OpenClaw: Архитектура Автономных ИИ-Агентов
Данное произведение является техническим руководством. Все упомянутые конфигурации, IP-адреса и архитектурные схемы приведены исключительно в образовательных целях. Любые совпадения с реальными закрытыми инфраструктурами случайны.
Глава 1. Local-First ассистенты
Платформа OpenClaw возникла как ответ на зависимость от проприетарных облачных чат-ботов. Это фреймворк для создания персонализированных ИИ-ассистентов, которые запускаются на вашем собственном оборудовании (Node.js/TypeScript). Основная идея – полный контроль над данными (Local-First). Ваш агент может читать вашу личную почту, анализировать финансовые документы на диске и управлять умным домом, при этом ни один байт информации не покинет пределы вашей локальной сети без явного разрешения.
Глава 2. AgentSkills и мульти-канальный Gateway
Мощь OpenClaw кроется в системе навыков (AgentSkills). С помощью простых Markdown-файлов вы можете научить агента пользоваться API любых сервисов (например, заказывать билеты или делать коммиты в Git). Для связи с внешним миром используется Gateway – процесс-маршрутизатор. Он позволяет вам общаться со своим домашним ИИ-агентом через привычные интерфейсы: Telegram, Slack или WhatsApp. При этом Gateway поддерживает долгосрочную память (Persistent Memory), поэтому агент всегда помнит контекст ваших разговоров даже спустя месяцы.
Приложение: Справочная документация BASH
BASH(1) General Commands Manual BASH(1)
NAME bash – GNU Bourne-Again SHell
SYNOPSIS bash [options] [command_string | file]
COPYRIGHT Bash is Copyright (C) 1989-2020 by the Free Software Foundation, Inc.
DESCRIPTION Bash is an sh-compatible command language interpreter that executes commands read from the standard input or from a file. Bash also incorporates useful features from the Korn and C shells (ksh and csh).
Bash is intended to be a conformant implementation of the Shell and Utilities portion of the IEEE POSIX specification (IEEE Standard 1003.1). Bash can be configured to be POSIX-conformant by default.
OPTIONS All of the single-character shell options documented in the description of the set builtin command, including -o, can be used as options when the shell is invoked. In addition, bash interprets the following options when it is invoked:
–c If the -c option is present, then commands are read from the first non-option argument command_string. If there are arguments after the command_string, the first argument is assigned to $0 and any remaining arguments are assigned to the positional parameters. The assignment to $0 sets the name of the shell, which is used in warning and error messages. -i If the -i option is present, the shell is interactive. -l Make bash act as if it had been invoked as a login shell (see INVOCATION below). -r If the -r option is present, the shell becomes restricted (see RESTRICTED SHELL below). -s If the -s option is present, or if no arguments remain after option processing, then commands are read from the standard input. This op‐ tion allows the positional parameters to be set when invoking an interactive shell or when reading input through a pipe. -D A list of all double-quoted strings preceded by $ is printed on the standard output. These are the strings that are subject to language translation when the current locale is not C or POSIX. This implies the -n option; no commands will be executed. [-+]O [shopt_option] shopt_option is one of the shell options accepted by the shopt builtin (see SHELL BUILTIN COMMANDS below). If shopt_option is present, -O sets the value of that option; +O unsets it. If shopt_option is not supplied, the names and values of the shell options accepted by shopt are printed on the standard output. If the invocation option is +O, the output is displayed in a format that may be reused as input. – A – signals the end of options and disables further option processing. Any arguments after the – are treated as filenames and arguments. An argument of – is equivalent to –.
Bash also interprets a number of multi-character options. These options must appear on the command line before the single-character options to be recognized.
–-debugger Arrange for the debugger profile to be executed before the shell starts. Turns on extended debugging mode (see the description of the extde‐ bug option to the shopt builtin below). –dump-po-strings Equivalent to -D, but the output is in the GNU gettext po (portable object) file format. –dump-strings Equivalent to -D. –help Display a usage message on standard output and exit successfully. –init-file file –rcfile file Execute commands from file instead of the standard personal initialization file ~/.bashrc if the shell is interactive (see INVOCATION below).
–-login Equivalent to -l.
–-noediting Do not use the GNU readline library to read command lines when the shell is interactive.
–-noprofile Do not read either the system-wide startup file /etc/profile or any of the personal initialization files ~/.bash_profile, ~/.bash_login, or ~/.profile. By default, bash reads these files when it is invoked as a login shell (see INVOCATION below).
–-norc Do not read and execute the personal initialization file ~/.bashrc if the shell is interactive. This option is on by default if the shell is invoked as sh.
–-posix Change the behavior of bash where the default operation differs from the POSIX standard to match the standard (posix mode). See SEE ALSO be‐ low for a reference to a document that details how posix mode affects bash's behavior.
–-restricted The shell becomes restricted (see RESTRICTED SHELL below).
–-rpm-requires Produce the list of files that are required for the shell script to run. This implies '-n' and is subject to the same limitations as compile time error checking checking; Command substitutions, Conditional expressions and eval builtin are not parsed so some dependencies may be missed.
–-verbose Equivalent to -v.
–-version Show version information for this instance of bash on the standard output and exit successfully.
ARGUMENTS If arguments remain after option processing, and neither the -c nor the -s option has been supplied, the first argument is assumed to be the name of a file containing shell commands. If bash is invoked in this fashion, $0 is set to the name of the file, and the positional parameters are set to the remaining arguments. Bash reads and executes commands from this file, then exits. Bash's exit status is the exit status of the last command ex‐ ecuted in the script. If no commands are executed, the exit status is 0. An attempt is first made to open the file in the current directory, and, if no file is found, then the shell searches the directories in PATH for the script.
INVOCATION A login shell is one whose first character of argument zero is a -, or one started with the –login option.
An interactive shell is one started without non-option arguments (unless -s is specified) and without the -c option whose standard input and error are both connected to terminals (as determined by isatty(3)), or one started with the -i option. PS1 is set and $– includes i if bash is interac‐ tive, allowing a shell script or a startup file to test this state.
The following paragraphs describe how bash executes its startup files. If any of the files exist but cannot be read, bash reports an error. Tildes are expanded in filenames as described below under Tilde Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-interactive shell with the –login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The –noprofile option may be used when the shell is started to inhibit this behavior.
When an interactive login shell exits, or a non-interactive login shell executes the exit builtin command, bash reads and executes commands from the files ~/.bash_logout and /etc/bash.bash_logout, if the files exists.
When an interactive shell that is not a login shell is started, bash reads and executes commands from ~/.bashrc, if that file exists. This may be inhibited by using the –norc option. The –rcfile file option will force bash to read and execute commands from file instead of ~/.bashrc.
When bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command were executed: if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi but the value of the PATH variable is not used to search for the filename.
If bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as possible, while conforming to the POSIX standard as well. When invoked as an interactive login shell, or a non-interactive shell with the –login option, it first attempts to read and execute commands from /etc/profile and ~/.profile, in that order. The –noprofile option may be used to inhibit this behavior. When in‐ voked as an interactive shell with the name sh, bash looks for the variable ENV, expands its value if it is defined, and uses the expanded value as the name of a file to read and execute. Since a shell invoked as sh does not attempt to read and execute commands from any other startup files, the –rcfile option has no effect. A non-interactive shell invoked with the name sh does not attempt to read any other startup files. When invoked as sh, bash enters posix mode after the startup files are read.
When bash is started in posix mode, as with the –posix command line option, it follows the POSIX standard for startup files. In this mode, interac‐ tive shells expand the ENV variable and commands are read and executed from the file whose name is the expanded value. No other startup files are read.
Bash attempts to determine when it is being run with its standard input connected to a network connection, as when executed by the remote shell dae‐ mon, usually rshd, or the secure shell daemon sshd. If bash determines it is being run in this fashion, it reads and executes commands from ~/.bashrc, if that file exists and is readable. It will not do this if invoked as sh. The –norc option may be used to inhibit this behavior, and the –rcfile option may be used to force another file to be read, but neither rshd nor sshd generally invoke the shell with those options or allow them to be specified.
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, no startup files are read, shell functions are not inherited from the environment, the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE variables, if they appear in the environment, are ignored, and the effective user id is set to the real user id. If the -p option is supplied at invocation, the startup behavior is the same, but the effective user id is not reset.
DEFINITIONS The following definitions are used throughout the rest of this document. blank A space or tab. word A sequence of characters considered as a single unit by the shell. Also known as a token. name A word consisting only of alphanumeric characters and underscores, and beginning with an alphabetic character or an underscore. Also referred to as an identifier. metacharacter A character that, when unquoted, separates words. One of the following: | & ; ( ) < > space tab newline control operator A token that performs a control function. It is one of the following symbols: || & && ; ;; ;& ;;& ( ) | |& <newline>
RESERVED WORDS Reserved words are words that have a special meaning to the shell. The following words are recognized as reserved when unquoted and either the first word of a command (see SHELL GRAMMAR below), the third word of a case or select command (only in is valid), or the third word of a for command (only in and do are valid):
! case coproc do done elif else esac fi for function if in select then until while { } time [[ ]]
SHELL GRAMMAR Simple Commands A simple command is a sequence of optional variable assignments followed by blank-separated words and redirections, and terminated by a control oper‐ ator. The first word specifies the command to be executed, and is passed as argument zero. The remaining words are passed as arguments to the in‐ voked command.
The return value of a simple command is its exit status, or 128+n if the command is terminated by signal n.
Pipelines A pipeline is a sequence of one or more commands separated by one of the control operators | or |&. The format for a pipeline is:
[time [-p]] [ ! ] command [ [|⎪|&] command2 … ]
The standard output of command is connected via a pipe to the standard input of command2. This connection is performed before any redirections spec‐ ified by the command (see REDIRECTION below). If |& is used, command's standard error, in addition to its standard output, is connected to com‐ mand2's standard input through the pipe; it is shorthand for 2>&1 |. This implicit redirection of the standard error to the standard output is per‐ formed after any redirections specified by the command.
