LPIC-1 104.3 — Control Mounting and Unmounting of Filesystems

Exam weight: 3 — LPIC-1 v5, Exam 101

What You Need to Know

Mount and unmount filesystems manually.
Configure automatic mounting at boot via /etc/fstab.
Use labels and UUIDs to identify partitions.
Work with systemd mount unit files.

Key utilities: /etc/fstab, /media/, mount, umount, blkid, lsblk.

What is Mounting

A filesystem in Linux cannot be used directly. It must first be attached to a point in the directory tree — called a mount point. After mounting, the partition’s contents are accessible through that directory.

Mounting can be done manually with mount, automatically via /etc/fstab at boot, or through systemd unit files.

The mount Command

Basic Syntax

mount -t TYPE DEVICE MOUNTPOINT

TYPE — filesystem type (ext4, btrfs, exfat, ntfs, etc.)
DEVICE — partition with the filesystem, e.g. /dev/sdb1
MOUNTPOINT — directory to attach to; must already exist

mount -t exfat /dev/sdb1 ~/flash/

Any files previously in the mount point directory become inaccessible while another filesystem is mounted on top.

Viewing Mounted Filesystems

mount without arguments lists all mounted filesystems. Filter by type with -t:

mount -t ext4
# /dev/sda1 on / type ext4 (rw,noatime,errors=remount-ro)

mount -t ext4,fuseblk    # multiple types comma-separated

Output format: SOURCE on TARGET type TYPE OPTIONS.

Alternatives: cat /proc/self/mounts, cat /proc/mounts, findmnt.

Key Options

Option	Description
`-t TYPE`	specify filesystem type
`-o OPTS`	pass comma-separated mount options
`-r` / `-ro`	mount read-only
`-w` / `-rw`	mount read-write
`-a`	mount all filesystems in `/etc/fstab`
`--bind`	make a directory’s contents visible at another path

Remounting an already-mounted filesystem — no need to specify type:

mount -o remount,ro /dev/sdb1
mount -o remount,ro /mnt/data    # by mount point

mount --bind makes a directory’s contents accessible at another location without copying:

mount --bind /src /dst

The umount Command

Accepts either device name or mount point — both are equivalent:

umount /dev/sdb1
umount ~/flash

Option	Description
`-a`	unmount all filesystems in `/etc/fstab`
`-f`	force unmount (useful for unreachable network filesystems)
`-r`	if unmounting fails, remount read-only

Options combine: umount -fr /mnt/server.

Busy Filesystem: lsof

When you get target is busy, some process is holding files open:

lsof /dev/sdb1
# COMMAND  PID  USER  FD  TYPE  DEVICE  SIZE/OFF  NODE  NAME
# evince   3135 carol 16r REG   8,17    21881768  5195  /media/.../file.pdf

Close the program, then unmount.

/mnt vs /media

/mnt — conventional for manual, temporary mounts.
/media — standard for removable media. Modern distributions auto-mount there: /media/USER/LABEL.

/etc/fstab

Describes filesystems that can be mounted. Each line has exactly 6 fields:

FILESYSTEM  MOUNTPOINT  TYPE  OPTIONS  DUMP  PASS

Field	Description
`FILESYSTEM`	device, UUID, or label
`MOUNTPOINT`	mount point path
`TYPE`	filesystem type
`OPTIONS`	comma-separated mount options
`DUMP`	whether `dump` backs this up (usually `0`)
`PASS`	fsck check order on boot; `0` = skip

Example:

/dev/sda1  /  ext4  noatime,errors=remount-ro  0  1

Mount Options

Option	Description
`defaults`	= `rw,suid,dev,exec,auto,nouser,async`
`atime` / `noatime`	update access time on reads
`auto` / `noauto`	mount with `mount -a` or not
`exec` / `noexec`	allow / deny binary execution
`user` / `nouser`	allow / deny mounting by regular users
`group`	allow mounting by users in the device’s group
`owner`	allow mounting by the device’s owner
`suid` / `nosuid`	respect / ignore SUID/SGID bits
`ro` / `rw`	read-only / read-write
`sync` / `async`	synchronous / asynchronous I/O
`dev` / `nodev`	interpret block/character devices
`remount`	remount already-mounted filesystem (`mount -o` only, not fstab)

sync on flash media shortens device lifespan due to limited write cycles.

