bash Version 2.0.
Bash is the shell, or command language interpreter,
that will appear in the GNU operating system.
The name is an acronym for the `Bourne-Again SHell',
a pun on Steve Bourne, the author of the direct ancestor of the current
Unix shell /bin/sh,
which appeared in the Seventh Edition Bell Labs Research version
of Unix.
Bash is an sh-compatible shell that incorporates useful
features from the Korn shell ksh and the C shell csh.
It is ultimately intended to be a
conformant implementation of the IEEE POSIX Shell and Tools
specification (IEEE Working Group 1003.2). It offers functional
improvements over sh for both interactive and programming use.
While the GNU operating system will include a version
of csh, Bash will be the default shell.
Like other GNU software, Bash is quite portable. It currently runs
on nearly every version of Unix and a few other operating systems -
independently-supported ports exist for OS/2 and Windows NT.
At its base, a shell is simply a macro processor that executes commands. A Unix shell is both a command interpreter, which provides the user interface to the rich set of Unix utilities, and a programming language, allowing these utilitites to be combined. The shell reads commands either from a terminal or a file. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories like `/bin', allowing users or groups to establish custom environments.
A shell allows execution of Unix commands, both synchronously and
asynchronously. The redirection constructs permit
fine-grained control of the input and output of those commands,
and the shell allows control over the contents of their
environment. Unix shells also provide a small set of built-in
commands (builtins) implementing functionality impossible
(e.g., cd, break, continue, and
exec), or inconvenient (history, getopts,
kill, or pwd, for example) to obtain via separate
utilities. Shells may be used interactively or
non-interactively: they accept input typed from the keyboard or
from a file. All of the shell builtins are described in
subsequent sections.
While executing commands is essential, most of the power (and complexity) of shells is due to their embedded programming languages. Like any high-level language, the shell provides variables, flow control constructs, quoting, and functions.
Shells have begun offering features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, history and aliases. Each of these features is described in this manual.
These definitions are used throughout the remainder of this manual.
POSIX
blank
builtin
control operator
word that performs a control function. It is a newline
or one of the following:
`||', `&&', `&', `;', `;;',
`|', `(', or `)'.
exit status
field
filename
job
job control
metacharacter
blank or one of the following characters:
`|', `&', `;', `(', `)', `<', or
`>'.
name
word consisting solely of letters, numbers, and underscores,
and beginning with a letter or underscore. Names are used as
shell variable and function names.
Also referred to as an identifier.
operator
control operator or a redirection operator.
See section Redirections, for a list of redirection operators.
process group
process group ID
process group
during its lifetime.
reserved word
word that has a special meaning to the shell. Most reserved
words introduce shell flow control constructs, such as for and
while.
return status
exit status.
signal
special builtin
token
word or an operator.
word
token that is not an operator.
Bash is an acronym for `Bourne-Again SHell'. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, and the rules for evaluation and quoting are taken from the POSIX 1003.2 specification for the `standard' Unix shell.
This chapter briefly summarizes the shell's "building blocks": commands, control structures, shell functions, shell parameters, shell expansions, redirections, which are a way to direct input and output from and to named files, and how the shell executes commands.
The following is a brief description of the shell's operation when it reads and executes a command. Basically, the shell does the following:
metacharacters. Alias expansion is performed by this step
(see section Aliases).
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 shell metacharacters (see section 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 Bash 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 section Shell Parameter Expansion).
Words of the form $'string' are treated specially. The
word expands to string, with backslash-escaped characters replaced
as specifed by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:
\a
\b
\e
\f
\n
\r
\t
\v
\\
\nnn
ASCII code is nnn in octal
The result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign (`$') will cause
the string to be translated according to the current locale.
If the current locale is C or POSIX, the dollar sign
is ignored.
If the string is translated and replaced, the replacement is
double-quoted.
In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt
builtin is enabled (see section Bash Builtin Commands),
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.
A simple command is the kind of command you'll encounter most often.
It's just a sequence of words separated by blanks, terminated
by one of the shell control operators (see section Definitions). The
first word generally specifies a command to be executed.
The return status (see section Exit Status) of a simple command is
its exit status as provided
by the POSIX.1 waitpid function, or 128+n if the command
was terminated by signal n.
A pipeline is a sequence of simple commands separated by
`|'.
[time[-p]] [!] command1 [|command2 ...]
The output of each command in the pipeline is connected to the input of the next command. That is, each command reads the previous command's output.
The reserved word time causes timing statistics
to be printed for the pipeline once it finishes.
The `-p' option changes the output format to that specified
by POSIX.
The TIMEFORMAT variable may be set to a format string that
specifies how the timing information should be displayed.
See section Bash Variables, for a description of the available formats.
Each command in a pipeline is executed in its own subshell. The exit status of a pipeline is the exit status of the last command in the pipeline. If the reserved word `!' precedes the pipeline, the exit status is the logical NOT of the exit status of the last command.
A list is a sequence of one or more pipelines separated by one
of the operators `;', `&', `&&', or `||',
and optionally terminated by one of `;', `&', or a
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 (true). 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.
Note that wherever you see a `;' in the description of a command's syntax, it may be replaced indiscriminately with one or more newlines.
Bash supports the following looping constructs.
until
until command is:
until test-commands; do consequent-commands; doneExecute consequent-commands as long as the final command in test-commands has an exit status which is not zero.
while
while command is:
while test-commands; do consequent-commands; doneExecute consequent-commands as long as the final command in test-commands has an exit status of zero.
for
for command is:
for name [in words ...]; do commands; doneExecute commands for each member in words, with name bound to the current member. If `in words' is not present, `in "$@"' is assumed.
