[SC2252] False positive with POSIX-compliant prefix string match

[rassie]

For bugs

• Rule Id (if any, e.g. SC1000): SC2252
• My shellcheck version (shellcheck --version or "online"): 0.7.0
• [X] The rule's wiki page does not already cover this (e.g. https://shellcheck.net/wiki/SC2086)
• [X] I tried on shellcheck.net and verified that this is still a problem on the latest commit

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

#!/bin/sh
if [ "$A" != "${A#substring1}" ] ||
   [ "$A" != "${A#substring2}" ] ||
   [ "$A" != "${A#substring3}" ]; then
    echo "substring matched"
    exit 0
fi

Here's what shellcheck currently says:

SC2252: You probably wanted && here, otherwise it's always true.

Here's what I wanted or expected to see:

This expression is perfectly valid and not always true. The idea is to check for string prefix match in $A, which has to be either substring1, substring2 or substring3 to execute the code in the then block. Since I can't use bash pattern globs, a technique is used which compares the original string with the same string without a prefix to get the idea whether the original string actually starts with that prefix. I would expect shellcheck not to warn about this expression.

[matthewpersico]

Then don’t you want = instead of !=?

[rassie]

No, since != means "match" in this case. If I want to match for bar and compare foobar with foo (foobar without bar) and there aren't equal, then it's a match. If I instead compare fuzzbuzz with fuzzbuzz (fuzzbuzz without bar), those are equal, which means it's not a match. Complicated, I know :)