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3. Basic Shell Features

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, The rules for evaluation and quoting are taken from the POSIX 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.


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3.1 Shell Syntax

When the shell reads input, it proceeds through a sequence of operations. If the input indicates the beginning of a comment, the shell ignores the comment symbol (`#'), and the rest of that line.

Otherwise, roughly speaking, the shell reads its input and divides the input into words and operators, employing the quoting rules to select which meanings to assign various words and characters.

The shell then parses these tokens into commands and other constructs, removes the special meaning of certain words or characters, expands others, redirects input and output as needed, executes the specified command, waits for the command's exit status, and makes that exit status available for further inspection or processing.


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3.1.1 Shell Operation

The following is a brief description of the shell's operation when it reads and executes a command. Basically, the shell does the following:

  1. Reads its input from a file (see section Shell Scripts), from a string supplied as an argument to the `-c' invocation option (see section Invoking Bash), or from the user's terminal.
  2. Breaks the input into words and operators, obeying the quoting rules described in Quoting. These tokens are separated by metacharacters. Alias expansion is performed by this step (see section Aliases).
  3. Parses the tokens into simple and compound commands (see section Shell Commands).
  4. Performs the various shell expansions (see section Shell Expansions), breaking the expanded tokens into lists of filenames (see section Filename Expansion) and commands and arguments.
  5. Performs any necessary redirections (see section Redirections) and removes the redirection operators and their operands from the argument list.
  6. Executes the command (see section Executing Commands).
  7. Optionally waits for the command to complete and collects its exit status (see section Exit Status).

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3.1.2 Quoting

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 it is to represent itself. When the command history expansion facilities are being used (see section History Expansion), the history expansion character, usually `!', must be quoted to prevent history expansion. See section Bash History Facilities, for more details concerning history expansion.

There are three quoting mechanisms: the escape character, single quotes, and double quotes.


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3.1.2.1 Escape Character

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 itself is not quoted, the \newline is treated as a line continuation (that is, it is removed from the input stream and effectively ignored).


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3.1.2.2 Single Quotes

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.


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3.1.2.3 Double Quotes

Enclosing characters in double quotes (`"') preserves the literal value of all characters within the quotes, with the exception of `$', ``', `\', and, when history expansion is enabled, `!'. The characters `$' and ``' retain their special meaning within double quotes (see section Shell Expansions). The backslash retains its special meaning only when followed by one of the following characters: `$', ``', `"', `\', or newline. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an `!' appearing in double quotes is escaped using a backslash. The backslash preceding the `!' is not removed.

The special parameters `*' and `@' have special meaning when in double quotes (see section Shell Parameter Expansion).


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3.1.2.4 ANSI-C Quoting

Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:

\a

alert (bell)

\b

backspace

\e

an escape character (not ANSI C)

\f

form feed

\n

newline

\r

carriage return

\t

horizontal tab

\v

vertical tab

\\

backslash

\'

single quote

\nnn

the eight-bit character whose value is the octal value nnn (one to three digits)

\xHH

the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)

\cx

a control-x character

The expanded result is single-quoted, as if the dollar sign had not been present.


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3.1.2.5 Locale-Specific Translation

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.

Some systems use the message catalog selected by the LC_MESSAGES shell variable. Others create the name of the message catalog from the value of the TEXTDOMAIN shell variable, possibly adding a suffix of `.mo'. If you use the TEXTDOMAIN variable, you may need to set the TEXTDOMAINDIR variable to the location of the message catalog files. Still others use both variables in this fashion: TEXTDOMAINDIR/LC_MESSAGES/LC_MESSAGES/TEXTDOMAIN.mo.


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3.1.3 Comments

In a non-interactive shell, or an interactive shell in which the interactive_comments option to the shopt builtin is enabled (see section The Shopt Builtin), 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. See section Interactive Shells, for a description of what makes a shell interactive.


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3.2 Shell Commands

A simple shell command such as echo a b c consists of the command itself followed by arguments, separated by spaces.

More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping.


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3.2.1 Simple Commands

A simple command is the kind of command encountered most often. It's just a sequence of words separated by blanks, terminated by one of the shell's control operators (see section Definitions). The first word generally specifies a command to be executed, with the rest of the words being that command's arguments.

The return status (see section Exit Status) of a simple command is its exit status as provided by the POSIX 1003.1 waitpid function, or 128+n if the command was terminated by signal n.


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3.2.2 Pipelines

A pipeline is a sequence of simple commands separated by one of the control operators `|' or `|&'.