The return status of a pipeline is the exit status of the last command, unless the pipefail option is enabled. If pipefail is enabled, the pipe‐ line's return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved word ! precedes a pipeline, the exit status of that pipeline is the logical negation of the exit status as described above. The shell waits for all commands in the pipeline to terminate before returning a value.
If the time reserved word precedes a pipeline, the elapsed as well as user and system time consumed by its execution are reported when the pipeline terminates. The -p option changes the output format to that specified by POSIX. When the shell is in posix mode, it does not recognize time as a reserved word if the next token begins with a `-'. The TIMEFORMAT variable may be set to a format string that specifies how the timing information should be displayed; see the description of TIMEFORMAT under Shell Variables below.
When the shell is in posix mode, time may be followed by a newline. In this case, the shell displays the total user and system time consumed by the shell and its children. The TIMEFORMAT variable may be used to specify the format of the time information.
Each command in a pipeline is executed as a separate process (i.e., in a subshell). See COMMAND EXECUTION ENVIRONMENT for a description of a sub‐ shell environment. If the lastpipe option is enabled using the shopt builtin (see the description of shopt below), the last element of a pipeline may be run by the shell process.
Lists A list is a sequence of one or more pipelines separated by one of the operators ;, &, &&, or ||, and optionally terminated by one of ;, &, or <new‐ line>.
Of these list operators, && and || have equal precedence, followed by ; and &, which have equal precedence.
A sequence of one or more newlines may appear in a list instead of a semicolon to delimit commands.
If a command is terminated by the control operator &, the shell executes the command in the background in a subshell. The shell does not wait for the command to finish, and the return status is 0. These are referred to as asynchronous commands. Commands separated by a ; are executed sequen‐ tially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.
AND and OR lists are sequences of one or more pipelines separated by the && and || control operators, respectively. AND and OR lists are executed with left associativity. An AND list has the form
command1 && command2
command2 is executed if, and only if, command1 returns an exit status of zero (success).
An OR list has the form
command1 || command2
command2 is executed if, and only if, command1 returns a non-zero exit status. The return status of AND and OR lists is the exit status of the last command executed in the list.
Compound Commands A compound command is one of the following. In most cases a list in a command's description may be separated from the rest of the command by one or more newlines, and may be followed by a newline in place of a semicolon.
(list) list is executed in a subshell environment (see COMMAND EXECUTION ENVIRONMENT below). Variable assignments and builtin commands that affect the shell's environment do not remain in effect after the command completes. The return status is the exit status of list.
{ list; } list is simply executed in the current shell environment. list must be terminated with a newline or semicolon. This is known as a group com‐ mand. The return status is the exit status of list. Note that unlike the metacharacters ( and ), { and } are reserved words and must occur where a reserved word is permitted to be recognized. Since they do not cause a word break, they must be separated from list by whitespace or another shell metacharacter.
((expression)) The expression is evaluated according to the rules described below under ARITHMETIC EVALUATION. If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to let "expression".
[[ expression ]] Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries de‐ scribed below under CONDITIONAL EXPRESSIONS. Word splitting and pathname expansion are not performed on the words between the [[ and ]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as -f must be unquoted to be recognized as primaries.
When used with [[, the < and > operators sort lexicographically using the current locale.
When the == and != operators are used, the string to the right of the operator is considered a pattern and matched according to the rules de‐ scribed below under Pattern Matching, as if the extglob shell option were enabled. The = operator is equivalent to ==. If the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters. The return value is 0 if the string matches (==) or does not match (!=) the pattern, and 1 otherwise. Any part of the pattern may be quoted to force the quoted portion to be matched as a string.
An additional binary operator, =~, is available, with the same precedence as == and !=. When it is used, the string to the right of the oper‐ ator is considered a POSIX extended regular expression and matched accordingly (using the POSIX regcomp and regexec interfaces usually de‐ scribed in regex(3)). The return value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the conditional expression's return value is 2. If the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters. Any part of the pattern may be quoted to force the quoted portion to be matched as a string. Bracket expressions in regular expressions must be treated carefully, since normal quoting characters lose their meanings between brackets. If the pattern is stored in a shell variable, quoting the variable expansion forces the entire pattern to be matched as a string.
The pattern will match if it matches any part of the string. Anchor the pattern using the ^ and $ regular expression operators to force it to match the entire string. The array variable BASH_REMATCH records which parts of the string matched the pattern. The element of BASH_REMATCH with index 0 contains the portion of the string matching the entire regular expression. Substrings matched by parenthesized subexpressions within the regular expression are saved in the remaining BASH_REMATCH indices. The element of BASH_REMATCH with index n is the portion of the string matching the nth parenthesized subexpression.
Expressions may be combined using the following operators, listed in decreasing order of precedence:
( expression ) Returns the value of expression. This may be used to override the normal precedence of operators. ! expression True if expression is false. expression1 && expression2 True if both expression1 and expression2 are true. expression1 || expression2 True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire con‐ ditional expression.
for name [ [ in [ word … ] ] ; ] do list ; done The list of words following in is expanded, generating a list of items. The variable name is set to each element of this list in turn, and list is executed each time. If the in word is omitted, the for command executes list once for each positional parameter that is set (see PA‐ RAMETERS below). The return status is the exit status of the last command that executes. If the expansion of the items following in results in an empty list, no commands are executed, and the return status is 0.
for (( expr1 ; expr2 ; expr3 )) ; do list ; done First, the arithmetic expression expr1 is evaluated according to the rules described below under ARITHMETIC EVALUATION. The arithmetic ex‐ pression expr2 is then evaluated repeatedly until it evaluates to zero. Each time expr2 evaluates to a non-zero value, list is executed and the arithmetic expression expr3 is evaluated. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in list that is executed, or false if any of the expressions is invalid.
select name [ in word ] ; do list ; done The list of words following in is expanded, generating a list of items. The set of expanded words is printed on the standard error, each pre‐ ceded by a number. If the in word is omitted, the positional parameters are printed (see PARAMETERS below). The PS3 prompt is then displayed and a line read from the standard input. If the line consists of a number corresponding to one of the displayed words, then the value of name is set to that word. If the line is empty, the words and prompt are displayed again. If EOF is read, the command completes. Any other value read causes name to be set to null. The line read is saved in the variable REPLY. The list is executed after each selection until a break command is executed. The exit status of select is the exit status of the last command executed in list, or zero if no commands were executed.
case word in [ [(] pattern [ | pattern ] … ) list ;; ] … esac A case command first expands word, and tries to match it against each pattern in turn, using the matching rules described under Pattern Match‐ ing below. The word is expanded using tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution and quote removal. Each pattern examined is expanded using tilde expansion, parameter and variable expansion, arithmetic expan‐ sion, command substitution, and process substitution. If the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters. When a match is found, the corresponding list is executed. If the ;; operator is used, no subsequent matches are attempted after the first pattern match. Using ;& in place of ;; causes execution to continue with the list associated with the next set of patterns. Using ;;& in place of ;; causes the shell to test the next pattern list in the statement, if any, and execute any asso‐ ciated list on a successful match, continuing the case statement execution as if the pattern list had not matched. The exit status is zero if no pattern matches. Otherwise, it is the exit status of the last command executed in list.
if list; then list; [ elif list; then list; ] … [ else list; ] fi The if list is executed. If its exit status is zero, the then list is executed. Otherwise, each elif list is executed in turn, and if its exit status is zero, the corresponding then list is executed and the command completes. Otherwise, the else list is executed, if present. The exit status is the exit status of the last command executed, or zero if no condition tested true.
while list-1; do list-2; done until list-1; do list-2; done The while command continuously executes the list list-2 as long as the last command in the list list-1 returns an exit status of zero. The until command is identical to the while command, except that the test is negated: list-2 is executed as long as the last command in list-1 re‐ turns a non-zero exit status. The exit status of the while and until commands is the exit status of the last command executed in list-2, or zero if none was executed.
Coprocesses A coprocess is a shell command preceded by the coproc reserved word. A coprocess is executed asynchronously in a subshell, as if the command had been terminated with the & control operator, with a two-way pipe established between the executing shell and the coprocess.
The format for a coprocess is:
coproc [NAME] command [redirections]
This creates a coprocess named NAME. If NAME is not supplied, the default name is COPROC. NAME must not be supplied if command is a simple command (see above); otherwise, it is interpreted as the first word of the simple command. When the coprocess is executed, the shell creates an array vari‐ able (see Arrays below) named NAME in the context of the executing shell. The standard output of command is connected via a pipe to a file descrip‐ tor in the executing shell, and that file descriptor is assigned to NAME[0]. The standard input of command is connected via a pipe to a file de‐ scriptor in the executing shell, and that file descriptor is assigned to NAME[1]. This pipe is established before any redirections specified by the command (see REDIRECTION below). The file descriptors can be utilized as arguments to shell commands and redirections using standard word expan‐ sions. Other than those created to execute command and process substitutions, the file descriptors are not available in subshells. The process ID of the shell spawned to execute the coprocess is available as the value of the variable NAME_PID. The wait builtin command may be used to wait for the coprocess to terminate.
Since the coprocess is created as an asynchronous command, the coproc command always returns success. The return status of a coprocess is the exit status of command.
