在shell中，内建（builtin）命令read，格式如下：
read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
1
read命令用于从标准输入或选项”-u“指定的文件描述符中读取一行文本，把第一个单词赋值给第一个名称name，第二个单词赋值给第二个名称name，以此类推，剩余的单词连同分隔符一起赋值给最后一个名称name，如果输入的名称name个数比读取的单词个数多，多出来的赋值为空，如果没有指定任何名称name，则把结果赋值给系统变量REPLY。
下面解释read命令各选项的含义。
”-a aname“：把各个单词依次赋值给数组aname中从0开始的连续下标，赋值之前aname被unset，使用了这个选项就会忽略其它的名称name。
”-d delim“：用分隔符delim的第一个字符来结束输入行，而不是换行符。
”-e“：如果标准输入来自shell终端，使用”readline“来读取输入行。
”-i text“：如果使用”readline“来读取输入行，文本text在编辑前被放到编辑缓冲中。
”-n nchars“：最多读取nchars个字符。
”-N nchars“：读取nchars个字符，转义字符不进行转义。
”-p prompt“：如果在shell终端读取输入，首先打印提示prompt，提示不换行。
”-r“：反斜线这个转义字符不作特殊处理，当作普通字符。
”-s“：安静模式，输入来自shell终端时，不进行回显echo。
”-t timeout“：如果在超时时间timeout指定的秒数内还没有读入完整的一行，则读取超时并返回false。timeout可以是个带有小数的十进制数。这个选项只有在read命令从终端、管道、或者其它特殊文件读取输入时才有效，从普通文件读取输入时没有作用。如果timeout为0，则当指定的文件描述符可用时返回true，不可用时返回fasle。如果超时，返回状态大于128。
“-u fd”：从文件描述符fd中读取输入。
help read
read: read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
Read a line from the standard input and split it into fields. Reads a single line from the standard input, or from file descriptor FD
if the -u option is supplied. The line is split into fields as with word
splitting, and the first word is assigned to the first NAME, the second
word to the second NAME, and so on, with any leftover words assigned to
the last NAME. Only the characters found in $IFS are recognized as word
delimiters. If no NAMEs are supplied, the line read is stored in the REPLY variable. Options:
-a array assign the words read to sequential indices of the array
variable ARRAY, starting at zero
-d delim continue until the first character of DELIM is read, rather
than newline
-e use Readline to obtain the line in an interactive shell
-i text Use TEXT as the initial text for Readline
-n nchars return after reading NCHARS characters rather than waiting
for a newline, but honor a delimiter if fewer than NCHARS
characters are read before the delimiter
-N nchars return only after reading exactly NCHARS characters, unless
EOF is encountered or read times out, ignoring any delimiter
-p prompt output the string PROMPT without a trailing newline before
attempting to read
-r do not allow backslashes to escape any characters
-s do not echo input coming from a terminal
-t timeout time out and return failure if a complete line of input is
not read withint TIMEOUT seconds. The value of the TMOUT
variable is the default timeout. TIMEOUT may be a
fractional number. If TIMEOUT is , read returns success only
if input is available on the specified file descriptor. The
exit status is greater than if the timeout is exceeded
-u fd read from file descriptor FD instead of the standard input Exit Status:
The return code is zero, unless end-of-file is encountered, read times out,
or an invalid file descriptor is supplied as the argument to -u.
readarray: readarray [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c quantum] [array]
Read lines from a file into an array variable. A synonym for `mapfile'.
readonly: readonly [-aAf] [name[=value] ...] or readonly -p
Mark shell variables as unchangeable. Mark each NAME as read-only; the values of these NAMEs may not be
changed by subsequent assignment. If VALUE is supplied, assign VALUE
before marking as read-only. Options:
-a refer to indexed array variables
-A refer to associative array variables
-f refer to shell functions
-p display a list of all readonly variables and functions An argument of `--' disables further option processing. Exit Status:
Returns success unless an invalid option is given or NAME is invalid.
man read
BASH_BUILTINS() General Commands Manual BASH_BUILTINS() 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() 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
- without 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 executed from filename. If filename does not
contain a slash, file names in PATH are used to find the directory containing filename. 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 positional 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 ( 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 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 ...]
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 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 con‐
trol. 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 sup‐
plied, 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.
-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
readline 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 READLINE_POINT, those new values will be reflected in the editing state. The return value is 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 ≥ . If n is greater than the number of enclosing loops, all
enclosing loops are exited. The return value is non-zero when n is ≤ ; Otherwise, break returns 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 functionality 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 function or a script executed with the . or source builtins). Without expr, caller displays the line number
and source filename of the current subroutine call. If a non-negative integer is supplied 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 . The return value is unless the shell is not executing a subroutine call or expr does not correspond to a valid position in the call stack. cd [-L|[-P [-e]]] [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 followed. If the -e option is supplied with -P, and the current working directory cannot be successfully deter‐
mined after a successful directory change, cd will return an unsuccessful status. 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 successfully 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 pro‐
duces a more verbose description. If the -V or -v option is supplied, the exit status is if command was found, and if not. If neither option is supplied and an
error occurred or command cannot be found, the exit status is . Otherwise, the exit status 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 programmable completion code had generated them directly from a completion specification with the same flags.
