no matches.
dirnames
Perform directory name completion if the comp-
spec generates no matches.
filenames
Tell readline that the compspec generates file-
names, so it can perform any filename-specific
processing (like adding a slash to directory
names, quoting special characters, or suppress-
ing trailing spaces). Intended to be used with
shell functions.
nospace Tell readline not to append a space (the
default) to words completed at the end of the
line.
plusdirs
After any matches defined by the compspec are
generated, directory name completion is
attempted and any matches are added to the
results of the other actions.
-A action
The action may be one of the following to generate a
list of possible completions:
alias Alias names. May also be specified as -a.
arrayvar
Array variable names.
binding Readline key binding names.
builtin Names of shell builtin commands. May also be
specified as -b.
command Command names. May also be specified as -c.
directory
Directory names. May also be specified as -d.
disabled
Names of disabled shell builtins.
enabled Names of enabled shell builtins.
export Names of exported shell variables. May also be
specified as -e.
file File names. May also be specified as -f.
function
Names of shell functions.
group Group names. May also be specified as -g.
helptopic
Help topics as accepted by the help builtin.
hostname
Hostnames, as taken from the file specified by
the HOSTFILE shell variable.
job Job names, if job control is active. May also
be specified as -j.
keyword Shell reserved words. May also be specified as
-k.
running Names of running jobs, if job control is active.
service Service names. May also be specified as -s.
setopt Valid arguments for the -o option to the set
builtin.
shopt Shell option names as accepted by the shopt
builtin.
signal Signal names.
stopped Names of stopped jobs, if job control is active.
user User names. May also be specified as -u.
variable
Names of all shell variables. May also be spec-
ified as -v.
-G globpat
The pathname expansion pattern globpat is expanded to
generate the possible completions.
-W wordlist
The wordlist is split using the characters in the IFS
special variable as delimiters, and each resultant word
is expanded. The possible completions are the members
of the resultant list which match the word being com-
pleted.
-C command
command is executed in a subshell environment, and its
output is used as the possible completions.
-F function
The shell function function is executed in the current
shell environment. When it finishes, the possible com-
pletions are retrieved from the value of the COMPREPLY
array variable.
-X filterpat
filterpat is a pattern as used for pathname expansion.
It is applied to the list of possible completions gener-
ated by the preceding options and arguments, and each
completion matching filterpat is removed from the list.
A leading ! in filterpat negates the pattern; in this
case, any completion not matching filterpat is removed.
-P prefix
prefix is added at the beginning of each possible com-
pletion after all other options have been applied.
-S suffix
suffix is appended to each possible completion after all
other options have been applied.
The return value is true unless an invalid option is supplied,
an option other than -p or -r is supplied without a name argu-
ment, an attempt is made to remove a completion specification
for a name for which no specification exists, or an error occurs
adding a completion specification.
compopt [-o option] [-DE] [+o option] [name]
Modify completion options for each name according to the
options, or for the currently-execution completion if no names
are supplied. If no options are given, display the completion
options for each name or the current completion. The possible
values of option are those valid for the complete builtin
described above. The -D option indicates that the remaining
options should apply to the ‘‘default’’ command completion; that
is, completion attempted on a command for which no completion
has previously been defined. The -E option indicates that the
remaining options should apply to ‘‘empty’’ command completion;
that is, completion attempted on a blank line.
The return value is true unless an invalid option is supplied, an
attempt is made to modify the options for a name for which no comple-
tion specification exists, or an output error occurs.
continue [n]
Resume the next iteration of the enclosing for, while, until, or
select 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. When continue is executed inside of loop, the
return value is non-zero when n is ≤ 0; Otherwise, continue
returns 0 value. When continue is executed outside of loop, the
return value is 0.
declare [-aAfFilrtux] [-p] [name[=value] ...]
typeset [-aAfFilrtux] [-p] [name[=value] ...]
Declare variables and/or give them attributes. If no names are
given then display the values of variables. The -p option will
display the attributes and values of each name. When -p is used
with name arguments, additional options are ignored. When -p is
supplied without name arguments, it will display the attributes
and values of all variables having the attributes specified by
the additional options. If no other options are supplied with
-p, declare will display the attributes and values of all shell
variables. The -f option will restrict the display to shell
functions. The -F option inhibits the display of function defi-
nitions; only the function name and attributes are printed. If
the extdebug shell option is enabled using shopt, the source
file name and line number where the function is defined are dis-
played as well. The -F option implies -f. The following
options can be used to restrict output to variables with the
specified attribute or to give variables attributes:
-a Each name is an indexed array variable (see Arrays
above).
-A Each name is an associative array variable (see Arrays
above).
-f Use function names only.
-i The variable is treated as an integer; arithmetic evalua-
tion (see ARITHMETIC EVALUATION above) is performed when
the variable is assigned a value.
-l When the variable is assigned a value, all upper-case
characters are converted to lower-case. The upper-case
attribute is disabled.
-r Make names readonly. These names cannot then be assigned
values by subsequent assignment statements or unset.
-t Give each name the trace attribute. Traced functions
inherit the DEBUG and RETURN traps from the calling
shell. The trace attribute has no special meaning for
variables.
-u When the variable is assigned a value, all lower-case
characters are converted to upper-case. The lower-case
attribute is disabled.
-x Mark names for export to subsequent commands via the
environment.
Using ‘+’ instead of ‘-’ turns off the attribute instead, with
the exceptions that +a may not be used to destroy an array vari-
able and +r will not remove the readonly attribute. When used
in a function, makes each name local, as with the local command.
