Mounting and Unmounting

One tree, many filesystems

Linux doesn't give each disk its own drive letter. Instead every filesystem is attached onto a directory somewhere in the single tree that starts at /. That directory is the filesystem's mount point, and attaching it is mounting. By convention, temporary disks are mounted under /mnt.

Attach the disk, make an empty directory for it, and mount it there. Afterwards df -h and findmnt show it in place:

sudo losetup -fP --show disk.img
sudo mkdir -p /mnt/data
sudo mount /dev/loop0 /mnt/data
df -h /mnt/data
findmnt /mnt/data

In the terminal

student@vulntech:~$ sudo losetup -fP --show disk.img
[sudo] password for student:
/dev/loop0
student@vulntech:~$ sudo mkdir -p /mnt/data
student@vulntech:~$ sudo mount /dev/loop0 /mnt/data
student@vulntech:~$ df -h /mnt/data
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0       55M   14K   51M   1% /mnt/data
student@vulntech:~$ findmnt /mnt/data
TARGET    SOURCE     FSTYPE OPTIONS
/mnt/data /dev/loop0 ext4   rw,relatime

Use it like any other folder

Once mounted, the disk is just a directory. Files you create under the mount point are stored on the disk, not on the root filesystem:

echo "hello from my disk" | sudo tee /mnt/data/notes.txt
cat /mnt/data/notes.txt
ls -l /mnt/data

In the terminal

student@vulntech:~$ echo "hello from my disk" | sudo tee /mnt/data/notes.txt
[sudo] password for student:
hello from my disk
student@vulntech:~$ cat /mnt/data/notes.txt
hello from my disk
student@vulntech:~$ ls -l /mnt/data
total 13
drwx------ 2 root root 12288 Oct 18 05:08 lost+found
-rw-r--r-- 1 root root    19 Oct 18 05:09 notes.txt

Unmounting: umount

When you're finished, umount (note: no ‘n’) detaches the filesystem. Always unmount before unplugging a real disk, so everything still in memory is written out first:

sudo umount /mnt/data
findmnt /mnt/data
lsblk /dev/loop0

In the terminal (after umount, /mnt/data is an empty directory again)

student@vulntech:~$ sudo umount /mnt/data
[sudo] password for student:
student@vulntech:~$ findmnt /mnt/data
student@vulntech:~$ lsblk /dev/loop0
NAME  MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
loop0   7:0    0  64M  0 loop

Common mistake: umount: target is busy means something is still using the disk — often your own shell sitting inside it. cd out of the mount point first, then unmount. sudo fuser -m /mnt/data shows what's holding it.

Tip: mount with no arguments, or findmnt, lists everything currently mounted. mount -o loop FILE DIR is a shortcut that attaches a loop device and mounts it in one step.

Remember: A filesystem must be mounted onto a directory to be used: sudo mount DEVICE DIR. df -h and findmnt show what's mounted; sudo umount DIR detaches it safely.