If word is specified, 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 globpat] [-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 specification for each name, or, if no names are supplied, all completion specifica‐
tions. 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 Programmable 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 expansion before the complete builtin is invoked.
-o comp-option
The comp-option controls several aspects of the compspec's behavior beyond the simple generation of completions. 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 compspec generates no matches.
filenames
Tell readline that the compspec generates filenames, so it can perform any filename-specific processing (like adding a slash to directory names, quot‐
ing special characters, or suppressing 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 specified as -v.
-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 completions are retrieved from the value of the COM‐
PREPLY array variable.
-G globpat
The pathname expansion pattern globpat is expanded to generate the possible completions.
-P prefix
prefix is added at the beginning of each possible completion after all other options have been applied.
-S suffix
suffix is appended to each possible completion after all other options have been applied.
-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 completed.
-X filterpat
filterpat is a pattern as used for pathname expansion. It is applied to the list of possible completions generated 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. The return value is true unless an invalid option is supplied, an option other than -p or -r is supplied without a name argument, an attempt is made to remove a com‐
pletion 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-executing 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 previ‐
ously 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 completion 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 ≥ . 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 ≤ ; Otherwise, continue returns value. When continue is executed outside of loop, the return value is . declare [-aAfFgilrtux] [-p] [name[=value] ...]
typeset [-aAfFgilrtux] [-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 definitions; 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 displayed
as well. The -F option implies -f. The -g option forces variables to be created or modified at the global scope, even when declare is executed in a shell function.
It is ignored in all other cases. 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 evaluation (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 variable and +r will not remove the readonly
attribute. When used in a function, makes each name local, as with the local command, unless the -g option is supplied, If a variable name is followed by =value, the
value of the variable is set to value. The return value is 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 readonly variable, an attempt is made to turn
off array status for an array variable, or an attempt is made to display a non-existent function with -f. dirs [+n] [-n] [-clpv]
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, prefixing each entry with its index in the stack. The return value is 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 cur‐
rent job is used. If the -h option is given, each jobspec is not removed from the table, 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 unless a jobspec does not specify a valid job. echo [-neE] [arg ...]
Output the args, separated by spaces, followed by a newline. The return status is always . If -n is specified, the trailing newline is suppressed. If the -e option
is given, interpretation of the following backslash-escaped characters is enabled. The -E option disables the interpretation of these escape characters, even on sys‐
tems 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
\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)
\uHHHH the Unicode (ISO/IEC ) character whose value is the hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC ) character whose value is the hexadecimal value HHHHHHHH (one to eight 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 arguments, the list consists of all enabled shell builtins. If -n
is supplied, only disabled builtins are printed. If -a is supplied, the list printed includes all builtins, with an indication of whether or not each is enabled. If
-s is supplied, the output is restricted to the POSIX special builtins. The return value is 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 command. 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 . 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 beginning of the zeroth argument passed to command. This is what login() does. The -c option causes command to be executed with an empty envi‐
ronment. If -a is supplied, the shell passes name as the zeroth argument to the executed command. If command cannot 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 redirections take effect in the current shell, and the return status is . If there is a redirection error, the return status is . 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 termi‐
nates. export [-fn] [name[=word]] ...
export -p
The supplied names are marked for automatic export to the environment 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 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. 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 current command number).
If last is not specified it is set to the current command for listing (so that ``fc -l -'' prints the last commands) and to first otherwise. If first is not
specified it is set to the previous command for editing and - 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. Otherwise, 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 command. If the first form is used, the return value is unless an invalid option is encountered or first or last specify history lines out of range. If the -e option is sup‐
plied, 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 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 specify 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 parameters. optstring contains the option characters to be recognized; 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 charac‐
ters. 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 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. 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 , no error messages will be displayed, 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]
Each time hash is invoked, the full pathname of the command name is determined by searching the directories in $PATH and remembered. Any previously-remembered path‐
name is discarded. 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 path‐
name 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 format 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 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 numbers. Lines listed with a * have been modified. An argument of n lists only the last n lines. If the
shell variable HISTTIMEFORMAT is set and not null, it is used as a format string for strftime() to display the time stamp associated with each displayed 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, overwriting 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 information associated with each history entry is written to the history 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 unless an invalid option is encountered, an error occurs while reading or writing the history file, an invalid offset is
supplied 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 following meanings:
-l List process IDs in addition to the normal information.
-n Display information only about jobs that have changed status since the user was last notified of their status.
-p List only the process ID of the job's process group leader.
-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 unless an invalid option is encountered or an invalid jobspec is sup‐
plied. 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, return‐
ing 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 . 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 ARITHMETIC EVALUATION above). If the last arg evaluates to , let returns ; 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 function and its children. With no operands, local writes a list of local variables
to the standard output. It is an error to use local when not within a function. The return status is 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 variable array, or from file descriptor fd if the -u option is supplied. The variable MAPFILE is the default
array. Options, if supplied, have the following meanings:
-n Copy at most count lines. If count is , all lines are copied.
