Add shellcheck to pre-commit and fix warnings

[gforsyth]

Description

shellcheck is a fast, static analysis tool for shell scripts. It's good at
flagging up unused variables, unintentional glob expansions, and other potential
execution and security headaches that arise from the wonders of bash (and
other shlangs).

This PR adds a pre-commit hook to run shellcheck on all of the sh-lang
files in the ci/ directory, and the changes requested by shellcheck to make
the existing files pass the check.

xref: rapidsai/build-planning#135

[dantegd]

lgtm, nice improvement!

[jameslamb]

Overall I support this! But left one question and one suggestion.

[jameslamb]

Why not run this on all shell scripts in the repo?

I especially would love to have this coverage on build.sh (at the repo root), as that's a public interface into building the components of this project. It's be nice to have shellcheck catch issues in changes there.

[gforsyth]

Yeah, build.sh coverage is definitely on my list -- I wanted to start with ci scripts to get shellcheck in place, then we can start to expand coverage to other shell scripts in the repository.

[jameslamb]

ok just wanted to be sure I understood the reasoning. As long as we'll come back again and do the rest, that's fine.

[jameslamb]

Suggested change

	cd "$(dirname "$(realpath "${BASH_SOURCE[0]}")")"/../python/cuml/cuml/tests || exit
	cd "$(dirname "$(realpath "${BASH_SOURCE[0]}")")"/../python/cuml/cuml/tests || exit 1

The || half of this will only be reached if the path doesn't exist. In that case, I think we want a non-0 exit code, so this fails loudly. Otherwise, we risk silently not running any tests in CI until someone notices.

A bare exit returns exit code 0.

cat > ./out.sh <<EOF
exit
EOF

bash ./out.sh
echo $?
# 0

[gforsyth]

yeah, that's a very good point and suggestion

[jameslamb]

I put that suggestion on just this line to show the point, but there are other || exit in the diff that I think should be || exit 1, can you update those as well?

[gforsyth]

Updated