The format for a pipeline is

 
[time [-p]] [!] command1 [ [| or |&] command2 …]

The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command's output. This connection is performed before any redirections specified by the command.

If `|&' is used, the standard error of command1 is connected to command2's standard input through the pipe; it is shorthand for 2>&1 |. This implicit redirection of the standard error is performed after any redirections specified by the command.

The reserved word time causes timing statistics to be printed for the pipeline once it finishes. The statistics currently consist of elapsed (wall-clock) time and user and system time consumed by the command's execution. 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. 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.

If the pipeline is not executed asynchronously (see section Lists of Commands), the shell waits for all commands in the pipeline to complete.

Each command in a pipeline is executed in its own subshell (see section Command Execution Environment). The exit status of a pipeline is the exit status of the last command in the pipeline, unless the pipefail option is enabled (see section The Set Builtin). If pipefail is enabled, the pipeline's return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved word `!' precedes the pipeline, the exit status is the logical negation of the exit status as described above. The shell waits for all commands in the pipeline to terminate before returning a value.


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3.2.3 Lists of Commands

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.

A sequence of one or more newlines may appear in a list to delimit commands, equivalent to a semicolon.

If a command is terminated by the control operator `&', the shell executes the command asynchronously in a subshell. This is known as executing the command in the background. The shell does not wait for the command to finish, and the return status is 0 (true). When job control is not active (see section Job Control), the standard input for asynchronous commands, in the absence of any explicit redirections, is redirected from /dev/null.

Commands separated by a `;' are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.

AND and OR lists are sequences of one or more pipelines separated by the control operators `&&' and `||', respectively. AND and OR lists are executed with left associativity.

An AND list has the form

 
command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form

 
command1 || command2

command2 is executed if, and only if, command1 returns a non-zero exit status.

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


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3.2.4 Compound Commands

Compound commands are the shell programming constructs. Each construct begins with a reserved word or control operator and is terminated by a corresponding reserved word or operator. Any redirections (see section Redirections) associated with a compound command apply to all commands within that compound command unless explicitly overridden.

Bash provides looping constructs, conditional commands, and mechanisms to group commands and execute them as a unit.


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3.2.4.1 Looping Constructs

Bash supports the following looping constructs.

Note that wherever a `;' appears in the description of a command's syntax, it may be replaced with one or more newlines.

until

The syntax of the until command is:

 
until test-commands; do consequent-commands; done

Execute consequent-commands as long as test-commands has an exit status which is not zero. The return status is the exit status of the last command executed in consequent-commands, or zero if none was executed.

while

The syntax of the while command is:

 
while test-commands; do consequent-commands; done

Execute consequent-commands as long as test-commands has an exit status of zero. The return status is the exit status of the last command executed in consequent-commands, or zero if none was executed.

for

The syntax of the for command is:

 
for name [in words …]; do commands; done

Expand words, and execute commands once for each member in the resultant list, with name bound to the current member. If `in words' is not present, the for command executes the commands once for each positional parameter that is set, as if `in "$@"' had been specified (see section Special Parameters). The return status is the exit status of the last command that executes. If there are no items in the expansion of words, no commands are executed, and the return status is zero.

An alternate form of the for command is also supported:

 
for (( expr1 ; expr2 ; expr3 )) ; do commands ; done

First, the arithmetic expression expr1 is evaluated according to the rules described below (see section Shell Arithmetic). The arithmetic expression expr2 is then evaluated repeatedly until it evaluates to zero. Each time expr2 evaluates to a non-zero value, commands are executed and the arithmetic expression expr3 is evaluated. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in list that is executed, or false if any of the expressions is invalid.

The break and continue builtins (see section Bourne Shell Builtins) may be used to control loop execution.


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3.2.4.2 Conditional Constructs

if

The syntax of the if command is:

 
if test-commands; then
  consequent-commands;
[elif more-test-commands; then
  more-consequents;]
[else alternate-consequents;]
fi

The test-commands list is executed, and if its return status is zero, the consequent-commands list is executed. If test-commands returns a non-zero status, 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 alternate-consequents is executed. The return status is the exit status of the last command executed, or zero if no condition tested true.

case

The syntax of the case command is:

 
case word in [ [(] pattern [| pattern]…) command-list ;;]… esac

case will selectively execute the command-list corresponding to the first pattern that matches word. If the shell option nocasematch (see the description of shopt in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. The `|' is used to separate multiple patterns, and the `)' operator terminates a pattern list. A list of patterns and an associated command-list is known as a clause.