Shell Function Definitions A shell function is an object that is called like a simple command and executes a compound command with a new set of positional parameters. Shell functions are declared as follows:
fname () compound-command [redirection] function fname [()] compound-command [redirection] This defines a function named fname. The reserved word function is optional. If the function reserved word is supplied, the parentheses are optional. The body of the function is the compound command compound-command (see Compound Commands above). That command is usually a list of commands between { and }, but may be any command listed under Compound Commands above, with one exception: If the function reserved word is used, but the parentheses are not supplied, the braces are required. compound-command is executed whenever fname is specified as the name of a simple command. When in posix mode, fname must be a valid shell name and may not be the name of one of the POSIX special builtins. In de‐ fault mode, a function name can be any unquoted shell word that does not contain $. Any redirections (see REDIRECTION below) specified when a function is defined are performed when the function is executed. The exit status of a function definition is zero unless a syntax error oc‐ curs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body. (See FUNCTIONS below.)
COMMENTS In a non-interactive shell, or an interactive shell in which the interactive_comments option to the shopt builtin is enabled (see SHELL BUILTIN COM‐ MANDS below), a word beginning with # causes that word and all remaining characters on that line to be ignored. An interactive shell without the in‐ teractive_comments option enabled does not allow comments. The interactive_comments option is on by default in interactive shells.
QUOTING Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for spe‐ cial characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion.
Each of the metacharacters listed above under DEFINITIONS has special meaning to the shell and must be quoted if it is to represent itself.
When the command history expansion facilities are being used (see HISTORY EXPANSION below), the history expansion character, usually !, must be quoted to prevent history expansion.
There are three quoting mechanisms: the escape character, single quotes, and double quotes.
A non-quoted backslash (\) is the escape character. It preserves the literal value of the next character that follows, with the exception of <new‐ line>. If a \<newline> pair appears, and the backslash is not itself quoted, the \<newline> is treated as a line continuation (that is, it is re‐ moved from the input stream and effectively ignored).
Enclosing characters in single quotes preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.
Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when his‐ tory expansion is enabled, !. When the shell is in posix mode, the ! has no special meaning within double quotes, even when history expansion is en‐ abled. The characters $ and ` retain their special meaning within double quotes. The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or <newline>. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash preceding the ! is not removed.
The special parameters * and @ have special meaning when in double quotes (see PARAMETERS below).
Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows: \a alert (bell) \b backspace \e \E an escape character \f form feed \n new line \r carriage return \t horizontal tab \v vertical tab \\ backslash \' single quote \" double quote \? question mark \nnn the eight-bit character whose value is the octal value nnn (one to three octal digits) \xHH the eight-bit character whose value is the hexadecimal value HH (one or two hex digits) \uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits) \UHHHHHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits) \cx a control-x character
The expanded result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign ($"string") will cause the string to be translated according to the current locale. The gettext in‐ frastructure performs the message catalog lookup and translation, using the LC_MESSAGES and TEXTDOMAIN shell variables. If the current locale is C or POSIX, or if there are no translations available, the dollar sign is ignored. If the string is translated and replaced, the replacement is dou‐ ble-quoted.
PARAMETERS A parameter is an entity that stores values. It can be a name, a number, or one of the special characters listed below under Special Parameters. A variable is a parameter denoted by a name. A variable has a value and zero or more attributes. Attributes are assigned using the declare builtin command (see declare below in SHELL BUILTIN COMMANDS).
A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the un‐ set builtin command (see SHELL BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null string. All values undergo tilde expansion, parameter and variable expansion, command sub‐ stitution, arithmetic expansion, and quote removal (see EXPANSION below). If the variable has its integer attribute set, then value is evaluated as an arithmetic expression even if the $((…)) expansion is not used (see Arithmetic Expansion below). Word splitting is not performed, with the ex‐ ception of "$@" as explained below under Special Parameters. Pathname expansion is not performed. Assignment statements may also appear as argu‐ ments to the alias, declare, typeset, export, readonly, and local builtin commands (declaration commands). When in posix mode, these builtins may appear in a command after one or more instances of the command builtin and retain these assignment statement properties.
In the context where an assignment statement is assigning a value to a shell variable or array index, the += operator can be used to append to or add to the variable's previous value. This includes arguments to builtin commands such as declare that accept assignment statements (declaration com‐ mands). When += is applied to a variable for which the integer attribute has been set, value is evaluated as an arithmetic expression and added to the variable's current value, which is also evaluated. When += is applied to an array variable using compound assignment (see Arrays below), the variable's value is not unset (as it is when using =), and new values are appended to the array beginning at one greater than the array's maximum in‐ dex (for indexed arrays) or added as additional key-value pairs in an associative array. When applied to a string-valued variable, value is expanded and appended to the variable's value.
A variable can be assigned the nameref attribute using the -n option to the declare or local builtin commands (see the descriptions of declare and local below) to create a nameref, or a reference to another variable. This allows variables to be manipulated indirectly. Whenever the nameref variable is referenced, assigned to, unset, or has its attributes modified (other than using or changing the nameref attribute itself), the operation is actually performed on the variable specified by the nameref variable's value. A nameref is commonly used within shell functions to refer to a variable whose name is passed as an argument to the function. For instance, if a variable name is passed to a shell function as its first argument, running declare -n ref=$1 inside the function creates a nameref variable ref whose value is the variable name passed as the first argument. References and assignments to ref, and changes to its attributes, are treated as references, assignments, and attribute modifications to the variable whose name was passed as $1. If the control variable in a for loop has the nameref attribute, the list of words can be a list of shell variables, and a name reference will be estab‐ lished for each word in the list, in turn, when the loop is executed. Array variables cannot be given the nameref attribute. However, nameref vari‐ ables can reference array variables and subscripted array variables. Namerefs can be unset using the -n option to the unset builtin. Otherwise, if unset is executed with the name of a nameref variable as an argument, the variable referenced by the nameref variable will be unset.
Positional Parameters A positional parameter is a parameter denoted by one or more digits, other than the single digit 0. Positional parameters are assigned from the shell's arguments when it is invoked, and may be reassigned using the set builtin command. Positional parameters may not be assigned to with assign‐ ment statements. The positional parameters are temporarily replaced when a shell function is executed (see FUNCTIONS below).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces (see EXPANSION below).
Special Parameters The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed. * Expands to the positional parameters, starting from one. When the expansion is not within double quotes, each positional parameter expands to a separate word. In contexts where it is performed, those words are subject to further word splitting and pathname expansion. When the ex‐ pansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS special variable. That is, "$*" is equivalent to "$1c$2c…", where c is the first character of the value of the IFS variable. If IFS is un‐ set, the parameters are separated by spaces. If IFS is null, the parameters are joined without intervening separators. @ Expands to the positional parameters, starting from one. In contexts where word splitting is performed, this expands each positional parame‐ ter to a separate word; if not within double quotes, these words are subject to word splitting. In contexts where word splitting is not per‐ formed, this expands to a single word with each positional parameter separated by a space. When the expansion occurs within double quotes, each parameter expands to a separate word. That is, "$@" is equivalent to "$1" "$2" … If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word. When there are no positional parameters, "$@" and $@ expand to nothing (i.e., they are removed). # Expands to the number of positional parameters in decimal. ? Expands to the exit status of the most recently executed foreground pipeline. – Expands to the current option flags as specified upon invocation, by the set builtin command, or those set by the shell itself (such as the -i option). $ Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the current shell, not the subshell. ! Expands to the process ID of the job most recently placed into the background, whether executed as an asynchronous command or using the bg builtin (see JOB CONTROL below). 0 Expands to the name of the shell or shell script. This is set at shell initialization. If bash is invoked with a file of commands, $0 is set
to the name of that file. If bash is started with the -c option, then $0 is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the filename used to invoke bash, as given by argument zero.
Shell Variables The following variables are set by the shell:
_ At shell startup, set to the pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, expands to the last argument to the previous simple command executed in the foreground, after expansion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the mail file currently being checked. BASH Expands to the full filename used to invoke this instance of bash. BASHOPTS A colon-separated list of enabled shell options. Each word in the list is a valid argument for the -s option to the shopt builtin command (see SHELL BUILTIN COMMANDS below). The options appearing in BASHOPTS are those reported as on by shopt. If this variable is in the environ‐ ment when bash starts up, each shell option in the list will be enabled before reading any startup files. This variable is read-only. BASHPID Expands to the process ID of the current bash process. This differs from $$ under certain circumstances, such as subshells that do not re‐ quire bash to be re-initialized. Assignments to BASHPID have no effect. If BASHPID is unset, it loses its special properties, even if it is subsequently reset. BASH_ALIASES An associative array variable whose members correspond to the internal list of aliases as maintained by the alias builtin. Elements added to this array appear in the alias list; however, unsetting array elements currently does not cause aliases to be removed from the alias list. If BASH_ALIASES is unset, it loses its special properties, even if it is subsequently reset. BASH_ARGC An array variable whose values are the number of parameters in each frame of the current bash execution call stack. The number of parameters to the current subroutine (shell function or script executed with . or source) is at the top of the stack. When a subroutine is executed, the number of parameters passed is pushed onto BASH_ARGC. The shell sets BASH_ARGC only when in extended debugging mode (see the description of
the extdebug option to the shopt builtin below). Setting extdebug after the shell has started to execute a script, or referencing this vari‐ able when extdebug is not set, may result in inconsistent values. BASH_ARGV An array variable containing all of the parameters in the current bash execution call stack. The final parameter of the last subroutine call is at the top of the stack; the first parameter of the initial call is at the bottom. When a subroutine is executed, the parameters supplied are pushed onto BASH_ARGV. The shell sets BASH_ARGV only when in extended debugging mode (see the description of the extdebug option to the shopt builtin below). Setting extdebug after the shell has started to execute a script, or referencing this variable when extdebug is not set, may result in inconsistent values. BASH_ARGV0 When referenced, this variable expands to the name of the shell or shell script (identical to $0; see the description of special parameter 0 above). Assignment to BASH_ARGV0 causes the value assigned to also be assigned to $0. If BASH_ARGV0 is unset, it loses its special proper‐ ties, even if it is subsequently reset. BASH_CMDS An associative array variable whose members correspond to the internal hash table of commands as maintained by the hash builtin. Elements added to this array appear in the hash table; however, unsetting array elements currently does not cause command names to be removed from the hash table. If BASH_CMDS is unset, it loses its special properties, even if it is subsequently reset. BASH_COMMAND The command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap. If BASH_COMMAND is unset, it loses its special properties, even if it is subsequently reset. BASH_EXECUTION_STRING
The command argument to the -c invocation option. BASH_LINENO An array variable whose members are the line numbers in source files where each corresponding member of FUNCNAME was invoked. ${BASH_LINENO[$i]} is the line number in the source file (${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]} was called (or ${BASH_LINENO[$i-1]} if referenced within another shell function). Use LINENO to obtain the current line number. BASH_LOADABLES_PATH A colon-separated list of directories in which the shell looks for dynamically loadable builtins specified by the enable command. BASH_REMATCH An array variable whose members are assigned by the =~ binary operator to the [[ conditional command. The element with index 0 is the portion of the string matching the entire regular expression. The element with index n is the portion of the string matching the nth parenthesized subexpression. BASH_SOURCE An array variable whose members are the source filenames where the corresponding shell function names in the FUNCNAME array variable are de‐ fined. The shell function ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]} and called from ${BASH_SOURCE[$i+1]}. BASH_SUBSHELL Incremented by one within each subshell or subshell environment when the shell begins executing in that environment. The initial value is 0. If BASH_SUBSHELL is unset, it loses its special properties, even if it is subsequently reset. BASH_VERSINFO A readonly array variable whose members hold version information for this instance of bash. The values assigned to the array members are as follows: BASH_VERSINFO[0] The major version number (the release).
