/usr/bin/bash: Difference between revisions

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 stan‐

       dard input or from a file.  Bash also incorporates useful features from the  Korn  and  C  shells

       fault.

       All  of  the single-character shell options documented in the description of the set builtin com‐

       mand, including -o, can be used as options when the shell is invoked.  In addition,  bash  inter‐

               cessfully.

       If  arguments  remain after option processing, and neither the -c nor the -s option has been sup‐

       plied, the first argument is assumed to be the name of a file containing shell commands.  If bash

       script.

       A login shell is one whose first character of argument zero is a -, or one started with the --lo‐

       gin option.

       not reset.

       The following definitions are used throughout the rest of this document.

       blank  A space or tab.

               || & && ; ;; ;& ;;& ( ) | |& <newline>

       Reserved  words are words that have a special meaning to the shell.  The following words are rec‐

       ognized as reserved when unquoted and either the first word of a command (see SHELL  GRAMMAR  be‐

       ]]

       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

       signal n.

       A  pipeline  is a sequence of one or more commands separated by one of the control operators | or

       |&.  The format for a pipeline is:

       line may be run by the shell process.

       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 AND and OR lists is the exit status of the last command executed in the list.

       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 new‐

               is the exit status of the last command executed in list-2, or zero if none was executed.

       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,

       cess.  The return status of a coprocess is the exit status of command.

       A  shell  function is an object that is called like a simple command and executes a compound com‐

       mand with a new set of positional parameters.  Shell functions are declared as follows:

               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

       is on by default in interactive shells.

       Quoting is used to remove the special meaning of certain characters or words to the shell.  Quot‐

       ing can be used to disable special treatment for special characters, to  prevent  reserved  words

       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 nameref variable as an argument, the variable referenced by the nameref variable will be unset.

       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

       closed in braces (see EXPANSION below).

       The shell treats several parameters specially.  These parameters may only be referenced;  assign‐

       ment to them is not allowed.

               given by argument zero.

       The following variables are set by the shell:

               comment.

       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

       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,

       plained above (see PARAMETERS).

       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

       COMMANDS below).

       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-

       not do this, except for the declaration commands listed above, when in posix mode.

       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

               scribed below.

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

       forms:

       performed on the results.

       Arithmetic expansion allows the evaluation of an arithmetic expression and  the  substitution  of

       the result.  The format for arithmetic expansion is:

       expression is invalid, bash prints a message indicating failure and no substitution occurs.

       Process  substitution  allows a process's input or output to be referred to using a filename.  It

       takes the form of <(list) or >(list).  The process list is run asynchronously, and its  input  or

       pansion, command substitution, and arithmetic expansion.

       The shell scans the results of parameter expansion, command substitution, and  arithmetic  expan‐

       sion that did not occur within double quotes for word splitting.

       Note that if no expansion occurs, no splitting is performed.

       After  word splitting, unless the -f option has been set, bash scans each word for the characters

       *, ?, and [.  If one of these characters appears, and is not quoted, then the word is regarded as

       set.  The pattern matching honors the setting of the extglob shell option.

       Any character that appears in a pattern, other than the special pattern characters described  be‐

       shorter strings, or using arrays of strings instead of a single long string, may be faster.

       After the preceding expansions, all unquoted occurrences of the characters \, ', and "  that  did

       not result from one of the above expansions are removed.

       Before a command is executed, its input and output may be redirected using a special notation in‐

       terpreted by the shell.  Redirection allows commands' file  handles  to  be  duplicated,  opened,

       Note that the exec builtin command can make redirections take effect in the current shell.

       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 speci‐

               [n]<word

       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 speci‐

       the file named by word exists.

       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)

               [n]>>word

       This construct allows both the standard output (file descriptor 1) and the standard error  output

       (file descriptor 2) to be redirected to the file whose name is the expansion of word.

       operators apply (see Duplicating File Descriptors below) for compatibility reasons.

       This construct allows both the standard output (file descriptor 1) and the standard error  output

       (file descriptor 2) to be appended to the file whose name is the expansion of word.

       (see Duplicating File Descriptors below).

       This  type  of redirection instructs the shell to read input from the current source until a line

       containing only delimiter (with no trailing blanks) is seen.  All of the lines read  up  to  that

       dented in a natural fashion.

       A variant of here documents, the format is:

       its standard input (or file descriptor n if n is specified).

       The redirection operator

       output and standard error are redirected as described previously.

       The redirection operator

       if n is not specified.

       The redirection operator

       is created.

       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 una‐

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

       A shell function, defined as described above under SHELL GRAMMAR, 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

       the number of recursive calls.

       The shell allows arithmetic expressions to be evaluated, under certain circumstances (see the let

       and  declare builtin commands, the (( compound command, and Arithmetic Expansion).  Evaluation is

       first and may override the precedence rules above.

       Conditional  expressions  are used by the [[ compound command and the test and [ builtin commands

       to test file attributes and perform string and arithmetic comparisons.  The test and  [  commands

               pressions (see ARITHMETIC EVALUATION above).

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

       redirections, from left to right, in the following order.

       were no command substitutions, the command exits with a status of zero.

       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.

       lowed by the name of the program, followed by the command arguments, if any.

       The shell has an execution environment, which consists of the following:

       tors of the calling shell as modified by redirections.

       When a program is invoked it is given an array of strings called the environment.  This is a list

       of name-value pairs, of the form name=value.

       and passed to that command in its environment.

       The exit status of an executed command is the value returned by the waitpid system call or equiv‐

       alent function.  Exit statuses fall between 0 and 255, though, as explained below, the shell  may

       in which case it exits with a non-zero value.  See also the exit builtin command below.