If a variable name is followed by =value, the value of the vari-
able is set to value. The return value is 0 unless an invalid
option is encountered, an attempt is made to define a function
using ‘‘-f foo=bar’’, an attempt is made to assign a value to a
readonly variable, an attempt is made to assign a value to an
array variable without using the compound assignment syntax (see
Arrays above), one of the names is not a valid shell variable
name, an attempt is made to turn off readonly status for a read-
only variable, an attempt is made to turn off array status for
an array variable, or an attempt is made to display a non-exis-
tent function with -f.
dirs [+n] [-n] [-cplv]
Without options, displays the list of currently remembered
directories. The default display is on a single line with
directory names separated by spaces. Directories are added to
the list with the pushd command; the popd command removes
entries from 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.
-c Clears the directory stack by deleting all of the
entries.
-l Produces a longer listing; the default listing format
uses a tilde to denote the home directory.
-p Print the directory stack with one entry per line.
-v Print the directory stack with one entry per line, pre-
fixing each entry with its index in the stack.
The return value is 0 unless an invalid option is supplied or n
indexes beyond the end of the directory stack.
disown [-ar] [-h] [jobspec ...]
Without options, each jobspec is removed from the table of
active jobs. If jobspec is not present, and neither -a nor -r
is supplied, the shell’s notion of the current job is used. If
the -h option is given, each jobspec is not removed from the ta-
ble, but is marked so that SIGHUP is not sent to the job if the
shell receives a SIGHUP. If no jobspec is present, and neither
the -a nor the -r option is supplied, the current job is used.
If no jobspec is supplied, the -a option means to remove or mark
all jobs; the -r option without a jobspec argument restricts
operation to running jobs. The return value is 0 unless a job-
spec does not specify a valid job.
echo [-neE] [arg ...]
Output the args, separated by spaces, followed by a newline.
The return status is always 0. If -n is specified, the trailing
newline is suppressed. If the -e option is given, interpreta-
tion of the following backslash-escaped characters is enabled.
The -E option disables the interpretation of these escape char-
acters, even on systems where they are interpreted by default.
The xpg_echo shell option may be used to dynamically determine
whether or not echo expands these escape characters by default.
echo does not interpret -- to mean the end of options. echo
interprets the following escape sequences:
\a alert (bell)
\b backspace
\c suppress further output
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\0nnn the eight-bit character whose value is the octal value
nnn (zero to three octal digits)
\xHH the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
enable [-a] [-dnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Disabling a builtin
allows a disk command which has the same name as a shell builtin
to be executed without specifying a full pathname, even though
the shell normally searches for builtins before disk commands.
If -n is used, each name is disabled; otherwise, names are
enabled. For example, to use the test binary found via the PATH
instead of the shell builtin version, run ‘‘enable -n test’’.
The -f option means to load the new builtin command name from
shared object filename, on systems that support dynamic loading.
The -d option will delete a builtin previously loaded with -f.
If no name arguments are given, or if the -p option is supplied,
a list of shell builtins is printed. With no other option argu-
ments, the list consists of all enabled shell builtins. If -n
is supplied, only disabled builtins are printed. If -a is sup-
plied, the list printed includes all builtins, with an indica-
tion of whether or not each is enabled. If -s is supplied, the
output is restricted to the POSIX special builtins. The return
value is 0 unless a name is not a shell builtin or there is an
error loading a new builtin from a shared object.
eval [arg ...]
The args are read and concatenated together into a single com-
mand. This command is then read and executed by the shell, and
its exit status is returned as the value of eval. If there are
no args, or only null arguments, eval returns 0.
exec [-cl] [-a name] [command [arguments]]
If command is specified, it replaces the shell. No new process
is created. The arguments become the arguments to command. If
the -l option is supplied, the shell places a dash at the begin-
ning of the zeroth argument passed to command. This is what
login(1) does. The -c option causes command to be executed with
an empty environment. If -a is supplied, the shell passes name
as the zeroth argument to the executed command. If command can-
not be executed for some reason, a non-interactive shell exits,
unless the shell option execfail is enabled, in which case it
returns failure. An interactive shell returns failure if the
file cannot be executed. If command is not specified, any redi-
rections take effect in the current shell, and the return status
is 0. If there is a redirection error, the return status is 1.
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 [-fn] [name[=word]] ...
export -p
The supplied names are marked for automatic export to the envi-
ronment of subsequently executed commands. 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 each name. If a variable
name is followed by =word, the value of the variable is set to
word. export returns an exit status of 0 unless an invalid
option is encountered, one of the names is not a valid shell
variable name, or -f is supplied with a name that is not a func-
tion.
fc [-e ename] [-lnr] [first] [last]
fc -s [pat=rep] [cmd]
Fix Command. In the first form, a range of commands from first
to last is selected from the history 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 history
list, where a negative number is used as an offset from the cur-
rent 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 spec-
ified it is set to the previous command for editing and -16 for
listing.
The -n option suppresses the command numbers when listing. The
-r option reverses the order of the commands. If the -l option
is given, the commands are listed on standard output. Other-
wise, the editor given by ename is invoked on a file containing
those commands. If ename is not given, the value of the FCEDIT
variable is used, and the value of EDITOR if FCEDIT is not set.
