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

Question

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

Answer 1

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

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

Answer 2

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

Answer 3

You are looking for the return value of diff and not the output of diff that you are using in your example code.

Try this:

diff a b
if [ $? -ne 0 ]; then
    echo "The directory was modified";
fi