[mzs]

  rm -rf "$TSTDIR"/etc

is pretty dangerous when you forget to set the env var

[sfink]

Fair! Upvoted.

I guess I'm not likely to type that into the shell, or if I do, I then tab-complete to expand it.

I could definitely see myself using that in a shell script, though. I tend to do validity checks there:

    if ! [ -d "$TSTDIR" ]; echo "$TSTDIR not found, stupid" >&2; exit 1; fi

but that's kind of irrelevant, since if I need it to exist then I won't be removing it. Plus, I could totally see myself doing

    if [ -d "$TESTDIR" ]; then
      rm -rf "$TSTDIR"/etc
    fi

[oneshtein]

In bash, `set -u` or `"${TSTDIR:?Error: TSTSDIR is required.}/etc"` protects from such errors.