Bash is ultimately intended to be a conformant implementation of the IEEE Posix Shell and Tools specification (IEEE Working Group 1003.2).
| & ; ( ) < > space tab
|| & && ; ;; ( ) | <newline>
! case do done elif else esac fi for function if in select then until while { }
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. 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 .
A pipeline is a sequence of one or more commands separated by the character | . The format for a pipeline is:
[ ! ] command [ | command2 ... ]
The standard output of command is connected to the standard input of command2 . This connection is performed before any redirections specified by the command (see REDIRECTION below).
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.
Each command in a pipeline is executed as a separate process (i.e., in a subshell).
A list is a sequence of one or more pipelines separated by one of the operators ; , & , && , or || , and terminated by one of ; , & , or <newline> .
Of these list operators, && and || have equal precedence, followed by ; and &, which have equal precedence.
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
command && command2
command2 is executed if, and only if, command returns an exit status of zero.
An OR list has the form
command || command2
command2 is executed if and only if command 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.
A compound command is one of the following:
Each of the metacharacters listed above under DEFINITIONS has special meaning to the shell and must be quoted if they are to represent themselves. 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 quoted, the \<newline> is treated as a line continuation (that is, it is 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 (see PARAMETERS below).
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. If the variable has its -i attribute set (see declare below in "SHELL BUILTIN COMMANDS" ) then value is subject to arithmetic expansion even if the $[...] syntax does not appear. Word splitting is not performed, with the exception of "$@" as explained below under "Special Parameters" . Pathname expansion is not performed.
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).
The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.
The following variables are set by the shell:
MAILPATH='/usr/spool/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
Bash supplies a default value for this variable, but the location of the user mail files that it uses is system dependent (e.g., /usr/spool/mail/$USER).
The order of expansions is: brace expansion, tilde expansion, parameter, variable, command, and arithmetic 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 single exception to this is the expansion of ``$@'' as explained 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 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 postamble . The preamble is prepended to each string contained within the braces, and the postamble 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.
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}
Brace expansion introduces a slight incompatibility with traditional versions of sh , the Bourne shell. sh does not treat opening or closing braces specially when they appear as part of a word, and preserves them in the output. Bash removes braces from words as a consequence of brace expansion. For example, a word entered to sh as file{1,2} appears identically in the output. The same word is output as file1 file2 after expansion by bash . If strict compatibility with sh is desired, start bash with the -nobraceexpansion flag (see OPTIONS above) or disable brace expansion with the +o braceexpand option to the set command (see SHELL BUILTIN COMMANDS below).
If a word begins with a tilde character (`~'), all of the characters preceding the first slash (or all characters, if there is no slash) are treated as a possible login name. If this login name is the null string, the tilde is replaced with the value of the parameter HOME . If HOME is unset, the home directory of the user executing the shell is substituted instead.
If a `+' follows the tilde, the value of PWD replaces the tilde and `+'. If a `-' follows, the value of OLDPWD is substituted. If the value following the tilde is a valid login name, the tilde and login name are replaced with the home directory associated with that name. If the name is invalid, or the tilde expansion fails, the word is unchanged.
Each variable assignment is checked for unquoted instances of tildes following a : or = . In these cases, tilde substitution is also performed. Consequently, one may use pathnames with tildes in assignments to PATH , MAILPATH , and CDPATH , and the shell assigns the expanded value.
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.
Command substitution allows the output of a command to replace the command name. There are two forms:
$(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.
When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by $ , ` , or \ . 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 old 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 allows the evaluation of an arithmetic expression and the substitution of the result. There are two formats for arithmetic expansion:
$[expression]
$((expression))
The expression is treated as if it were within double quotes, but a double quote inside the braces or parentheses is not treated specially. All tokens in the expression undergo parameter expansion, command substitution, and quote removal. Arithmetic substitutions may be nested.
The evaluation is performed according to the rules listed below under "ARITHMETIC EVALUATION" . If expression is invalid, bash prints a message indicating failure and no substitution occurs.
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.
On systems that support it, process substitution is performed simultaneously with "parameter and variable expansion" , "command substitution" , and "arithmetic expansion" .
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 the value of IFS 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. IFS cannot be unset.
Explicit null arguments ("" or '') are retained. Implicit null arguments, resulting from the expansion of parameters that have no values, are removed.
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, then the word is regarded as a pattern , and replaced with an alphabetically sorted list of pathnames matching the pattern. If no matching pathnames are found, and the shell variable allow_null_glob_expansion is unset, the word is left unchanged. If the variable is set, and no matches are found, the word is removed. When a pattern is used for pathname generation, the character ``.'' at the start of a name or immediately following a slash must be matched explicitly, unless the shell variable glob_dot_filenames is set. The slash character must always be matched explicitly. In other cases, the ``.'' character is not treated specially.
