neulab/tldr huggingface dataset:
This manual page was written by Chris Lamb <chris@chris-lamb.co.uk>, for the Debian project (but may be used by others).
BASH(1)                      General Commands Manual                     BASH(1)
bash - GNU Bourne-Again SHell
bash [options] [command_string | file]
Bash is Copyright (C) 1989-2020 by the Free Software Foundation, Inc.
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.
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 option 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 extdebug 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 below for a reference to a document that details how posix mode affects bash's behavior.
--restricted The shell becomes restricted (see RESTRICTED SHELL below).
--verbose Equivalent to -v.
--version Show version information for this instance of bash on the standard output and exit successfully.
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.
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 $- includes i if bash is interactive, allowing a shell script or a startup file to test this state.
The following paragraphs describe how bash executes its startup files. If any of the files exist but cannot be read, bash reports an error. Tildes are expanded in filenames as described below under Tilde Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non- interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists.  After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable.  The --noprofile option may be used when the shell is started to inhibit this behavior.
When an interactive login shell exits, or a non-interactive login shell executes the exit builtin command, bash reads and executes commands from the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started, bash reads and executes commands from ~/.bashrc, if that file exists.  This may be inhibited by using the --norc option.  The --rcfile file option will force bash to read and execute commands from file instead of ~/.bashrc.
When bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute.  Bash behaves as if the following command were executed: if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi but the value of the PATH variable is not used to search for the 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 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 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, 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 with its standard input connected to a network connection, as when executed by the remote shell daemon, 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.
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 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 [[ ]]
Simple Commands A 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 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 specified by the command (see REDIRECTION below).  If |& is used, command's standard error, in addition to its standard output, is connected to command2's standard input through the pipe; it is shorthand for 2>&1 |.  This implicit redirection of the standard error to the standard output is performed 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 pipeline'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 subshell 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 <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.  These are referred to as asynchronous commands.  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.
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 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 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 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.  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 described 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 operator is considered a POSIX extended regular expression and matched accordingly (using the POSIX regcomp and regexec interfaces usually described 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 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 (see PARAMETERS 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 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 (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 Matching 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 expansion, 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 associated 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 returns 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 variable (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 descriptor 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 descriptor 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 expansions.  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 default 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 occurs 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.)
In a non-interactive shell, or an interactive shell in which the interactive_comments option to the shopt builtin is enabled (see SHELL BUILTIN COMMANDS below), 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 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 (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 <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, when history expansion is enabled, !.  When the shell is in posix mode, the ! has no special meaning within double quotes, even when history expansion is enabled.  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 infrastructure 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 double-quoted.
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 unset 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 substitution, 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 $((...)) expansion is not used (see Arithmetic Expansion below).  Word splitting is not performed, with the exception of "$@" as explained below under Special Parameters.  Pathname expansion is not performed.  Assignment statements may also appear as arguments 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 commands).  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 index (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 $1.  If the control variable in a for loop has the nameref attribute, the list of words can be a list of shell variables, and a name reference will be established for each word in the list, in turn, when the loop is executed.  Array variables cannot be given the nameref attribute.  However, nameref variables can reference array variables and subscripted array variables.  Namerefs can be unset using the -n option to the unset builtin.  Otherwise, if unset is executed with the name of a nameref variable as an argument, the variable referenced by the nameref variable will be unset.
Positional Parameters A positional parameter is a parameter denoted by one or more digits, other than the single digit 0.  Positional parameters are assigned from the shell's arguments when it is invoked, and may be reassigned using the set builtin command.  Positional parameters may not be assigned to with assignment statements.  The positional parameters are temporarily replaced when a shell function is executed (see FUNCTIONS below).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces (see EXPANSION below).
Special Parameters The shell treats several parameters specially.  These parameters may only be referenced; assignment to them is not allowed. *      Expands to the positional parameters, starting from one.  When the expansion is not within double quotes, each positional parameter expands to a separate word.  In contexts where it is performed, those words are subject to further word splitting and pathname expansion.  When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS special variable. That is, "$*" is equivalent to "$1c$2c...", where c is the first character of the value of the IFS variable.  If IFS is unset, the parameters are separated by spaces.  If IFS is null, the parameters are joined without intervening separators. @      Expands to the positional parameters, starting from one.  In contexts where word splitting is performed, this expands each positional parameter to a separate word; if not within double quotes, these words are subject to word splitting.  In contexts where word splitting is not performed, this expands to a single word with each positional parameter separated by a space.  When the expansion occurs within double quotes, each parameter expands to a separate word.  That is, "$@" 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:
The following variables are used by the shell.  In some cases, bash assigns a default value to a variable; these cases are noted below.
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 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 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 history expansion character, the character which signals the start of a history expansion, normally `!'.  The second character is the quick substitution character, which is used as shorthand for re-running the previous command entered, substituting one string for another in the command.  The default is `^'.  The optional third character is the 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 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 indexed and associative array variables. Any variable may be used as an indexed 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.  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 below).  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.  Indexed 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 identically 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 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.  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 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, parameter, 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 removal).
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 generated 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 expands into `ade ace abe'.