General cleanup of shell bits

[charles-dyfis-net]

• Avoid sed use for functionality built into shell parameter expansion
• Avoid piping into a while read loop, which introduces the surprising behaviors discussed in BashFAQ #24
• Switch from nonstandard external command which to built-in, POSIX-standardized command -v
• Avoid using pushd and popd (which are interactive-extension options, not guaranteed to be compiled into a noninteractive shell at all).

With this applied, shellcheck scripts/* runs clean.

Not yet addressed:

• Failure to sanitize or escape strings substituted into JSON (base image does not provide jq or this would be trivial)
• Use of set -e

[taylorsilva]

I fixed up the scripts in #134 and in some other commits/PRs as well. Not sure if you want to keep this open still or close it now.

[taylorsilva]

Closing because stale and no response from OP.