The special pattern characters have the following meanings:
After the preceding expansions, all unquoted occurrences of the characters \ , ` , and " are removed.
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 that follows the redirection operator in the following descriptions is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, and pathname expansion. If it expands to more than one word, bash reports an error.
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1
directs both standard output and standard error to the file dirlist , while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist , because the standard error was duplicated as standard output before the standard output was redirected to dirlist .
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
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 >| , then the value of the -C option to the set builtin command is not tested, and file creation is attempted. (See also the description of noclobber under "Shell Variables" above.)
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
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
Of the two forms, the first is preferred. This is semantically equivalent to
>word 2>&1
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 as follows:
<<[-]word here-document delimiter
No parameter expansion, command substitution, pathname expansion, or arithmetic 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. Otherwise, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion. In the latter case, the pair \<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.
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 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. 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.
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 as the standard input and standard output if n is not specified. If the file does not exist, it is created.
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 function execution.
Function names and definitions may be listed with the -f option to the declare or typeset builtin commands. 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.
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, as in csh . If arguments are needed, a shell function should be used.
Aliases are not expanded when the shell is not interactive.
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. This means that 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 the 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.
Note that for almost every purpose, aliases are superseded by shell functions.
The shell associates a job with each pipeline. It keeps a table of currently executing jobs, which may be listed with the jobs command. When bash starts a job asynchronously (in the background ), it prints a line that looks like:
[1] 25647
indicating that this job is job number 1 and that the process ID of the last process in the pipeline associated with this job is 25647. All of the processes in a single pipeline are members of the same job. Bash uses the job abstraction as the basis for job control.
To facilitate the implementation of the user interface to job control, the system maintains the notion of a current terminal process group ID. Members of this process group (processes whose process group ID is equal to the current terminal process group ID) receive keyboard-generated signals such as SIGINT . These processes are said to be in the foreground . Background processes are those whose process group ID differs from the terminal's; such processes are immune to keyboard-generated signals. Only foreground processes are allowed to read from or write to the terminal. Background processes which attempt to read from (write to) the terminal are sent a SIGTTIN (SIGTTOU) signal by the terminal driver, which, unless caught, suspends the process.
If the operating system on which bash is running supports job control, bash allows you to use it. Typing the suspend character (typically ^Z , Control-Z) while a process is running causes that process to be stopped and returns you to bash . Typing the "delayed suspend" character (typically ^Y , Control-Y) causes the process to be stopped when it attempts to read input from the terminal, and control to be returned to bash . You may then manipulate the state of this job, using the bg command to continue it in the background, the fg command to continue it in the foreground, or the kill command to kill it. A ^Z takes effect immediately, and has the additional side effect of causing pending output and typeahead to be discarded.
There are a number of ways to refer to a job in the shell. The character % introduces a job name. Job number n may be referred to as %n . A job may also be referred to using a prefix of the name used to start it, or using a substring that appears in its command line. For example, %ce refers to a stopped ce job. If a prefix matches more than one job, bash reports an error. Using %?ce , on the other hand, refers to any job containing the string ce in its command line. If the substring matches more than one job, bash reports an error. The symbols %% and %+ refer to the shell's notion of the "current job" , which is the last job stopped while it was in the foreground. The "previous job" may be referenced using %- . In output pertaining to jobs (e.g., the output of the jobs command), the current job is always flagged with a + , and the previous job with a - .
Simply naming a job can be used to bring it into the foreground: %1 is a synonym for ``fg %1'', bringing job 1 from the background into the foreground. Similarly, ``%1 &'' resumes job 1 in the background, equivalent to ``bg %1''.
The shell learns immediately whenever a job changes state. Normally, bash waits until it is about to print a prompt before reporting changes in a job's status so as to not interrupt any other output. If the -b option to the set builtin command is set, bash reports such changes immediately. (See also the description of notify variable under "Shell Variables" above.)
If you attempt to exit bash while jobs are stopped, the shell prints a message warning you. You may then use the jobs command to inspect their status. If you do this, or try to exit again immediately, you are not warned again, and the stopped jobs are terminated.
Synchronous jobs started by bash have signals set to the values inherited by the shell from its parent. When job control is not in effect, background jobs (jobs started with & ) ignore SIGINT and SIGQUIT . Commands run as a result of command substitution ignore the keyboard-generated job control signals SIGTTIN , SIGTTOU , and SIGTSTP .
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 as described above in FUNCTIONS . 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. If the search is unsuccessful, the shell prints an error message and returns a nonzero exit status.
