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lacygoill edited this page Sep 26, 2022 · 4 revisions

Shells disambiguate $(( differently or not at all. For $(command substitution), add space after $( . For $((arithmetics)), fix parsing errors.

Problematic code:

echo "$((cmd "$@") 2>&1)"

Correct code:

echo "$( (cmd "$@") 2>&1)"

Rationale:

You appear to be using $(( with two (or more) parentheses in a row, where the first $( should open a subshell.

This is an ill-defined structure that is parsed differently between different shells and shell versions. Prefer adding spaces to make it unambiguous, both to shells and humans.

Consider the $((( in $(((1)) ):

Ash, dash and Bash 1 parses it as $(( ( and subsequently fail to find the matching )). Zsh and Bash 2+ looks ahead and parses it as $( ((. Ksh parses it as $( ( (.

Exceptions:

Alternatively, you may indeed have correctly spaced your parentheses, but ShellCheck failed to parse $(( as an arithmetic expression while accidentally succeeding in parsing it as $( + (.

In these cases, double check the syntax to ensure ShellCheck can parse the $((, or ignore this error and hope that it won't affect analysis too severely.

ShellCheck

Each individual ShellCheck warning has its own wiki page like SC1000. Use GitHub Wiki's "Pages" feature above to find a specific one, or see Checks.

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