BASH_VERSINFO[1] The minor version number (the version). BASH_VERSINFO[2] The patch level. BASH_VERSINFO[3] The build version. BASH_VERSINFO[4] The release status (e.g., beta1). BASH_VERSINFO[5] The value of MACHTYPE. BASH_VERSION Expands to a string describing the version of this instance of bash. COMP_CWORD An index into ${COMP_WORDS} of the word containing the current cursor position. This variable is available only in shell functions invoked by the programmable completion facilities (see Programmable Completion below). COMP_KEY The key (or final key of a key sequence) used to invoke the current completion function. COMP_LINE The current command line. This variable is available only in shell functions and external commands invoked by the programmable completion fa‐ cilities (see Programmable Completion below). COMP_POINT The index of the current cursor position relative to the beginning of the current command. If the current cursor position is at the end of the current command, the value of this variable is equal to ${#COMP_LINE}. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion below). COMP_TYPE Set to an integer value corresponding to the type of completion attempted that caused a completion function to be called: TAB, for normal com‐
pletion, ?, for listing completions after successive tabs, !, for listing alternatives on partial word completion, @, to list completions if the word is not unmodified, or %, for menu completion. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion below). COMP_WORDBREAKS The set of characters that the readline library treats as word separators when performing word completion. If COMP_WORDBREAKS is unset, it loses its special properties, even if it is subsequently reset. COMP_WORDS An array variable (see Arrays below) consisting of the individual words in the current command line. The line is split into words as readline would split it, using COMP_WORDBREAKS as described above. This variable is available only in shell functions invoked by the programmable com‐ pletion facilities (see Programmable Completion below). COPROC An array variable (see Arrays below) created to hold the file descriptors for output from and input to an unnamed coprocess (see Coprocesses above). DIRSTACK An array variable (see Arrays below) containing the current contents of the directory stack. Directories appear in the stack in the order they are displayed by the dirs builtin. Assigning to members of this array variable may be used to modify directories already in the stack, but the pushd and popd builtins must be used to add and remove directories. Assignment to this variable will not change the current direc‐ tory. If DIRSTACK is unset, it loses its special properties, even if it is subsequently reset. EPOCHREALTIME Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see time(3)) as a floating point value with micro-second granularity. Assignments to EPOCHREALTIME are ignored. If EPOCHREALTIME is unset, it loses its special properties, even if it is subsequently reset.
EPOCHSECONDS Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see time(3)). Assignments to EPOCHSECONDS are ignored. If EPOCHSECONDS is unset, it loses its special properties, even if it is subsequently reset. EUID Expands to the effective user ID of the current user, initialized at shell startup. This variable is readonly. FUNCNAME An array variable containing the names of all shell functions currently in the execution call stack. The element with index 0 is the name of any currently-executing shell function. The bottom-most element (the one with the highest index) is "main". This variable exists only when a shell function is executing. Assignments to FUNCNAME have no effect. If FUNCNAME is unset, it loses its special properties, even if it is subsequently reset.
This variable can be used with BASH_LINENO and BASH_SOURCE. Each element of FUNCNAME has corresponding elements in BASH_LINENO and BASH_SOURCE to describe the call stack. For instance, ${FUNCNAME[$i]} was called from the file ${BASH_SOURCE[$i+1]} at line number ${BASH_LINENO[$i]}. The caller builtin displays the current call stack using this information. GROUPS An array variable containing the list of groups of which the current user is a member. Assignments to GROUPS have no effect. If GROUPS is unset, it loses its special properties, even if it is subsequently reset. HISTCMD The history number, or index in the history list, of the current command. Assignments to HISTCMD are ignored. If HISTCMD is unset, it loses its special properties, even if it is subsequently reset. HOSTNAME Automatically set to the name of the current host. HOSTTYPE Automatically set to a string that uniquely describes the type of machine on which bash is executing. The default is system-dependent. LINENO Each time this parameter is referenced, the shell substitutes a decimal number representing the current sequential line number (starting with 1) within a script or function. When not in a script or function, the value substituted is not guaranteed to be meaningful. If LINENO is un‐ set, it loses its special properties, even if it is subsequently reset. MACHTYPE Automatically set to a string that fully describes the system type on which bash is executing, in the standard GNU cpu-company-system format. The default is system-dependent. MAPFILE An array variable (see Arrays below) created to hold the text read by the mapfile builtin when no variable name is supplied. OLDPWD The previous working directory as set by the cd command.
OPTARG The value of the last option argument processed by the getopts builtin command (see SHELL BUILTIN COMMANDS below). OPTIND The index of the next argument to be processed by the getopts builtin command (see SHELL BUILTIN COMMANDS below). OSTYPE Automatically set to a string that describes the operating system on which bash is executing. The default is system-dependent. PIPESTATUS An array variable (see Arrays below) containing a list of exit status values from the processes in the most-recently-executed foreground pipe‐ line (which may contain only a single command). PPID The process ID of the shell's parent. This variable is readonly. PWD The current working directory as set by the cd command. RANDOM Each time this parameter is referenced, it expands to a random integer between 0 and 32767. Assigning a value to RANDOM initializes (seeds) the sequence of random numbers. If RANDOM is unset, it loses its special properties, even if it is subsequently reset. READLINE_LINE The contents of the readline line buffer, for use with "bind -x" (see SHELL BUILTIN COMMANDS below). READLINE_MARK The position of the mark (saved insertion point) in the readline line buffer, for use with "bind -x" (see SHELL BUILTIN COMMANDS below). The characters between the insertion point and the mark are often called the region. READLINE_POINT The position of the insertion point in the readline line buffer, for use with "bind -x" (see SHELL BUILTIN COMMANDS below). REPLY Set to the line of input read by the read builtin command when no arguments are supplied. SECONDS Each time this parameter is referenced, the number of seconds since shell invocation is returned. If a value is assigned to SECONDS, the value returned upon subsequent references is the number of seconds since the assignment plus the value assigned. The number of seconds at
shell invocation and the current time is always determined by querying the system clock. If SECONDS is unset, it loses its special proper‐ ties, even if it is subsequently reset. SHELLOPTS A colon-separated list of enabled shell options. Each word in the list is a valid argument for the -o option to the set builtin command (see SHELL BUILTIN COMMANDS below). The options appearing in SHELLOPTS are those reported as on by set -o. If this variable is in the environment when bash starts up, each shell option in the list will be enabled before reading any startup files. This variable is read-only. SHLVL Incremented by one each time an instance of bash is started. SRANDOM This variable expands to a 32-bit pseudo-random number each time it is referenced. The random number generator is not linear on systems that support /dev/urandom or arc4random, so each returned number has no relationship to the numbers preceding it. The random number generator can‐ not be seeded, so assignments to this variable have no effect. If SRANDOM is unset, it loses its special properties, even if it is subse‐ quently reset. UID Expands to the user ID of the current user, initialized at shell startup. This variable is readonly.
The following variables are used by the shell. In some cases, bash assigns a default value to a variable; these cases are noted below.
