Understanding the Linux/Unix Command bash

Everything you ever need to know about bash and then some

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The Linux command 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 Options

In addition to the single-character shell options documented in the description of the set builtin command, bash interprets the following options when it is invoked:

  • -c string: If the -c option is present, then commands are read from string. If there are arguments after the string, they are assigned to the positional parameters, starting with $0.
  • -i: If the -i option is present, the shell is interactive.
  • -l: Make bash act as if it had been invoked as a login shell.
  • -r: If the -r option is present, the shell becomes restricted.
  • -s: If the -s option is present, or if no arguments remain after option processing, then commands are read from the standard input. This option allows the positional parameters to be set when invoking an interactive shell.
  • -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. If shopt_option is present, -O sets the value of that option, and +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.
  • --: -- 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:

  • --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-filefile and --rcfile file: Execute commands from file instead of the standard personal initialization file ~/.bashrc if the shell is interactive.
  • --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 the systemwide 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.
  • --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).
  • --restricted: The shell becomes restricted.
  • --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; Backticks, [] tests, and evals 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 executed 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

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 and without the -c option whose standard input and output are both connected to terminals as determined by isatty(3), or one started with the -i option. PS1 is set and $- includes if bash is interactive, 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 file names as described in Tilde Expansion.

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 a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it 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 file name.

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 invoked 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 shbash 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, interactive 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 by the remote shell daemon, usually rshd. If bash determines it is being run by rshd, 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 rshd does not 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 variable, if it appears in the environment, is 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: word consisting only of alphanumeric characters and underscores, and beginning with an alphabetic character or an underscore. Also referred to as an identifier.
  • Metacharacter: A character that, when unquoted, separates words. One of the following: | & ; ( ) < > space tab
  • Control operator: A token that performs a control function. It is one of the following symbols: || & && ; ;; ( ) | <newline>

Reserved Words

Reserved words are words that have a special meaning to the shell. The following words are recognized as reserved when unquoted and either the first word of a simple command or the third word of a case or for command:

! case do done elif else esac fi for function if in select then until while { } time [[ ]]  

Shell Grammar: Simple Commands

simple command is a sequence of optional variable assignments followed by blank-separated words and redirections and terminated by a control operator. The first word specifies the command to be executed, and is passed as argument zero. The remaining words are passed as arguments to the invoked command.

The return value of a simple command is its exit status, or 128+n if the command is terminated by signal n.  

Pipelines

pipeline is a sequence of one or more commands separated by the character |. 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 specified by the command.

If the reserved word ! precedes a pipeline, the exit status of that pipeline is the logical NOT of the exit status of the last command. Otherwise, the status of the pipeline is the exit status of the last command. The shell waits for all commands in the pipeline to terminate before returning a value.

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. The TIMEFORMAT variable may be set to a format string that specifies how the timing information should be displayed.

Each command in a pipeline is executed as a separate process (in a subshell, for example).  

Lists

list is a sequence of one or more pipelines separated by one of the operators ;&&&, or ||, and optionally terminated by one of ;&, or <newline>.

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. Commands separated by a ; are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.

The control operators && and || denote AND lists and OR lists, respectively. An AND list has the form

command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

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

compound command is one of the following:

(list): list is executed in a subshell. 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 command. 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.

((expression)): The expression is evaluated according to the rules described 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 expressionexpression. Expressions are composed of the primaries described 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.

When the == and != operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described under Pattern Matching. The return value is 0 if the string matches or does not match the pattern, respectively, and 1 otherwise. Any part of the pattern may be quoted to force it to be matched as a string.

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 conditional 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. 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 under Arithmetic Evaluation. The arithmetic expression 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 preceded by a number. If the in word is omitted, the positional parameters are printed. 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 ]: A case command first expands word, and tries to match it against each pattern in turn, using the same matching rules as for pathname expansion. When a match is found, the corresponding list is executed. After the first match, no subsequent matches are attempted. The exit status is zero if no pattern matches. Otherwise, it is the exit status of the last command executed in list.
  • if listthen list; and elif listthen 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 listdo listdone and until listdo listdone: The while command continuously executes the do list as long as the last command in list returns an exit status of zero. The until command is identical to the while command, except that the test is negated; the do list is executed as long as the last command in list returns a non-zero exit status. The exit status of the while and until commands is the exit status of the last do list command executed, or zero if none was executed.
  • function ] name () { list; }: This defines a function named name. The body of the function is the list of commands between { and }. This list is executed whenever name is specified as the name of a simple command. The exit status of a function is the exit status of the last command executed in the body.