Each clause must be terminated with `;;', `,&', or `;;&'. The word undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before matching is attempted. Each pattern undergoes tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

There may be an arbitrary number of case clauses, each terminated by a `;;', `;&', or `;;&'. The first pattern that matches determines the command-list that is executed.

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."

If the `;;' operator is used, no subsequent matches are attempted after the first pattern match. Using `;&' in place of `;;' causes execution to continue with the command-list associated with the next clause, if any. Using `;;&' in place of `;;' causes the shell to test the patterns in the next clause, if any, and execute any associated command-list on a successful match.

The return status is zero if no pattern is matched. Otherwise, the return status is the exit status of the command-list executed.

select

The select construct allows the easy generation of menus. It has almost the same syntax as the for command:

 
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 output stream, each preceded by a number. If the `in words' is omitted, the positional parameters are printed, as if `in "$@"' had been specified. 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 command is executed, at which point the select command completes.

Here is an example that allows the user to pick a filename from the current directory, and displays the name and index of the file selected.

 
select fname in *;
do
	echo you picked $fname \($REPLY\)
	break;
done
((…))
 
(( expression ))

The arithmetic expression is evaluated according to the rules described below (see section Shell Arithmetic). 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"

See section Bash Builtin Commands, for a full description of the let builtin.

[[…]]
 
[[ expression ]]

Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below in Bash Conditional Expressions. Word splitting and filename expansion are not performed on the words between the `[[' and `]]'; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as `-f' must be unquoted to be recognized as primaries.

When the `==' and `!=' operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below in Pattern Matching. If the shell option nocasematch (see the description of shopt in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. The return value is 0 if the string matches (`==') or does not match (`!=')the pattern, and 1 otherwise. Any part of the pattern may be quoted to force it to be matched as a string.

An additional binary operator, `=~', is available, with the same precedence as `==' and `!='. When it is used, the string to the right of the operator is considered an extended regular expression and matched accordingly (as in regex3)). The return value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the conditional expression's return value is 2. If the shell option nocasematch (see the description of shopt in The Shopt Builtin) is enabled, the match is performed without regard to the case of alphabetic characters. Any part of the pattern may be quoted to force it to be matched as a string. Substrings matched by parenthesized subexpressions within the regular expression are saved in the array variable BASH_REMATCH. The element of BASH_REMATCH with index 0 is the portion of the string matching the entire regular expression. The element of BASH_REMATCH with index n is the portion of the string matching the nth parenthesized subexpression.

Expressions may be combined using the following operators, listed in decreasing order of precedence:

( expression )

Returns the value of expression. This may be used to override the normal precedence of operators.

! expression

True if expression is false.

expression1 && expression2

True if both expression1 and expression2 are true.

expression1 || expression2

True if either expression1 or expression2 is true.

The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.


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3.2.4.3 Grouping Commands

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 environment to be created (see section Command Execution Environment), and each of the commands in list 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 (or newline) 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 or other shell metacharacters. 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.


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3.2.5 Coprocesses

A coprocess is a shell command preceded by the coproc reserved word. A coprocess is executed asynchronously in a subshell, as if the command had been terminated with the `&' control operator, with a two-way pipe established between the executing shell and the coprocess.

The format for a coprocess is:

 
coproc [NAME] command [redirections]

This creates a coprocess named NAME. If NAME is not supplied, the default name is COPROC. NAME must not be supplied if command is a simple command (see section Simple Commands); otherwise, it is interpreted as the first word of the simple command.

When the coproc is executed, the shell creates an array variable (see section Arrays) named NAME in the context of the executing shell. The standard output of command is connected via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned to NAME[0]. The standard input of command is connected via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned to NAME[1]. This pipe is established before any redirections specified by the command (see section Redirections). The file descriptors can be utilized as arguments to shell commands and redirections using standard word expansions.

The process id of the shell spawned to execute the coprocess is available as the value of the variable NAME_PID. The wait builtin command may be used to wait for the coprocess to terminate.

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


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3.3 Shell Functions

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. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. 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 () compound-command [ redirections ]

This defines a shell function named name. The reserved word function is optional. If the function reserved word is supplied, the parentheses are optional. The body of the function is the compound command compound-command (see section Compound Commands). That command is usually a list enclosed between { and }, but may be any compound command listed above. compound-command is executed whenever name is specified as the name of a command. Any redirections (see section Redirections) associated with the shell function are performed when the function is executed.

A function definition may be deleted using the `-f' option to the unset builtin (see section Bourne Shell Builtins).

The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body.

