[[ expression ]]
[...] Expressions are composed of the primaries described below under CONDITIONAL EXPRESSIONS. Word splitting and pathname expansion are not performed on the words between the [[ and ]];
[...] 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 under Pattern Matching.
In the whole section the case of a single
= is not mentioned.
string1 == string2
string1 = string2
True if the strings are equal. = should be used with the test command for POSIX conformance.
From this description I would expect that
[[ a = $cmpstring ]]
checks for equal strings and
[[ a == $cmpstring ]]
checks for a pattern match. But that is not the case:
> [[ a == ? ]]; echo $? 0 > [[ a = ? ]]; echo $? 0 > [[ a == "?" ]]; echo $? 1
Do I misunderstand something or has the bash man page just forgotten to mention