If the search is successful, or if the command name contains one or more slashes, the shell executes the named program. 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 (see hash below under SHELL BUILTIN COMMANDS) 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.
The shell allows you to manipulate the environment in several ways. On invocation, the shell scans its own environment and creates a parameter for each name found, automatically marking it for export to child processes. Executed commands inherit the environment. The export and declare -x commands allow parameters and functions to be added to and deleted from the environment. If the value of a parameter in the environment is modified, the new value becomes part of the environment, replacing the old. The environment inherited by any executed command consists of the shell's initial environment, whose values may be modified in the shell, less any pairs removed by the unset command, plus any additions via the export and declare -x commands.
The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments, as described above in PARAMETERS . These assignment statements affect only the environment seen by that command.
If the -k flag is set (see the set builtin command below), then all parameter assignments are placed in the environment for a command, not just those that precede the command name.
When bash invokes an external command, the variable _ is set to the full path name of the command and passed to that command in its environment.
If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.
Bash itself returns the exit status of the last command executed, unless a syntax error occurs, in which case it exits with a non-zero value. See also the exit builtin command below.
The command number and the history number are usually different: the history number of a command is its position in the history list, which may include commands restored from the history file (see HISTORY below), while the command number is the position in the sequence of commands executed during the current shell session. After the string is decoded, it is expanded via parameter expansion, command substitution, arithmetic expansion, and word splitting.
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 Meta-X. (On keyboards without a meta key, M-x means ESC x, i.e., press the Escape key then the x key. This makes ESC the meta prefix. The combination M-C-x means ESC-Control-x, or press the Escape key then hold the Control key while pressing the x key.)
The default key-bindings may be changed with an ~/.inputrc file. The value of the shell variable INPUTRC , if set, is used instead of ~/.inputrc . Other programs that use this library may add their own commands and bindings.
For example, placing
M-Control-u: universal-argument
The following symbolic character names are recognized: RUBOUT , DEL , ESC , LFD , NEWLINE , RET , RETURN , SPC , SPACE , and TAB . In addition to command names, readline allows keys to be bound to a string that is inserted when the key is pressed (a macro).
Readline is customized by putting commands in an initialization file. The name of this file is taken from the value of the INPUTRC variable. If that variable is unset, the default is ~/.inputrc . When a program which uses the readline library starts up, the init file is read, and the key bindings and variables are set. There are only a few basic constructs allowed in the readline init file. Blank lines are ignored. Lines beginning with a # are comments. Lines beginning with a $ indicate conditional constructs. Other lines denote key bindings and variable settings.
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. The name may be specified in one of two ways: as a symbolic key name, possibly with Meta- or Control- prefixes, or as a key sequence. When using the form keyname:function-name or macro, keyname is the name of a key spelled out in English. For example:
In the above example, C-u is bound to the function universal-argument , M-DEL is bound to the function backward-kill-word , and C-o is bound to run the macro expressed on the right hand side (that is, to insert the text >&output into the line).
In the second form, "keyseq":function-name or macro, keyseq differs from keyname above in that strings denoting an entire key sequence may be specified by placing the sequence within double quotes. Some GNU Emacs style key escapes can be used, as in the following example.
In this example, C-u is again bound to the function universal-argument . "C-x C-r" is bound to the function re-read-init-file , and "ESC [ 1 1 ~" is bound to insert the text "Function Key 1" . The full set of escape sequences is
When entering the text of a macro, single or double quotes should be used to indicate a macro definition. Unquoted text is assumed to be a function name. Backslash will quote any character in the macro text, including " and '.
Bash allows the current readline key bindings to be displayed or modified with the bind builtin command. The editing mode may be switched during interactive use by using the -o option to the 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
set variable-name value
Except where noted, readline variables can take the values On or Off . The variables and their default values are:
$if Bash # Quote the current or previous word "\C-xq": "\eb\"\ef\"" $endif
Readline commands may be given numeric arguments , which normally act as a repeat count. Sometimes, however, it is the sign of the argument that is significant. Passing a negative argument to a command that acts in the forward direction (e.g., kill-line) causes that command to act in a backward direction. Commands whose behavior with arguments deviates from this are noted.
When a command is described as killing text, the text deleted is saved for possible future retrieval (yanking). The killed text is saved in a kill-ring. Consecutive kills cause the text to be accumulated into one unit, which can be yanked all at once. Commands which do not kill text separate the chunks of text on the kill-ring.
The following is a list of the names of the commands and the default key sequences to which they are bound.
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 for interactive shells, and can be disabled using the \+H option to the set builtin command (see SHELL BUILTIN COMMANDS below). Non-interactive shells do not perform history expansion.