Note that for historical reasons, in the most common usage the curly braces that surround the body of the function must be separated from the body by blanks or newlines. This is because the braces are reserved words and are only recognized as such when they are separated from the command list by whitespace or another shell metacharacter. Also, when using the braces, the list must be terminated by a semicolon, a `&', or a newline.

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. Special parameter 0 is unchanged. The first element of the FUNCNAME variable is set to the name of the function while the function is executing. All other aspects of the shell execution environment are identical between a function and its caller with the exception that the DEBUG and RETURN traps are not inherited unless the function has been given the trace attribute using the declare builtin or the -o functrace option has been enabled with the set builtin, (in which case all functions inherit the DEBUG and RETURN traps). See section Bourne Shell Builtins, for the description of the trap builtin.

If the builtin command return is executed in a function, the function completes and execution resumes with the next command after the function call. Any command associated with the RETURN trap is executed before execution resumes. When a function completes, the values of the positional parameters and the special parameter `#' are restored to the values they had prior to the function's execution. If a numeric argument is given to return, that is the function's return status; otherwise the function's return status is the exit status of the last command executed before the return.

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.

Function names and definitions may be listed with the `-f' option to the declare or typeset builtin commands (see section Bash Builtin Commands). The `-F' option to declare or typeset will list the function names only (and optionally the source file and line number, if the extdebug shell option is enabled). Functions may be exported so that subshells automatically have them defined with the `-f' option to the export builtin (see section Bourne Shell Builtins). Note that shell functions and variables with the same name may result in multiple identically-named entries in the environment passed to the shell's children. Care should be taken in cases where this may cause a problem.

Functions may be recursive. No limit is placed on the number of recursive calls.


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3.4 Shell Parameters

A parameter is an entity that stores values. It can be a name, a number, or one of the special characters listed below. A variable is a parameter denoted by a name. A variable has a value and zero or more attributes. Attributes are assigned using the declare builtin command (see the description of the declare builtin in Bash Builtin Commands).

A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the unset builtin command.

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 integer attribute set, then value is evaluated as an arithmetic expression even if the $((…)) expansion is not used (see section Arithmetic Expansion). Word splitting is not performed, with the exception of "$@" as explained below. Filename expansion is not performed. Assignment statements may also appear as arguments to the alias, declare, typeset, export, readonly, and local builtin commands.

In the context where an assignment statement is assigning a value to a shell variable or array index (see section Arrays), the `+=' operator can be used to append to or add to the variable's previous value. When `+=' is applied to a variable for which the integer attribute has been set, value is evaluated as an arithmetic expression and added to the variable's current value, which is also evaluated. When `+=' is applied to an array variable using compound assignment (see section Arrays), the variable's value is not unset (as it is when using `='), and new values are appended to the array beginning at one greater than the array's maximum index (for indexed arrays), or added as additional key-value pairs in an associative array. When applied to a string-valued variable, value is expanded and appended to the variable's value.


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3.4.1 Positional Parameters

A positional parameter is a parameter denoted by one or more digits, other than the single digit 0. Positional parameters are assigned from the shell's arguments when it is invoked, and may be reassigned using the set builtin command. Positional parameter N may be referenced as ${N}, or as $N when N consists of a single digit. Positional parameters may not be assigned to with assignment statements. The set and shift builtins are used to set and unset them (see section Shell Builtin Commands). 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.


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3.4.2 Special Parameters

The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.

*

Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS special variable. That is, "$*" is equivalent to "$1c$2c…", where c is the first character of the value of the IFS variable. If IFS is unset, the parameters are separated by spaces. If IFS is null, the parameters are joined without intervening separators.

@

Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each parameter expands to a separate word. That is, "$@" is equivalent to "$1" "$2" …. If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expansion of the last parameter is joined with the last part of the original word. When there are no positional parameters, "$@" and $@ expand to nothing (i.e., they are removed).

#

Expands to the number of positional parameters in decimal.

?

Expands to the exit status of the most recently executed foreground pipeline.

-

(A hyphen.) Expands to the current option flags as specified upon invocation, by the set builtin command, or those set by the shell itself (such as the `-i' option).

$

Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the invoking shell, not the subshell.

!

Expands to the process ID of the most recently executed background (asynchronous) command.

0

Expands to the name of the shell or shell script. This is set at shell initialization. If Bash is invoked with a file of commands (see section Shell Scripts), $0 is set to the name of that file. If Bash is started with the `-c' option (see section Invoking Bash), 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.

_

(An underscore.) At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, expands to the last argument to the previous command, after expansion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, this parameter holds the name of the mail file.


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3.5 Shell Expansions

Expansion is performed on the command line after it has been split into tokens. There are seven kinds of expansion performed:

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 arithmetic 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.