UUID and Labels

Device names are unstable — /dev/sdb1 may become /dev/sdc1 after reconnection. UUIDs and labels don’t change between reconnections.

lsblk -f

lsblk -f /dev/sda1
# NAME  FSTYPE  LABEL  UUID                                  FSAVAIL  FSUSE%  MOUNTPOINT
# sda1  ext4           6e2c12e3-472d-4bac-a257-c49ac07f3761  64.9G    33%     /

blkid

blkid
blkid /dev/sda1
# /dev/sda1: UUID="6e2c12e3-..." TYPE="ext4" PARTUUID="..."

Both utilities are listed in the 104.3 objectives.

Using UUID and LABEL

In /etc/fstab:

UUID=6e2c12e3-472d-4bac-a257-c49ac07f3761  /      ext4  noatime,errors=remount-ro  0  1
LABEL=homedisk                              /home  ext4  defaults                   0  2

On the command line:

mount UUID=56C11DCC5D2E1334 /mnt/external
mount LABEL=Backup /mnt/backup

Mounting via systemd

systemd manages mounting through unit files in /etc/systemd/system/.

If you mount a filesystem manually without a fstab entry or mount unit, systemd automatically generates a temporary mount unit and tracks the mount point.

Mount Unit (.mount)

[Unit]
Description=External data disk

[Mount]
What=/dev/disk/by-uuid/56C11DCC5D2E1334
Where=/mnt/external
Type=ntfs
Options=defaults

[Install]
WantedBy=multi-user.target

Field	Description
`What=`	device (typically via `/dev/disk/by-uuid/`)
`Where=`	full mount point path
`Type=`	filesystem type
`Options=`	mount options
`WantedBy=`	target (`multi-user.target` for normal boot)

File Naming

File name = mount point with / replaced by -, plus .mount extension:

Mount point	File name
`/mnt/external`	`mnt-external.mount`
`/var/log/db`	`var-log-db.mount`
`/`	`-.mount`

File goes in /etc/systemd/system/.

Managing Units

systemctl daemon-reload
systemctl start  mnt-external.mount
systemctl status mnt-external.mount
systemctl enable mnt-external.mount    # persist across reboots

Automount Unit (.automount)

Mounts on demand — when the mount point is first accessed:

[Unit]
Description=Automount for the external data disk

[Automount]
Where=/mnt/external

[Install]
WantedBy=multi-user.target

File name: mnt-external.automount. Same naming rule.

systemctl daemon-reload
systemctl start  mnt-external.automount
systemctl enable mnt-external.automount

Quick Reference

# View mounted filesystems
mount
mount -t ext4,ntfs
findmnt

# Mount
mount -t ext4 /dev/sdb1 /mnt/data
mount -o ro,noatime /dev/sdb1 /mnt/data
mount -o remount,ro /mnt/data
mount UUID=... /mnt/data
mount LABEL=Backup /mnt/backup
mount -a
mount --bind /src /dst

# Unmount
umount /dev/sdb1
umount /mnt/data
umount -fr /mnt/server
umount -a

# Device info
lsblk -f /dev/sdb1
blkid /dev/sda1

# Who is using the filesystem
lsof /dev/sdb1

# systemd
systemctl daemon-reload
systemctl start  mnt-external.mount
systemctl enable mnt-external.mount

Exam Questions

How many fields in a valid /etc/fstab line? → 6: FILESYSTEM, MOUNTPOINT, TYPE, OPTIONS, DUMP, PASS.
Two ways besides device name to identify a partition in fstab? → UUID= and LABEL=.
What does mount --bind do? → Makes a directory’s contents visible at another path.
What happens if you mount manually without fstab/unit entry? → systemd auto-generates a temporary mount unit.
Which commands list all mounted filesystems? → mount, cat /proc/self/mounts, cat /proc/mounts, findmnt.
What does noauto in fstab mean? → Entry is skipped by mount -a; must be mounted manually.
What does nouser mean? → Regular users cannot mount this filesystem; only root can.
How to force-unmount an unreachable network filesystem? → umount -f -r /mnt/server.
Where does a mount unit go and what is it named? → /etc/systemd/system/, name = mount point with slashes→dashes + .mount.
Must you specify type when remounting? → No, device or mount point is sufficient.
What does defaults include? → rw, suid, dev, exec, auto, nouser, async.
How to disable binary execution while keeping defaults? → Add noexec: defaults,noexec.
Difference between lsblk -f and blkid? → lsblk -f shows a table with free space and mount point; blkid gives compact output suitable for scripts.
How to find processes holding files on a busy filesystem? → lsof /dev/sdXN.
Why is sync not recommended for flash storage? → Writes happen synchronously, wearing out flash cells faster due to limited write cycles.