If neither variable is set, is used. When editing is complete,
the 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 typing ‘‘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
invalid 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-exe-
cuted, unless cmd does not specify a valid history line, in
which case fc returns failure.
fg [jobspec]
Resume 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 spec-
ify a valid job or jobspec specifies a job that was started
without job control.
getopts optstring name [args]
getopts is used by shell procedures to parse positional parame-
ters. optstring contains the option characters to be recog-
nized; if a character is followed by a colon, the option is
expected to have an argument, which should be separated from it
by white space. The colon and question mark characters may not
be used as option characters. Each time it is invoked, getopts
places the next option in the shell variable name, initializing
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 automati-
cally; it must be manually reset between multiple calls to
getopts within the same shell invocation if a new set of parame-
ters is to be used.
When the end of options is encountered, getopts exits with a
return value greater than zero. OPTIND is set to the index of
the first non-option argument, and name is set to ?.
getopts normally parses the positional parameters, but if more
arguments are given in args, getopts parses those instead.
getopts can report errors in two ways. If the first character
of optstring is a colon, silent error reporting is used. In
normal operation diagnostic messages are printed when invalid
options or missing option arguments are encountered. If the
variable OPTERR is set to 0, no error messages will be dis-
played, even if the first character of optstring is not a colon.
If an invalid 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 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 [-lr] [-p filename] [-dt] [name]
For each name, the full file name of the command is determined
by searching the directories in $PATH and remembered. If the -p
option is supplied, no path search is performed, and filename is
used as the full file name of the command. The -r option causes
the shell to forget all remembered locations. The -d option
causes the shell to forget the remembered location of each name.
If the -t option is supplied, the full pathname to which each
name corresponds is printed. If multiple name arguments are
supplied with -t, the name is printed before the hashed full
pathname. The -l option causes output to be displayed in a for-
mat that may be reused as input. If no arguments are given, or
if only -l is supplied, information about remembered commands is
printed. The return status is true unless a name is not found
or an invalid option is supplied.
help [-dms] [pattern]
Display helpful information about builtin commands. If pattern
is specified, help gives detailed help on all commands matching
pattern; otherwise help for all the builtins and shell control
structures is printed.
-d Display a short description of each pattern
-m Display the description of each pattern in a manpage-like
format
-s Display only a short usage synopsis for each pattern
The return status is 0 unless no command matches pattern.
history [n]
history -c
history -d offset
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]
With no options, display the command history list with line num-
bers. Lines listed with a * have been modified. An argument of
n lists only the last n lines. If the shell variable HISTTIME-
FORMAT is set and not null, it is used as a format string for
strftime(3) to display the time stamp associated with each dis-
played history entry. No intervening blank is printed between
the formatted time stamp and the history line. If filename 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:
-c Clear the history list by deleting all the entries.
-d offset
Delete the history entry at position offset.
-a Append the ‘‘new’’ history lines (history lines entered
since the beginning of the current bash session) to the
history file.
-n Read the history lines not already read from the history
file into the current history list. These are lines
appended to the history 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, overwrit-
ing the history file’s contents.
-p Perform history substitution on the following args and
display the result on the standard output. Does not
store the results in the history list. Each arg must be
quoted to disable normal history expansion.
-s Store the args in the history list as a single entry.
The last command in the history list is removed before
the args are added.
If the HISTTIMEFORMAT variable is set, the time stamp informa-
tion associated with each history entry is written to the his-
tory file, marked with the history comment character. When the
history file is read, lines beginning with the history comment
character followed immediately by a digit are interpreted as
timestamps for the previous history line. The return value is 0
unless an invalid option is encountered, an error occurs while
reading or writing the history file, an invalid offset is sup-
plied as an argument to -d, or the history expansion supplied as
an argument to -p fails.
jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The options have the fol-
lowing meanings:
-l List process IDs in addition to the normal information.
-p List only the process ID of the job’s process group
leader.
-n Display information only about jobs that have changed
status since the user was last notified of their status.
-r Restrict output to running jobs.
-s Restrict output to stopped jobs.
If jobspec is given, output is restricted to information about
that job. The return status is 0 unless an invalid option is
encountered or an invalid jobspec is supplied.
If the -x option is supplied, jobs replaces any jobspec found in
command or args with the corresponding process group ID, and
executes command passing it args, returning its exit status.
kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...
kill -l [sigspec | exit_status]
Send the signal named by sigspec or signum to the processes
named by pid or jobspec. sigspec is either a case-insensitive
signal name such as SIGKILL (with or without the SIG prefix) or
a signal number; signum is a signal number. 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 signals corresponding to the arguments are
listed, and the return status is 0. The exit_status argument to
-l is a number specifying either a signal number or the exit
status of a process terminated by a signal. kill returns true
if at least one signal was successfully sent, or false if an
error occurs or an invalid option is encountered.
let arg [arg ...]
Each arg is an arithmetic expression to be evaluated (see ARITH-
METIC EVALUATION above). If the last arg evaluates to 0, let
returns 1; 0 is returned otherwise.
local [option] [name[=value] ...]
For each argument, a local variable named name is created, and
assigned value. The option can be any of the options accepted
by declare. When local is used within a function, it causes the
variable name to have a visible scope restricted to that func-
tion and its children. With no operands, local writes a list of
local variables to the standard output. It is an error to use
local when not within a function. The return status is 0 unless
local is used outside a function, an invalid name is supplied,
or name is a readonly variable.
logout Exit a login shell.
mapfile [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback]
[-c quantum] [array]
readarray [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback]
[-c quantum] [array]
Read lines from the standard input into the indexed array vari-
able array, or from file descriptor fd if the -u option is sup-
plied. The variable MAPFILE is the default array. Options, if
supplied, have the following meanings:
-n Copy at most count lines. If count is 0, all lines are
copied.
