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Joachim Ansorg edited this page Nov 12, 2021 · 2 revisions

In POSIX sh, let is undefined.

(or "In dash, ... is not supported." when using dash)

Problematic code:

#!/bin/sh
let x=2+2

Correct code:

Use standard $((arithmetic expansion)) instead of legacy let:

#!/bin/sh
x=$((2+2))

Rationale:

let is a command used for arithmetic in some shells, but it's not a standard command. Use $((..)) instead.

Exceptions:

If you only intend to target shells that supports this feature, you can change the shebang to a shell that guarantees support, or ignore this warning.

You can use # shellcheck disable=SC3000-SC4000 to ignore all such compatibility warnings.

Related resources:

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

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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