Difference between --rbind and --bind in mounting

Question

I am confused. Linux filesystem is a tree structure, with the root node(starting node) as the root directory. Now let's suppose I have a folder abc at location /home/abc and another folder xyz at location /home/xyz

Folder xyz consists of some other folders and files inside it. (ex def and mno are folders inside it)

/
├── home/
│   ├── abc
│   └── xyz
│       ├── def
│       └── mno

When I run the command

mount --rbind /home/xyz /home/abc

(rbind is recursively bind) I see all the contents of the folder xyz in abc. Now, when I just run the command

mount --bind /home/xyz /home/abc

I still see all the contents of xyz in abc.

Why is that?

--bind to work just as similarly to --rbind.

Answer 1

You've rightly observed that, with both --bind and --rbind, you see directories under the bind mount.

The difference is that, with --rbind but not with --bind, you see the contents of other bind mounts under the bind mount.

---

As applied to your example, suppose for simplicity that /home/xyz/def and /home/xyz/mno are empty directories. Suppose further that you then use them as bind mounts, i.e., you use them as mount points in mount --bind or mount --rbind. This causes them to appear nonempty. Then, suppose you run:

mount --bind /home/xyz /home/abc

Then, /home/abc/def and /home/abc/mno both exist, but they appear empty, because you used mount --bind, which is nonrecursive.

In contrast, suppose you instead made /home/abc a bind mount by running this command:

mount --rbind /home/xyz /home/abc

Then, /home/abc/def and /home/abc/mno both exist and they appear nonempty--they have the contents of the /home/xyz/def and /home/xyz/mno bind mounts--because you used mount --rbind, which is recursive.

---

Here's a fully worked-out example:

ek@Gnar:~$ mkdir original
ek@Gnar:~$ touch original/a-regular-file
ek@Gnar:~$ mkdir original/a-directory
ek@Gnar:~$ mkdir parent-of-mountpoint
ek@Gnar:~$ mkdir parent-of-mountpoint/mountpoint
ek@Gnar:~$ sudo mount --bind ~/original ~/parent-of-mountpoint/mountpoint
ek@Gnar:~$ mkdir nonrecursive-other-mountpoint recursive-other-mountpoint
ek@Gnar:~$ sudo mount --bind ~/parent-of-mountpoint ~/nonrecursive-other-mountpoint
ek@Gnar:~$ sudo mount --rbind ~/parent-of-mountpoint ~/recursive-other-mountpoint

ek@Gnar:~$ tree -F original/
original/
├── a-directory/
└── a-regular-file

1 directory, 1 file
ek@Gnar:~$ tree -F parent-of-mountpoint/
parent-of-mountpoint/
└── mountpoint/
    ├── a-directory/
    └── a-regular-file

2 directories, 1 file
ek@Gnar:~$ tree -F nonrecursive-other-mountpoint/
nonrecursive-other-mountpoint/
└── mountpoint/

1 directory, 0 files
ek@Gnar:~$ tree -F recursive-other-mountpoint/
recursive-other-mountpoint/
└── mountpoint/
    ├── a-directory/
    └── a-regular-file

2 directories, 1 file

ek@Gnar:~$ mount | grep ~
/dev/sda2 on /home/ek/parent-of-mountpoint/mountpoint type ext4 (rw,relatime)
/dev/sda2 on /home/ek/nonrecursive-other-mountpoint type ext4 (rw,relatime)
/dev/sda2 on /home/ek/recursive-other-mountpoint type ext4 (rw,relatime)
/dev/sda2 on /home/ek/recursive-other-mountpoint/mountpoint type ext4 (rw,relatime)

Answer 2

Read man 8 mount. It says (emphasis added):

Bind mounts

Remount part of the file hierarchy somewhere else. The call is:

mount --bind olddir newdir

or by using this fstab entry:

/olddir /newdir none bind

After this call the same contents are accessible in two places. One can also remount a single file (on a single file). It's also possible to use the bind mount to create a mountpoint from a regular directory, for example:

mount --bind foo foo

The bind mount call attaches only (part of) a single filesystem, not possible submounts. The entire file hierarchy including submounts is attached a second place by using:

mount --rbind olddir newdir

Note that the filesystem mount options will remain the same as those on the original mount point.

Answer 3

The reason being is bind and rbind solve different problems.

For example, my son wants to access my media database from windows across the country. So I grant him an ssh login, and tell him to install sshfs. Only sshfs in windows acts like it is in a chroot jail. So to give him access I do:

sudo mkdir -p /home/myson/share
sudo mount --rbind /share /home/myson/share

Now he can travers all the drives I have mounted for my share folder.

But suppose I want to make a backup of /share to a remote host, with out all the other drives mounted underneath. I can certainly tell rsync just to backup one file system. But then it will also skip the mount points. If I actually want to copy the mount points as well I should do something like:

sudo mkdir -p /tmp/share
sudo mount --bind /share /tmp/share

Now I have a version of the share folder free of any mounts.

Most people assume that mount points are empty directories. But sometimes it is useful to populate files there. For example, if your website is stored on a separate drive, maybe you want a set of files that will redirect users to your backup site should your mount fail.