    Comments

    In a non-interactive shell or an interactive shell in which the interactive_comments option to the shopt builtin is enabled, a word beginning with # causes that word and all remaining characters on that line to be ignored. An interactive shell without the interactive_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 special 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, 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 <newline>. If a \<newline> pair appears and the backslash is not itself quoted, the \<newline> is treated as a line continuation (that is, it is removed 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 \. 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.

    The special parameters * and @ have special meaning when in double quotes.

    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: an escape character
    • \f: form feed
    • \n: new line
    • \r: carriage return
    • \t: horizontal tab
    • \v: vertical tab
    • \\: backslash
    • \': single quote
    • \nnn: the eight-bit character whose value is the octal value nnn (one to three digits)
    • \xHH: the eight-bit character whose value is the hexadecimal value HH (one or two 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 ($) will cause the string to be translated according to the current locale. If the current locale is C or POSIX, the dollar sign is ignored. If the string is translated and replaced, the replacement is double-quoted.  

    Parameters

    parameter is an entity that stores values. It can be a name, a number, or one of the special characters listed under Special Parameters. For the shell's purposes, 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.

    A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the unset builtin command.

    variable may be assigned to by a statement of the form: name=[value]

    If value is not given, the variable is assigned the null string. All values undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal. If the variable has its integer attribute set, then value is subject to arithmetic expansion even if the $((...)) expansion is not used. Word splitting is not performed, with the exception of "$@" as explained under Special Parameters. Pathname expansion is not performed. Assignment statements may also appear as arguments to the declaretypesetexportreadonly, and local builtin commands.  

    Positional Parameters

    positional parameter is a parameter denoted by one or more digits, other than the single digit 0. Positional parameters are assigned from the shell's arguments when it is invoked, and may be reassigned using the set builtin command. Positional parameters may not be assigned to with assignment statements. The positional parameters are temporarily replaced when a shell function is executed.

    When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.  

    Special Parameters

    The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.

    • *:Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS special variable. That is, "$*" is equivalent to "$1c$2c...", where c is the first character of the value of the IFS variable. If IFS is unset, the parameters are separated by spaces. If IFS is null, the parameters are joined without intervening separators.
    • @: Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each parameter expands to a separate word. That is, "$@" is equivalent to "$1" "$2" ... When there are no positional parameters, "$@" and $@expand to nothing (they are removed).
    • #: Expands to the number of positional parameters in decimal.
    • ?: Expands to the status of the most recently executed foreground pipeline.
    • -: Expands to the current option flags as specified upon invocation, by the set builtin command or those set by the shell itself, such as the -i 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 most recently executed background (asynchronous) command.
    • 0: Expands to the name of the shell or shell script. This is set at shell initialization. If bash is invoked with a file of commands, $0 is set to the name of that file. If bash is started with the -c option, then $0 is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the file name used to invoke bash, as given by argument zero.
    • _: At shell startup, set to the absolute file name of the shell or shell script being executed as passed in the argument list. Subsequently, expands to the last argument to the previous command, after expansion. Also set to the full file name of 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.

      Shell Variables

      The following variables are set by the shell:

      BASH: Expands to the full file name used to invoke this instance of bash.

      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 (beta1, for example)
      • 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.

      COMP_LINE: The current command line. This variable is available only in shell functions and external commands invoked by the programmable completion facilities.

      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.

      COMP_WORDS: An array variable consisting of the individual words in the current command line. This variable is available only in shell functions invoked by the programmable completion facilities.

      DIRSTACK: An array variable 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 directory. If DIRSTACK 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: The name of any currently executing shell function. This variable exists only when a shell function is executing. Assignments to FUNCNAME have no effect and return an error status. If FUNCNAME is unset, it loses its special properties, even if it is subsequently reset.

      GROUPS: An array variable containing the list of groups of which the current user is a member. Assignments to GROUPS have no effect and return an error status. 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. 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 unset, 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.

      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.

      OPTIND: The index of the next argument to be processed by the getopts builtin command.

      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 containing a list of exit status values from the processes in the most-recently-executed foreground pipeline, 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, a random integer between 0 and 32767 is generated. The sequence of random numbers may be initialized by assigning a value to RANDOM. If RANDOM is unset, it loses its special properties, even if it is subsequently reset.

