SC3018

In POSIX sh, ++ is undefined.

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

Problematic code:

#!/bin/sh
i=1
echo "$((i++))"

Correct code:

Write out the increment/decrement operation explicitly:

#!/bin/sh
i=1
echo "$i"
i=$((i+1))

Rationale:

Prefix and postfix increment and decrement are extensions in bash and ksh. They're not supported in dash or POSIX sh, which is what your shebang says you're using.

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!