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 blank
s, 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 blank
s. The parentheses are operators
, and are
recognized as separate tokens by the shell even if they are not separated
from the 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 expands to 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$2c..."
, where c
is the first character of the value of the IFS
variable.
If IFS
is unset, the parameters are separated by spaces.
If IFS
is null, the parameters are joined without intervening
separators.
@
"$@"
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 $0
is set to the first argument after the string to be
executed, if one is present. Otherwise, it is set
to the filename used to invoke Bash, as given by argument zero.
_
Expansion is performed on the command line after it has been split into
token
s. There are seven kinds of expansion performed:
Brace expansion, tilde expansion, and arithmetic expansion are described in other sections. For brace expansion, see section Brace Expansion; for tilde expansion, see section Tilde Expansion; and for arithmetic expansion, see section Arithmetic Expansion.
The order of expansions is: brace expansion, tilde expansion, parameter, variable, and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and filename expansion.
On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as parameter, variable, and arithemtic expansion and command substitution.
Only brace expansion, word splitting, and filename expansion
can change the number of words of the expansion; other expansions
expand a single word to a single word.
The only exceptions to this are the expansions of
"$@"
(see section Special Parameters) and "${name[@]}"
(see section Arrays).
After all expansions, quote removal
(see section Quote Removal)
is performed.
The `$' 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 is 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.
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