      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. If SECONDS is unset, it loses its special properties, 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. 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 readonly.

      SHLVL: Incremented by one each time an instance of bash is started.

      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.

      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 being interpreted as a file name. PATH is not used to search for the resultant file name.

      CDPATH: The search path for the cd command. This is a colon-separated list of directories in which the shell looks for destination directories specified by the cd command. A sample value is ".:~:/usr".

      COLUMNS: Used by the select builtin command to determine the terminal width when printing selection lists. Automatically set 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.

      FCEDIT: The default editor for the fc builtin command.

      FIGNORE: A colon-separated list of suffixes to ignore when performing filename completion. A filename whose suffix matches one of the entries in FIGNORE is excluded from the list of matched filenames. A sample value is ".o:~".

      GLOBIGNORE: A colon-separated list of patterns defining the set of filenames to be ignored by pathname expansion. If a filename matched by a pathname expansion pattern also matches one of the patterns in GLOBIGNORE, it is removed from the list of matches.

      HISTCONTROL: If set to a value of ignorespace, lines that begin with a space character are not entered on the history list. If set to a value of ignoredups, lines matching the last history line are not entered. A value of ignoreboth combines the two options. If unset or if set to any other value than those above, all lines read by the parser are saved on the history list, subject to the value of HISTIGNORE. This variable's function is superseded by HISTIGNORE. 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. The default value is ~/.bash_history. If unset, the command history is not saved when an interactive 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 necessary, to contain no more than that number of lines. The default value is 500. The history file is also truncated to this size after writing it when an interactive shell exits.

      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.

      HISTSIZE: The number of commands to remember in the command history. The default value is 500.

      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 performing 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, 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 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 that must be typed as the first characters on an input line before bash exits. If the variable exists but does not have a numeric value or has no value, the default value is 10. If it does not exist, EOF signifies the end of input to the shell.

      INPUTRC: The filename for the readline startup file, overriding the default of ~/.inputrc.

      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 expressions, 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 matching.

      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.

      LINES: Used by the select builtin command to determine the column length for printing selection lists. Automatically set upon receipt of a SIGWINCH.

      MAIL: If this parameter is set to a file name and the MAILPATH variable is not set, bashinforms the user of the arrival of mail in the specified file.

      MAILCHECK: Specifies how often (in seconds) bash checks for mail. The default is 60 seconds. When it is time to check for mail, the shell does so before 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 file names to be checked for mail. The message to be printed when mail arrives in a particular file may be specified by separating the file name 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 supplies a default value for this variable, but the location of the user mail files that it uses is system dependent (/var/mail/$USER).

      OPTERR: If set to the value 1, bash displays error messages generated by the getopts builtin command. 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. The default path is system dependent and is set by the administrator who installs bash. A common value is "/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin:.''.

      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 invocation 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.

      PROMPT_COMMAND: If set, the value is executed as a command prior to issuing each primary prompt.

      PS1: The value of this parameter is expanded 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.

      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 PS4 is replicated multiple times, as necessary, to indicate multiple levels of indirection. The default is "''.

      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 %3lS'. If the value is null, no timing information 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 input after issuing the primary prompt. Bash terminates after waiting for that number of seconds if input does not arrive.

      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. If set to any other value, the supplied string must be a prefix of a stopped job's name; this provides functionality analogous to the % job identifier.

      histchars: The two or three characters that control history expansion and tokenization. The first character is the history expansion character, the character that signals the start of a history expansion, normally '!'. The second character is the quick substitution character, which is used as shorthand for rerunning the previous command entered, substituting one string for another in the command. The default is '^'. The optional third character is the character that 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 remaining 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 array variables. Any variable may be used as an array; the declare builtin will explicitly declare an array. There is no maximum limit on the size of an array nor any requirement that members be indexed or assigned contiguously. Arrays are indexed using integers and are zero-based.

      An 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 greater than or equal to zero. To explicitly declare an array, use declare -a namedeclare -a name[subscript] is also accepted; the subscript is ignored. 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 is of the form [subscript]=string. Only string is required. 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. This syntax is also accepted by the declare builtin. Individual array elements may be assigned to using the name[subscript]=value syntax.

