The du command estimates the amount of space taken up by a directory tree, I usually use it when I’m trying to figure out why something is taking up so much space, piped to a ‘sort -n’ to see what the largest subdirectories of a given start point are.

As a first approximation, du prints the directory tree under the starting point annotated with sizes, but that’s not exactly correct. Take the following example run on a Debian GNU/Linux box:

$ mkdir dir
$ ln -s dir link
$ touch dir/foo
$ du
4       ./dir
8       .

As you can see du did not follow the symlink, since this is the default behavior, unless the -L option is provided to make it dereference symlinks, as such:

$ du -L
4       ./dir
8       .

Hmm, ok that still didn’t work. It turns out that since the purpose of du is to determine how much space is used up by a directory tree, it tries really hard not to account for the same space twice, and realizes that the ./link directory has already been accounted for and skips it. In fact I ran this test a few times and was even to sometimes get it to skip the ./dir and process the ./link instead, probably something to do with inode number ordering I’m guessing.

Bind mounts are another alternative to symlinks in Linux that do sort of the same thing in a very different way and with different consequences. How does ‘du’ handle them?

$ mkdir mnt
$ sudo mount --bind dir mnt
$ du
4       ./dir
4       ./mnt
12      .

It appears that using a bind mount successfully tricks du into counting the space twice, but that’s not entirely true:

$ du -L
4       ./dir
8       .

The man page says that -L, “Dereferences all symbolic links”, but it appears that the man page is not telling the whole story. It appears that -L also turns on the “make sure we don’t count the same space twice” feature. Presumably without that option enabled, du assumes it doesn’t have to worry about such a situation and doesn’t run that code, allowing the bind mount to sneak in the back door.

You can leave off the -L option to tell du not to follow symlinks, but what if you don’t want to follow bind mounts? There is a convenient -x option which will “skip directories on different filesystems”, which sounds like it would help:

$ du -x
4       ./dir
4       ./mnt
12      .

But nope, the tool doesn’t see the bind mount as a separate filesystem, since its really part of the same filesystem mounted at a new point.

Unlike the Linux version of du, which is part of GNU Core Utils, the du from FreeBSD is a different program, originally from Version 1 AT&T Unix, and it does not appear to make this attempt to avoid double counting space. On FreeBSD all three directories are shown with the -L option is enabled, and the ./dir and the ./mnt when it is not enabled. (Using nullfs instead of bind mounts on FreeBSD, since it doesn’t support the latter by default)