-O Begin assigning to array at index origin. The default
index is 0.
-s Discard the first count lines read.
-t Remove a trailing newline from each line read.
-u Read lines from file descriptor fd instead of the stan-
dard input.
-C Evaluate callback each time quantum lines are read. The
-c option specifies quantum.
-c Specify the number of lines read between each call to
callback.
If -C is specified without -c, the default quantum is 5000.
When callback is evaluated, it is supplied the index of the next
array element to be assigned as an additional argument. call-
back is evaluated after the line is read but before the array
element is assigned.
If not supplied with an explicit origin, mapfile will clear
array before assigning to it.
mapfile returns successfully unless an invalid option or option
argument is supplied, array is invalid or unassignable, or if
array is not an indexed array.
popd [-n] [+n] [-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. Arguments, if supplied, have the follow-
ing meanings:
-n Suppresses the normal change of directory when removing
directories from the stack, so that only the stack is
manipulated.
+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 performed as well,
and the return status is 0. popd returns false if an invalid
option is encountered, the directory stack is empty, a non-exis-
tent directory stack entry is specified, or the directory change
fails.
printf [-v var] format [arguments]
Write the formatted arguments to the standard output under the
control of the format. The format is a character string which
contains three types of objects: plain characters, which are
simply copied to standard output, character escape sequences,
which are converted and copied to the standard output, and for-
mat specifications, each of which causes printing of the next
successive argument. In addition to the standard printf(1) for-
mats, %b causes printf to expand backslash escape sequences in
the corresponding argument (except that \c terminates output,
backslashes in \', \", and \? are not removed, and octal escapes
beginning with \0 may contain up to four digits), and %q causes
printf to output the corresponding argument in a format that can
be reused as shell input.
The -v option causes the output to be assigned to the variable
var rather than being printed to the standard output.
The format is reused as necessary to consume all of the argu-
ments. If the format requires more arguments than are supplied,
the extra format specifications behave as if a zero value or
null string, as appropriate, had been supplied. The return
value is zero on success, non-zero on failure.
pushd [-n] [+n] [-n]
pushd [-n] [dir]
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 arguments, exchanges the top two directories
and returns 0, unless the directory stack is empty. Arguments,
if supplied, have the following meanings:
-n Suppresses the normal change of directory when adding
directories to the stack, so that only the stack is
manipulated.
+n Rotates the stack so that the nth directory (counting
from the left of the list shown by dirs, starting with
zero) is at the top.
-n Rotates the stack so that the nth directory (counting
from the right of the list shown by dirs, starting with
zero) 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 performed as well.
If the first form is used, pushd returns 0 unless the cd to dir
fails. With the second form, pushd returns 0 unless the direc-
tory stack is empty, a non-existent directory stack element is
specified, or the directory change to the specified new current
directory fails.
pwd [-LP]
Print the absolute pathname of the current working directory.
The pathname printed contains no symbolic links if the -P option
is supplied or the -o physical option to the set builtin command
is enabled. If the -L option is used, the pathname printed may
contain symbolic links. The return status is 0 unless an error
occurs while reading the name of the current directory or an
invalid option is supplied.
read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p
prompt] [-t timeout] [-u fd] [name ...]
One line is read from the standard input, or from the file
descriptor fd supplied as an argument to the -u option, and the
first word is assigned to the first name, the second word to the
second name, and so on, with leftover words and their interven-
ing separators assigned to the last name. If there are fewer
words read from the input stream than names, the remaining names
are assigned empty values. The characters in IFS are used to
split the line into words. The backslash character (\) may be
used to remove any special meaning for the next character read
and for line continuation. Options, if supplied, have the fol-
lowing meanings:
-a aname
The words are assigned to sequential indices of the array
variable aname, starting at 0. aname is unset before any
new values are assigned. Other name arguments are
ignored.
-d delim
The first character of delim is used to terminate the
input line, rather than newline.
-e If the standard input is coming from a terminal, readline
(see READLINE above) is used to obtain the line. Read-
line uses the current (or default, if line editing was
not previously active) editing settings.
-i text
If readline is being used to read the line, text is
placed into the editing buffer before editing begins.
-n nchars
read returns after reading nchars characters rather than
waiting for a complete line of input, but honor a delim-
iter if fewer than nchars characters are read before the
delimiter.
-N nchars
read returns after reading exactly nchars characters
rather than waiting for a complete line of input, unless
EOF is encountered or read times out. Delimiter charac-
ters encountered in the input are not treated specially
and do not cause read to return until nchars characters
are read.
-p prompt
Display prompt on standard error, without a trailing new-
line, before attempting to read any input. The prompt is
displayed only if input is coming from a terminal.
-r Backslash does not act as an escape character. The back-
slash is considered to be part of the line. In particu-
lar, a backslash-newline pair may not be used as a line
continuation.
-s Silent mode. If input is coming from a terminal, charac-
ters are not echoed.
-t timeout
Cause read to time out and return failure if a complete
line of input is not read within timeout seconds. time-
out may be a decimal number with a fractional portion
following the decimal point. This option is only effec-
tive if read is reading input from a terminal, pipe, or
other special file; it has no effect when reading from
regular files. If timeout is 0, read returns success if
input is available on the specified file descriptor,
failure otherwise. The exit status is greater than 128
if the timeout is exceeded.
-u fd Read input from file descriptor fd.
