How to test if location is a btrfs subvolume?

Question

In bash scripting, how could I check elegantly if a specific location is a btrfs subvolume?

I do NOT want to know if the given location is in a btrfs file system (or subvolume). I want to know if the given location is the head of a subvolume.

Ideally, the solution could be written in a bash function so I could write:

if is_btrfs_subvolume $LOCATION; then
    # ... stuff ...
fi 

An 'elegant' solution would be readable, small in code, small in resource consumption.

Answer 1

The subvolume is identified by inode number 256, so you can check it simply by

if [ `stat --format=%i /path` -eq 256 ]; then ...; fi

There's also a so called empty-subvolume, ie. if a nested subvolume is snapshotted, this entity will exist in place of the original subvolume. Its inode number is 2.

For a generally reliable check wheter any directory is a subvolume, the filesystem type should be verified as well

stat -f --format=%T /path

Answer 2

Solution1: Using @kdave suggestions:

is_btrfs_subvolume() {
    local dir=$1
    [ "$(stat -f --format="%T" "$dir")" == "btrfs" ] || return 1
    inode="$(stat --format="%i" "$dir")"
    case "$inode" in
        2|256)
            return 0;;
        *)
            return 1;;
    esac
}

Solution2: What I used before (only one call, but probably brittle):

is_btrfs_subvolume() {
    btrfs subvolume show "$1" >/dev/null 2>&1
}

EDIT: Corrected and replaced list by show as the behavior of list would not answer correctly on any normal btrfs directory.

EDIT2: as @kdave didn't post a full version of his superior answer, I added it to my answer.