3.2.5 Conditional Constructs
The syntax of the
if command is:
if test-commands; then
[elif more-test-commands; then
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
is executed in turn, and if its exit status is zero,
the corresponding more-consequents is executed and the
If 'else alternate-consequents' is present, and
the final command in the final
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.
The syntax of the
case command is:
case word in
[ [(] pattern [| pattern]...) command-list ;;]
case will selectively execute the command-list corresponding to
the first pattern that matches word.
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 ';;'.
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 ';;'. 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: "
echo -n "The $ANIMAL has "
case $ANIMAL in
horse | dog | cat) echo -n "four";;
man | kangaroo ) echo -n "two";;
*) echo -n "an unknown number of";;
echo " legs."
The return status is zero if no pattern is matched. Otherwise, the
return status is the exit status of the command-list executed.
select construct allows the easy generation of menus.
It has almost the same syntax as the
select name [in words ...]; do
The list of words following
in is expanded, generating a list
of items. The set of expanded words is printed on the standard
error output stream, each preceded by a number. If the
'in words' is omitted, the positional parameters are printed,
as if 'in "$@"' had been specified.
PS3 prompt is then displayed and a line is read from the
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.
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
The commands are executed after each selection until a
break command is executed, at which
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
select fname in *;
echo you picked $fname \($REPLY\)
(( expression ))
The arithmetic expression is evaluated according to the rules
described below (see section 6.5 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
See section 4.2 Bash Builtin Commands, for a full description of the
[[ 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
section 6.4 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.
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 section 188.8.131.52 Pattern Matching.
The return value is 0 if the string matches or does not match
the pattern, respectively, and 1 otherwise.
Any part of the pattern may be quoted to force it to be matched as a
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.
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.
|| operators do not evaluate expression2 if the
value of expression1 is sufficient to determine the return
value of the entire conditional expression.