If no names are supplied, the line read is assigned to the vari-
able REPLY. The return code is zero, unless end-of-file is
encountered, read times out (in which case the return code is
greater than 128), or an invalid file descriptor is supplied as
the argument to -u.
readonly [-aApf] [name[=word] ...]
The given names are marked readonly; 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. The -a option restricts the variables to indexed
arrays; the -A option restricts the variables to associative
arrays. If no name arguments are given, or if the -p option is
supplied, a list of all readonly names is printed. The -p
option causes output to be displayed in a format that may be
reused as input. If a variable name is followed by =word, the
value of the variable is set to word. The return status is 0
unless an invalid option is encountered, one of the names is not
a valid 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 function 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 status of the script. If used outside a
function and not during execution of a script by ., the return
status is false. Any command associated with the RETURN trap is
executed before execution resumes after the function or script.
set [--abefhkmnptuvxBCEHPT] [-o option] [arg ...]
set [+abefhkmnptuvxBCEHPT] [+o option] [arg ...]
Without options, the name and value of each shell variable are
displayed in a format that can be reused as input for setting or
resetting the currently-set variables. Read-only variables can-
not be reset. In posix mode, only shell variables are listed.
The output is sorted according to the current locale. When
options are specified, they set or unset shell attributes. Any
arguments remaining after option processing are treated as val-
ues for the positional parameters and are assigned, in order, to
$1, $2, ... $n. Options, if specified, have the following
meanings:
-a Automatically mark variables and functions which are
modified or created for export to the environment of
subsequent commands.
-b Report the status of terminated background jobs immedi-
ately, rather than before the next primary prompt. This
is effective only when job control is enabled.
-e Exit immediately if a pipeline (which may consist of a
single simple command), a subshell command enclosed in
parentheses, or one of the commands executed as part of
a command list enclosed by braces (see SHELL GRAMMAR
above) exits with a non-zero status. The shell does not
exit if the command that fails is part of the command
list immediately following a while or until keyword,
part of the test following the if or elif reserved
words, part of any command executed in a && or ││ list
except the command following the final && or ││, any
command in a pipeline but the last, or if the command’s
return value is being inverted with !. A trap on ERR,
if set, is executed before the shell exits. This option
applies to the shell environment and each subshell envi-
ronment separately (see COMMAND EXECUTION ENVIRONMENT
above), and may cause subshells to exit before executing
all the commands in the subshell.
-f Disable pathname expansion.
-h Remember the location of commands as they are looked up
for execution. This is enabled by default.
-k All arguments in the form of assignment statements are
placed in the environment for a command, not just those
that precede the command name.
-m Monitor mode. Job control is enabled. This option is
on by default for interactive shells on systems that
support it (see JOB CONTROL above). Background pro-
cesses run in a separate process group and a line con-
taining their exit status is printed upon their comple-
tion.
-n Read commands but do not execute them. This may be used
to check a shell script for syntax errors. This is
ignored by interactive shells.
-o option-name
The option-name can be one of the following:
allexport
Same as -a.
braceexpand
Same as -B.
emacs Use an emacs-style command line editing inter-
face. This is enabled by default when the shell
is interactive, unless the shell is started with
the --noediting option. This also affects the
editing interface used for read -e.
errexit Same as -e.
errtrace
Same as -E.
functrace
Same as -T.
hashall Same as -h.
histexpand
Same as -H.
history Enable command history, as described above under
HISTORY. This option is on by default in inter-
active shells.
ignoreeof
The effect is as if the shell command
‘‘IGNOREEOF=10’’ had been executed (see Shell
Variables above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f.
nolog Currently ignored.
notify Same as -b.
nounset Same as -u.
onecmd Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline is the
value of the last (rightmost) command to exit
with a non-zero status, or zero if all commands
in the pipeline exit successfully. This option
is disabled by default.
posix Change the behavior of bash where the default
operation differs from the POSIX standard to
match the standard (posix mode).
privileged
Same as -p.
verbose Same as -v.
vi Use a vi-style command line editing interface.
This also affects the editing interface used for
read -e.
xtrace Same as -x.
If -o is supplied with no option-name, the values of the
current options are printed. If +o is supplied with no
option-name, a series of set commands to recreate the
current option settings is displayed on the standard
output.
-p Turn on privileged mode. In this mode, the $ENV and
$BASH_ENV files are not processed, shell functions are
not inherited from the environment, and the SHELLOPTS,
BASHOPTS, CDPATH, and GLOBIGNORE variables, if they
appear in the environment, are ignored. If the shell is
started with the effective user (group) id not equal to
the real user (group) id, and the -p option is not sup-
plied, these actions are taken and the effective user id
is set to the real user id. If the -p option is sup-
plied at startup, the effective user id is not reset.
Turning this option off causes the effective user and
group ids to be set to the real user and group ids.
-t Exit after reading and executing one command.
-u Treat unset variables and parameters other than the spe-
cial parameters "@" and "*" as an error when performing
parameter expansion. If expansion is attempted on an
unset variable or parameter, 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, for command, case
command, select command, or arithmetic for command, dis-
play the expanded value of PS4, followed by the command
and its expanded arguments or associated word list.
-B The shell performs brace expansion (see Brace Expansion
above). This is on by default.
-C If set, bash does not overwrite an existing file with
the >, >&, and <> redirection operators. This may be
overridden when creating output files by using the redi-
rection operator >| instead of >.