The break and continue builtins (see section Bourne Shell Builtins)
may be used to control loop execution.
if
if command is:
if test-commands; then consequent-commands; [elif more-test-commands; then more-consequents;] [else alternate-consequents;] fiExecute consequent-commands only if the final command in test-commands has an exit status of zero. Otherwise, each
elif list is executed in turn,
and if its exit status is zero,
the corresponding more-consequents is executed and the
command completes.
If `else alternate-consequents' is present, and
the final command in the final if or elif clause
has a non-zero exit status, then execute alternate-consequents.
case
case command is:
case word in [pattern [| pattern]...) commands ;;]... esac
Selectively execute commands based upon word matching
pattern. The `|' is used to separate multiple patterns.
Here is an example using case in a script that could be used to
describe one interesting feature of an animal:
echo -n "Enter the name of an animal: " read ANIMAL echo -n "The $ANIMAL has " case $ANIMAL in horse | dog | cat) echo -n "four";; man | kangaroo ) echo -n "two";; *) echo -n "an unknown number of";; esac echo " legs."
((...))
(( expression ))The expression is evaluated according to the rules described below ((see section 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"
The select construct, which allows users to choose from a list
of items presented as a menu, is also available.
See section Korn Shell Constructs, for a full description of select.
Bash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream.
()
( list )Placing a list of commands between parentheses causes a subshell to be created, and each of the commands to be executed in that subshell. Since the list is executed in a subshell, variable assignments do not remain in effect after the subshell completes.
{}
{ list; }
Placing a list of commands between curly braces causes the list to
be executed in the current shell context. No subshell is created.
The semicolon following list is required.
In addition to the creation of a subshell, there is a subtle difference
between these two constructs due to historical reasons. The braces
are reserved words, so they must be separated from the list
by blanks. The parentheses are operators, and are
recognized as separate tokens by the shell even if they are not separated
from list by whitespace.
The exit status of both of these constructs is the exit status of list.
Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. Shell functions are executed in the current shell context; no new process is created to interpret them.
Functions are declared using this syntax:
[ function ] name () { command-list; }
This defines a shell function named name. The reserved
word function is optional. The body of the
function is the command-list between { and }. This list
is executed whenever name is specified as the
name of a command. The exit status of a function is
the exit status of the last command executed in the body.
When a function is executed, the arguments to the
function become the positional parameters
during its execution (see section Positional Parameters).
The special parameter
`#' that gives the number of positional parameters
is updated to reflect the change. Positional parameter 0
is unchanged.
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. If a numeric argument is given to return,
that is the function return status.
Variables local to the function may be declared with the
local builtin. These variables are visible only to
the function and the commands it invokes.
Functions may be recursive. No limit is placed on the number of recursive calls.
A parameter is an entity that stores values.
It can be a name, a number, or one of the special characters
listed below.
For the shell's purposes, a variable is a parameter denoted by a
name.
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.
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 (detailed below). If the variable has its `-i' attribute
set (see the description of the declare builtin in
section Bash Builtin Commands), then value
is subject to arithmetic expansion even if the $((...))
syntax does not appear (see section Arithmetic Expansion).
Word splitting is not performed, with the exception
of "$@" as explained below.
Filename 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 section Shell Functions).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.
The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.
*
IFS
special variable. That is, "$*" is equivalent
to "$1c$2c...", where c
is the first character of the value of the IFS
variable. If IFS
is null or unset, the parameters are separated by spaces.
@
"$@" is equivalent to
"$1" "$2" ....
When there are no positional parameters, "$@" and
$@
expand to nothing (i.e., they are removed).
#
?
-
set
builtin command, or those set by the shell itself
(such as the `-i' option).
$
() subshell, it
expands to the process ID of the current shell, not the
subshell.
!
0
$0 is set to the name of that file. If Bash
is started with the `-c' option, then $0
is set to the first argument after the string to be
executed, if one is present. Otherwise, it is set
to the filename used to invoke Bash, as given by argument zero.
_
Expansion is performed on the command line after it has been split into
tokens. There are seven kinds of expansion performed:
Brace expansion, tilde expansion, and arithmetic expansion are described in other sections. For brace expansion, see section Brace Expansion; for tilde expansion, see section Tilde Expansion; and for arithmetic expansion, see section Arithmetic Expansion.
The order of expansions is: brace expansion, tilde expansion, parameter, variable, and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and filename expansion.
On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as parameter, variable, and arithemtic expansion and command substitution.
Only brace expansion, word splitting, and filename expansion
can change the number of words of the expansion; other expansions
expand a single word to a single word.
The only exceptions to this are the expansions of
"$@" (see section Special Parameters) and "${[@]}"
(see section Arrays).
After all expansions, quote removal (see section Quote Removal)
is performed.
The `$' 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.
The basic form of parameter expansion is ${parameter}. The value of parameter is substituted. The braces are required when parameter is a positional parameter with more than one digit, or when parameter is followed by a character that is not to be interpreted as part of its name.
If the first character of parameter is an exclamation point,
a level of variable indirection is introduced.
Bash uses the value of the variable formed from the rest of
parameter as the name of the variable; this variable is then
expanded and that value used in the rest of the substitution, rather
than the value of parameter itself.
This is known as indirect expansion.
In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. When not performing substring expansion, Bash tests for a parameter that is unset or null; omitting the colon results in a test only for a parameter that is unset.
