Warn on empty local assignment

[rowlap]

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

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

#!/bin/bash

set -u

myfunc() {
        local a=
        a=apple
        echo "a is $a"
}

myfunc

Here's what I wanted or expected to see:

shellcheck currently says nothing about this. The issue is that the empty assignment local a= defeats the purpose of set -u, which won't be triggered on empty string.

If the developer really means empty string this can be written as local a='', otherwise local a is safer in that set -u is still effective against typos.

Other forms of assignment with modifiers are alias a=b / declare a=b / typeset a=b / export a=b / readonly a=b but I'm less sure that any of those are likely to appear with missing values.

[apenwarr]

Unfortunately a lot of shells, including dash, ksh, and busybox, do not seem to clear a automatically when you write "local a", only when you write "local a=". So it would probably be dangerous to advise people to write it without the =. In fact, maybe we should have the opposite warning.

[akinomyoga]

In fact, maybe we should have the opposite warning.

Bash 5.0+ now has the shell options shopt -s localvar_inherit to make the behavior closer to some other shells, so the declarations without values generally cause undefined results. It would be useful to have the warning about the declarations in function scopes without values such as local a.