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In POSIX sh, $[..] in place of $((..)) is undefined.

Problematic code:

var=$[1 + 2]

Correct code:

var=$((1 + 2))

Rationale:

Your script uses a shell feature not supported by the shebang. Either rewrite the script to be portable, or change the shebang to explicitly require a shell like Bash. In this case, $[ .. ] can easily be replaced by the POSIX standard arithmetic expansion $(( .. )).

Exceptions:

None

Related resources:

• Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!

ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.