${parameter:-word}
${parameter:=word}
${parameter:?word}
${parameter:+word}
${parameter:offset}
${parameter:offset:length}
${#parameter}
${parameter#word}
${parameter##word}
${parameter%word}
${parameter%%word}
${parameter/pattern/string}
${parameter//pattern/string}
/ following pattern may be omitted.
If parameter is `@' or `*',
the substitution operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If parameter
is an array variable subscripted with `@' or `*',
the substitution operation is applied to each member of the
array in turn, and the expansion is the resultant list.
Command substitution allows the output of a command to replace the command name. There are two forms:
$(command)
or
`command`
Bash performs the expansion by executing command and replacing the command substitution with the standard output of the command, with any trailing newlines deleted.
When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed by
`$', ``', 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 filename expansion are not performed on the results.
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
IFS is unset, or its value is exactly <space><tab><newline>,
the default, then any sequence of IFS
characters serves to delimit words. If IFS
has a value other than the default, then sequences of
the whitespace characters space and tab
are ignored at the beginning and end of the
word, as long as the whitespace character is in the
value of IFS (an IFS whitespace character).
Any character in IFS that is not IFS
whitespace, along with any adjacent IFS
whitespace characters, delimits a field. A sequence of IFS
whitespace characters is also treated as a delimiter.
If the value of IFS is null, no word splitting occurs.
Explicit null arguments ("" or ") are retained.
Unquoted implicit null arguments, resulting from the expansion of
parameters
that have no values, are removed.
If a parameter with no value is expanded within double quotes, a
null argument results and is retained.
Note that if no expansion occurs, no splitting is performed.
After word splitting,
unless the `-f'
option has been set (see section The Set Builtin),
Bash scans each word for the characters
`*', `?', and `['.
If one of these characters appears, then the word is
regarded as a pattern,
and replaced with an alphabetically sorted list of
file names matching the pattern. If no matching file names are found,
and the shell option nullglob is disabled, the word is left
unchanged. If the option is set, and no matches are found, the word
is removed. When a pattern is used for filename generation,
the character `.'
at the start of a filename or immediately following a slash
must be matched explicitly, unless the shell option dotglob
is set. The slash character must always be matched explicitly.
In other cases, the `.' character is not treated specially.
See the description of shopt in section Bash Builtin Commands,
for a description of the nullglob and dotglob options.
The GLOBIGNORE
shell variable may be used to restrict the set of filenames matching a
pattern. If GLOBIGNORE
is set, each matching filename that also matches one of the patterns in
GLOBIGNORE is removed from the list of matches. The filenames
`.' and `..'
are always ignored, even when GLOBIGNORE.
is set. However, setting GLOBIGNORE has the effect of
enabling the dotglob
shell option, so all other filenames beginning with a
`.' will match.
To get the old behavior of ignoring filenames beginning with a
`.', make `.*' one of the patterns in GLOBIGNORE.
The dotglob option is disabled when GLOBIGNORE
is unset.
The special pattern characters have the following meanings:
*
?
[...]
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 interpreted by the shell. Redirection may also be used to open and close files for the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right.
In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is `<', the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is `>', the redirection refers to the standard output (file descriptor 1).
The word 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 filename 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 `>', and the `-C' option to the
set builtin has been enabled, the redirection will fail if the
filename whose name results from the expansion of word exists.
If the redirection operator is `>|',
then the value of the `-C' option to the set
builtin command is not tested, and the redirection is attempted even
if 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) 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
and
>&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, filename
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 on file descriptor 0 if n
is not specified. If the file does not exist, it is created.
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.
$PATH for a directory containing an executable file
by that name. Bash uses a hash table to remember the full
filenames of executable files (see the description of
hash in section Bourne Shell Builtins) to avoid multiple
PATH searches.
A full search of the directories in $PATH
is performed only if the command is not found in the hash table.
If the search is unsuccessful, the shell prints an error
message and returns a nonzero exit status.
When a program is invoked it is given an array of strings
called the environment.
This is a list of name-value pairs, of the form name=value.
Bash allows you to manipulate the environment in several
ways. On invocation, the shell scans its own environment and
creates a parameter for each name found, automatically marking
it for export
to child processes. Executed commands inherit the environment.
The export and `declare -x'
commands allow parameters and functions to be added to and
deleted from the environment. If the value of a parameter
in the environment is modified, the new value becomes part
of the environment, replacing the old. The environment
inherited by any executed command consists of the shell's
initial environment, whose values may be modified in the shell,
less any pairs removed by the unset command, plus any
additions via the export and `declare -x' commands.
The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments, as described in section Shell Parameters. These assignment statements affect only the environment seen by that command.
If the `-k' flag is set (see section The Set Builtin, then all parameter assignments are placed in the environment for a command, not just those that precede the command name.
When Bash invokes an external command, the variable `$_' is set to the full path name of the command and passed to that command in its environment.
For the purposes of the shell, a command which exits with a zero exit status has succeeded. A non-zero exit status indicates failure. This seemingly counter-intuitive scheme is used so there is one well-defined way to indicate success and a variety of ways to indicate various failure modes. When a command terminates on a fatal signal whose number is n, Bash uses the value 128+n as the exit status.
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.
The exit status is used by the Bash conditional commands (see section Conditional Constructs) and some of the list constructs (see section Lists of Commands).
All of the Bash builtins return an exit status of zero if they succeed and a non-zero status on failure, so they may be used by the conditional and list constructs.
When Bash is interactive, 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 interruptible).
When Bash receives a SIGINT, it breaks out of any executing loops.
