How to check if a file is empty in Bash?

[ -s file.name ] || echo "file is empty"

Misspellings are irritating, aren't they? Check your spelling of empty, but then also 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

I like shell scripting a lot, but one disadvantage of it is that the shell cannot help you when you misspell, whereas a compiler like your C++ compiler can help you.

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

[ -s file ] # Checks if file has size greater than 0

[ -s diff.txt ] && echo "file has something" || echo "file is empty"

If needed, this checks all the *.txt files in the current directory; and reports all the empty file:

for file in *.txt; do if [ ! -s $file ]; then echo $file; fi; done