BASH_COMPAT The value is used to set the shell's compatibility level. See SHELL COMPATIBILITY MODE below for a description of the various compatibility levels and their effects. The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42) corresponding to the desired compatibility level. If BASH_COMPAT is unset or set to the empty string, the compatibility level is set to the default for the current version. If BASH_COMPAT is set to a value that is not one of the valid compatibility levels, the shell prints an error message and sets the compatibility level to the default for the current version. The valid values correspond to the compatibility levels described below under BSHELLCOMPATIBIL‐ ITYMODE. For example, 4.2 and 42 are valid values that correspond to the compat42 shopt option and set the compatibility level to 42. The current version is also a valid value. BASH_ENV If this parameter is set when bash is executing a shell script, its value is interpreted as a filename containing commands to initialize the shell, as in ~/.bashrc. The value of BASH_ENV is subjected to parameter expansion, command substitution, and arithmetic expansion before be‐ ing interpreted as a filename. PATH is not used to search for the resultant filename. BASH_XTRACEFD If set to an integer corresponding to a valid file descriptor, bash will write the trace output generated when set -x is enabled to that file descriptor. The file descriptor is closed when BASH_XTRACEFD is unset or assigned a new value. Unsetting BASH_XTRACEFD or assigning it the empty string causes the trace output to be sent to the standard error. Note that setting BASH_XTRACEFD to 2 (the standard error file descrip‐ tor) and then unsetting it will result in the standard error being closed. CDPATH The search path for the cd command. This is a colon-separated list of directories in which the shell looks for destination directories speci‐ fied by the cd command. A sample value is ".:~:/usr". CHILD_MAX Set the number of exited child status values for the shell to remember. Bash will not allow this value to be decreased below a POSIX-mandated
minimum, and there is a maximum value (currently 8192) that this may not exceed. The minimum value is system-dependent. COLUMNS Used by the select compound command to determine the terminal width when printing selection lists. Automatically set if the checkwinsize op‐ tion is enabled or in an interactive shell upon receipt of a SIGWINCH. COMPREPLY An array variable from which bash reads the possible completions generated by a shell function invoked by the programmable completion facility (see Programmable Completion below). Each array element contains one possible completion. EMACS If bash finds this variable in the environment when the shell starts with value "t", it assumes that the shell is running in an Emacs shell buffer and disables line editing. ENV Expanded and executed similarly to BASH_ENV (see INVOCATION above) when an interactive shell is invoked in posix mode. EXECIGNORE A colon-separated list of shell patterns (see Pattern Matching) defining the list of filenames to be ignored by command search using PATH. Files whose full pathnames match one of these patterns are not considered executable files for the purposes of completion and command execu‐ tion via PATH lookup. This does not affect the behavior of the [, test, and [[ commands. Full pathnames in the command hash table are not subject to EXECIGNORE. Use this variable to ignore shared library files that have the executable bit set, but are not executable files. The pattern matching honors the setting of the extglob shell option. FCEDIT The default editor for the fc builtin command. FIGNORE A colon-separated list of suffixes to ignore when performing filename completion (see READLINE below). A filename whose suffix matches one of the entries in FIGNORE is excluded from the list of matched filenames. A sample value is ".o:~". FUNCNEST
If set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that exceed this nesting level will cause the current command to abort. GLOBIGNORE A colon-separated list of patterns defining the set of file names to be ignored by pathname expansion. If a file name matched by a pathname expansion pattern also matches one of the patterns in GLOBIGNORE, it is removed from the list of matches. HISTCONTROL A colon-separated list of values controlling how commands are saved on the history list. If the list of values includes ignorespace, lines which begin with a space character are not saved in the history list. A value of ignoredups causes lines matching the previous history entry to not be saved. A value of ignoreboth is shorthand for ignorespace and ignoredups. A value of erasedups causes all previous lines matching the current line to be removed from the history list before that line is saved. Any value not in the above list is ignored. If HISTCONTROL is unset, or does not include a valid value, all lines read by the shell parser are saved on the history list, subject to the value of HISTIG‐ NORE. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of HISTCONTROL. HISTFILE The name of the file in which command history is saved (see HISTORY below). The default value is ~/.bash_history. If unset, the command his‐ tory is not saved when a shell exits. HISTFILESIZE The maximum number of lines contained in the history file. When this variable is assigned a value, the history file is truncated, if neces‐ sary, to contain no more than that number of lines by removing the oldest entries. The history file is also truncated to this size after writing it when a shell exits. If the value is 0, the history file is truncated to zero size. Non-numeric values and numeric values less than zero inhibit truncation. The shell sets the default value to the value of HISTSIZE after reading any startup files.
HISTIGNORE A colon-separated list of patterns used to decide which command lines should be saved on the history list. Each pattern is anchored at the beginning of the line and must match the complete line (no implicit `*' is appended). Each pattern is tested against the line after the checks specified by HISTCONTROL are applied. In addition to the normal shell pattern matching characters, `&' matches the previous history line. `&' may be escaped using a backslash; the backslash is removed before attempting a match. The second and subsequent lines of a multi- line compound command are not tested, and are added to the history regardless of the value of HISTIGNORE. The pattern matching honors the setting of the extglob shell option. HISTSIZE The number of commands to remember in the command history (see HISTORY below). If the value is 0, commands are not saved in the history list. Numeric values less than zero result in every command being saved on the history list (there is no limit). The shell sets the default value to 500 after reading any startup files. HISTTIMEFORMAT If this variable is set and not null, its value is used as a format string for strftime(3) to print the time stamp associated with each his‐ tory entry displayed by the history builtin. If this variable is set, time stamps are written to the history file so they may be preserved across shell sessions. This uses the history comment character to distinguish timestamps from other history lines. HOME The home directory of the current user; the default argument for the cd builtin command. The value of this variable is also used when per‐ forming tilde expansion. HOSTFILE Contains the name of a file in the same format as /etc/hosts that should be read when the shell needs to complete a hostname. The list of possible hostname completions may be changed while the shell is running; the next time hostname completion is attempted after the value is changed, bash adds the contents of the new file to the existing list. If HOSTFILE is set, but has no value, or does not name a readable file,
bash attempts to read /etc/hosts to obtain the list of possible hostname completions. When HOSTFILE is unset, the hostname list is cleared. IFS The Internal Field Separator that is used for word splitting after expansion and to split lines into words with the read builtin command. The default value is ``<space><tab><newline>''. IGNOREEOF Controls the action of an interactive shell on receipt of an EOF character as the sole input. If set, the value is the number of consecutive EOF characters which must be typed as the first characters on an input line before bash exits. If the variable exists but does not have a nu‐ meric value, or has no value, the default value is 10. If it does not exist, EOF signifies the end of input to the shell. INPUTRC The filename for the readline startup file, overriding the default of ~/.inputrc (see READLINE below). INSIDE_EMACS If this variable appears in the environment when the shell starts, bash assumes that it is running inside an Emacs shell buffer and may dis‐ able line editing, depending on the value of TERM. LANG Used to determine the locale category for any category not specifically selected with a variable starting with LC_. LC_ALL This variable overrides the value of LANG and any other LC_ variable specifying a locale category. LC_COLLATE This variable determines the collation order used when sorting the results of pathname expansion, and determines the behavior of range expres‐ sions, equivalence classes, and collating sequences within pathname expansion and pattern matching. LC_CTYPE This variable determines the interpretation of characters and the behavior of character classes within pathname expansion and pattern match‐ ing. LC_MESSAGES
This variable determines the locale used to translate double-quoted strings preceded by a $. LC_NUMERIC This variable determines the locale category used for number formatting. LC_TIME This variable determines the locale category used for data and time formatting. LINES Used by the select compound command to determine the column length for printing selection lists. Automatically set if the checkwinsize option is enabled or in an interactive shell upon receipt of a SIGWINCH. MAIL If this parameter is set to a file or directory name and the MAILPATH variable is not set, bash informs the user of the arrival of mail in the specified file or Maildir-format directory. MAILCHECK Specifies how often (in seconds) bash checks for mail. The default is 60 seconds. When it is time to check for mail, the shell does so be‐ fore displaying the primary prompt. If this variable is unset, or set to a value that is not a number greater than or equal to zero, the shell disables mail checking. MAILPATH A colon-separated list of filenames to be checked for mail. The message to be printed when mail arrives in a particular file may be specified by separating the filename from the message with a `?'. When used in the text of the message, $_ expands to the name of the current mailfile. Example: MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"' Bash can be configured to supply a default value for this variable (there is no value by default), but the location of the user mail files that it uses is system dependent (e.g., /var/mail/$USER). OPTERR If set to the value 1, bash displays error messages generated by the getopts builtin command (see SHELL BUILTIN COMMANDS below). OPTERR is
initialized to 1 each time the shell is invoked or a shell script is executed. PATH The search path for commands. It is a colon-separated list of directories in which the shell looks for commands (see COMMAND EXECUTION be‐ low). A zero-length (null) directory name in the value of PATH indicates the current directory. A null directory name may appear as two ad‐ jacent colons, or as an initial or trailing colon. The default path is system-dependent, and is set by the administrator who installs bash. A common value is ``/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin''. POSIXLY_CORRECT If this variable is in the environment when bash starts, the shell enters posix mode before reading the startup files, as if the –posix invo‐ cation option had been supplied. If it is set while the shell is running, bash enables posix mode, as if the command set -o posix had been executed. When the shell enters posix mode, it sets this variable if it was not already set. PROMPT_COMMAND If this variable is set, and is an array, the value of each set element is executed as a command prior to issuing each primary prompt. If this is set but not an array variable, its value is used as a command to execute instead. PROMPT_DIRTRIM If set to a number greater than zero, the value is used as the number of trailing directory components to retain when expanding the \w and \W prompt string escapes (see PROMPTING below). Characters removed are replaced with an ellipsis. PS0 The value of this parameter is expanded (see PROMPTING below) and displayed by interactive shells after reading a command and before the com‐ mand is executed. PS1 The value of this parameter is expanded (see PROMPTING below) and used as the primary prompt string. The default value is ``\s-\v\$ ''. PS2 The value of this parameter is expanded as with PS1 and used as the secondary prompt string. The default is ``> ''. PS3 The value of this parameter is used as the prompt for the select command (see SHELL GRAMMAR above). PS4 The value of this parameter is expanded as with PS1 and the value is printed before each command bash displays during an execution trace. The
first character of the expanded value of PS4 is replicated multiple times, as necessary, to indicate multiple levels of indirection. The de‐ fault is ``+ ''. SHELL This variable expands to the full pathname to the shell. If it is not set when the shell starts, bash assigns to it the full pathname of the current user's login shell. TIMEFORMAT The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed. The % character introduces an escape sequence that is expanded to a time value or other information. The escape sequences and their meanings are as follows; the braces denote optional portions. %% A literal %. %[p][l]R The elapsed time in seconds. %[p][l]U The number of CPU seconds spent in user mode. %[p][l]S The number of CPU seconds spent in system mode. %P The CPU percentage, computed as (%U + %S) / %R.