In all cases, Bash ignores SIGQUIT.
If job control is in effect (see section Job Control), Bash
ignores SIGTTIN, SIGTTOU, and SIGTSTP.
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 (commands terminated with `&')
ignore SIGINT and SIGQUIT.
Commands run as a result of command substitution ignore the
keyboard-generated job control signals
SIGTTIN, SIGTTOU, and SIGTSTP.
The shell exits by default upon receipt of a SIGHUP.
Before exiting, it resends the SIGHUP
to all jobs, running or stopped. To prevent the shell from
sending the SIGHUP signal to a particular job, remove it
from the jobs table with the disown builtin
(see section Job Control Builtins)
or use disown -h to mark it to not receive SIGHUP.
A shell script is a text file containing shell commands. When such
a file is used as the first non-option argument when invoking Bash,
and neither the `-c' nor `-s' option is supplied
(see section Invoking Bash),
Bash reads and executes commands from the file, then exits. This
mode of operation creates a non-interactive shell. When Bash runs
a shell script, it sets the special parameter 0 to the name
of the file, rather than the name of the shell, and the positional
parameters are set to the remaining arguments, if any are given.
If no additional arguments are supplied, the positional parameters
are unset.
A shell script may be made executable by using the chmod command
to turn on the execute bit. When Bash finds such a file while
searching the $PATH for a command, it spawns a subshell to
execute it. In other words, executing
filename arguments
is equivalent to executing
bash filename arguments
if filename is an executable shell script.
This subshell reinitializes itself, so that the effect is as if a
new shell had been invoked to interpret the script.
Most versions of Unix make this a part of the kernel's command execution mechanism. If the first line of a script begins with the two characters `#!', the remainder of the line specifies an interpreter for the program. The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the script file, followed by the name of the script file, followed by the rest of the arguments. Bash will perform this action on operating systems that do not handle it themselves. Note that some older versions of Unix limit the interpreter name and argument to a maximum of 32 characters.
This section briefly summarizes things which Bash inherits from the Bourne Shell: builtins, variables, and other features. It also lists the significant differences between Bash and the Bourne Shell.
The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX 1003.2 standard.
:
: [arguments]Do nothing beyond expanding arguments and performing redirections.
.
. filenameRead and execute commands from the filename argument in the current shell context.
break
break [n]Exit from a
for, while, until, or select loop.
If n is supplied, the nth enclosing loop is exited.
cd
cd [-LP] [directory]Change the current working directory to directory. If directory is not given, the value of the
HOME shell variable is used. If the
shell variable CDPATH exists, it is used as a search path. If
directory begins with a slash, CDPATH is not used.
The `-P' option means
to not follow symbolic links; symlinks are followed by default or with the
`-L' option.
continue
continue [n]Resume the next iteration of an enclosing
for, while,
until, or select loop.
If n is supplied, the execution of the
nth enclosing loop is resumed.
eval
eval [arguments]The arguments are concatenated together into a single command, which is then read and executed.
exec
exec [-cl] [-a name] [command] [arguments]If command is supplied, it replaces the shell. If the `-l' option is supplied, the shell places a dash in the zeroth arg passed to command. This is what the
login program does.
The `-c' option causes command
to be executed with an empty environment.
If `-a' is supplied, the shell passes name
as the zeroth argument to command.
If no command is specified, redirections may be used to affect
the current shell environment.
exit
exit [n]Exit the shell, returning a status of n to the shell's parent.
export
export [-fn] [-p] [name[=value]]Mark each name to be passed to child processes in the environment. If the `-f' option is supplied, the names refer to shell functions. The `-n' option means to no longer mark each name for export. If no names are supplied, or if the `-p' option is given, a list of exported names is displayed.
getopts
getopts optstring name [args]
getopts is used by shell scripts to parse positional parameters.
optstring contains the option letters to be recognized; if a letter
is followed by a colon, the option is expected to have an
argument, which should be separated from it by white space.
Each time it is invoked, getopts
places the next option in the shell variable name, initializing
name if it does not exist,
and the index of the next argument to be processed into the
variable OPTIND. OPTIND
is initialized to 1 each time the shell or a shell script
is invoked. When an option requires an argument,
getopts places that argument into the variable OPTARG.
The shell does not reset OPTIND
automatically; it must be manually reset between multiple
calls to getopts
within the same shell invocation if a new set of parameters
is to be used.
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.
hash
hash [-r] [-p filename] [name]Remember the full filenames of commands specified as arguments, so they need not be searched for on subsequent invocations. The commands are found by searching through the directories listed in
$PATH. The `-p' option inhibits the path search, and
filename is used as the location of name.
The `-r' option causes the shell to forget
all remembered locations. If no arguments are given, information
about remembered commands is printed.
pwd
pwd [-LP]Print the current working directory. If the `-P' option is supplied, the path printed will not contain symbolic links. If the `-L' option is supplied, the path printed may contain symbolic links.
readonly
readonly [-apf] [name] ...Mark each name as unchangable. The values of these names may not be changed by subsequent assignment. If the `-f' option is supplied, each name refers to a shell function. The `-a' option means each name refers to an array variable. If no name arguments are given, or if the `-p' option is supplied, a list of all readonly names is printed.
return
return [n]Cause a shell function to exit with value n. This may also be used to terminate execution of a script being executed with the
.
builtin.
shift
shift [n]Shift positional parameters to the left by n. The positional parameters from n+1 ... are renamed to
$1 ... .