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3.5.1 Brace 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 either a series of comma-separated strings or a seqeunce expression between a pair of braces, followed by an optional postscript. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right.

Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example,

 
bash$ echo a{d,c,b}e
ade ace abe

A sequence expression takes the form {x..y[incr]}, where x and y are either integers or single characters, and incr, an optional increment, is an integer. When integers are supplied, the expression expands to each number between x and y, inclusive. Supplied integers may be prefixed with `0' to force each term to have the same width. When either x or y begins with a zero, the shell attempts to force all generated terms to contain the same number of digits, zero-padding where necessary. When characters are supplied, the expression expands to each character lexicographically between x and y, inclusive. Note that both x and y must be of the same type. When the increment is supplied, it is used as the difference between each term. The default increment is 1 or -1 as appropriate.

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. To avoid conflicts with parameter expansion, the string `${' is not considered eligible for brace expansion.

A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged.

A { or `,' may be quoted with a backslash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string `${' is not considered eligible for brace expansion.

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}}

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3.5.2 Tilde Expansion

If a word begins with an unquoted tilde character (`~'), all of the characters up to the first unquoted slash (or all characters, if there is no unquoted slash) are considered a tilde-prefix. If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following the tilde are treated as a possible login name. If this login name is the null string, the tilde is replaced with the value of the HOME shell variable. If HOME is unset, the home directory of the user executing the shell is substituted instead. Otherwise, the tilde-prefix is replaced with the home directory associated with the specified login name.

If the tilde-prefix is `~+', the value of the shell variable PWD replaces the tilde-prefix. If the tilde-prefix is `~-', the value of the shell variable OLDPWD, if it is set, is substituted.

If the characters following the tilde in the tilde-prefix consist of a number N, optionally prefixed by a `+' or a `-', the tilde-prefix is replaced with the corresponding element from the directory stack, as it would be displayed by the dirs builtin invoked with the characters following tilde in the tilde-prefix as an argument (see section The Directory Stack). If the tilde-prefix, sans the tilde, consists of a number without a leading `+' or `-', `+' is assumed.

If the login name is invalid, or the tilde expansion fails, the word is left unchanged.

Each variable assignment is checked for unquoted tilde-prefixes immediately following a `:' or the first `='. In these cases, tilde expansion is also performed. Consequently, one may use file names with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the expanded value.

The following table shows how Bash treats unquoted tilde-prefixes:

~

The value of $HOME

~/foo

`$HOME/foo'

~fred/foo

The subdirectory foo of the home directory of the user fred

~+/foo

`$PWD/foo'

~-/foo

`${OLDPWD-'~-'}/foo'

~N

The string that would be displayed by `dirs +N'

~+N

The string that would be displayed by `dirs +N'

~-N

The string that would be displayed by `dirs -N'


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3.5.3 Shell Parameter Expansion

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.

When braces are used, the matching ending brace is the first `}' not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion.

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. The exceptions to this are the expansions of ${!prefix*} and ${!name[@]} described below. The exclamation point must immediately follow the left brace in order to introduce indirection.

In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

When not performing substring expansion, using the form described below, Bash tests for a parameter that is unset or null. Omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both parameter's existence and that its value is not null; if the colon is omitted, the operator tests only for existence.

${parameter:-word}

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

${parameter:=word}

If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.

${parameter:?word}

If parameter is null or unset, the expansion of word (or a message to that effect if word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of parameter is substituted.

${parameter:+word}

If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted.

${parameter:offset}
${parameter:offset:length}

Expands to up to length characters of parameter starting at the character specified by offset. If length is omitted, expands to the substring of parameter starting at the character specified by offset. length and offset are arithmetic expressions (see section Shell Arithmetic). This is referred to as Substring Expansion.

length must evaluate to a number greater than or equal to zero. If offset evaluates to a number less than zero, the value is used as an offset from the end of the value of parameter. If parameter is `@', the result is length positional parameters beginning at offset. If parameter is an indexed array name subscripted by `@' or `*', the result is the length members of the array beginning with ${parameter[offset]}. A negative offset is taken relative to one greater than the maximum index of the specified array. Substring expansion applied to an associative array produces undefined results.

Note that a negative offset must be separated from the colon by at least one space to avoid being confused with the `:-' expansion. Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1 by default. If offset is 0, and the positional parameters are used, $@ is prefixed to the list.

${!prefix*}
${!prefix@}

Expands to the names of variables whose names begin with prefix, separated by the first character of the IFS special variable. When `@' is used and the expansion appears within double quotes, each variable name expands to a separate word.