      Any element of an array may be referenced using ${name[subscript]}. The braces are required to avoid conflicts with pathname expansion. If subscript 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. This is analogous to the expansion of the special parameters * and @. ${#name[subscript]} expands to the length of ${name[subscript]}. If subscript is * or @, the expansion is the number of elements in the array. Referencing an array variable without a subscript is equivalent to referencing element zero.

      The unset builtin is used to destroy arrays. Unset name[subscript] destroys the array element at index subscriptUnset name, where name is an array, or unset name[subscript], wheresubscript is * or @, removes the entire array.

      The declarelocal, and readonly builtins each accept a -a option to specify an array. 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.

      Only brace expansion, word splitting, and pathname expansion can change 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[@]}".

      Brace Expansion

      Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to pathname expansion, but the filenames generated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by a series of comma-separated strings between a pair of braces, followed by an optional 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 expands into "ade ace abe."

      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. Any incorrectly formed brace expansion is left unchanged. A { or , may be quoted with a backslash to prevent it from being considered part of a brace expression. To avoid conflicts with parameter expansion, the string ${ is not considered eligible for brace expansion.

      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, which 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 compatibility with sh is desired, start bash with the +B option or disable brace expansion with the +B option to the set command.  

      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 following 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 parameter 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 =. In these cases, tilde expansion is also performed. Consequently, one may use file names with tildes in assignments to PATHMAILPATH, and CDPATH, and the shell assigns the expanded value.  

      Parameter Expansion

      The $ character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it, which could be interpreted as part of the name.

      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 parameter is followed by a character that is not to be interpreted as part of its name.

      If the first character of parameter is an exclamation point, a level of variable indirection is introduced. Bash uses the value of the variable formed from the rest of parameter as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of parameter itself. This is known as indirect expansion. The exception to this is the expansion of ${!prefix*} described below.

      In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. When not performing substring expansion, bash tests for a parameter that is unset or null; omitting the colon results in a test only for a parameter that is unset.

      • ${parameter:-word}: Use default values. If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.
      • ${parameter:=word}: Assign default values. If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.
      • ${parameter:?word}: Display error if null or unset. If parameter is null or unset, the expansion of word (or a message to that effect if word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of 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} and ${parameter:offset:length}: Substring Expansion. Expands to up to length characters of parameter starting at the character specified by offset. If length is omitted, expands to the substring of parameter starting at the character specified by offsetLength and offset are arithmetic expressions. Length must evaluate to a number greater than or equal to zero. If offset evaluates to a number less than zero, the value is used as an offset from the end of the value of parameter. If parameter is @, the result is length positional parameters beginning at offset. If parameter is an array name indexed by @ or *, the result is the length members of the array beginning with ${parameter[offset]}. Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1.
      • ${!prefix*}: Expands to the names of variables whose names begin with prefix, separated by the first character of the IFS special variable.
      • ${#parameter}: 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.
      • ${parameter#word} and ${parameter##word}: The word is expanded to produce a pattern just as in pathname expansion. If the pattern matches the beginning of the value of parameter, then the 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 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} and ${parameter%%word}: The word is expanded to produce a pattern just as in pathname expansion. 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 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/pattern/string} and ${parameter//pattern/string}: 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. In the first form, only the first match is replaced. The second form causes all matches of pattern to be replaced with string. 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 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.

        Command Substitution

        Command substitution allows the output of a command to replace the command name. There are two forms:

        $(command) and command

        Bash performs the expansion by executing command 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 substitution $(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 backquote 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 expansion, string expansion, command substitution, and quote removal. Arithmetic substitutions may be nested.

        The evaluation is performed according to the rules listed under Arithmetic Evaluation. If expression is invalid, bash prints a message indicating failure, and no substitution occurs.  

        Process Substitution

        Process substitution is supported on systems that support named pipes (FIFOs) or the/dev/fd method of naming open files. It takes the form of <(list) or >(list). The process list is run with its input or output connected to a FIFO or some file in /dev/fd. The name of this file is passed as an argument to the current command as the result of the expansion. If the >(list) form is used, writing to the file will provide input for list. If the <(list) form is used, the file passed as an argument should be read to obtain the output of list.

        When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.  

        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 on these characters. If IFS is unset or its value is exactly <space><tab><newline>, the default, then any sequence of IFS characters serves to delimit words. If IFS has a value other than the default, then sequences of the whitespace characters space and tab are ignored at the beginning and end of the word, as long as the whitespace character is in the value of IFS (an IFS whitespace 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 delimiter. If the value of IFS is null, no word splitting occurs.

