I would not trust things like `ls -sf` to be thread-safe either.

For shell scripts, a reasonable approach is to use a (correctly implemented) lock file at the start, then a bunch of `test` (aka `[`) blocks to check things. That does a decent job of documenting, too. Vs. everyone who looks at the script needing to know the less-common command line options.

Then, in the bigger picture, make sure that nobody else's script - maybe with its own lock file - starts overlapping with your script.