-O Begin assigning to array at index origin. The default index is .
-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 standard 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 . When callback is evaluated, it is supplied the index of the next array element to be assigned and the line
to be assigned to that element as additional arguments. callback 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 sup‐
plied, have the following 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 +'' removes the first directory, ``popd +''
the second.
-n Removes the nth entry counting from the right of the list shown by dirs, starting with zero. For example: ``popd -'' removes the last directory, ``popd -''
the next to last. If the popd command is successful, a dirs is performed as well, and the return status is . popd returns false if an invalid option is encountered, the directory
stack is empty, a non-existent 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 -v option causes the output to be assigned to the variable var rather than
being printed to the standard output. 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 format specifications, each of which causes printing of the next successive argument. In addition to the standard
printf() format specifications, printf interprets the following extensions:
%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 \ may contain up to four digits).
%q causes printf to output the corresponding argument in a format that can be reused as shell input.
%(datefmt)T
causes printf to output the date-time string resulting from using datefmt as a format string for strftime(). The corresponding argument is an integer repre‐
senting the number of seconds since the epoch. Two special argument values may be used: - represents the current time, and - represents the time the shell
was invoked. Arguments to non-string format specifiers are treated as C constants, except that a leading plus or minus sign is allowed, and if the leading character is a single or
double quote, the value is the ASCII value of the following character. The format is reused as necessary to consume all of the arguments. 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 , 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 unless the cd to dir fails. With the second form, pushd
returns unless the directory 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 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 intervening 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 following meanings:
-a aname
The words are assigned to sequential indices of the array variable aname, starting at . aname is unset before any new values are assigned. Other name argu‐
ments 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. Readline uses the current (or default, if line edit‐
ing 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 delimiter 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
characters 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 newline, before attempting to read any input. The prompt is displayed only if input is coming from a ter‐
minal.
-r Backslash does not act as an escape character. The backslash is considered to be part of the line. In particular, a backslash-newline pair may not be used as
a line continuation.
-s Silent mode. If input is coming from a terminal, characters 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. timeout may be a decimal number with a fractional
portion following the decimal point. This option is only effective 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 , read returns success if input is available on the specified file descriptor, failure otherwise. The exit status
is greater than 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 variable REPLY. The return code is zero, unless end-of-file is encountered, read times out (in which case
the return code is greater than ), or an invalid file descriptor is supplied as the argument to -u. readonly [-aAf] [-p] [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 both options are
supplied, -A takes precedence. If no name arguments are given, or if the -p option is supplied, a list of all readonly names is printed. The other options may be
used to restrict the output to a subset of the set of readonly names. The -p option causes output to be displayed in a format that may be reused as input. If a vari‐
able name is followed by =word, the value of the variable is set to word. The return status is 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 sta‐
tus 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-name] [arg ...]
set [+abefhkmnptuvxBCEHPT] [+o option-name] [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 cannot be reset. In posix mode, only shell variables are listed. The output is sorted according to the current locale. When options are speci‐
fied, they set or unset shell attributes. Any arguments remaining after option processing are treated as values for the positional parameters and are assigned, in
order, to $, $, ... $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 immediately, 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 environment 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
processes run in a separate process group and a line containing 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 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 interface. 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 interactive shells.
ignoreeof
The effect is as if the shell command ``IGNOREEOF='' 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 SHEL‐
LOPTS, 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 supplied, these actions are taken and the effective user id is set to the real user id. If the
-p option is supplied 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 special 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, display 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
redirection operator >| instead of >.
-E If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The ERR trap is normally not
inherited 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 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 positional parameters. The -x and -v options are turned off. If there are no args,
the positional 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 encountered. shift [n]
The positional parameters from n+ ... are renamed to $ .... Parameters represented by the numbers $# down to $#-n+ are unset. n must be a non-negative number less
than or equal to $#. If n is , no parameters are changed. If n is not given, it is assumed to be . 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 . shopt [-pqsu] [-o] [optname ...]
Toggle the values of variables controlling optional shell behavior. With no options, or with the -p option, a list of all settable options is displayed, with an indi‐
cation 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 following 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 multiple optname arguments are given with -q, the
return status is zero if all optnames are enabled; non-zero otherwise.
-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 display is limited to those options which are set or unset, respectively. 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 command. 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 component in a cd command will be corrected. The errors checked for are transposed characters, a missing
$ read foo
hello world
$ echo $foo
hello world
$ read foo bar
hello world
$ echo $foo
hello
$ echo $bar
world
$ read
hello world
$ echo $REPLY
hello world
$ read -a foo
hello a b c
$ echo ${foo[@]}
hello a b c
$ echo ${#foo[@]} $ echo ${foo[]}
hello
$ echo ${foo[]}
c
$ read -p "Please input a string:" foo
Please input a string:hello
$ echo $foo
hello