String Test Operators
=
, ==
Return true if strings are equal. Double-equals is bash-specific and not supported in other shells.
if [[ "str1" = "str2" ]]; then echo "Strings are equal"; fi
-z
Return true if string is null (has a zero length). Always quote the tested string.
STR=""
if [[ -z "$STR" ]]; then echo "String is null"; fi
-n
Return true if string is not null. Always quote the tested string.
STR="foo"
if [[ -n "$STR" ]]; then echo "String is not null"; fi