        Explicit null arguments ("" or '') are retained. Unquoted implicit null arguments, resulting from the expansion 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.

        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, then the word is regarded as a pattern and replaced with an alphabetically sorted list of file names matching the pattern. If no matching file names are found and the shell option nullglob is disabled, the word is left unchanged. If the nullglob option is set and no matches are found, the word is removed. If the shell option nocaseglob is enabled, the match is performed without regard to the case of alphabetic 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. When matching a pathname, the slash character must always be matched explicitly. In other cases, the ".'' character is not treated specially. See the description of shopt under Shell Builtin Commands for a description of the nocaseglobnullglob, 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. The file names ".'' and "..''are always ignored, even when GLOBIGNORE is set. However, setting GLOBIGNORE has the effect of enabling the dotglob shell option, so all other file names beginning with a ".'' will match. To get the old behavior of ignoring file names beginning with a ".'', make ".*'' one of the patterns in GLOBIGNORE. The dotglob option is disabled when GLOBIGNORE is unset.

        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. 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

        ?: 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 sorts 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 value of the LC_COLLATE shell variable, if set. 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.2 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 description, 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 exactly one of the given patterns
        • !(pattern-list): Matches anything except one of the given patterns

        Quote Removal

        After the preceding expansions, all unquoted occurrences of the characters \', and " that did not result from one of the above expansions are removed.  

        Redirection

        Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection may also be used to open and close files for the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear from left to right.

        In the following descriptions, if the file descriptor 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 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>&> dirlist

        directs only the standard output to file dirlist, because the standard error was duplicated as standard output before the standard output was redirected to dirlist.

        Bash handles several filenames specially when they are used in redirections:

        • /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 a TCP connection to the corresponding 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 a UDP connection to the corresponding socket.

        A failure to open or create a file causes the redirection to fail.  

        Redirecting Input

        Redirection of input causes the file whose name results from the expansion of word to be opened for reading on file descriptor n or the standard input (file descriptor 0) if n is not specified.

        The general format for redirecting input is:

        [n]<word

        Redirecting Output

        Redirection of output causes the file whose name results from the expansion of word to be opened for writing on file descriptor n or the standard output (file descriptor 1) if n is not specified. If the file does not exist, it is created; if it does exist it is truncated to zero size.

        The general format for redirecting output is:

        [n]>word

        If the redirection operator is >, and the noclobber option to the set builtin has been enabled, the redirection will fail if the file whose name results 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

        Bash allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of word with this construct.

        There are two formats for redirecting standard output and standard error:

        &>word

        and

        >&word

        Of the two forms, the first is preferred. This is semantically equivalent to

        >word 2>&1

        Here Documents

        This type of redirection instructs the shell to read input from the current source until a line containing only word with no trailing blanks is seen. All of the lines read up to that point are then used as the standard input for a command.

        The format of here-documents is:

        <<[-]word here-document
        delimiter

        No parameter expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word. If any characters in word are quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion. In the latter case, 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

        For here-strings, a variant of here-documents, the format is:

        <<<word

        The word is expanded and supplied to the command on its standard input.  

        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. As a special case, if n is omitted and word does not expand to one or more digits, the standard output and standard error are redirected as described 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. The first word of each command, if unquoted, is checked to see if it has an alias. If so, that word is replaced by the text of the alias. The alias name and the replacement text may contain any valid shell input, including the metacharacters listed above, with the exception that the alias name may not contain =. The first word of the replacement text is tested for aliases, but a word that is identical to an alias being expanded is not expanded a second time. This means that one may alias ls to ls -F, for example, 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.

        Aliases are not expanded when the shell is not interactive, unless the expand_aliases shell option is set using shopt.

        The rules concerning the definition and use of aliases are somewhat confusing. Bash always reads at least one complete line of input before executing any of the commands on that line. Aliases are expanded when a command is read, not when it is executed. Therefore, an alias definition appearing on the same line as another command does not take effect until the next line of input is read. The commands following the alias definition on that line are not affected by the new alias. This behavior is also an issue when functions are executed. Aliases are expanded when a function definition is read, not when the function is executed, because a function definition is itself a compound command. As a consequence, aliases defined in a function are not available until after that function is executed. To be safe, always put alias definitions on a separate line and do not use alias in compound commands.

