How to determine the mount point for a device on Linux

While reinstalling a server and moving it to Debian, I ran into the problem that I need to sync the EFI partitions. The system is using a RAID on two disks (RAID 1), but the EFI partitions can’t be a RAID and need to exist on both disks. Or rather it is useful to have the EFI partition on every disk: if one disk fails, the system can boot from the other disk.

grub only installs the necessary files on one partition. However one can use a script as grub hook, and sync all partitions.

This script needs to determine if a device (the EFI partition) is already mounted somewhere, or needs to be mounted. Turns out there is a little and not well-known tool for this.

Mounting a device on Linux

Devices on Linux can be mounted in several ways. One is by manually specifying the device name and the mount point:

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mount /dev/sda2 /mnt

The above commandline will mount the device /dev/sda2 on the mount pont /mnt (which must already exist). The mount command will determine the filesystem on the partition, if that doesn’t work it can be specified using the -t option:

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mount -t vfat /dev/sda2 /mnt

A second way to mount devices is by pre-defining which device will be mounted on which mount point. This information is stored in the file /etc/fstab, and this file contains one line per device and mount point.

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# grep sda2 /etc/fstab
/dev/sda2  /boot/efi       vfat    umask=0077      0       1

This allows to mount the mount point /boot/efi by issuing the following command:

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mount /boot/efi

It is also possible to use the label of the disk:

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# grep efi /etc/fstab
UUID=48F7-B093  /boot/efi       vfat    umask=0077      0       1

This is safer. The device name can change if a disk configuration changes. The label however is unique and identifies this specific device and partition.

Identify the mount point

What if I don’t know if the device is already mounted somewhere or not, and if I don’t know the exact mount point. The findmnt tool will help with that:

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# findmnt /dev/sda2
TARGET    SOURCE    FSTYPE OPTIONS
/boot/efi /dev/sda2 vfat   rw,relatime,fmask=0077,dmask=0077,codepage=437,iocharset=ascii,shortname=mixed,utf8,errors=remount-ro

That’s very helpful!

If the device is currently not mounted, the findmnt tool will not output anything, and also exit with a return code RC=1. This comes handy in scripts.

The output of the tool can be formatted. I need the following options:

• --first-only: Only show the first find - not that I expect more mount points, but play it safe
• --noheadings: Skip the headers (like TARGET, SOURCE, FSTYPE, and OPTIONS)
• --output=TARGET: Only output the TARGET column, that is the actual mount point

Using these options together:

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# findmnt --first-only --noheadings --output=TARGET /dev/sda2
/boot/efi

The device /dev/sda2 is mounted on /boot/efi.

See this blog posting and this repository with scripts how the findmnt tool is used to identify mount points for grub.

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