Suggest -z/-n for comparison against empty string (= ''/!= '')

[e-kwsm]

For bugs

• Rule Id (if any, e.g. SC1000): SC2244?
• My shellcheck version (shellcheck --version or 'online'): online
• I tried on shellcheck.net and verified that this is still a problem on the latest commit
• It's not reproducible on shellcheck.net, but I think that's because it's an OS, configuration or encoding issue

For new checks and feature suggestions

• shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related shellcheck should encourage explicit -z & -n string tests #1664?

Here's a snippet or screenshot that shows the problem:

#!/bin/sh
# shellcheck enable=avoid-nullary-conditions
[ "$1" = '' ]
[ "$1" != '' ]

Here's what shellcheck currently says:

Nothing.

Here's what I wanted or expected to see:

In - line 3:
[ "$1" = '' ]
  ^-------^ SCXXXX (style): Prefer -z to check empty string

Did you mean:
[ -z "$1" ]


In - line 4:
[ "$1" != '' ]
  ^--------^ SCXXXX (style): Prefer -n to check non-empty string

Did you mean:
[ -n "$1" ]