The optional p is a digit specifying the precision, the number of fractional digits after a decimal point. A value of 0 causes no decimal point or fraction to be output. At most three places after the decimal point may be specified; values of p greater than 3 are changed to 3. If p is not specified, the value 3 is used.
The optional l specifies a longer format, including minutes, of the form MMmSS.FFs. The value of p determines whether or not the fraction is included.
If this variable is not set, bash acts as if it had the value $'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'. If the value is null, no timing infor‐ mation is displayed. A trailing newline is added when the format string is displayed. TMOUT If set to a value greater than zero, TMOUT is treated as the default timeout for the read builtin. The select command terminates if input does not arrive after TMOUT seconds when input is coming from a terminal. In an interactive shell, the value is interpreted as the number of seconds to wait for a line of input after issuing the primary prompt. Bash terminates after waiting for that number of seconds if a complete line of input does not arrive. TMPDIR If set, bash uses its value as the name of a directory in which bash creates temporary files for the shell's use. auto_resume This variable controls how the shell interacts with the user and job control. If this variable is set, single word simple commands without redirections are treated as candidates for resumption of an existing stopped job. There is no ambiguity allowed; if there is more than one job beginning with the string typed, the job most recently accessed is selected. The name of a stopped job, in this context, is the command line used to start it. If set to the value exact, the string supplied must match the name of a stopped job exactly; if set to substring, the string supplied needs to match a substring of the name of a stopped job. The substring value provides functionality analogous to the %? job identifier (see JOB CONTROL below). If set to any other value, the supplied string must be a prefix of a stopped job's name; this provides functionality analogous to the %string job identifier. histchars The two or three characters which control history expansion and tokenization (see HISTORY EXPANSION below). The first character is the his‐ tory expansion character, the character which signals the start of a history expansion, normally `!'. The second character is the quick sub‐ stitution character, which is used as shorthand for re-running the previous command entered, substituting one string for another in the com‐ mand. The default is `^'. The optional third character is the character which indicates that the remainder of the line is a comment when found as the first character of a word, normally `#'. The history comment character causes history substitution to be skipped for the remain‐
ing words on the line. It does not necessarily cause the shell parser to treat the rest of the line as a comment.
Arrays Bash provides one-dimensional indexed and associative array variables. Any variable may be used as an indexed array; the declare builtin will ex‐ plicitly declare an array. There is no maximum limit on the size of an array, nor any requirement that members be indexed or assigned contiguously. Indexed arrays are referenced using integers (including arithmetic expressions) and are zero-based; associative arrays are referenced using arbitrary strings. Unless otherwise noted, indexed array indices must be non-negative integers.
An indexed array is created automatically if any variable is assigned to using the syntax name[subscript]=value. The subscript is treated as an arithmetic expression that must evaluate to a number. To explicitly declare an indexed array, use declare -a name (see SHELL BUILTIN COMMANDS be‐ low). declare -a name[subscript] is also accepted; the subscript is ignored.
Associative arrays are created using declare -A name.
Attributes may be specified for an array variable using the declare and readonly builtins. Each attribute applies to all members of an array.
Arrays are assigned to using compound assignments of the form name=(value1 … valuen), where each value may be of the form [subscript]=string. In‐ dexed array assignments do not require anything but string. Each value in the list is expanded using all the shell expansions described below under EXPANSION. When assigning to indexed arrays, if the optional brackets and subscript are supplied, that index is assigned to; otherwise the index of the element assigned is the last index assigned to by the statement plus one. Indexing starts at zero.
When assigning to an associative array, the words in a compound assignment may be either assignment statements, for which the subscript is required, or a list of words that is interpreted as a sequence of alternating keys and values: name=( key1 value1 key2 value2 …). These are treated identi‐ cally to name=( [key1]=value1 [key2]=value2 …). The first word in the list determines how the remaining words are interpreted; all assignments in a list must be of the same type. When using key/value pairs, the keys may not be missing or empty; a final missing value is treated like the empty string.
This syntax is also accepted by the declare builtin. Individual array elements may be assigned to using the name[subscript]=value syntax introduced above. When assigning to an indexed array, if name is subscripted by a negative number, that number is interpreted as relative to one greater than the maximum index of name, so negative indices count back from the end of the array, and an index of -1 references the last element.
Any element of an array may be referenced using ${name[subscript]}. The braces are required to avoid conflicts with pathname expansion. If sub‐ script is @ or *, the word expands to all members of name. These subscripts differ only when the word appears within double quotes. If the word is double-quoted, ${name[*]} expands to a single word with the value of each array member separated by the first character of the IFS special variable, and ${name[@]} expands each element of name to a separate word. When there are no array members, ${name[@]} expands to nothing. If the double- quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word. This is analogous to the expansion of the special parameters * and @ (see Special Parameters above). ${#name[subscript]} expands to the length of ${name[subscript]}. If subscript is * or @, the expansion is the number of elements in the array. If the subscript used to reference an element of an indexed array evaluates to a number less than zero, it is interpreted as relative to one greater than the maximum index of the array, so negative indices count back from the end of the array, and an index of -1 references the last element.
Referencing an array variable without a subscript is equivalent to referencing the array with a subscript of 0. Any reference to a variable using a valid subscript is legal, and bash will create an array if necessary.
An array variable is considered set if a subscript has been assigned a value. The null string is a valid value.
It is possible to obtain the keys (indices) of an array as well as the values. ${!name[@]} and ${!name[*]} expand to the indices assigned in array variable name. The treatment when in double quotes is similar to the expansion of the special parameters @ and * within double quotes.
The unset builtin is used to destroy arrays. unset name[subscript] destroys the array element at index subscript, for both indexed and associative arrays. Negative subscripts to indexed arrays are interpreted as described above. Unsetting the last element of an array variable does not unset the variable. unset name, where name is an array, or unset name[subscript], where subscript is * or @, removes the entire array.
When using a variable name with a subscript as an argument to a command, such as with unset, without using the word expansion syntax described above, the argument is subject to pathname expansion. If pathname expansion is not desired, the argument should be quoted.
The declare, local, and readonly builtins each accept a -a option to specify an indexed array and a -A option to specify an associative array. If both options are supplied, -A takes precedence. The read builtin accepts a -a option to assign a list of words read from the standard input to an array. The set and declare builtins display array values in a way that allows them to be reused as assignments.
EXPANSION Expansion is performed on the command line after it has been split into words. There are seven kinds of expansion performed: brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, word splitting, and pathname expansion.
The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; and pathname expansion.
On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as tilde, pa‐ rameter, variable, and arithmetic expansion and command substitution.
After these expansions are performed, quote characters present in the original word are removed unless they have been quoted themselves (quote re‐ moval).
Only brace expansion, word splitting, and pathname expansion can increase the number of words of the expansion; other expansions expand a single word to a single word. The only exceptions to this are the expansions of "$@" and "${name[@]}", and, in most cases, $* and ${name[*]} as explained above (see PARAMETERS).
Brace Expansion Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to pathname expansion, but the filenames gen‐ erated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by either a series of comma-separated strings or a sequence expression between a pair of braces, followed by an optional postscript. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example, a{d,c,b}e ex‐ pands into `ade ace abe'.
A sequence expression takes the form {x..y[..incr]}, where x and y are either integers or single characters, and incr, an optional increment, is an integer. When integers are supplied, the expression expands to each number between x and y, inclusive. Supplied integers may be prefixed with 0 to force each term to have the same width. When either x or y begins with a zero, the shell attempts to force all generated terms to contain the same number of digits, zero-padding where necessary. When characters are supplied, the expression expands to each character lexicographically between x and y, inclusive, using the default C locale. Note that both x and y must be of the same type. When the increment is supplied, it is used as the difference between each term. The default increment is 1 or -1 as appropriate.
Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces.
A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged. A { or , may be quoted with a backslash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string ${ is not considered eligible for brace expansion, and inhibits brace expansion until the closing }.
This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs} or chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with historical versions of sh. sh does not treat opening or closing braces specially when they appear as part of a word, and preserves them in the output. Bash removes braces from words as a consequence of brace expansion. For example, a word entered to sh as file{1,2} appears identically in the output. The same word is output as file1 file2 after expansion by bash. If strict compatibil‐ ity with sh is desired, start bash with the +B option or disable brace expansion with the +B option to the set command (see SHELL BUILTIN COMMANDS below).
Tilde Expansion If a word begins with an unquoted tilde character (`~'), all of the characters preceding the first unquoted slash (or all characters, if there is no unquoted slash) are considered a tilde-prefix. If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix follow‐ ing the tilde are treated as a possible login name. If this login name is the null string, the tilde is replaced with the value of the shell parame‐ ter HOME. If HOME is unset, the home directory of the user executing the shell is substituted instead. Otherwise, the tilde-prefix is replaced with the home directory associated with the specified login name.
If the tilde-prefix is a `~+', the value of the shell variable PWD replaces the tilde-prefix. If the tilde-prefix is a `~-', the value of the shell variable OLDPWD, if it is set, is substituted. If the characters following the tilde in the tilde-prefix consist of a number N, optionally prefixed by a `+' or a `-', the tilde-prefix is replaced with the corresponding element from the directory stack, as it would be displayed by the dirs builtin invoked with the tilde-prefix as an argument. If the characters following the tilde in the tilde-prefix consist of a number without a leading `+' or `-', `+' is assumed.
If the login name is invalid, or the tilde expansion fails, the word is unchanged.
Each variable assignment is checked for unquoted tilde-prefixes immediately following a : or the first =. In these cases, tilde expansion is also performed. Consequently, one may use filenames with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the expanded value.
Bash also performs tilde expansion on words satisfying the conditions of variable assignments (as described above under PARAMETERS) when they appear as arguments to simple commands. Bash does not do this, except for the declaration commands listed above, when in posix mode.
Parameter Expansion The `$' character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be in‐ terpreted as part of the name.