        For almost every purpose, aliases are superseded by shell functions.  

        Functions

        A shell function stores a series of commands for later execution. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. Functions are executed in the context of the current shell; no new process is created to interpret them (contrast this with the execution of a shell script). When a function is executed, the arguments to the function become the positional parameters during its execution. The special parameter # is updated to reflect the change. Positional parameter 0 is unchanged. The FUNCNAME variable is set to the name of the function while the function is executing. All other aspects of the shell execution environment are identical between a function and its caller with the exception that the DEBUG trap is not inherited unless the function has been given the trace attribute.

        Variables local to the function may be declared with the local builtin command. Ordinarily, variables and their values are shared between the function and its caller.

        If the builtin command return is executed in a function, the function completes and execution resumes with the next command after the function call. When a function completes, the values of the positional parameters and the special parameter # are restored to the values they had prior to the function's execution.

        Function names and definitions may be listed with the -f option to the declare or typeset builtin commands. The -F option to declare or typeset will list the function names only. Functions may be exported so that subshells automatically have them defined with the -f option to the export builtin.

        Functions may be recursive. No limit is imposed on the number of recursive calls.  

        Arithmetic Evaluation

        The shell allows arithmetic expressions to be evaluate, under certain circumstances (see the let builtin command and Arithmetic Expansion). Evaluation is done in fixed-width integers with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence and associativity are the same as in the C language. The following list of operators is grouped into levels of equal-precedence operators. The levels are listed in order of decreasing precedence.

        • id++ id--: variable post-increment and post-decrement
        • ++id --id: variable pre-increment and pre-decrement
        • - +: unary minus and plus
        • ! ~: logical and bitwise negation
        • **: exponentiation
        • * / %: multiplication, division, remainder
        • + -: addition, subtraction
        • << >>: left and right bitwise shifts
        • <= >= < >: comparison
        • == !=: equality and inequality
        • &: bitwise AND
        • ^: bitwise exclusive OR
        • |: bitwise OR
        • &&: logical AND
        • ||: logical OR
        • expr?expr:expr: conditional evaluation
        • = *= /= %= += -= <<= >>= &= ^= |=: assignment
        • expr1 , expr2: comma

        Shell variables are allowed as operands; parameter expansion is performed before the expression is evaluated. Within an expression, shell variables may also be referenced by name without using the parameter expansion syntax. The value of a variable is evaluated as an arithmetic expression when it is referenced. A shell variable need not have its integer attribute turned on to be used in an expression.

        Constants with a leading 0 are interpreted as octal numbers. A leading 0x or 0X denotes hexadecimal. Otherwise, numbers take the form [base#]n, where base is a decimal number between 2 and 64 representing the arithmetic base, and n is a number in that base. If base# is omitted, then base 10 is used. The digits greater than 9 are represented by the lowercase letters, the uppercase letters, @, and _, in that order. If base is less than or equal to 36, lowercase and uppercase letters may be used interchangeably to represent numbers between 10 and 35.

        Operators are evaluated in order of precedence. Subexpressions in parentheses are evaluated first and may override the precedence rules above.  

        Conditional Expressions

        Conditional expressions are used by the [[ compound command and the test and [ builtin commands to test file attributes and perform string and arithmetic comparisons. Expressions are formed from the following unary or binary primaries. If any file argument to one of the primaries is of the form /dev/fd/n, then file descriptor n is checked. If the file argument to one of the primaries is one of /dev/stdin/dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively, is checked.

