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

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

If you don't need to know what the changes are, cmp is enough. Plus you can play with the syntactical trick provided by && and || :

cmp a b || echo 'The directory was modified'

The instruction may be interpreted as: "either a and b are equal, or i echo the message".

(The semantic of && and || must be handled with care, but here it's intuitive).

Just for the sake of readability, i actually prefer to put it on two lines:

cmp a b \
  || echo 'The directory was modified'

DIFF=$(diff -u <(find dir1/ -type f -printf '%P\n' | sort) <(find dir2/ -type f -printf '%P\n' | sort))
if [ "$DIFF" ]; then
  echo "Directories differ"
  # Do other stuff here
fi

This uses one of my favorite bashisms, the <() process substitution.

The $DIFF variable holds a printable difference. If you want to show it to the end user, be sure to double-quote it, e.g. echo "$DIFF".

If you want to only tell the user there was any difference, if can be shortened to something like [ "$(diff ...)" ] && echo "Difference found"

Note: I'm assuming the original question meant to have dir1 and dir2 to make a little more sense. If it was dir at time 0 and then dir at time 1, this approach obviously wouldn't work.