Parameters represented by the numbers
$# to n+1 are unset. n
must be a non-negative number less than or equal to $#.
test
[
times
timesPrint out the user and system times used by the shell and its children.
trap
trap [-lp] [arg] [sigspec]The commands in arg are to be read and executed when the shell receives signal sigspec. If arg is absent or equal to `-', all specified signals are reset to the values they had when the shell was started. If arg is the null string, then sigspec is ignored by the shell and commands it invokes. If arg is `-p', the shell displays the trap commands associated with each sigspec. If no arguments are supplied, or only `-p' is given,
trap prints the list of commands
associated with each signal number. sigspec is either a signal
name such as SIGINT or a signal number. If sigspec is
0 or EXIT, arg is executed when the shell exits.
If sigspec is DEBUG, the command arg is executed
after every simple command.
The `-l' option causes the shell to print a list of signal names
and their corresponding numbers.
Signals ignored upon entry to the shell cannot be trapped or reset.
Trapped signals are reset to their original values in a child
process when it is created.
umask
umask [-S] [mode]Set the shell process's file creation mask to mode. If mode begins with a digit, it is interpreted as an octal number; if not, it is interpreted as a symbolic mode mask similar to that accepted by the
chmod command. If mode is
omitted, the current value of the mask is printed. If the `-S'
option is supplied without a mode argument, the mask is printed
in a symbolic format.
unset
unset [-fv] [name]Each variable or function name is removed. If no options are supplied, or the `-v' option is given, each name refers to a shell variable. If the `-f' option is given, the names refer to shell functions, and the function definition is removed. Read-only variables and functions may not be unset.
Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable.
IFS
PATH
HOME
cd builtin
command.
CDPATH
cd command.
MAILPATH
$_ stands for the name of the current mailfile.
MAIL
MAILPATH variable
is not set, Bash informs the user of the arrival of mail in
the specified file.
PS1
PS2
OPTIND
getopts builtin.
OPTARG
getopts builtin.
Bash implements essentially the same grammar, parameter and variable expansion, redirection, and quoting as the Bourne Shell. Bash uses the POSIX 1003.2 standard as the specification of how these features are to be implemented. There are some differences between the traditional Bourne shell and the POSIX standard; this section quickly details the differences of significance. A number of these differences are explained in greater depth in subsequent sections.
Bash is POSIX-conformant, even where the POSIX specification
differs from traditional sh behavior.
Bash has multi-character invocation options (see section Invoking Bash).
Bash has command-line editing (see section Command Line Editing) and
the bind builtin.
Bash has command history (see section Bash History Facilities) and the
history and fc builtins to manipulate it.
Bash implements csh-like history expansion (see section Interactive History Expansion).
Bash has one-dimensional array variables (see section Arrays), and the appropriate variable expansions and assignment syntax to use them. Some of the Bash builtins take options to act on arrays. Bash provides some built-in array variables.
Bash implements the ! keyword to negate the return value of
a pipeline (see section Pipelines).
Very useful when an if statement needs to act only if a test fails.
Bash includes the select compound command, which allows the
generation of simple menus (see section Korn Shell Constructs).
Bash includes brace expansion (see section Brace Expansion) and tilde expansion (see section Tilde Expansion).
Bash implements command aliases and the alias and unalias
builtins (see section Aliases).
Bash provides shell arithmetic and arithmetic expansion (see section Shell Arithmetic).
The POSIX and ksh-style $() form of command substitution
is implemented (see section Command Substitution),
and preferred to the Bourne shell's " (which
is also implemented for backwards compatibility).
Variables present in the shell's initial environment are automatically
exported to child processes. The Bourne shell does not normally do
this unless the variables are explicitly marked using the export
command.
Bash includes the POSIX and ksh-style pattern removal
`%', `#', `%%' and `##' constructs to remove
leading or trailing substrings from variable values
(see section Shell Parameter Expansion).
The expansion ${#xx}, which returns the length of $xx,
is supported (see section Shell Parameter Expansion).
The $'...' quoting syntax, which expands ANSI-C
backslash-escaped characters in the text between the single quotes,
is supported (see section ANSI-C Quoting).
Bash supports the $"..." quoting syntax to do
locale-specific translation of the characters between the double
quotes. The `-D' and `--dump-strings' invocation options
list the translatable strings found in a script
(see section Locale-Specific Translation).
The expansion ${var:length[:offset]},
which expands to the substring of var's value of length
length, optionally beginning at offset, is present
(see section Shell Parameter Expansion).
The expansion
${var/[/]pattern[/replacement]},
which matches pattern and replaces it with replacement in
the value of var, is available (see section Shell Parameter Expansion).
Bash has indirect variable expansion using ${!word}
(see section Shell Parameter Expansion).
Bash can expand positional parameters beyond $9 using
${num}.
Bash has process substitution (see section Process Substitution).
Bash automatically assigns variables that provide information about the
current user (UID and EUID), the current host
(HOSTTYPE, OSTYPE, MACHTYPE, and HOSTNAME),
and the instance of Bash that is running (BASH,
BASH_VERSION, and BASH_VERSINFO. See section Bash Variables,
for details.
The IFS variable is used to split only the results of expansion,
not all words (see section Word Splitting).
This closes a longstanding shell security hole.
It is possible to have a variable and a function with the same name;
sh does not separate the two name spaces.
Bash functions are permitted to have local variables using the
local builtin, and thus useful recursive functions may be written.
Variable assignments preceding commands affect only that command, even
builtins and functions. In sh, all variable assignments
preceding commands are global unless the command is executed from the
file system.
Bash performs filename expansion on filenames specified as operands to output redirection operators.
