==. For assignment, use
=. For comparison, use
[ "$var" = value ]
ShellCheck has noticed that you're using
== in an unexpected way. The two most common reasons for this is:
You wanted to assign a value but accidentally used
== instead of
You wanted to compare two values, but neglected to use
[ .. ] or
[[ .. ]].
If you wanted to assign a literal equals sign, use quotes to make this clear:
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.