Bash Conditional Expressions String comparison and matching
Example
String comparison uses the == operator between quoted strings. The != operator negates the comparison.
if [[ "$string1" == "$string2" ]]; then
echo "\$string1 and \$string2 are identical"
fi
if [[ "$string1" != "$string2" ]]; then
echo "\$string1 and \$string2 are not identical"
fi
If the right-hand side is not quoted then it is a wildcard pattern that $string1 is matched against.
string='abc'
pattern1='a*'
pattern2='x*'
if [[ "$string" == $pattern1 ]]; then
# the test is true
echo "The string $string matches the pattern $pattern"
fi
if [[ "$string" != $pattern2 ]]; then
# the test is false
echo "The string $string does not match the pattern $pattern"
fi
The < and > operators compare the strings in lexicographic order (there are no less-or-equal or greater-or-equal operators for strings).
There are unary tests for the empty string.
if [[ -n "$string" ]]; then
echo "$string is non-empty"
fi
if [[ -z "${string// }" ]]; then
echo "$string is empty or contains only spaces"
fi
if [[ -z "$string" ]]; then
echo "$string is empty"
fi
Above, the -z check may mean $string is unset, or it is set to an empty string. To distinguish between empty and unset, use:
if [[ -n "${string+x}" ]]; then
echo "$string is set, possibly to the empty string"
fi
if [[ -n "${string-x}" ]]; then
echo "$string is either unset or set to a non-empty string"
fi
if [[ -z "${string+x}" ]]; then
echo "$string is unset"
fi
if [[ -z "${string-x}" ]]; then
echo "$string is set to an empty string"
fi
where x is arbitrary. Or in table form:
+-------+-------+-----------+
$string is: | unset | empty | non-empty |
+-----------------------+-------+-------+-----------+
| [[ -z ${string} ]] | true | true | false |
| [[ -z ${string+x} ]] | true | false | false |
| [[ -z ${string-x} ]] | false | true | false |
| [[ -n ${string} ]] | false | false | true |
| [[ -n ${string+x} ]] | false | true | true |
| [[ -n ${string-x} ]] | true | false | true |
+-----------------------+-------+-------+-----------+
Alternatively, the state can be checked in a case statement:
case ${var+x$var} in
(x) echo empty;;
("") echo unset;;
(x*[![:blank:]]*) echo non-blank;;
(*) echo blank
esac
Where [:blank:] is locale specific horizontal spacing characters (tab, space, etc).