        • -a file: True if file exists
        • -b file: True if file exists and is a block special file
        • -c file: True if file exists and is a character special file
        • -d file: True if file exists and is a directory
        • -e file: True if file exists
        • -f file: True if file exists and is a regular file
        • -g file: True if file exists and is set-group-id
        • -h file: True if file exists and is a symbolic link
        • -k file: True if file exists and its "sticky'' bit is set
        • -p file: True if file exists and is a named pipe (FIFO)
        • -r file: True if file exists and is readable
        • -s file: True if file exists and has a size greater than zero
        • -t fd: True if file descriptor fd is open and refers to a terminal
        • -u file: True if file exists and its set-user-id bit is set
        • -w file: True if file exists and is writable
        • -x file: True if file exists and is executable
        • -O file: True if file exists and is owned by the effective user id
        • -G file: True if file exists and is owned by the effective group id
        • -L file: True if file exists and is a symbolic link
        • -S file: True if file exists and is a socket
        • -N file: True if file exists and has been modified since it was last read
        • file1 -nt file2: True if file1 is newer (according to modification date) than file2 or if file1 exists and file2 does not
        • file1 -ot file2: True if file1 is older than file2, or if file2 exists and file1 does not
        • file1 -ef file2: True if file1 and file2 refer to the same device and inode numbers
        • -o optname: True if shell option optname is enabled.
        • -z string: True if the length of string is zero
        • string -n string: True if the length of string is non-zero
        • string1 == string2: True if the strings are equal. = may be used in place of == for strict POSIX compliance.
        • string1 != string2: True if the strings are not equal.
        • string1 < string2: True if string1 sorts before string2 lexicographically in the current locale
        • string1 > string2: True if string1 sorts after string2 lexicographically in the current locale
        • arg1 OP arg2: OP is one of -eq-ne-lt-le-gt, or -ge. These arithmetic binary operators return true if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively. Arg1 and arg2 may be positive or negative integers.

          Simple Command Expansion

          When a simple command is executed, the shell performs the following expansions, assignments, and redirections.

          • 1.: The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing.
          • 2.: The words that are not variable assignments or redirections are expanded. If any words remain after expansion, the first word is taken to be the name of the command and the remaining words are the arguments.
          • 3.: Redirections are performed.
          • 4.: The text after the = in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable.

          If no command name results, the variable assignments affect the current shell environment. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status.

          If no command name results, redirections are performed but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status.

          If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. If there were no command substitutions, the command exits with a status of zero.  

          Command Execution

          After a command has been split into words, if it results in a simple command and an optional list of arguments, the following actions are taken.

          If the command name contains no slashes, the shell attempts to locate it. If there exists a shell function by that name, that function is invoked. If the name does not match a function, the shell searches for it in the list of shell builtins. If a match is found, that builtin is invoked.

          If the name is neither a shell function nor a builtin and contains no slashes, bash searches each element of the PATH for a directory containing an executable file by that name. Bash uses a hash table to remember the full pathnames of executable files. A full search of the directories in PATH is performed only if the command is not found in the hash table. If the search is unsuccessful, the shell prints an error message and returns an exit status of 127.

          If the search is successful or if the command name contains one or more slashes, the shell executes the named program in a separate execution environment. Argument 0 is set to the name given, and the remaining arguments to the command are set to the arguments given, if any.

          If this execution fails because the file is not in executable format and the file is not a directory, it is assumed to be a shell script, a file containing shell commands. A subshell is spawned to execute it. This subshell reinitializes itself, so that the effect is as if a new shell had been invoked to handle the script with the exception that the locations of commands remembered by the parent are retained by the child.

          If the program is a file beginning with #!, the remainder of the first line specifies an interpreter for the program. The shell executes the specified interpreter on operating systems that do not handle this executable format themselves. The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the program, followed by the name of the program, followed by the command arguments, if any. 

          Command Execution Environment

          The shell has an execution environment that consists of the following:

          • Open files inherited by the shell at invocation, as modified by redirections supplied to the exec builtin
          • The current working directory as set by cdpushd, or popd, or inherited by the shell at invocation
          • The file creation mode mask as set by umask or inherited from the shell's parent
          • Current traps set by trap
          • Shell parameters that are set by variable assignment or with set or inherited from the shell's parent in the environment
          • Shell functions defined during execution or inherited from the shell's parent in the environment
          • Options enabled at invocation, either by default or with command-line arguments, or by set
          • Options enabled by shopt
          • Shell aliases defined with alias
          • Various process IDs, including those of background jobs, the value of $$, and the value of $PPID

          When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell.

          • The shell's open files, plus any modifications and additions specified by redirections to the command
          • The current working directory
          • The file creation mode mask
          • Shell variables marked for export, along with variables exported for the command, passed in the environment
          • Traps caught by the shell are reset to the values the inherited from the shell's parent, and traps ignored by the shell are ignored

          A command invoked in this separate environment cannot affect the shell's execution environment.

          Command substitution and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline are also executed in a subshell environment. Changes made to the subshell environment cannot affect the shell's execution environment.

          If a command is followed by an & and job control is not active, the default standard input for the command is the empty file /dev/null. Otherwise, the invoked command inherits the file descriptors of the calling shell as modified by redirections.