       When bash is interactive, in the absence of any traps, it ignores SIGTERM (so that  kill  0  does

       not kill an interactive shell), and SIGINT is caught and handled (so that the wait builtin is in‐

       after which the trap is executed.

       Job  control  refers  to the ability to selectively stop (suspend) the execution of processes and

       continue (resume) their execution at a later point.  A user typically employs this  facility  via

       job or process terminates before returning.

       When  executing  interactively,  bash  displays the primary prompt PS1 when it is ready to read a

       command, and the secondary prompt PS2 when it needs more input to complete a command.  Bash  dis‐

       appear within command substitution or contain characters special to word expansion.

       This  is  the  library  that  handles  reading  input when using an interactive shell, unless the

       --noediting option is given at shell invocation.  Line editing is also used when using the -e op‐

       builtin.

       In this section, the Emacs-style notation is used to denote keystrokes.  Control keys are denoted

       by C-key, e.g., C-n means Control-N.  Similarly, meta keys are denoted by  M-key,  so  M-x  means

       separate the chunks of text on the kill ring.

       Readline  is  customized  by  putting commands in an initialization file (the inputrc file).  The

       name of this file is taken from the value of the INPUTRC variable.  If that  variable  is  unset,

       the key is pressed (a macro).

       The  syntax  for controlling key bindings in the inputrc file is simple.  All that is required is

       the name of the command or the text of a macro and a key sequence to which it  should  be  bound.

       set builtin command (see SHELL BUILTIN COMMANDS below).

       Readline has variables that can be used to further customize its behavior.  A variable may be set

       in the inputrc file with a statement of the form

               filename when listing possible completions.

       Readline implements a facility similar in spirit to the conditional compilation features of the C

       preprocessor which allows key bindings and variable settings to be performed  as  the  result  of

               $include  /etc/inputrc

       Readline  provides  commands  for  searching  through the command history (see HISTORY below) for

       lines containing a specified string.  There are two search modes: incremental  and  non-incremen‐

       rent line.

       The  following is a list of the names of the commands and the default key sequences to which they

       are bound.  Command names without an accompanying key sequence are unbound by  default.  In  the

               Refresh the current line.

       accept-line (Newline, Return)

               Accept  the  line regardless of where the cursor is.  If this line is non-empty, add it to

               Bash attempts to invoke $VISUAL, $EDITOR, and emacs as the editor, in that order.

       end-of-file (usually C-d)

               The character indicating end-of-file as set, for example, by ``stty''.  If this  character

               mand is unbound.

       kill-line (C-k)

               Kill the text from point to the end of the line.

               Rotate the kill ring, and yank the new top.  Only works following yank or yank-pop.

       digit-argument (M-0, M-1, ..., M--)

               Add this digit to the argument already accumulating, or start a new argument.  M--  starts

               ment count sixteen, and so on.

       complete (TAB)

               Attempt to perform completion on the text before point.  Bash attempts completion treating

               braces so the list is available to the shell (see Brace Expansion above).

       start-kbd-macro (C-x ()

               Begin saving the characters typed into the current keyboard macro.

               Print the last keyboard macro defined in a format suitable for the inputrc file.

       re-read-init-file (C-x C-r)

               Read in the contents of the inputrc file, and incorporate any bindings or variable assign‐

               Display version information about the current instance of bash.

       When  word completion is attempted for an argument to a command for which a completion specifica‐

       tion (a compspec) has been defined using the complete builtin (see SHELL BUILTIN COMMANDS below),

       complete -D -F _completion_loader -o bashdefault -o default

       When the -o history option to the set builtin is enabled, the shell provides access to  the  com‐

       mand  history, the list of commands previously typed.  The value of the HISTSIZE variable is used

       COMMANDS for information on setting and unsetting shell options.

       The shell supports a history expansion feature that is similar to the history expansion  in  csh.

       This  section  describes  what syntax features are available.  This feature is enabled by default

       character to mark history timestamps when writing the history file.

       An event designator is a reference to a command line entry in the history list.  Unless the  ref‐

       erence is absolute, events are relative to the current position in the history list.

       !#    The entire command line typed so far.

       Word designators are used to select desired words from the event.  A : separates the event speci‐

       fication from the word designator.  It may be omitted if the word designator begins with a ^,  $,

       the event.

       After the optional word designator, there may appear a sequence of one or more of  the  following

       modifiers,  each  preceded  by a `:'.  These modify, or edit, the word or words selected from the

       G      Apply the following `s' or `&' modifier once to each word in the event line.

       Unless otherwise noted, each builtin command documented in this section as accepting options pre‐

       ceded by - accepts -- to signify the end of the options.  The :, true, false, and test/[ builtins

               turn status is the exit status of the last process or job waited for.

       Bash-4.0  introduced  the concept of a `shell compatibility level', specified as a set of options

       to the shopt builtin compat31, compat32, compat40, compat41, and so on).  There is only one  cur‐

                     as input. Bash-5.1 suppresses that message when the -l option is supplied.

       If bash is started with the name rbash, or the -r option is supplied at invocation, the shell be‐

       comes restricted.  A restricted shell is used to set up an environment more controlled  than  the

       rbash turns off any restrictions in the shell spawned to execute the script.

       Bash Reference Manual, Brian Fox and Chet Ramey

       The Gnu Readline Library, Brian Fox and Chet Ramey

       readline(3)

       /bin/bash

               The bash executable

               Individual readline initialization file

       Brian Fox, Free Software Foundation

       bfox@gnu.org

       chet.ramey@case.edu

       If  you find a bug in bash, you should report it.  But first, you should make sure that it really

       is a bug, and that it appears in the latest version of bash.  The latest version is always avail‐

       Comments and bug reports concerning this manual page should be directed to chet.ramey@case.edu.

       It's too big and too slow.