Warn about using grep outside conditionals when set -e is in effect

[baumanj]

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

Using grep in the context of set -e, it is not generally the intent to exit the script on non-match situations. When the grep is in the context of a conditional, this is avoided, but sometimes it's used to assign variables and the result is unexpected.

Real world example: this line caused habitat-sh/habitat#4834

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

#!/bin/sh

set -e

matches=$(grep foo bar)
echo "${matches:-no matches}"

Here's what shellcheck currently says:

Nothing

Here's what I wanted or expected to see:

In test.sh line 5:
matches=$(grep foo bar)
          ^-- SCXXXX: Consider 'grep ... || true' or else script will exit on no match due to set -e