History expansion is performed immediately after a complete line is read, before the shell breaks it into words. It takes place in two parts. The first is to determine which line from the previous history to use during substitution. The second is to select portions of that line for inclusion into the current one. The line selected from the previous history is the event, and the portions of that line that are acted upon are words. The line is broken into words in the same fashion as when reading input, so that several metacharacter-separated words surrounded by quotes are considered as one word. Only backslash (\) and single quotes can quote the history escape character, which is ! by default.
The shell allows control of the various characters used by the history expansion mechanism (see the description of histchars above under "Shell Variables" ).
An event designator is a reference to a command line entry in the history list.
A : separates the event specification from the word designator. It can be omitted if the word designator begins with a ^ , $ , * , or % . Words are numbered from the beginning of the line, with the first word being denoted by a 0 (zero).
After the optional word designator, you can add a sequence of one or more of the following modifiers, each preceded by a `:'.
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 36 representing the arithmetic base, and n is a number in that base. If base is omitted, then base 10 is used.
Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first and may override the precedence rules above.
The -n flag suppresses the command numbers when listing. The -r flag reverses the order of the commands. If the -l flag is given, the commands are listed on standard output. Otherwise, the editor given by ename is invoked on a file containing those commands. If ename is not given, the value of the FCEDIT variable is used, and the value of EDITOR if FCEDIT is not set. If neither variable is set, vi is used. When editing is complete, the edited commands are echoed and executed.
In the second form, command is re-executed after each instance of pat is replaced by rep. A useful alias to use with this is ``r=fc -s'', so that typing ``r cc'' runs the last command beginning with ``cc'' and typing ``r'' re-executes the last command.
If the first form is used, the return value is 0 unless an illegal option is encountered or first or last specify history lines out of range. If the -e option is supplied, the return value is the value of the last command executed or failure if an error occurs with the temporary file of commands. If the second form is used, the return status is that of the command re-executed, unless cmd does not specify a valid history line, in which case fc returns failure.
getopts can report errors in two ways. If the first character of optstring is a colon, silent error reporting is used. In normal operation diagnostic messages are printed when illegal options or missing option arguments are encountered. If the variable OPTERR is set to 0, no error message will be displayed, even if the first character of optstring is not a colon.
If an illegal option is seen, getopts places ? into name and, if not silent, prints an error message and unsets OPTARG . If getopts is silent, the option character found is placed in OPTARG and no diagnostic message is printed.
If a required argument is not found, and getopts is not silent, a question mark (?) is placed in name , OPTARG is unset, and a diagnostic message is printed. If getopts is silent, then a colon (:) is placed in name and OPTARG is set to the option character found.
getopts normally parses the positional parameters, but if more arguments are given in args , getopts parses those instead. getopts returns true if an option, specified or unspecified, is found. It returns false if the end of options is encountered or an error occurs.
If the -x option is supplied, jobs replaces any jobspec found in command or args with the corresponding process group ID, and executes command passing it args , returning its exit status.
If no option-name is supplied, the values of the current options are printed.
An interactive shell is one 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 $- includes i if bash is interactive, allowing a shell script or a startup file to test this state.
Login shells: On login (subject to the -noprofile option): if /etc/profile exists, source it.
if ~/.bash_profile exists, source it, else if ~/.bash_login exists, source it, else if ~/.profile exists, source it.
On exit: if ~/.bash_logout exists, source it.
Non-login interactive shells: On startup (subject to the -norc and -rcfile options): if ~/.bashrc exists, source it.
Non-interactive shells: On startup: if the environment variable ENV is non-null, expand it and source the file it names, as if the command if [ "$ENV" ]; then . $ENV; fi had been executed, but do not use PATH to search for the pathname. When not started in Posix mode, bash looks for BASH_ENV before ENV.
If Bash is invoked as sh , it tries to mimic the behavior of sh as closely as possible. For a login shell, it attempts to source only /etc/profile and ~/.profile , in that order. The -noprofile option may still be used to disable this behavior. A shell invoked as sh does not attempt to source any other startup files.
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, the ENV variable is expanded and that file sourced; no other startup files are read.
Chet Ramey, Case Western Reserve University
chet@ins.CWRU.Edu
Once you have determined that a bug actually exists, use the bashbug command to submit a bug report. If you have a fix, you are welcome to mail that as well! Suggestions and `philosophical' bug reports may be mailed to bug-bash@prep.ai.MIT.Edu or posted to the Usenet newsgroup gnu.bash.bug .
ALL bug reports should include:
Comments and bug reports concerning this manual page should be directed to chet@ins.CWRU.Edu .
It's too big and too slow.
There are some subtle differences between bash and traditional versions of sh , mostly because of the POSIX specification.
Aliases are confusing in some uses.