Testing if two directory trees overlap

Question

Let $foo and $bar be two absolute directory names (output by realpath). I want to test if either is contained in the other one, at any depth, and execute commands accordingly.

I thought I'd use find, but its exit status doesn't tell whether something has been found or not. So I've come to:

if [ x$(find -H "$foo" -path "$bar" -printf y -quit) == xy \
    -o x$(find -H "$bar" -path "$foo" -printf y -quit) == xy ] ; then
    ...
fi

Is it correct? Is there something simpler?

Context: my home dir contains an encrypted folder (ecryptfs). I'm writing a script that will sync its argument to the cloud. I don't want the decrypted files to be synced, so I'll test whether the argument overlaps the encrypted folder to unmount the ecryptfs before syncing.

Answer 1

I'm not sure I fully understand the question and the context. But maybe this works:

foo="$(realpath "...")"
bar="$(realpath "...")"

if [[ $foo = $bar/* || $bar = $foo/* ]]; then
    echo "Overlap"
fi

Note that this won't work if one tree is sym-linked into the other.