Used but not assigned not warning when in same line

[rugk]

For bugs

• Rule Id (if any, e.g. SC1000): SC2034
• 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

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

#!/bin/sh
hu="x$hu"
number=$(( number+1 ))

Here's what shellcheck currently says:

nothing

The problem is, it does consider that the right part of the = is evaluated first and thus the variable is first used and later assigned to itself. Especially with arithmetic operations, this is highly problematic.

Here's what I wanted or expected to see:

Warning: "Variable hu referenced before assigned."
Warning: "Variable number referenced before assigned."

[nihilus]

How is that problematic in arithmetic operations? It will implicity get the value 0.

[rugk]

If you think of multiplication (var*5) it will get worse (syntax errored in my small test) out dividing through zero it so…

[koalaman]

You're right, of course. This is something I really want to fix in the general case, but it requires some significant infrastructure improvements that I haven't gotten around to (specifically, building a useful control flow graph of the script).

[NateEag]

I spent a few minutes yesterday scratching my head until I realized I had made this error.

It surprised me that Shellcheck didn't catch it for me, as it does amazingly well at detecting my stupid mistakes overall.

@koalaman , when you say you want to fix this "in the general case", do you mean to imply there's a simpler, dumber fix for this that you don't want to use, because analogous problems will exist elsewhere?