Exercises

Exercise 1 — Mount ext4 read-only with options

Mount ext4 on /dev/sdc1 to /mnt/external read-only, with options noatime and async.

Answer

mount -t ext4 -o noatime,async,ro /dev/sdc1 /mnt/external

Exercise 2 — Find what is holding a busy filesystem

Unmounting /dev/sdd2 returns target is busy. How do you find which files are open?

Answer

lsof /dev/sdd2

Output shows: process name, PID, user, and the open file. Close the program, then unmount.

Exercise 3 — noauto and mount -a

/etc/fstab contains:

/dev/sdb1  /data  ext4  noatime,noauto,async

Will this filesystem be mounted by mount -a?

Answer

No. The noauto option tells mount -a to skip this entry. The filesystem must be mounted manually.

Exercise 4 — Finding a filesystem’s UUID

How do you find the UUID of the filesystem on /dev/sdb1?

Answer

lsblk -f /dev/sdb1
# or
blkid /dev/sdb1

lsblk -f shows a table with type, label, UUID, free space and mount point. blkid gives compact output, convenient for scripts. Both are in the 104.3 objectives.

Exercise 5 — Remount as read-only

An exFAT filesystem is mounted at /mnt/data. How do you remount it read-only?

Answer

When remounting, type and UUID are not needed — mount point alone is sufficient:

mount -o remount,ro /mnt/data

Exercise 6 — List ext3 and ntfs mounts

How do you list all mounted filesystems of type ext3 and ntfs?

Answer

mount -t ext3,ntfs

Exercise 7 — nouser and regular user mounting

/etc/fstab contains:

/dev/sdc1  /backup  ext4  noatime,nouser,async

Can a regular user mount this filesystem with mount /backup?

Answer

No. The nouser option allows only root to mount this filesystem.

To allow regular users: use user (any user) or group (users in the device’s group).

Exercise 8 — Force-unmount an unreachable network filesystem

A network filesystem at /mnt/server has become unreachable due to connection loss. How do you force-unmount it, or fall back to read-only if that fails?

Answer

umount -fr /mnt/server

-f forces unmount; -r falls back to remounting read-only if unmounting fails.

Exercise 9 — fstab entry for btrfs Backup

Write an /etc/fstab line that mounts a btrfs volume with label Backup at /mnt/backup with default options but no binary execution.

Answer

LABEL=Backup  /mnt/backup  btrfs  defaults,noexec  0  0

defaults includes exec. Adding noexec overrides it — the last matching option wins.

DUMP and PASS are 0 0: skip dump backup, skip fsck on boot.

Exercise 10 — fstab equivalent of a mount unit

Given this systemd mount unit:

[Unit]
Description=External data disk

[Mount]
What=/dev/disk/by-uuid/56C11DCC5D2E1334
Where=/mnt/external
Type=ntfs
Options=defaults

[Install]
WantedBy=multi-user.target

What is the equivalent /etc/fstab line, and what must the unit file be named and where should it go?

Answer

fstab equivalent:

UUID=56C11DCC5D2E1334  /mnt/external  ntfs  defaults  0  0

What=/dev/disk/by-uuid/UUID is a symlink path created by udev. In fstab, the same UUID is written as UUID=.

Unit file: mnt-external.mount in /etc/systemd/system/.

Naming rule: replace each / in the mount point path with -:

Mount point	File name
`/mnt/external`	`mnt-external.mount`
`/var/log/db`	`var-log-db.mount`
`/`	`-.mount`

LPIC-1 Study Notes | Topic 104: Devices, Linux Filesystems, Filesystem Hierarchy Standard