The loop device is a block device that maps its data to the blocks of a regular file or another block device. Loop devices are mounted as file systems on Unix-like operating systems.
losetup is a handy command-line utility to set up and control loop devices in Linux. It also allows you to detach loop devices and display the information of used loop devices.
How to use losetup command
The syntax to use losetup command is as follows:
losetup [options] [loopdev] [file]
Common Options:
-a,--all: Show the status of all loop devices.-d,--detach loopdev: Detach the specified loop device.-D,--detach-all: Detach all associated loop devices.-f,--find: Find the first unused loop device.-j file,--associated file: Show status of all loop devices associated with a given file.-P,--partscan: Force kernel to scan partition table on the loop device.-r,--read-only: Set up a read-only loop device.-v,--verbose: Verbose mode.-o offset,--offset offset: The data start is moved offset bytes into the specified file or device.-s size,--sizelimit size: The data end is limited to size bytes after the data start.
Here is a full example of creating, setting up, and using a loop device:
# Create a 100MB disk image file
dd if=/dev/zero of=disk.img bs=1M count=100
# Set up a loop device
losetup /dev/loop0 disk.img
# Create an ext4 filesystem on the loop device
mkfs.ext4 /dev/loop0
# Mount the loop device
mount /dev/loop0 /mnt
# Use the filesystem (e.g., create a file)
echo "Hello, World!" > /mnt/hello.txt
# Unmount the filesystem
umount /mnt
# Detach the loop device
losetup -d /dev/loop0Let's explore this with individual examples.
1. Create a loop device with losetup
You can use the losetup command to create a new loop device in Linux.
First, you need to create a file that will be used as a block device. The following command creates a file named loopfile which is 1GB in size.
$ sudo dd if=/dev/zero of=loopfile bs=100M count=10
Sample Output:
golinux@ubuntu-PC:~$ sudo dd if=/dev/zero of=loopfile bs=100M count=10
10+0 records in
10+0 records out
1048576000 bytes (1.0 GB, 1000 MiB) copied, 1.40495 s, 746 MB/s
Next, run the losetup command specifying the loop device and file you want to map it to.
$ sudo losetup /dev/loop13 loopfile
If the loop device is already in use, you will get the following error.
losetup: loopfile: failed to set up loop device: Device or resource busy
You can use the -f option to avoid errors. The available loop device will be automatically used.
$ sudo losetup -f loopfile
To verify whether the loop device is created with a file, run this command.
$ losetup -j loopfile
Sample Output:
golinux@ubuntu-PC:~$ losetup -j loopfile
/dev/loop13: []: (/home/golinux/loopfile)
2. Display information about all loop devices
The losetup command can be executed with -a argument to list all loop devices.
$ losetup -a
Sample Output:
Output columns:
- NAME: loop device name
- SIZELIMIT: size limit of the file in bytes
- OFFSET: offset from the beginning
- AUTOCLEAR: autoclear flag set
- RO: read-only device
- BACK-FILE: device backing file
- DIO: access backing file with direct-io
- LOG-SEC: logical sector size in bytes
3. Detach loop devices
The -d or --detach option is used to detach the file or device associated with the loop device. You can specify one or more loop devices.
$ losetup -d loopdev
OR
$ losetup --detach loopdev
These are the available loop devices on my server:
server:~ # losetup -a
/dev/loop0: [64772]:46 (/Images/CentOS-8.2.2004-x86_64-dvd1.iso)
/dev/loop1: [64772]:38 (/Images/OneNDS-XX-IS-20.0.0.05_dvd.iso)
/dev/loop2: [64773]:131302 (//opt/secure/loopback.img)
I will detach /dev/loop2 as the other 2 devices are basically image mounted on the system so we cannot detach them using losetup command:
server:~ # losetup -d /dev/loop2
server:~ # losetup -a
/dev/loop0: [64772]:46 (/Images/CentOS-8.2.2004-x86_64-dvd1.iso)
/dev/loop1: [64772]:38 (/Images/Dummy-XX-IS-20.0.0.05_dvd.iso)
4. Detach all used devices
If you want to detach all loop devices, you can run the losetup command with -D or --detach-all option.
$ losetup -D
OR
$ losetup --detach-all
5. Find first unused loop device
The -f or --find option helps to find the first unused loop device.
$ losetup -f
OR
$ losetup --find
Sample Output:
golinux@ubuntu-PC:~$ losetup -f
/dev/loop13Â
6. List all devices associated with a file
The -j or --associated option instructs losetup to display the list of all loop devices associated with a given file.
$ losetup -j file
OR
$ losetup --associated file
Sample Output:
The following command prints the loop device associated with a file named blockfile in the current directory
golinux@ubuntu-PC:~$ losetup -j blockfile
/dev/loop14: []: (/home/golinux/blockfile)
7. Print the list in JSON format
The -J or --json option uses the JSON output format to display all devices.
$ losetup -J
OR
$ losetup --json
Sample Output:
8. Set up a read-only loop device
To set up a read-only loop device, you have to use the losetup command with -r or --read-only option.
The following command creates a read-only loop device using the file named loopfile.
$ sudo losetup -r /dev/loop13 loopfile
Sample Output:
9. Create a loop device with the size limit
You can set the size limit in the loop device with --sizelimit option. The following command creates a new loop device with a size limit of 200M.
$ sudo losetup --sizelimit <num> loopdev loopfile
Sample Output:
10. Set offset value in loop device
The -o or --offset option helps to create a loop device with a specified offset value.
$ sudo losetup -o <num> loopdev loopfile
OR
$ sudo losetup --offset <num> loopdev loopfile
Sample Output:
11. Create a partitioned loop device
The -P or --partscan can be used to create a partitioned loop device.
$ sudo losetup -Pf loopfile
OR
$ sudo losetup --partscan -f loopfile
Conclusion
Now you know how to set up and manage loop devices in Linux. You have also learned to delete loop devices and get detailed information about used loop devices in the system.
We hope you find this article helpful. If you have any confusion, please let us know in the comment section.
References