Bash contains the `<>' redirection operator, allowing a file to be opened for both reading and writing, and the `&>' redirection operator, for directing standard output and standard error to the same file (see section Redirections).
The noclobber option is available to avoid overwriting existing
files with output redirection (see section The Set Builtin).
The `>|' redirection operator may be used to override noclobber.
Bash interprets special backslash-escaped characters in the prompt strings when interactive (see section Controlling the Prompt).
Bash allows you to write a function to override a builtin, and provides
access to that builtin's functionality within the function via the
builtin and command builtins (see section Bash Builtin Commands).
The command builtin allows selective disabling of functions
when command lookup is performed (see section Bash Builtin Commands).
Individual builtins may be enabled or disabled using the enable
builtin (see section Bash Builtin Commands).
The Bash hash builtin allows a name to be associated with
an arbitrary filename, even when that filename cannot be found by
searching the $PATH, using `hash -p'.
Shell functions may be exported to children via the environment (see section Shell Functions).
Bash includes a help builtin for quick reference to shell
facilities (see section Bash Builtin Commands).
The Bash read builtin (see section Bash Builtin Commands)
will read a line ending in `\' with
the `-r' option, and will use the REPLY variable as a
default if no arguments are supplied. The Bash read builtin
also accepts a prompt string with the `-p' option and will use
Readline to obtain the line when given the `-e' option.
Bash includes the shopt builtin, for finer control of shell
optional capabilities (see section Bash Builtin Commands).
Bash has much more optional behavior controllable with the set
builtin (see section The Set Builtin).
The disown builtin can remove a job from the internal shell
job table (see section Job Control Builtins).
The return builtin may be used to abort execution of scripts
executed with the . or source builtins
(see section Bourne Shell Builtins).
The test builtin (see section Bourne Shell Builtins)
is slightly different, as it implements the
POSIX 1003.2 algorithm, which specifies the behavior based on the
number of arguments.
The trap builtin (see section Bourne Shell Builtins)
allows a DEBUG pseudo-signal specification,
similar to EXIT. Commands specified with a DEBUG trap are
executed after every simple command. The DEBUG trap is not
inherited by shell functions.
The Bash export, readonly, and declare builtins can
take a `-f' option to act on shell functions, a `-p' option to
display variables with various attributes set in a format that can be
used as shell input, a `-n' option to remove various variable
attributes, and `name=value' arguments to set variable attributes
and values simultaneously.
The Bash cd and pwd builtins each take `-L' and
`-P' builtins to switch between logical and physical modes.
The Bash type builtin is more extensive and gives more information
about the names it finds.
Bash implements a csh-like directory stack, and provides the
pushd, popd, and dirs builtins to manipulate it.
Bash also makes the directory stack visible as the value of the
DIRSTACK shell variable.
The Bash restricted mode is more useful (see section The Restricted Shell); the SVR4.2 shell restricted mode is too limited.
Bash has the time reserved word and command timing (see section Pipelines).
The display of the timing statistics may be controlled with the
TIMEFORMAT variable.
The SVR4.2 shell has two privilege-related builtins
(mldmode and priv) not present in Bash.
Bash does not have the stop or newgrp builtins.
Bash does not use the SHACCT variable or perform shell accounting.
The SVR4.2 sh uses a TIMEOUT variable like Bash uses
TMOUT.
More features unique to Bash may be found in section Bash Features.
Since Bash is a completely new implementation, it does not suffer from many of the limitations of the SVR4.2 shell. For instance:
if or while
statement.
EOF under certain circumstances.
This can be the cause of some hard-to-find errors.
SIGSEGV. If the shell is started from a process with
SIGSEGV blocked (e.g., by using the system() C library
function call), the shell misbehaves badly.
SIGALRM or
SIGCHLD.
MAILCHECK
variable to be unset.
-x -v);
the SVR4.2 shell allows only one option argument (-xv). In
fact, some versions of the shell dump core if the second argument begins
with a `-'.
POSIX.2 standard.
jsh
(it turns on job control).
The C-Shell (csh) was created by Bill Joy at The
University of California at Berkeley. It
is generally considered to have better features for interactive use than
the original Bourne shell. Some of the csh features present in
Bash include job control, history expansion, `protected' redirection, and
several variables to control the interactive behaviour of the shell
(e.g., IGNOREEOF).
See section Using History Interactively, for details on history expansion.
Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to filename expansion (see section Filename Expansion), but the file names 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,
bash$ echo a{d,c,b}e
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}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Bash has tilde (~) expansion, similar, but not identical, to that of
csh. The following table shows what unquoted words beginning
with a tilde expand to.
~
$HOME.
~/foo
~fred/foo
foo of the home directory of the user
fred.
~+/foo
~-/foo
Bash will also tilde expand words following redirection operators and words following `=' in assignment statements.
Bash has several builtin commands whose definition is very similar
to csh.
pushd
pushd [dir | +N | -N] [-n]Save the current directory on a list and then
cd to
dir. With no
arguments, exchanges the top two directories.
+N
dirs) to the top of the list by rotating
the stack.
-N
dirs) to the top of the list by rotating
the stack.
-n
dir
cds to dir. You can see the saved directory list
with the dirs command.
popd
popd [+N | -N] [-n]Pop the directory stack, and
cd to the new top directory. When
no arguments are given, popd
removes the top directory from the stack and
performs a cd to the new top directory. The
elements are numbered from 0 starting at the first directory listed with
dirs; i.e., popd is equivalent to popd +0.
+N
dirs), starting with zero.
