Added condition testing docs

condition-testing/README.md

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+## Testing a Condition
+
+There are a number of ways to test conditions like this. My go-to is generally as follows:
+
+`[ -s /path/to/some/test/file ] && echo "condition was true" || echo "condition was false"`
+
+It is worth pointing out that it is possible to use `if` statements from the command line, just as you can in a script. It is definitely more cumbersome though.
+
+One thing to look out for is quoting issues with an unquoted shell variable. It might be prudent to wrap that in quotes (so `"$FILE"`) so as to avoid these issues. As such, I'd also test by setting the variable `FILE` in my shell and running the conditional as-written in the script (rather than substituting a test file path directly) if using a test file directly didn't show any issues.
+
+If that still doesn't work as expected, I'd start looking at the output of the `ls` command on the test file(s). Specifically, I'd be using `ls -lh`. Since this condition is checking for the existence and non-zero size of the given file, I want to make sure that the input data is what I'm expecting.
+
+If the files exist but are empty, and this is expected (for example, they exist as flags created with `touch`), I'd probably change to a different condition (e.g. `-f` or `-e`), but ultimately anything beyond the above conditional statement would be more situation-dependent, and I don't currently have enough information on the nature of the problem this conditional is being used to solve to give a more specific example.