Safely Deleting a Directory Only If It ISN'T a Mount Point

Question

Unfortunately I'm being forced to work with a piece of software that handles automatically mounting and unmounting a network volume very poorly; it has an annoying tendency to leave a directory where the mount point was, but can't cope properly with that being the case when mounting the volume again later (it's supposed to tidy up but often doesn't).

Naturally I'm on at the developers to get that fixed, but in the meantime I need to do something to tidy up the mount point(s) myself.

So essentially what I need to do is remove the directory, but only if it isn't currently a mount point (as I don't want to delete the volume's contents by accident).

Now, I can get the device ID of the directory and compare it to the device ID of root easily enough, but there's a possibility of a race-condition if I use such a comparison, i.e- if the volume is mounted between checking device IDs and calling rm -r /mnt/point.

Are there any alternatives? I was intrigued by the possibility of using the find command's -xdev option, but I'm not sure how I would actually provide a point of comparison, as find /mnt/point -xdev won't work as the target and its contents are the same device.

Also, using rmdir on the assumption that the leftover folder will always be empty seems unreliable, as on some systems a mount point may have a file inside; macOS for example leaves an .autodiskmounted file inside. While I could create a list of such cases and handle them, it'd be nice (and hopefully useful to others) to have a more general purpose solution for future reference, if such a thing is possible.

Answer 1

If the directory is a mount point, it will be busy and you shouldn't be able to rename it.

$ sudo mv /mnt /mnt.old
mv: cannot move '/mnt' to '/mnt.old': Device or resource busy

If it's just a regular directory, you should be able to rename it.

$ sudo mv /mnt /mnt.old

If the move succeeds, re-create the mount directory and delete the renamed directory. Optionally, you can validate the renamed directory is part of the filesystem you expect before removal.

Answer 2

If you are using Linux

You can test if a directory is a mount point using mountpoint.

To avoid mount point appearing/disappearing between test and rm -r, you need to run the script in a separate mount namespace with private subtree (mounts do not propagate from/to new namespace). This can be accomplished with unshare.

unshare -m --propagation private -- "<delete script>"

Everything in same script:

#!/bin/sh

unshare -m --propagation private -- sh -e <<EOF

if ! mountpoint -q "<path>"; then
    rm -r "<path>"
fi

EOF

Answer 3

You cannot rmdir a directory that's not empty or one that's a mountpoint. So this will satisfy your requirement:

rm -f mountpoint/.autodiskmounted    # MacOS cookie
rmdir mountpoint 2>/dev/null

If you're running in a script with exit on error set, you can use rmdir ... || true. (Note that --ignore-fail-on-non-empty is of little use, because we will get an uncaught error when attempting to remove a mountpoint.)

I should point out that I don't have access to a Mac system so I cannot test whether the .autodiskmounted file is present when the filesystem really is mounted, or whether it's a placeholder for when the filesystem is unmounted.