${!name[@]}
${!name[*]}

If name is an array variable, expands to the list of array indices (keys) assigned in name. If name is not an array, expands to 0 if name is set and null otherwise. When `@' is used and the expansion appears within double quotes, each key expands to a separate word.

${#parameter}

The length in characters of the expanded value of parameter is substituted. If parameter is `*' or `@', the value substituted is the number of positional parameters. If parameter is an array name subscripted by `*' or `@', the value substituted is the number of elements in the array.

${parameter#word}
${parameter##word}

The word is expanded to produce a pattern just as in filename expansion (see section Filename Expansion). If the pattern matches the beginning of the expanded value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the `#' case) or the longest matching pattern (the `##' case) deleted. If parameter is `@' or `*', the pattern removal 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 pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.

${parameter%word}
${parameter%%word}

The word is expanded to produce a pattern just as in filename expansion. If the pattern matches a trailing portion of the expanded value of parameter, then the result of the expansion is the value of parameter with the shortest matching pattern (the `%' case) or the longest matching pattern (the `%%' case) deleted. If parameter is `@' or `*', the pattern removal 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 pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.

${parameter/pattern/string}

The pattern is expanded to produce a pattern just as in filename expansion. Parameter is expanded and the longest match of pattern against its value is replaced with string. If pattern begins with `/', all matches of pattern are replaced with string. Normally only the first match is replaced. If pattern begins with `#', it must match at the beginning of the expanded value of parameter. If pattern begins with `%', it must match at the end of the expanded value of parameter. If string is null, matches of pattern are deleted and the / 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.

${parameter^pattern}
${parameter^^pattern}
${parameter,pattern}
${parameter,,pattern}

This expansion modifies the case of alphabetic characters in parameter. The pattern is expanded to produce a pattern just as in pathname expansion. The `^' operator converts lowercase letters matching pattern to uppercase; the `,' operator converts matching uppercase letters to lowercase. The `^^' and `,,' expansions convert each matched character in the expanded value; the `^' and `,' expansions match and convert only the first character in the expanded value. If pattern is omitted, it is treated like a `?', which matches every character. If parameter is `@' or `*', the case modification 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 case modification operation is applied to each member of the array in turn, and the expansion is the resultant list.


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3.5.4 Command Substitution

Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed as follows:

 
$(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. Embedded newlines are not deleted, but they may be removed during word splitting. The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).

When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by `$', ``', or `\'. The first backquote not preceded by a backslash terminates the command substitution. 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 backquoted 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.


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3.5.5 Arithmetic Expansion

Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is:

 
$(( expression ))

The expression is treated as if it were within double quotes, but a double quote inside the parentheses is not treated specially. All tokens in the expression undergo parameter expansion, command substitution, and quote removal. Arithmetic expansions may be nested.

The evaluation is performed according to the rules listed below (see section Shell Arithmetic). If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs.


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3.5.6 Process Substitution

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. Note that no space may appear between the < or > and the left parenthesis, otherwise the construct would be interpreted as a redirection.

When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.


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3.5.7 Word Splitting

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 sequences of <space>, <tab>, and <newline> at the beginning and end of the results of the previous expansions are ignored, and any sequence of IFS characters not at the beginning or end 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.


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3.5.8 Filename Expansion

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 nullglob option is set, and no matches are found, the word is removed. If the failglob shell option is set, and no matches are found, an error message is printed and the command is not executed. If the shell option nocaseglob is enabled, the match is performed without regard to the case of alphabetic characters.

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. When matching a file name, the slash character must always be matched explicitly. In other cases, the `.' character is not treated specially.

See the description of shopt in The Shopt Builtin, for a description of the nocaseglob, nullglob, failglob, 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 when GLOBIGNORE is set and not null. However, setting GLOBIGNORE to a non-null value 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.


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3.5.8.1 Pattern Matching

Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The NUL character may not occur in a pattern. A backslash escapes the following character; the escaping backslash is discarded when matching. The special pattern characters must be quoted if they are to be matched literally.

The special pattern characters have the following meanings:

*

Matches any string, including the null string. When the globstar shell option is enabled, and `*' is used in a filename expansion context, two adjacent `*'s used as a single pattern will match all files and zero or more directories and subdirectories. If followed by a `/', two adjacent `*'s will match only directories and subdirectories.

?

Matches any single character.