-E If set, any trap on ERR is inherited by shell functions,
command substitutions, and commands executed in a sub-
shell environment. The ERR trap is normally not inher-
ited in such cases.
-H Enable ! style history substitution. This option is on
by default when the shell is interactive.
-P If set, the shell does not follow symbolic links when
executing commands such as cd that change the current
working directory. It uses the physical directory
structure instead. By default, bash follows the logical
chain of directories when performing commands which
change the current directory.
-T If set, any traps on DEBUG and RETURN are inherited by
shell functions, command substitutions, and commands
executed in a subshell environment. The DEBUG and
RETURN traps are normally not inherited in such cases.
-- If no arguments follow this option, then the positional
parameters are unset. Otherwise, the positional parame-
ters 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 positional parameters. The -x and -v
options are turned off. If there are no args, the posi-
tional parameters remain unchanged.
The options are off by default unless otherwise noted. Using +
rather than - causes these options to be turned off. The
options can also be specified as arguments to an invocation of
the shell. The current set of options may be found in $-. The
return status is always true unless an invalid option is encoun-
tered.
shift [n]
The positional parameters from n+1 ... are renamed to $1 ....
Parameters represented by the numbers $# down to $#-n+1 are
unset. n must be a non-negative number less than or equal to
$#. If n is 0, no parameters are changed. If n is not given,
it is assumed to be 1. If n is greater than $#, the positional
parameters are not changed. The return status is greater than
zero if n is greater than $# or less than zero; otherwise 0.
shopt [-pqsu] [-o] [optname ...]
Toggle the values of variables controlling optional shell behav-
ior. With no options, or with the -p option, a list of all set-
table options is displayed, with an indication of whether or not
each is set. The -p option causes output to be displayed in a
form that may be reused as input. Other options have the fol-
lowing meanings:
-s Enable (set) each optname.
-u Disable (unset) each optname.
-q Suppresses normal output (quiet mode); the return status
indicates whether the optname is set or unset. If multi-
ple optname arguments are given with -q, the return sta-
tus is zero if all optnames are enabled; non-zero other-
wise.
-o Restricts the values of optname to be those defined for
the -o option to the set builtin.
If either -s or -u is used with no optname arguments, the dis-
play is limited to those options which are set or unset, respec-
tively. Unless otherwise noted, the shopt options are disabled
(unset) by default.
The return status when listing options is zero if all optnames
are enabled, non-zero otherwise. When setting or unsetting
options, the return status is zero unless an optname is not a
valid shell option.
The list of shopt options is:
autocd If set, a command name that is the name of a directory
is executed as if it were the argument to the cd com-
mand. This option is only used by interactive shells.
cdable_vars
If set, an argument to the cd builtin command that is
not a directory is assumed to be the name of a variable
whose value is the directory to change to.
cdspell If set, minor errors in the spelling of a directory com-
ponent in a cd command will be corrected. The errors
checked for are transposed characters, a missing charac-
ter, and one character too many. If a correction is
found, the corrected file name is printed, and the com-
mand proceeds. This option is only used by interactive
shells.
checkhash
If set, bash checks that a command found in the hash ta-
ble exists before trying to execute it. If a hashed
command no longer exists, a normal path search is per-
formed.
checkjobs
If set, bash lists the status of any stopped and running
jobs before exiting an interactive shell. If any jobs
are running, this causes the exit to be deferred until a
second exit is attempted without an intervening command
(see JOB CONTROL above). The shell always postpones
exiting if any jobs are stopped.
checkwinsize
If set, bash checks the window size after each command
and, if necessary, updates the values of LINES and
COLUMNS.
cmdhist If set, bash attempts to save all lines of a multiple-
line command in the same history entry. This allows
easy re-editing of multi-line commands.
compat31
If set, bash changes its behavior to that of version 3.1
with respect to quoted arguments to the conditional com-
mand’s =~ operator.
compat32
If set, bash changes its behavior to that of version 3.2
with respect to locale-specific string comparison when
using the conditional command’s < and > operators.
compat40
If set, bash changes its behavior to that of version 4.0
with respect to locale-specific string comparison when
using the conditional command’s < and > operators and
the effect of interrupting a command list.
dirspell
If set, bash attempts spelling correction on directory
names during word completion if the directory name ini-
tially supplied does not exist.
dotglob If set, bash includes filenames beginning with a ‘.’ in
the results of pathname expansion.
execfail
If set, a non-interactive shell will not exit if it can-
not execute the file specified as an argument to the
exec builtin command. An interactive shell does not
exit if exec fails.
expand_aliases
If set, aliases are expanded as described above under
ALIASES. This option is enabled by default for interac-
tive shells.
extdebug
If set, behavior intended for use by debuggers is
enabled:
1. The -F option to the declare builtin displays the
source file name and line number corresponding to
each function name supplied as an argument.
2. If the command run by the DEBUG trap returns a
non-zero value, the next command is skipped and
not executed.
3. If the command run by the DEBUG trap returns a
value of 2, and the shell is executing in a sub-
routine (a shell function or a shell script exe-
cuted by the . or source builtins), a call to
return is simulated.
4. BASH_ARGC and BASH_ARGV are updated as described
in their descriptions above.
5. Function tracing is enabled: command substitu-
tion, shell functions, and subshells invoked with
( command ) inherit the DEBUG and RETURN traps.
