Bash: using the result of a diff in a if statement

I am writing a simple Bash script to detect when a folder has been modified.

It is something very close to:

ls -lR $dir > a
ls -lR $dir > b

DIFF=$(diff a b) 
if [ $DIFF -ne 0 ] 
then
    echo "The directory was modified"

Unfortunately, the if statement prints an error: [: -ne: unary operator expected

I am not sure what is wrong with my script, would anyone please be able to help me?

Thank you very much!

Jary

ls -lR $dir > a
ls -lR $dir > b

DIFF=$(diff a b) 
if [ "$DIFF" != "" ] 
then
    echo "The directory was modified"
fi

if ! diff -q a b &>/dev/null; then
  >&2 echo "different"
fi

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