bashref.texi

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Commands separated by a @samp{;} 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.

@sc{and} and @sc{or} lists are sequences of one or more pipelines
separated by the control operators @samp{&&} and @samp{||},
respectively.  @sc{and} and @sc{or} lists are executed with left
associativity.

An @sc{and} list has the form
@example
@var{command1} && @var{command2}
@end example

@noindent
@var{command2} is executed if, and only if, @var{command1}
returns an exit status of zero.

An @sc{or} list has the form
@example
@var{command1} || @var{command2}
@end example

@noindent
@var{command2} is executed if, and only if, @var{command1}
returns a non-zero exit status.

The return status of
@sc{and} and @sc{or} lists is the exit status of the last command
executed in the list.

@node Compound Commands
@subsection Compound Commands
@cindex commands, compound

@menu
* Looping Constructs::		Shell commands for iterative action.
* Conditional Constructs::	Shell commands for conditional execution.
* Command Grouping::		Ways to group commands.
@end menu

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 (@pxref{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.

@node Looping Constructs
@subsubsection Looping Constructs
@cindex commands, looping

Bash supports the following looping constructs.

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

@table @code
@item until
@rwindex until
@rwindex do
@rwindex done
The syntax of the @code{until} command is:
@example
until @var{test-commands}; do @var{consequent-commands}; done
@end example
Execute @var{consequent-commands} as long as
@var{test-commands} has an exit status which is not zero.
The return status is the exit status of the last command executed
in @var{consequent-commands}, or zero if none was executed.

@item while
@rwindex while
The syntax of the @code{while} command is:
@example
while @var{test-commands}; do @var{consequent-commands}; done
@end example

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

@item for
@rwindex for
The syntax of the @code{for} command is:

@example
for @var{name} [ [in [@var{words} @dots{}] ] ; ] do @var{commands}; done
@end example
Expand @var{words}, and execute @var{commands} once for each member
in the resultant list, with @var{name} bound to the current member.
If @samp{in @var{words}} is not present, the @code{for} command
executes the @var{commands} once for each positional parameter that is
set, as if @samp{in "$@@"} had been specified
(@pxref{Special Parameters}).
The return status is the exit status of the last command that executes.
If there are no items in the expansion of @var{words}, no commands are
executed, and the return status is zero.

An alternate form of the @code{for} command is also supported:

@example
for (( @var{expr1} ; @var{expr2} ; @var{expr3} )) ; do @var{commands} ; done
@end example
First, the arithmetic expression @var{expr1} is evaluated according
to the rules described below (@pxref{Shell Arithmetic}).
The arithmetic expression @var{expr2} is then evaluated repeatedly
until it evaluates to zero.   
Each time @var{expr2} evaluates to a non-zero value, @var{commands} are
executed and the arithmetic expression @var{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 @var{commands}
that is executed, or false if any of the expressions is invalid.

@end table

The @code{break} and @code{continue} builtins (@pxref{Bourne Shell Builtins})
may be used to control loop execution.

@node Conditional Constructs
@subsubsection Conditional Constructs
@cindex commands, conditional

@table @code
@item if
@rwindex if
@rwindex then
@rwindex else
@rwindex elif
@rwindex fi
The syntax of the @code{if} command is:

@example
if @var{test-commands}; then
  @var{consequent-commands};
[elif @var{more-test-commands}; then
  @var{more-consequents};]
[else @var{alternate-consequents};]
fi
@end example

The @var{test-commands} list is executed, and if its return status is zero,
the @var{consequent-commands} list is executed.
If @var{test-commands} returns a non-zero status, each @code{elif} list
is executed in turn, and if its exit status is zero,
the corresponding @var{more-consequents} is executed and the   
command completes.
If @samp{else @var{alternate-consequents}} is present, and
the final command in the final @code{if} or @code{elif} clause
has a non-zero exit status, then @var{alternate-consequents} is executed.
The return status is the exit status of the last command executed, or
zero if no condition tested true.

@item case
@rwindex case
@rwindex in
@rwindex esac
The syntax of the @code{case} command is:

@example
@code{case @var{word} in [ [(] @var{pattern} [| @var{pattern}]@dots{}) @var{command-list} ;;]@dots{} esac}
@end example

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

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

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

Here is an example using @code{case} in a script that could be used to
describe one interesting feature of an animal:

@example
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."
@end example

@noindent

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

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

@item select
@rwindex select

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

@example
select @var{name} [in @var{words} @dots{}]; do @var{commands}; done
@end example

The list of words following @code{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
@samp{in @var{words}} is omitted, the positional parameters are printed,
as if @samp{in "$@@"} had been specified.
The @env{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 @var{name} is set to that word.
If the line is empty, the words and prompt are displayed again.
If @code{EOF} is read, the @code{select} command completes.
Any other value read causes @var{name} to be set to null.
The line read is saved in the variable @env{REPLY}.

The @var{commands} are executed after each selection until a
@code{break} command is executed, at which
point the @code{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.

@example
select fname in *;
do
	echo you picked $fname \($REPLY\)
	break;
done
@end example

@item ((@dots{}))
@example
(( @var{expression} ))
@end example

The arithmetic @var{expression} is evaluated according to the rules
described below (@pxref{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
@example
let "@var{expression}"
@end example
@noindent
@xref{Bash Builtins}, for a full description of the @code{let} builtin.

@item [[@dots{}]]
@rwindex [[
@rwindex ]]
@example
[[ @var{expression} ]]
@end example

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

When used with @samp{[[}, the @samp{<} and @samp{>} operators sort
lexicographically using the current locale.

When the @samp{==} and @samp{!=} operators are used, the string to the
right of the operator is considered a pattern and matched according
to the rules described below in @ref{Pattern Matching}.
If the shell option @code{nocasematch}
(see the description of @code{shopt} in @ref{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 (@samp{==}) or does not
match (@samp{!=})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, @samp{=~}, is available, with the same
precedence as @samp{==} and @samp{!=}.
When it is used, the string to the right of the operator is considered
an extended regular expression and matched accordingly (as in @i{regex}3)).  
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 @code{nocasematch}
(see the description of @code{shopt} in @ref{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 @code{BASH_REMATCH}.
The element of @code{BASH_REMATCH} with index 0 is the portion of the string
matching the entire regular expression.
The element of @code{BASH_REMATCH} with index @var{n} is the portion of the
string matching the @var{n}th parenthesized subexpression.

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

@table @code
@item ( @var{expression} )
Returns the value of @var{expression}.
This may be used to override the normal precedence of operators.

@item ! @var{expression}
True if @var{expression} is false.

@item @var{expression1} && @var{expression2}
True if both @var{expression1} and @var{expression2} are true.

@item @var{expression1} || @var{expression2}
True if either @var{expression1} or @var{expression2} is true.
@end table
@noindent
The @code{&&} and @code{||} operators do not evaluate @var{expression2} if the
value of @var{expression1} is sufficient to determine the return
value of the entire conditional expression.

@end table

@node Command Grouping
@subsubsection Grouping Commands
@cindex commands, grouping

Bash provides two ways to group a list of commands to be executed