[[ ]]to prevent glob matching.
[[ $a = $b ]]
[[ $a = "$b" ]]
When the right-hand side of
!= is unquoted in
[[ .. ]], it will be treated like a glob.
This has some unexpected consequences like
[[ $var = $var ]] being false (for
[[ $foo = $bar ]] giving a different result from
[[ $bar = $foo ]].
The most common intention is to compare one variable to another as strings, in which case the right-hand side must be quoted.
If you explicitly want to match against a pattern, you can ignore this warning.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.