When braces are used, the matching ending brace is the first `}' not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion.
${parameter} The value of parameter is substituted. The braces are required when parameter is a positional parameter with more than one digit, or when pa‐ rameter is followed by a character which is not to be interpreted as part of its name. The parameter is a shell parameter as described above PARAMETERS) or an array reference (Arrays).
If the first character of parameter is an exclamation point (!), and parameter is not a nameref, it introduces a level of indirection. Bash uses the value formed by expanding the rest of parameter as the new parameter; this is then expanded and that value is used in the rest of the expansion, rather than the expansion of the original parameter. This is known as indirect expansion. The value is subject to tilde expansion, parameter expan‐ sion, command substitution, and arithmetic expansion. If parameter is a nameref, this expands to the name of the parameter referenced by parameter instead of performing the complete indirect expansion. The exceptions to this are the expansions of ${!prefix*} and ${!name[@]} described below. The exclamation point must immediately follow the left brace in order to introduce indirection.
In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, using the forms documented below (e.g., :-), bash tests for a parameter that is unset or null. Omitting the colon results in a test only for a parameter that is unset.
${parameter:-word} Use Default Values. If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted. ${parameter:=word} Assign Default Values. If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substi‐ tuted. Positional parameters and special parameters may not be assigned to in this way. ${parameter:?word} Display Error if Null or Unset. If parameter is null or unset, the expansion of word (or a message to that effect if word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of parameter is substituted. ${parameter:+word} Use Alternate Value. If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted. ${parameter:offset} ${parameter:offset:length} Substring Expansion. Expands to up to length characters of the value of parameter starting at the character specified by offset. If parame‐ ter is @, an indexed array subscripted by @ or *, or an associative array name, the results differ as described below. If length is omitted, expands to the substring of the value of parameter starting at the character specified by offset and extending to the end of the value. length and offset are arithmetic expressions (see ARITHMETIC EVALUATION below).
If offset evaluates to a number less than zero, the value is used as an offset in characters from the end of the value of parameter. If length evaluates to a number less than zero, it is interpreted as an offset in characters from the end of the value of parameter rather than a number of characters, and the expansion is the characters between offset and that result. Note that a negative offset must be separated from the colon by at least one space to avoid being confused with the :– expansion.
If parameter is @, the result is length positional parameters beginning at offset. A negative offset is taken relative to one greater than the greatest positional parameter, so an offset of -1 evaluates to the last positional parameter. It is an expansion error if length evalu‐ ates to a number less than zero.
If parameter is an indexed array name subscripted by @ or *, the result is the length members of the array beginning with ${parameter[off‐ set]}. A negative offset is taken relative to one greater than the maximum index of the specified array. It is an expansion error if length evaluates to a number less than zero.
Substring expansion applied to an associative array produces undefined results.
Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1 by default. If offset is 0, and the positional parameters are used, $0 is prefixed to the list.
${!prefix*} ${!prefix@} Names matching prefix. Expands to the names of variables whose names begin with prefix, separated by the first character of the IFS special variable. When @ is used and the expansion appears within double quotes, each variable name expands to a separate word.
${!name[@]} ${!name[*]} List of array keys. If name is an array variable, expands to the list of array indices (keys) assigned in name. If name is not an array, ex‐ pands to 0 if name is set and null otherwise. When @ is used and the expansion appears within double quotes, each key expands to a separate word.
${#parameter} Parameter length. The length in characters of the value of parameter is substituted. If parameter is * or @, the value substituted is the number of positional parameters. If parameter is an array name subscripted by * or @, the value substituted is the number of elements in the array. If parameter is an indexed array name subscripted by a negative number, that number is interpreted as relative to one greater than the maximum index of parameter, so negative indices count back from the end of the array, and an index of -1 references the last element.
${parameter#word} ${parameter##word} Remove matching prefix pattern. The word is expanded to produce a pattern just as in pathname expansion, and matched against the expanded value of parameter using the rules described under Pattern Matching below. If the pattern matches the beginning of the value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the ``#'' case) or the longest match‐ ing pattern (the ``##'' case) deleted. If parameter is @ or *, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter%word} ${parameter%%word} Remove matching suffix pattern. The word is expanded to produce a pattern just as in pathname expansion, and matched against the expanded value of parameter using the rules described under Pattern Matching below. If the pattern matches a trailing portion of the expanded value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the ``%'' case) or the longest matching pattern (the ``%%'' case) deleted. If parameter is @ or *, the pattern removal operation is applied to each positional pa‐ rameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the pattern removal op‐ eration is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter/pattern/string} Pattern substitution. The pattern is expanded to produce a pattern just as in pathname expansion, Parameter is expanded and the longest match of pattern against its value is replaced with string. The match is performed using the rules described under Pattern Matching below. If pat‐ tern begins with /, all matches of pattern are replaced with string. Normally only the first match is replaced. If pattern begins with #, it must match at the beginning of the expanded value of parameter. If pattern begins with %, it must match at the end of the expanded value of parameter. If string is null, matches of pattern are deleted and the / following pattern may be omitted. If the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters. If parameter is @ or *, the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter^pattern} ${parameter^^pattern} ${parameter,pattern} ${parameter,,pattern} Case modification. This expansion modifies the case of alphabetic characters in parameter. The pattern is expanded to produce a pattern just as in pathname expansion. Each character in the expanded value of parameter is tested against pattern, and, if it matches the pattern, its case is converted. The pattern should not attempt to match more than one character. The ^ operator converts lowercase letters matching pat‐ tern to uppercase; the , operator converts matching uppercase letters to lowercase. The ^^ and ,, expansions convert each matched character in the expanded value; the ^ and , expansions match and convert only the first character in the expanded value. If pattern is omitted, it is treated like a ?, which matches every character. If parameter is @ or *, the case modification operation is applied to each positional param‐ eter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the case modification oper‐ ation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter@operator} Parameter transformation. The expansion is either a transformation of the value of parameter or information about parameter itself, depending on the value of operator. Each operator is a single letter:
U The expansion is a string that is the value of parameter with lowercase alphabetic characters converted to uppercase. u The expansion is a string that is the value of parameter with the first character converted to uppercase, if it is alphabetic. L The expansion is a string that is the value of parameter with uppercase alphabetic characters converted to lowercase. Q The expansion is a string that is the value of parameter quoted in a format that can be reused as input. E The expansion is a string that is the value of parameter with backslash escape sequences expanded as with the $'…' quoting mechanism. P The expansion is a string that is the result of expanding the value of parameter as if it were a prompt string (see PROMPTING below). A The expansion is a string in the form of an assignment statement or declare command that, if evaluated, will recreate parameter with its attributes and value. K Produces a possibly-quoted version of the value of parameter, except that it prints the values of indexed and associative arrays as a sequence of quoted key-value pairs (see Arrays above). a The expansion is a string consisting of flag values representing parameter's attributes.
If parameter is @ or *, the operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the operation is applied to each member of the array in turn, and the expansion is the resultant list.
The result of the expansion is subject to word splitting and pathname expansion as described below.
Command Substitution Command substitution allows the output of a command to replace the command name. There are two forms:
$(command) or `command`
Bash performs the expansion by executing command in a subshell environment and replacing the command substitution with the standard output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word splitting. The command substi‐ tution $(cat file) can be replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by $, `, or \. The first back‐ quote not preceded by a backslash terminates the command substitution. When using the $(command) form, all characters between the parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and pathname expansion are not performed on the results.
Arithmetic Expansion Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is:
$((expression))
The expression is treated as if it were within double quotes, but a double quote inside the parentheses is not treated specially. All tokens in the expression undergo parameter and variable expansion, command substitution, and quote removal. The result is treated as the arithmetic expression to be evaluated. Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below under ARITHMETIC EVALUATION. If expression is invalid, bash prints a message indi‐ cating failure and no substitution occurs.
Process Substitution Process substitution allows a process's input or output to be referred to using a filename. It takes the form of <(list) or >(list). The process list is run asynchronously, and its input or output appears as a filename. This filename is passed as an argument to the current command as the re‐ sult of the expansion. If the >(list) form is used, writing to the file will provide input for list. If the <(list) form is used, the file passed as an argument should be read to obtain the output of list. Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.
When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expan‐ sion.
Word Splitting The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting.
The shell treats each character of IFS as a delimiter, and splits the results of the other expansions into words using these characters as field ter‐ minators. If IFS is unset, or its value is exactly <space><tab><newline>, the default, then sequences of <space>, <tab>, and <newline> at the begin‐ ning and end of the results of the previous expansions are ignored, and any sequence of IFS characters not at the beginning or end serves to delimit words. If IFS has a value other than the default, then sequences of the whitespace characters space, tab, and newline are ignored at the beginning and end of the word, as long as the whitespace character is in the value of IFS (an IFS whitespace character). Any character in IFS that is not IFS whitespace, along with any adjacent IFS whitespace characters, delimits a field. A sequence of IFS whitespace characters is also treated as a delim‐ iter. If the value of IFS is null, no word splitting occurs.
Explicit null arguments ("" or '') are retained and passed to commands as empty strings. Unquoted implicit null arguments, resulting from the expan‐ sion of parameters that have no values, are removed. If a parameter with no value is expanded within double quotes, a null argument results and is retained and passed to a command as an empty string. When a quoted null argument appears as part of a word whose expansion is non-null, the null ar‐ gument is removed. That is, the word -d'' becomes -d after word splitting and null argument removal.
Note that if no expansion occurs, no splitting is performed.
