Closes #2149

I'm using shellcheck to check compatibility with bourne shell explicitly. I confirmed that this catches the incompatibility that was missed in the previous release (#2148):

In tests/playTests.sh line 270:
if [[ $file2timestamp -ge $file1timestamp ]]; then
   ^-- SC2039: In POSIX sh, [[ ]] is undefined.

It also discovered 3 other warnings. Two of them were good suggestions so I fixed them. One of them isn't worth fixing imo so I suppressed it using shellcheck --exclude. This is the warning I excluded

In tests/playTests.sh line 401:
    ls -las $INTOVOID | grep "rw-rw-rw-"
    ^-- SC2010: Don't use ls | grep. Use a glob or a for loop with a condition to allow non-alphanumeric filenames.