SC3017

In POSIX sh, unary -a in place of -e is undefined.

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

Problematic code:

#!/bin/sh
if [ -a file.txt ]
then
  echo "file.txt exists"
fi

Correct code:

#!/bin/sh
if [ -e file.txt ]
then
  echo "file.txt exists"
fi

Rationale:

-a was used in some shells to check for the existence of a file, but the POSIX standard and better supported equivalent is -e.

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!