Pathname Expansion After word splitting, unless the -f option has been set, bash scans each word for the characters *, ?, and [. If one of these characters appears, and is not quoted, then the word is regarded as a pattern, and replaced with an alphabetically sorted list of filenames matching the pattern (see Pattern Matching below). If no matching filenames are found, and the shell option nullglob is not enabled, the word is left unchanged. If the null‐ glob option is set, and no matches are found, the word is removed. If the failglob shell option is set, and no matches are found, an error message is printed and the command is not executed. If the shell option nocaseglob is enabled, the match is performed without regard to the case of alpha‐ betic characters. When a pattern is used for pathname expansion, the character ``.'' at the start of a name or immediately following a slash must be matched explicitly, unless the shell option dotglob is set. The filenames ``.'' and ``..'' must always be matched explicitly, even if dotglob is set. In other cases, the ``.'' character is not treated specially. When matching a pathname, the slash character must always be matched explic‐ itly by a slash in the pattern, but in other matching contexts it can be matched by a special pattern character as described below under Pattern Matching. See the description of shopt below under SHELL BUILTIN COMMANDS for a description of the nocaseglob, nullglob, failglob, and dotglob shell options.
The GLOBIGNORE shell variable may be used to restrict the set of file names matching a pattern. If GLOBIGNORE is set, each matching file name that also matches one of the patterns in GLOBIGNORE is removed from the list of matches. If the nocaseglob option is set, the matching against the pat‐ terns in GLOBIGNORE is performed without regard to case. The filenames ``.'' and ``..'' are always ignored when GLOBIGNORE is set and not null. However, setting GLOBIGNORE to a non-null value has the effect of enabling the dotglob shell option, so all other filenames beginning with a ``.'' will match. To get the old behavior of ignoring filenames beginning with a ``.'', make ``.*'' one of the patterns in GLOBIGNORE. The dotglob op‐ tion is disabled when GLOBIGNORE is unset. The pattern matching honors the setting of the extglob shell option.
Pattern Matching
Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The NUL character may not occur in a pattern. A backslash escapes the following character; the escaping backslash is discarded when matching. The special pattern characters must be quoted if they are to be matched literally.
The special pattern characters have the following meanings:
* Matches any string, including the null string. When the globstar shell option is enabled, and * is used in a pathname expansion con‐ text, two adjacent *s used as a single pattern will match all files and zero or more directories and subdirectories. If followed by a /, two adjacent *s will match only directories and subdirectories. ? Matches any single character. […] Matches any one of the enclosed characters. A pair of characters separated by a hyphen denotes a range expression; any character that falls between those two characters, inclusive, using the current locale's collating sequence and character set, is matched. If the first character following the [ is a ! or a ^ then any character not enclosed is matched. The sorting order of characters in range expressions is determined by the current locale and the values of the LC_COLLATE or LC_ALL shell variables, if set. To obtain the tra‐ ditional interpretation of range expressions, where [a-d] is equivalent to [abcd], set value of the LC_ALL shell variable to C, or en‐ able the globasciiranges shell option. A – may be matched by including it as the first or last character in the set. A ] may be matched by including it as the first character in the set.
Within [ and ], character classes can be specified using the syntax [:class:], where class is one of the following classes defined in the POSIX standard: alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit A character class matches any character belonging to that class. The word character class matches letters, digits, and the character _.
Within [ and ], an equivalence class can be specified using the syntax [=c=], which matches all characters with the same collation weight (as defined by the current locale) as the character c.
Within [ and ], the syntax [.symbol.] matches the collating symbol symbol.
If the extglob shell option is enabled using the shopt builtin, several extended pattern matching operators are recognized. In the following de‐ scription, a pattern-list is a list of one or more patterns separated by a |. Composite patterns may be formed using one or more of the following sub-patterns:
?(pattern-list) Matches zero or one occurrence of the given patterns *(pattern-list) Matches zero or more occurrences of the given patterns +(pattern-list) Matches one or more occurrences of the given patterns @(pattern-list) Matches one of the given patterns !(pattern-list) Matches anything except one of the given patterns
Complicated extended pattern matching against long strings is slow, especially when the patterns contain alternations and the strings contain multi‐ ple matches. Using separate matches against shorter strings, or using arrays of strings instead of a single long string, may be faster.
Quote Removal After the preceding expansions, all unquoted occurrences of the characters \, ', and " that did not result from one of the above expansions are re‐ moved.
REDIRECTION Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection allows commands' file handles to be duplicated, opened, closed, made to refer to different files, and can change the files the command reads from and writes to. Re‐ direction may also be used to modify file handles in the current shell execution environment. The following redirection operators may precede or ap‐ pear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right.
Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname}. In this case, for each redirection operator except >&– and <&-, the shell will allocate a file descriptor greater than or equal to 10 and assign it to varname. If >&– or <&– is preceded by {varname}, the value of varname defines the file descriptor to close. If {varname} is supplied, the redirection persists beyond the scope of the command, allowing the shell programmer to manage the file descriptor himself.
In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is <, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is >, the redirection refers to the standard output (file descriptor 1).
The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, quote removal, pathname expansion, and word splitting. If it expands to more than one word, bash reports an error.
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1
directs both standard output and standard error to the file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error was duplicated from the standard output before the standard output was redirected to dirlist.
Bash handles several filenames specially when they are used in redirections, as described in the following table. If the operating system on which bash is running provides these special files, bash will use them; otherwise it will emulate them internally with the behavior described below.
/dev/fd/fd If fd is a valid integer, file descriptor fd is duplicated. /dev/stdin File descriptor 0 is duplicated. /dev/stdout File descriptor 1 is duplicated. /dev/stderr File descriptor 2 is duplicated. /dev/tcp/host/port If host is a valid hostname or Internet address, and port is an integer port number or service name, bash attempts to open the corre‐ sponding TCP socket. /dev/udp/host/port If host is a valid hostname or Internet address, and port is an integer port number or service name, bash attempts to open the corre‐ sponding UDP socket.
A failure to open or create a file causes the redirection to fail.
Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally.
Redirecting Input Redirection of input causes the file whose name results from the expansion of word to be opened for reading on file descriptor n, or the standard in‐ put (file descriptor 0) if n is not specified.
The general format for redirecting input is:
[n]<word
Redirecting Output Redirection of output causes the file whose name results from the expansion of word to be opened for writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size.
The general format for redirecting output is:
[n]>word
If the redirection operator is >, and the noclobber option to the set builtin has been enabled, the redirection will fail if the file whose name re‐ sults from the expansion of word exists and is a regular file. If the redirection operator is >|, or the redirection operator is > and the noclobber option to the set builtin command is not enabled, the redirection is attempted even if the file named by word exists.
Appending Redirected Output Redirection of output in this fashion causes the file whose name results from the expansion of word to be opened for appending on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created.
The general format for appending output is:
[n]>>word
Redirecting Standard Output and Standard Error This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of word.
There are two formats for redirecting standard output and standard error:
&>word and >&word
Of the two forms, the first is preferred. This is semantically equivalent to
>word 2>&1
When using the second form, word may not expand to a number or -. If it does, other redirection operators apply (see Duplicating File Descriptors below) for compatibility reasons.
Appending Standard Output and Standard Error This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be appended to the file whose name is the expansion of word.
The format for appending standard output and standard error is:
&>>word
This is semantically equivalent to
>>word 2>&1
(see Duplicating File Descriptors below).
Here Documents This type of redirection instructs the shell to read input from the current source until a line containing only delimiter (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input (or file descriptor n if n is specified) for a command.
The format of here-documents is:
[n]<<[-]word here-document delimiter
No parameter and variable expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word. If any part of word is quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion, the character sequence \<newline> is ignored, and \ must be used to quote the characters \, $, and `.
If the redirection operator is <<-, then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.
Here Strings A variant of here documents, the format is:
[n]<<<word
The word undergoes tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal. Pathname expan‐ sion and word splitting are not performed. The result is supplied as a single string, with a newline appended, to the command on its standard input (or file descriptor n if n is specified).
Duplicating File Descriptors The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to one or more digits, the file descriptor denoted by n is made to be a copy of that file descriptor. If the digits in word do not specify a file descriptor open for input, a redirection error occurs. If word evaluates to -, file descriptor n is closed. If n is not specified, the standard input (file descriptor 0) is used.
The operator
[n]>&word
is used similarly to duplicate output file descriptors. If n is not specified, the standard output (file descriptor 1) is used. If the digits in word do not specify a file descriptor open for output, a redirection error occurs. If word evaluates to -, file descriptor n is closed. As a spe‐ cial case, if n is omitted, and word does not expand to one or more digits or -, the standard output and standard error are redirected as described previously.
Moving File Descriptors The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n is not specified. digit is closed after being duplicated to n.
Similarly, the redirection operator
[n]>&digit-
moves the file descriptor digit to file descriptor n, or the standard output (file descriptor 1) if n is not specified.
Opening File Descriptors for Reading and Writing The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be opened for both reading and writing on file descriptor n, or on file descriptor 0 if n is not specified. If the file does not exist, it is created.
ALIASES Aliases allow a string to be substituted for a word when it is used as the first word of a simple command. The shell maintains a list of aliases that may be set and unset with the alias and unalias builtin commands (see SHELL BUILTIN COMMANDS below). The first word of each simple command, if unquoted, is checked to see if it has an alias. If so, that word is replaced by the text of the alias. The characters /, $, `, and = and any of the shell metacharacters or quoting characters listed above may not appear in an alias name. The replacement text may contain any valid shell input, in‐ cluding shell metacharacters. The first word of the replacement text is tested for aliases, but a word that is identical to an alias being expanded is not expanded a second time. This means that one may alias ls to ls -F, for instance, and bash does not try to recursively expand the replacement text. If the last character of the alias value is a blank, then the next command word following the alias is also checked for alias expansion.
Aliases are created and listed with the alias command, and removed with the unalias command.
There is no mechanism for using arguments in the replacement text. If arguments are needed, a shell function should be used (see FUNCTIONS below).
Aliases are not expanded when the shell is not interactive, unless the expand_aliases shell option is set using shopt (see the description of shopt under SHELL BUILTIN COMMANDS below).