List only bind mounts

This is similar to the other findmnt answer, but avoids the formatting issue.

To show all submounts:

findmnt --kernel -n --list | grep '['

To show all submounts of filesystems of type ext4:

findmnt --kernel -t ext4 -n --list | grep '['

To show all mounts excluding submounts:

findmnt --kernel -n --list | grep -v '['

To show all mounts of filesystems of type ext4 excluding submounts:

findmnt --kernel -t ext4 -n --list | grep -v '['

The "-n" removes the headers and the "--list" removes the lines of the "tree" format.

Tested on Debian stretch.

The kernel doesn't handle bind mounts different from normal mounts after the fact. The only differ in what happens while mount runs.

When you mount a filesystem (eg. with mount -t ext4 /dev/sda1 /mnt) the kernel (a bit simplified) performs three steps:

1. The kernel looks for a filesystem driver for the specified filesystem type (if you omit -t or use -t auto mount guesses the type for you and provides the guessed type to the kernel)
2. The kernel instructs the filesystem driver to access the filesystem using the source path and any provided options. At this point the filesystem is only identified by a major:minor number pair.
3. The filesystem is bound to a path (the mountpoint). The kernel also uses some of the mount options here. (nodev for example is an option on the mountpoint, not on the filesystem. You can have a bind mount with nodev and one without)

If you perform a bind mount (eg. with mount --bind /a /b) the following happens:

1. The kernel resolves which filesystem contains the source path and the relative path from the mountpoint to the directory.
2. The filesystem is bound to the new mountpoint using the options and the relative path.

(I'll skip mount --move, because it's not relevant to the question.)

This quite is similar to how files are created on Linux:

1. The kernel resolves which filesystem is responsible for the directory in which the file should be created.
2. A new file in the filesystem is created. At this point the file only has an inode number.
3. The new file is linked to a filename in the directory.

If you make a hard link the following happens:

1. The kernel resolves the inode number of the source file.
2. The file is linked to the destination filename.

As you can see, the created file and the hard link are indistinguishable:

$ touch first
$ ln first second
$ ls -li
1184243 -rw-rw-r-- 2 cg909 cg909 0 Feb 20 23:56 /tmp/first
1184243 -rw-rw-r-- 2 cg909 cg909 0 Feb 20 23:56 /tmp/second

But, as you can identify all hardlinks to a file by comparing the inode numbers, you can identify all mounts to a filesystem by comparing the major:minor numbers of mounts.

You can do this with findmnt -o TARGET,MAJ:MIN or by directly looking at /proc/self/mountinfo (see the Linux kernel documentation for more information).

The following Python script lists all bind mounts. It assumes that the oldest mount point with the shortest relative path to the root of the mounted file system is the original mount.

#!/usr/bin/python3

import os.path, re
from collections import namedtuple

MountInfo = namedtuple('MountInfo', ['mountid', 'parentid', 'devid', 'root', 'mountpoint', 'mountoptions', 'extra', 'fstype', 'source', 'fsoptions'])

mounts = {}

def unescape(string):
    return re.sub(r'\([0-7]{3})', (lambda m: chr(int(m.group(1), 8))), string)

with open('/proc/self/mountinfo', 'r') as f:
    for line in f:
        # Parse line
        mid, pid, devid, root, mp, mopt, *tail = line.rstrip().split(' ')
        extra = []
        for item in tail:
            if item != '-':
                extra.append(item)
            else:
                break
        fstype, src, fsopt = tail[len(extra)+1:]
        # Save mount info
        mount = MountInfo(int(mid), int(pid), devid, unescape(root), unescape(mp), mopt, extra, fstype, unescape(src), fsopt)
        mounts.setdefault(devid, []).append(mount)

for devid, mnts in mounts.items():
    # Skip single mounts
    if len(mnts) <= 1:
        continue
    # Sort list to get the first mount of the device's root dir (if still mounted)
    mnts.sort(key=lambda x: x.root)
    src, *binds = mnts
    # Print bind mounts
    for bindmount in binds:
        if src.root == bindmount.root:
            srcstring = src.mountpoint
        else:
            srcstring = src.mountpoint+':/'+os.path.relpath(bindmount.root, src.root)
        print('{0} -> {1.mountpoint} ({1.mountoptions})'.format(srcstring, bindmount))

unset DONE1FSES
FSES=$(findmnt -vUPno SOURCE,FSROOT,TARGET,MAJ:MIN)
FSES=${FSES//MAJ:MIN/MAJ_MIN}
while read SEARCH1FS
do
  unset DONE2FSES
  eval "$SEARCH1FS"
  SEARCH1SOURCE=$SOURCE
  SEARCH1FSROOT=$FSROOT
  SEARCH1TARGET=$TARGET
  SEARCH1MAJMIN=$MAJ_MIN

  FS1WASHANDLED=0
  while read DONE1FS 
  do
    if [[ $DONE1FS == $MAJ_MIN ]]
    then
      FS1WASHANDLED=1
      break
    fi
  done < <(echo "$DONE1FSES")


  if [[ ($SEARCH1FSROOT == /) && ($FS1WASHANDLED == 0) ]]
  then
  DONE1FSES+=$MAJ_MIN$'n'
  while read SEARCH2FS
  do
    eval "$SEARCH2FS"
    SEARCH2SOURCE=$SOURCE
    SEARCH2FSROOT=$FSROOT
    SEARCH2TARGET=$TARGET
    SEARCH2MAJMIN=$MAJ_MIN

    FS2WASHANDLED=0
    while read DONE2FS 
    do
      if [[ $DONE2FS == $SEARCH2FS ]]
      then
        FS2WASHANDLED=1
        break
      fi
    done < <(echo "$DONE2FSES")

    if [[ ($SEARCH1MAJMIN == $SEARCH2MAJMIN)  && ($SEARCH1TARGET != $SEARCH2TARGET )  && ($FS2WASHANDLED == 0 ) ]]
    then
      DONE2FSES+=$SEARCH2FS$'n'
      echo "$SEARCH1TARGET$SEARCH2FSROOT   --> $SEARCH2TARGET"
    fi

  done < <(echo "$FSES")


  fi
done   < <(echo "$FSES")

Maybe this could do the trick:

findmnt | grep  "["

Example:

$ mkdir /tmp/foo
$ sudo mount --bind /media/ /tmp/foo
$ findmnt | grep  "["
│ └─/tmp/foo                     /dev/sda2[/media] ext4            rw,relatime,data=ordered