new check: warn about using set with default values

[pabs3]

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

I can't check shellcheck.net because it requires JavaScript.

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

#!/bin/sh
set +x
echo foo

Here's what shellcheck currently says:

Nothing

Here's what I wanted or expected to see:

You have used set +x, which is the default, did you mean set -x instead?

Comment

ShellCheck could trace all use of the set builtin and determine what the current values are and where the set commands have no effect because they either set to the default or they set to the same value as an earlier set command.

[brother]

As long as the current script isn't sourced into something else that could have had the default bit flipped...

brother ~$ cat /tmp/a.sh 
#!/bin/sh
echo foo
set +x
echo bar
brother ~$ cat /tmp/b.sh 
#!/bin/sh
set -x
echo sourcing
. /tmp/a.sh
echo done
brother ~$ /tmp/b.sh
+ echo sourcing
sourcing
+ . /tmp/a.sh
+ echo foo
foo
+ set +x
bar
done