[…]

Matches any one of the enclosed characters. A pair of characters separated by a hyphen denotes a range expression; any character that sorts between those two characters, inclusive, using the current locale's collating sequence and character set, is matched. If the first character following the `[' is a `!' or a `^' then any character not enclosed is matched. A `-' may be matched by including it as the first or last character in the set. A `]' may be matched by including it as the first character in the set. The sorting order of characters in range expressions is determined by the current locale and the value of the LC_COLLATE shell variable, if set.

For example, in the default C locale, `[a-dx-z]' is equivalent to `[abcdxyz]'. Many locales sort characters in dictionary order, and in these locales `[a-dx-z]' is typically not equivalent to `[abcdxyz]'; it might be equivalent to `[aBbCcDdxXyYz]', for example. To obtain the traditional interpretation of ranges in bracket expressions, you can force the use of the C locale by setting the LC_COLLATE or LC_ALL environment variable to the value `C'.

Within `[' and `]', character classes can be specified using the syntax [:class:], where class is one of the following classes defined in the POSIX standard:

 
alnum   alpha   ascii   blank   cntrl   digit   graph   lower
print   punct   space   upper   word    xdigit

A character class matches any character belonging to that class. The word character class matches letters, digits, and the character `_'.

Within `[' and `]', an equivalence class can be specified using the syntax [=c=], which matches all characters with the same collation weight (as defined by the current locale) as the character c.

Within `[' and `]', the syntax [.symbol.] matches the collating symbol symbol.

If the extglob shell option is enabled using the shopt builtin, several extended pattern matching operators are recognized. In the following description, a pattern-list is a list of one or more patterns separated by a `|'. Composite patterns may be formed using one or more of the following sub-patterns:

?(pattern-list)

Matches zero or one occurrence of the given patterns.

*(pattern-list)

Matches zero or more occurrences of the given patterns.

+(pattern-list)

Matches one or more occurrences of the given patterns.

@(pattern-list)

Matches one of the given patterns.

!(pattern-list)

Matches anything except one of the given patterns.


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3.5.9 Quote Removal

After the preceding expansions, all unquoted occurrences of the characters `\', `'', and `"' that did not result from one of the above expansions are removed.


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3.6 Redirections

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 following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. 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 (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command

 
ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard error was made a copy of the standard output before the standard output was redirected to dirlist.

Bash handles several filenames specially when they are used in redirections, as described in the following table:

/dev/fd/fd

If fd is a valid integer, file descriptor fd is duplicated.

/dev/stdin

File descriptor 0 is duplicated.

/dev/stdout

File descriptor 1 is duplicated.

/dev/stderr

File descriptor 2 is duplicated.

/dev/tcp/host/port

If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open a TCP connection to the corresponding socket.

/dev/udp/host/port

If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open a UDP connection to the corresponding socket.

A failure to open or create a file causes the redirection to fail.

Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally.


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3.6.1 Redirecting Input

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

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3.6.2 Redirecting Output

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 noclobber option to the set builtin has been enabled, the redirection will fail if the file whose name results from the expansion of word exists and is a regular file. If the redirection operator is `>|', or the redirection operator is `>' and the noclobber option is not enabled, the redirection is attempted even if the file named by word exists.


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3.6.3 Appending Redirected Output

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

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3.6.4 Redirecting Standard Output and Standard Error

This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of word.

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

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3.6.5 Appending Standard Output and Standard Error

This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be appended to the file whose name is the expansion of word.

The format for appending standard output and standard error is:

 
&>>word

This is semantically equivalent to

 
>>word 2>&1

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3.6.6 Here Documents

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:

 
<<[-]word
        here-document
delimiter

No parameter expansion, command substitution, arithmetic expansion, or filename 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. If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion. In the latter case, the character sequence \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.


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3.6.7 Here Strings

A variant of here documents, the format is:

 
<<< word

The word is expanded and supplied to the command on its standard input.


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3.6.8 Duplicating File Descriptors

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 the digits in word do not specify a file descriptor open for input, a redirection error occurs. 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. If the digits in word do not specify a file descriptor open for output, a redirection error occurs. 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.


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3.6.9 Moving File Descriptors

The redirection operator

 
[n]<&digit-

moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n is not specified. digit is closed after being duplicated to n.

Similarly, the redirection operator

 
[n]>&digit-

moves the file descriptor digit to file descriptor n, or the standard output (file descriptor 1) if n is not specified.


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3.6.10 Opening File Descriptors for Reading and Writing

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.


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3.7 Executing Commands


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3.7.1 Simple Command Expansion

When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right.

  1. The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing.
  2. The words that are not variable assignments or redirections are expanded (see section Shell Expansions). If any words remain after expansion, the first word is taken to be the name of the command and the remaining words are the arguments.
  3. Redirections are performed as described above (see section Redirections).
  4. The text after the `=' in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable.