-N
dirs), starting with zero.
-n
dirs
dirs [+N | -N] [-clvp]Display the list of currently remembered directories. Directories find their way onto the list with the
pushd command; you can get
back up through the list with the popd command.
+N
dirs when invoked without options), starting
with zero.
-N
dirs when invoked without options), starting
with zero.
-c
-l
-p
dirs to print the directory stack with one entry per
line.
-v
dirs to print the directory stack with one entry per
line, prepending each entry with its index in the stack.
history
history [-c] [n] history [-anrw] [filename] history -ps argDisplay the history list with line numbers. Lines prefixed with with a `*' have been modified. An argument of n says to list only the last n lines. Options, if supplied, have the following meanings:
-w
-r
-a
-n
-c
-s
-p
HISTFILE variable is used.
logout
source
. (see section Bourne Shell Builtins).
IGNOREEOF
EOFs Bash will read before exiting. By default, Bash will exit
upon reading a single EOF. If IGNOREEOF is not set to
a numeric value, Bash acts as if its value were 10.
This section describes features primarily inspired by the
Korn Shell (ksh). In some cases, the POSIX 1003.2
standard has adopted these commands and variables from the
Korn Shell; Bash implements those features using the POSIX
standard as a guide.
Bash includes the Korn Shell select construct. This construct
allows the easy generation of menus. It has almost the same syntax as
the for command.
The syntax of the select command is:
select name [in words ...]; do commands; 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 words'
is omitted, the positional parameters are printed. The
PS3 prompt is then displayed and a line is 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 select
command completes. Any other value read causes name
to be set to null. The line read is saved in the variable
REPLY.
The commands are executed after each selection until a
break or return command is executed, at which
point the select command completes.
Bash also has adopted command timing from the Korn shell. If the
time reserved word precedes a pipeline or simple command,
timing statistics for the pipeline are displayed when it completes.
The statistics currently consist of elapsed (wall-clock) time and
user and system time consumed by the command's execution.
The use of time as a reserved word permits the timing of
shell builtins, shell functions, and pipelines. An external
time command cannot time these easily.
This section describes Bash builtin commands taken from ksh.
fc
Fix Command. In the first form, a range of commands from first to last is selected from the history list. Both first and last may be specified as a string (to locate the most recent command beginning with that string) or as a number (an index into the history list, where a negative number is used as an offset from the current command number). If last is not specified it is set to first. If first is not specified it is set to the previous command for editing and -16 for listing. If the `-l' flag is given, the commands are listed on standard output. The `-n' flag suppresses the command numbers when listing. The `-r' flag reverses the order of the listing. Otherwise, the editor given by ename is invoked on a file containing those commands. If ename is not given, the value of the following variable expansion is used:fc [-e ename] [-nlr] [first] [last]fc -s [pat=rep] [command]
${FCEDIT:-${EDITOR:-vi}}. This says to use the
value of the FCEDIT variable if set, or the value of the
EDITOR variable if that is set, or vi if neither is set.
When editing is complete, the edited commands are echoed and executed.
In the second form, command is re-executed after each instance
of pat in the selected command is replaced by rep.
A useful alias to use with the fc command is r='fc -s', so
that typing `r cc' runs the last command beginning with cc
and typing `r' re-executes the last command (see section Aliases).
let
let builtin allows arithmetic to be performed on shell variables.
For details, refer to section Arithmetic Builtins.
typeset
typeset command is supplied for compatibility with the Korn
shell; however, it has been deprecated in favor of the
declare command (see section Bash Builtin Commands).
REPLY
read builtin.
RANDOM
SECONDS
PS3
select command. If this variable is not set, the
select command prompts with `#? '
PS4
PWD
cd builtin.
OLDPWD
cd builtin.
TMOUT
LINENO
ENV
FCEDIT
fc builtin command.
The shell maintains a list of aliases
that may be set and unset with the alias and
unalias builtin commands.
The first word of each command, if unquoted,
is checked to see if it has an
alias. If so, that word is replaced by the text of the alias.
The alias name and the replacement text may contain any valid
shell input, including shell metacharacters, with the exception
that the alias name may not contain =.
The first word of the replacement text is tested for
aliases, but a word that is identical to an alias being expanded
is not expanded a second time. This means that one may alias
ls to "ls -F",
for instance, and Bash does not try to recursively expand the
replacement text. If the last character of the alias value is a
space or tab character, then the next command word following the
alias is also checked for alias expansion.
Aliases are created and listed with the alias
command, and removed with the unalias command.
There is no mechanism for using arguments in the replacement text,
as in csh.
If arguments are needed, a shell function should be used
(see section Shell Functions).
Aliases are not expanded when the shell is not interactive,
unless the expand_aliases shell option is set using
shopt (see section Bash Builtin Commands).
The rules concerning the definition and use of aliases are
somewhat confusing. Bash
always reads at least one complete line
of input before executing any
of the commands on that line. Aliases are expanded when a
command is read, not when it is executed. Therefore, an
alias definition appearing on the same line as another
command does not take effect until the next line of input is read.
The commands following the alias definition
on that line are not affected by the new alias.
This behavior is also an issue when functions are executed.
Aliases are expanded when 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.
alias
alias [-p] [name[=value] ...]
Without arguments or with the `-p' option, alias prints
the list of aliases on the standard output in a form that allows
them to be reused as input.
If arguments are supplied, an alias is defined for each name
whose value is given. If no value is given, the name
and value of the alias is printed.
unalias
unalias [-a] [name ... ]Remove each name from the list of aliases. If `-a' is supplied, all aliases are removed.
