How to get the complete and exact list of mounted filesystems in Linux?

The definitive list of mounted filesystems is in /proc/mounts.

If you have any form of containers on your system, /proc/mounts only lists the filesystems that are in your present container. For example, in a chroot, /proc/mounts lists only the filesystems whose mount point is within the chroot. (There are ways to escape the chroot, mind.)

There's also a list of mounted filesystems in /etc/mtab. This list is maintained by the mount and umount commands. That means that if you don't use these commands (which is pretty rare), your action (mount or unmount) won't be recorded. In practice, it's mostly in a chroot that you'll find /etc/mtab files that differ wildly from the state of the system. Also, mounts performed in the chroot will be reflected in the chroot's /etc/mtab but not in the main /etc/mtab. Actions performed while /etc/mtab is on a read-only filesystem are also not recorded there.

The reason why you'd sometimes want to consult /etc/mtab in preference to or in addition to /proc/mounts is that because it has access to the mount command line, it's sometimes able to present information in a way that's easier to understand; for example you see mount options as requested (whereas /proc/mounts lists the mount and kernel defaults as well), and bind mounts appear as such in /etc/mtab.

As of v. 2.18 (July 2010) util-linux includes a tool that allows you to display a list of currently mounted file systems:

findmnt

You can switch from the default tree view to list view with -l, define output columns with -o (similar to lsblk), filter results based on filesystem type with -t etc...

findmnt -lo source,target,fstype,label,options,used -t ext4

SOURCE    TARGET      FSTYPE LABEL OPTIONS                           USED
/dev/sda1 /           ext4   ARCH  rw,noatime,discard,data=ordered  17.6G
/dev/sdb2 /media/DATA ext4   DATA  rw,noatime,discard,data=ordered    44M

For more details read the man page (and findmnt --help to get the list of available columns)

Maybe because it has been 5 years since this question was answered things have changed. The cat /proc/mounts creates a lot of info you do not care about. Today, IMHO, I find this to be the ultimate solution.

df -h --output=source,target

when you read the man pages there are all kinds of options you can do but this is what you what. For example to clean up the results even more you can exclude file types of "tmpfs" with this command:

df -hx tmpfs --output=source,target

df works on the filesystem level and not the file level.

The commands above will include network mounts as well.

To see a little more information use this:

df -hT

NOTE With slow mounted network connections this can take several minutes!

If you don't have or care about mounted network connections (and you have root permissions) than this is even better:

sudo lsblk -f