Mounting Drives and /etc/fstab

What’s mounted now

me@garden-laptop:~$ findmnt
me@garden-laptop:~$ findmnt /media/me/backup
me@garden-laptop:~$ mount | grep ^/dev

findmnt shows every mounted file system as a tree, with its source device, type, and options ¹; give it a path to ask about one. The older mount with no arguments prints a flat list in the form device on mount_point type type (options). On Ubuntu it’s cluttered with snaps’ loop devices, so TLCL suggests mount | grep ^/dev for just the real disks ². TLCL’s example after plugging in a USB stick:

/dev/sdc on /media/me/C911-C314 type vfat (rw,nosuid,nodev,relatime,
uid=1000,gid=1000,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,
shortname=mixed,showexec,utf8,flush,errors=remount-ro)

The desktop mounted it at /media/me/ under a folder named after its label (or, with no label, its ID), and set it up so user 1000, you, owns the files ².

Mounting by hand

Any empty folder can be a mount point ². The convention is a folder under /mnt for things you mount yourself:

me@garden-laptop:~$ sudo mkdir /mnt/backup
me@garden-laptop:~$ sudo mount /dev/sdX1 /mnt/backup
me@garden-laptop:~$ ls /mnt/backup
me@garden-laptop:~$ sudo umount /mnt/backup

mount works out the file system type itself; -t ext4 names it explicitly ^{3 2}. Mounting on a folder that has files in it hides them until you unmount; they’re not lost ².

Why the desktop’s mounts aren’t enough

The automatic mounts under /media/<you>/ happen when you’re logged in to the desktop and plug something in. On a computer nobody’s logged in to, such as the home server this course ends with, or for last module’s backup timer running with lingering, nothing mounts the drive, and the backup script rightly refuses to run without it. And the folder name depends on the label, so a different drive or a re-format moves it.

The answer is /etc/fstab, the file system table: a list of what to mount at boot, and where ^{4 2}.

The six fields

Each line describes one file system, with six fields separated by spaces ^{4 2}:

# <what>        <where>        <type>  <options>                             <dump> <fsck>
LABEL=backup    /mnt/backup    ext4    nofail,x-systemd.device-timeout=5s    0      2

1. What to mount. A device, but better a LABEL= or UUID=, since device names “are often a coincidence of hardware detection order” and change when disks come and go ⁴.
2. Where: the mount point.
3. Type: ext4, vfat, exfat, ntfs, and so on.
4. Options, comma-separated; defaults if nothing special ⁴.
5. Dump: an old backup tool’s setting; use 0.
6. fsck order: 1 for the root file system, 2 for others to be checked at boot, 0 for never ⁴.

cat /etc/fstab on your system shows lines like this for / and /boot/efi, by UUID, written by the installer.

The options that matter

• defaults: the usual settings, read-write and mounted at boot ⁴.
• noauto: listed, but not mounted at boot; sudo mount /mnt/backup mounts it when you want, since mount fills in the rest from fstab ^{4 5}.
• nofail: boot doesn’t wait for this mount or fail without it ⁵. Essential for anything removable.
• x-systemd.device-timeout=5s: how long to wait for the device to appear. A missing drive otherwise holds up boot for 90 seconds, so the ArchWiki recommends pairing it with nofail ^{5 6}.
• ro: read-only. Useful for a Windows partition you only want to read ⁴.

If a label or mount point contains a space, write it as \040 ⁴; easier to choose names without spaces.

Setting up the backup drive

With the ext4 drive labelled backup from the last lesson:

me@garden-laptop:~$ lsblk -f
me@garden-laptop:~$ sudo mkdir -p /mnt/backup
me@garden-laptop:~$ sudo nano /etc/fstab

Add one line at the end, as above:

LABEL=backup  /mnt/backup  ext4  nofail,x-systemd.device-timeout=5s  0  2

Then, before rebooting, check your work. A broken line in fstab can stop the system booting normally, which is a bad time to find a typo:

me@garden-laptop:~$ sudo systemctl daemon-reload
me@garden-laptop:~$ sudo findmnt --verify
me@garden-laptop:~$ sudo mount /mnt/backup
me@garden-laptop:~$ findmnt /mnt/backup

• daemon-reload: systemd turns fstab into units, so after changing it, the manual recommends this ⁴.
• findmnt --verify checks that fstab parses and that its entries make sense ¹.
• mount /mnt/backup mounts it using the fstab line; sudo mount -a mounts everything in fstab that isn’t already mounted (except noauto lines) ³.

Last, make the drive’s top folder yours, once: sudo chown $USER: /mnt/backup.

fstab and systemd

systemd reads fstab and makes a mount unit for each line, named after the mount point: /mnt/backup becomes mnt-backup.mount ⁵. So the tools from module 3 work on it:

me@garden-laptop:~$ systemctl status mnt-backup.mount
me@garden-laptop:~$ journalctl -b -u mnt-backup.mount

If the drive wasn’t there at boot, that’s where it says so.

Pointing the backup at it

Last module’s backup service used /media/%u/backup. Edit ~/.config/systemd/user/snapshot-backup.service so the line reads:

ExecStart=%h/.local/bin/snapshot-backup /mnt/backup

then systemctl --user daemon-reload, and test with systemctl --user start snapshot-backup.service. Now the backup has a fixed place to go, whether or not anyone’s logged in.

Your turn

So

findmnt (or mount | grep ^/dev) shows what’s mounted; sudo mount and sudo umount attach and detach file systems by hand at any empty folder. The desktop’s /media mounts only happen for a logged-in user, so a drive that must always be there belongs in /etc/fstab: what (LABEL= or UUID=), where, type, options, dump, and fsck order. Use nofail and x-systemd.device-timeout=5s for removable drives. After editing, daemon-reload, findmnt --verify, and mount it before rebooting. Each line becomes a systemd mount unit, like mnt-backup.mount.