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+## In POSIX sh, $[..] in place of $((..)) is undefined.
+
+### Problematic code:
+
+```sh
+var=$[1 + 2]
+```
+
+### Correct code:
+
+```sh
+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!
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