6. Error tracing is enabled: command substitution,
shell functions, and subshells invoked with (
command ) inherit the ERROR trap.
extglob If set, the extended pattern matching features described
above under Pathname Expansion are enabled.
extquote
If set, $'string' and $"string" quoting is performed
within ${parameter} expansions enclosed in double
quotes. This option is enabled by default.
failglob
If set, patterns which fail to match filenames during
pathname expansion result in an expansion error.
force_fignore
If set, the suffixes specified by the FIGNORE shell
variable cause words to be ignored when performing word
completion even if the ignored words are the only possi-
ble completions. See SHELL VARIABLES above for a
description of FIGNORE. This option is enabled by
default.
globstar
If set, the pattern ** used in a pathname expansion con-
text will match a files and zero or more directories and
subdirectories. If the pattern is followed by a /, only
directories and subdirectories match.
gnu_errfmt
If set, shell error messages are written in the standard
GNU error message format.
histappend
If set, the history list is appended to the file named
by the value of the HISTFILE variable when the shell
exits, rather than overwriting the file.
histreedit
If set, and readline is being used, a user is given the
opportunity to re-edit a failed history substitution.
histverify
If set, and readline is being used, the results of his-
tory substitution are not immediately passed to the
shell parser. Instead, the resulting line is loaded
into the readline editing buffer, allowing further modi-
fication.
hostcomplete
If set, and readline is being used, bash will attempt to
perform hostname completion when a word containing a @
is being completed (see Completing under READLINE
above). This is enabled by default.
huponexit
If set, bash will send SIGHUP to all jobs when an inter-
active login shell exits.
interactive_comments
If set, 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). This
option is enabled by default.
lithist If set, and the cmdhist option is enabled, multi-line
commands are saved to the history with embedded newlines
rather than using semicolon separators where possible.
login_shell
The shell sets this option if it is started as a login
shell (see INVOCATION above). The value may not be
changed.
mailwarn
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 dis-
played.
no_empty_cmd_completion
If set, and readline is being used, bash will not
attempt to search the PATH for possible completions when
completion is attempted on an empty line.
nocaseglob
If set, bash matches filenames in a case-insensitive
fashion when performing pathname expansion (see Pathname
Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive
fashion when performing matching while executing case or
[[ conditional commands.
nullglob
If set, bash allows patterns which match no files (see
Pathname Expansion above) to expand to a null string,
rather than themselves.
progcomp
If set, the programmable completion facilities (see Pro-
grammable Completion above) are enabled. This option is
enabled by default.
promptvars
If set, prompt strings undergo parameter expansion, com-
mand substitution, arithmetic expansion, and quote
removal after being expanded as described in PROMPTING
above. This option is enabled by default.
restricted_shell
The shell sets this option if it is started in
restricted mode (see RESTRICTED SHELL below). The value
may not be changed. This is not reset when the startup
files are executed, allowing the startup files to dis-
cover whether or not a shell is restricted.
shift_verbose
If set, the shift builtin prints an error message when
the shift count exceeds the number of positional parame-
ters.
sourcepath
If set, the source (.) builtin uses the value of PATH to
find the directory containing the file supplied as an
argument. This option is enabled by default.
xpg_echo
If set, the echo builtin expands backslash-escape
sequences by default.
suspend [-f]
Suspend the execution of this shell until it receives a SIGCONT
signal. When the suspended shell is a background process, it can
be restarted by the fg command. For more information, read the
JOB CONTROL section. The suspend command can not suspend the
login shell. However, when -f option is specified, suspend com-
mand can suspend even login shell. The return status is 0
unless the shell is a login shell and -f is not supplied, or if
job control is not enabled.
test expr
[ expr ]
Return a status of 0 or 1 depending on the evaluation of the
conditional expression expr. Each operator and operand must be
a separate argument. Expressions are composed of the primaries
described above under CONDITIONAL EXPRESSIONS. test does not
accept any options, nor does it accept and ignore an argument of
-- as signifying the end of options.
Expressions may be combined using the following operators,
listed in decreasing order of precedence. The evaluation
depends on the number of arguments; see below.
! expr True if expr is false.
( expr )
Returns the value of expr. This may be used to override
the normal precedence of operators.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.
test and [ evaluate conditional expressions using a set of rules
based on the number of arguments.
0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument is not
null.
2 arguments
If the first argument is !, the expression is true if and
only if the second argument is null. If the first argu-
ment is one of the unary conditional operators listed
above under CONDITIONAL EXPRESSIONS, the expression is
true if the unary test is true. If the first argument is
not a valid unary conditional operator, the expression is
false.
3 arguments
If the second argument is one of the binary conditional
operators listed above under CONDITIONAL EXPRESSIONS, the
result of the expression is the result of the binary test
using the first and third arguments as operands. The -a
and -o operators are considered binary operators when
there are three arguments. If the first argument is !,
the value is the negation of the two-argument test using
the second and third arguments. If the first argument is
exactly ( and the third argument is exactly ), the result
is the one-argument test of the second argument. Other-
wise, the expression is false.
4 arguments
If the first argument is !, the result is the negation of
the three-argument expression composed of the remaining
arguments. Otherwise, the expression is parsed and eval-
uated according to precedence using the rules listed
above.
5 or more arguments
The expression is parsed and evaluated according to
precedence using the rules listed above.
times Print the accumulated user and system times for the shell and
for processes run from the shell. The return status is 0.
trap [-lp] [[arg] sigspec ...]
The command arg is to be read and executed when the shell
receives signal(s) sigspec. If arg is absent (and there is a
single sigspec) or -, each specified signal is reset to its
original disposition (the value it had upon entrance to the
shell). If arg is the null string the signal specified by each
sigspec is ignored by the shell and by the commands it invokes.
