How to check if a file is empty in Bash?

I have a file called diff.txt. I want to check whether it is empty.

I wrote a bash script something like below, but I couldn't get it work.

if [ -s diff.txt ]
then
        touch empty.txt
        rm full.txt
else
        touch full.txt
        rm empty.txt
fi

try this:

#!/bin/bash -e

if [ -s diff.txt ]; then
        # The file is not-empty.
        rm -f empty.txt
        touch full.txt
else
        # The file is empty.
        rm -f full.txt
        touch empty.txt
fi

Notice incidentally, that I have swapped the roles of empty.txt and full.txt, as @Matthias suggests.