This section describes features unique to Bash.
bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o option] [argument ...] bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o option] -c string [argument ...] bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o option] [argument ...]
In addition to the single-character shell command-line options (see section The Set Builtin), there are several multi-character options that you can use. These options must appear on the command line before the single-character options in order for them to be recognized.
--dump-strings
--help
--login
csh. If you wanted to replace your
current login shell with a Bash login shell, you would say
`exec bash --login'.
--noediting
--noprofile
--norc
sh.
--posix
--rcfile filename
--restricted
--verbose
--version
There are several single-character options you can give which are
not available with the set builtin.
-c string
$0.
-i
-r
-s
-D
C or POSIX (see section Locale-Specific Translation).
This implies the `-n' option; no commands will be executed.
An interactive shell is one whose input and output are both
connected to terminals (as determined by isatty()), or one
started with the `-i' option.
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 (see section Shell Scripts).
When Bash is invoked in this fashion, $0
is set to the name of the file, and the positional parameters
are set to the remaining arguments.
Bash reads and executes commands from this file, then exits.
Bash's exit status is the exit status of the last command executed
in the script. If no commands are executed, the exit status is 0.
This section describs how bash executes its startup files. If any of the files exist but cannot be read, bash reports an error. Tildes are expanded in file names as described above under Tilde Expansion (see section Tilde Expansion).
When Bash is invoked as a login shell, it first reads and executes commands from the file `/etc/profile', if that file exists. After reading that file, it looks for `~/.bash_profile', `~/.bash_login', and `~/.profile', in that order, and reads and executes commands from the first one that exists and is readable. The `--noprofile' option may be used when the shell is started to inhibit this behavior.
When a login shell exits, Bash reads and executes commands from the file `~/.bash_logout', if it exists.
When an interactive shell that is not a login shell is started, Bash reads and executes commands from `~/.bashrc', if that file exists. This may be inhibited by using the `--norc' option. The `--rcfile file' option will force Bash to read and execute commands from file instead of `~/.bashrc'.
So, typically, your `~/.bash_profile' contains the line
if [ -f `~/.bashrc' ]; then . `~/.bashrc'; fi
after (or before) any login-specific initializations.
When Bash is started non-interactively, to run a shell script,
for example, it looks for the variable BASH_ENV in the environment,
expands its value if it appears there, and uses the expanded value as
the name of a file to read and execute. Bash behaves as if the
following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the
file name.
If Bash is invoked with the name sh, it tries to mimic the
startup behavior of historical versions of sh as closely as
possible, while conforming to the POSIX standard as well.
When invoked as a login shell, 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 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, the ENV variable is expanded and commands are read
and executed from the file whose name is the expanded value.
No other startup files are read.
This is done by both interactive and non-interactive shells.
Bash attempts to determine when it is being run by the remote shell
daemon, usually rshd. If Bash determines it is being run by
rshd, it reads and executes commands from `~/.bashrc', if that
file exists and is readable.
It will not do this if invoked as sh.
The `--norc' option may be used to inhibit this behavior, and the
`--rcfile' option may be used to force another file to be read, but
rshd does not generally invoke the shell with those options or allow
them to be specified.
As defined in section Invoking Bash, an interactive shell
is one whose input and output are both
connected to terminals (as determined by isatty(3)),
or one started with the `-i' option.
You may wish to determine within a startup script whether Bash is
running interactively or not. To do this, examine the variable
$PS1; it is unset in non-interactive shells, and set in
interactive shells. Thus:
if [ -z "$PS1" ]; then
echo This shell is not interactive
else
echo This shell is interactive
fi
This section describes builtin commands which are unique to or have been extended in Bash.
bind
bind [-m keymap] [-lpsvPSV] [-q name] [-r keyseq] bind [-m keymap] -f filename bind [-m keymap] keyseq:function-nameDisplay current Readline (see section Command Line Editing) key and function bindings, or bind a key sequence to a Readline function or macro. The binding syntax accepted is identical to that of `.inputrc' (@xref{Readline Init File}), but each binding must be passed as a separate argument: e.g., `"\C-x\C-r":re-read-init-file'. Options, if supplied, have the following meanings:
-m keymap
emacs,
emacs-standard,
emacs-meta,
emacs-ctlx,
vi,
vi-command, and
vi-insert.
vi is equivalent to vi-command;
emacs is equivalent to emacs-standard.
-l
-p
-P
-v
-V
-s
-S
-f filename
-q
-r keyseq
builtin
builtin [shell-builtin [args]]Run a shell builtin. This is useful when you wish to rename a shell builtin to be a function, but need the functionality of the builtin within the function itself.
command
command [-pVv] command [args ...]Runs command with arg ignoring shell functions. If you have a shell function called
ls, and you wish to call
the command ls, you can say `command ls'. The
`-p' option means to use a default value for $PATH
that is guaranteed to find all of the standard utilities.
If either the `-V' or `-v' option is supplied, a
description of command is printed. The `-v' option
causes a single word indicating the command or file name used to
invoke command to be printed; the `-V' option produces
a more verbose description.
declare
declare [-afFrxi] [-p] [name[=value]]Declare variables and give them attributes. If no names are given, then display the values of variables instead. The `-p' option will display the attributes and values of each name. When `-p' is used, additional options are ignored. The `-F' option inhibits the display of function definitions; only the function name and attributes are printed. `-F' implies `-f'. The following options can be used to restrict output to variables with the specified attributes or to give variables attributes:
-a
-f
-i
-r
-x