If arg is not present and -p has been supplied, then the trap
commands associated with each sigspec are displayed. If no
arguments are supplied or if only -p is given, trap prints the
list of commands associated with each signal. The -l option
causes the shell to print a list of signal names and their cor-
responding numbers. Each sigspec is either a signal name
defined in
case insensitive and the SIG prefix is optional.
If a sigspec is EXIT (0) the command arg is executed on exit
from the shell. If a sigspec is DEBUG, the command arg is exe-
cuted before every simple command, for command, case command,
select command, every arithmetic for command, and before the
first command executes in a shell function (see SHELL GRAMMAR
above). Refer to the description of the extdebug option to the
shopt builtin for details of its effect on the DEBUG trap. If a
sigspec is RETURN, the command arg is executed each time a shell
function or a script executed with the . or source builtins fin-
ishes executing.
If a sigspec is ERR, the command arg is executed whenever a sim-
ple command has a non-zero exit status, subject to the following
conditions. The ERR trap is not executed if the failed command
is part of the command list immediately following a while or
until keyword, part of the test in an if statement, part of a
command executed in a && or ││ list, or if the command’s return
value is being inverted via !. These are the same conditions
obeyed by the errexit option.
Signals ignored upon entry to the shell cannot be trapped or
reset. Trapped signals that are not being ignored are reset to
their original values in a subshell or subshell environment when
one is created. The return status is false if any sigspec is
invalid; otherwise trap returns true.
type [-aftpP] name [name ...]
With no options, indicate how each name would be interpreted if
used as a command name. If the -t option is used, type prints a
string 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 -p option is used, type either returns the
name of the disk file that would be executed if name were speci-
fied as a command name, or nothing if ‘‘type -t name’’ would not
return file. The -P option forces a PATH search for each name,
even if ‘‘type -t name’’ would not return file. If a command is
hashed, -p and -P print the hashed value, not necessarily the
file that appears first in PATH. If the -a option is used, type
prints all of the places that contain an executable named name.
This includes aliases and functions, if and only if the -p
option is not also used. The table of hashed commands is not
consulted when using -a. The -f option suppresses shell func-
tion lookup, as with the command builtin. type returns true if
all of the arguments are found, false if any are not found.
ulimit [-HSTabcdefilmnpqrstuvx [limit]]
Provides control over the resources available to the shell and
to processes started by it, on systems that allow such control.
The -H and -S options specify that the hard or soft limit is set
for the given resource. A hard limit cannot be increased by a
non-root user once it is set; a soft limit may be increased up
to the value of the hard limit. If neither -H nor -S is speci-
fied, both the soft and hard limits are set. The value of limit
can be a number in the unit specified for the resource or one of
the special values hard, soft, or unlimited, which stand for the
current hard limit, the current soft limit, and no limit,
respectively. If limit is omitted, the current 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 are printed before the value. Other options are inter-
preted as follows:
-a All current limits are reported
-b The maximum socket buffer size
-c The maximum size of core files created
-d The maximum size of a process’s data segment
-e The maximum scheduling priority ("nice")
-f The maximum size of files written by the shell and its
children
-i The maximum number of pending signals
-l The maximum size that may be locked into memory
-m The maximum resident set size (many systems do not honor
this limit)
-n The maximum number of open file descriptors (most systems
do not allow this value to be set)
-p The pipe size in 512-byte blocks (this may not be set)
-q The maximum number of bytes in POSIX message queues
-r The maximum real-time scheduling priority
-s The maximum stack size
-t The maximum amount of cpu time in seconds
-u The maximum number of processes available to a single
user
-v The maximum amount of virtual memory available to the
shell
-x The maximum number of file locks
-T The maximum number of threads
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 increments, except for -t,
which is in seconds, -p, which is in units of 512-byte blocks,
and -T, -b, -n, and -u, which are unscaled values. The return
status is 0 unless an invalid option or argument is supplied, or
an error occurs while setting a new limit.
umask [-p] [-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 symbolic mode mask similar to that accepted by
chmod(1). If mode is omitted, the current value of the mask is
printed. The -S option causes the mask to be printed in sym-
bolic form; the default output is an octal number. If the -p
option is supplied, and mode is omitted, the output is in a form
that may be reused as input. The return status is 0 if the mode
was successfully changed or if no mode argument was supplied,
and false otherwise.
unalias [-a] [name ...]
Remove each name 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 function.
If no options are supplied, or the -v option is given, each name
refers to a shell variable. Read-only variables may not be
unset. If -f is specified, each name refers to a shell func-
tion, and the function definition is removed. Each unset vari-
able or function is removed from the environment passed to sub-
sequent commands. If any of COMP_WORDBREAKS, RANDOM, SECONDS,
LINENO, HISTCMD, FUNCNAME, GROUPS, or DIRSTACK are unset, they
lose their special properties, even if they are subsequently
reset. The exit status is true unless a name is readonly.
wait [n ...]
Wait for each specified process and return its termination sta-
tus. Each n may be a process ID or a job specification; if a
job spec is given, all processes in that job’s pipeline are
waited for. If n is not given, all currently active child pro-
cesses are waited for, and the return status is zero. If n
specifies a non-existent process or job, the return status is
127. Otherwise, the return status is the exit status of the
last process or job waited for.
SEE ALSO
bash(1), sh(1)
GNU Bash-4.0 2004 Apr 20 BASH_BUILTINS(1)