If no command name results, the variable assignments affect the current shell environment. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status.

If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status.

If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. If there were no command substitutions, the command exits with a status of zero.


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3.7.2 Command Search and Execution

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.

  1. If the command name contains no slashes, the shell attempts to locate it. If there exists a shell function by that name, that function is invoked as described in Shell Functions.
  2. If the name does not match a function, the shell searches for it in the list of shell builtins. If a match is found, that builtin is invoked.
  3. If the name is neither a shell function nor a builtin, and contains no slashes, Bash searches each element of $PATH for a directory containing an executable file by that name. Bash uses a hash table to remember the full pathnames of executable files to avoid multiple PATH searches (see the description of hash in Bourne Shell Builtins). 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 searches for a defined shell function named command_not_found_handle. If that function exists, it is invoked with the original command and the original command's arguments as its arguments, and the function's exit status becomes the exit status of the shell. If that function is not defined, the shell prints an error message and returns an exit status of 127.
  4. If the search is successful, or if the command name contains one or more slashes, the shell executes the named program in a separate execution environment. Argument 0 is set to the name given, and the remaining arguments to the command are set to the arguments supplied, if any.
  5. If this execution fails because the file is not in executable format, and the file is not a directory, it is assumed to be a shell script and the shell executes it as described in Shell Scripts.
  6. If the command was not begun asynchronously, the shell waits for the command to complete and collects its exit status.

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3.7.3 Command Execution Environment

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

When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell.

A command invoked in this separate environment cannot affect the shell's execution environment.

Command substitution, commands grouped with parentheses, and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline are also executed in a subshell environment. Changes made to the subshell environment cannot affect the shell's execution environment.

Subshells spawned to execute command substitutions inherit the value of the `-e' option from the parent shell. When not in POSIX mode, Bash clears the `-e' option in such subshells.

If a command is followed by a `&' and job control is not active, the default standard input for the command is the empty file `/dev/null'. Otherwise, the invoked command inherits the file descriptors of the calling shell as modified by redirections.


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3.7.4 Environment

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 provides several ways to manipulate the environment. 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 and `export -n' commands, 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 Shell Parameters. These assignment statements affect only the environment seen by that command.

If the `-k' option 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.


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3.7.5 Exit Status

The exit status of an executed command is the value returned by the waitpid system call or equivalent function. Exit statuses fall between 0 and 255, though, as explained below, the shell may use values above 125 specially. Exit statuses from shell builtins and compound commands are also limited to this range. Under certain circumstances, the shell will use special values to indicate specific failure modes.

For the shell's purposes, 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.

If a command fails because of an error during expansion or redirection, the exit status is greater than zero.

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. All builtins return an exit status of 2 to indicate incorrect usage.


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3.7.6 Signals

When Bash is interactive, in the absence of any traps, it ignores SIGTERM (so that `kill 0' does not kill an interactive shell), and SIGINT is caught and handled (so that the wait builtin is 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.

Non-builtin commands started by Bash have signal handlers set to the values inherited by the shell from its parent. When job control is not in effect, asynchronous commands ignore SIGINT and SIGQUIT in addition to these inherited handlers. 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, an interactive shell resends the SIGHUP to all jobs, running or stopped. Stopped jobs are sent SIGCONT to ensure that they receive the SIGHUP. To prevent the shell from sending the SIGHUP signal to a particular job, it should be removed from the jobs table with the disown builtin (see section Job Control Builtins) or marked to not receive SIGHUP using disown -h.

If the huponexit shell option has been set with shopt (see section The Shopt Builtin), Bash sends a SIGHUP to all jobs when an interactive login shell exits.

If Bash is waiting for a command to complete and receives a signal for which a trap has been set, the trap will not be executed until the command completes. When Bash is waiting for an asynchronous command via the wait builtin, the reception of a signal for which a trap has been set will cause the wait builtin to return immediately with an exit status greater than 128, immediately after which the trap is executed.


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3.8 Shell Scripts

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. The shell first searches for the file in the current directory, and looks in the directories in $PATH if not found there.

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, with the exception that the locations of commands remembered by the parent (see the description of hash in Bourne Shell Builtins) are retained by the child.

Most versions of Unix make this a part of the operating system'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. Thus, you can specify Bash, awk, Perl, or some other interpreter and write the rest of the script file in that language.

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.

Bash scripts often begin with #! /bin/bash (assuming that Bash has been installed in `/bin'), since this ensures that Bash will be used to interpret the script, even if it is executed under another shell.


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