Loop Device in Linux
Learn how to access the contents inside a new Linux distribution ISO image prior to repartitioning your disk and installing the operating system onto your local disk.
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If you have ever downloaded a new Linux distribution ISO image, you may have wondered how to access the contents inside the image prior to repartitioning your disk and installing the operating system onto your local disk. This can be done via a loop mount in Linux.
In Linux and other UNIX-like systems, it is possible to use a regular file as a block device. A loop device is a virtual or pseudo-device which enables a regular file to be accessed as a block device. Say you want to create a Linux file system but do not have a free disk partition available. In such a case, you can create a regular file on the disk and create a loop device using this file. The device node listing for the new pseudo-device can be seen under/dev. This loop device can then be used to create a new file system. The file system can be mounted, and its contents can be accessed using normal file system APIs.
Uses of Loop Device
As described above, one of the uses is creating a file system with a regular file when no disk partition is available.
Another common use of a loop device is with ISO images of installable operating systems. The contents of ISO images can be easily browsed by mounting the ISO image as a loop device.
Creating a Loop Device in Linux
These commands require root privilege.
1. Create a large regular file on disk that will be used to create the loop device.
# dd if=/dev/zero of=/loopfile bs=1024 count=51200 51200+0 records in 51200+0 records out 52428800 bytes (52 MB, 50 MiB) copied, 0.114882 s, 456 MB/s
This command creates a 50Mb file called loopfile filled with zeros.
If you already have an image file that you want to mount as a loop device, then you can skip this step.
2. Create a loop device with the large file created above.
There may be some loop devices already created. Run the following command to find the first available device node.
# losetup -f /dev/loop1
So we can safely use
/dev/loop1 to create our loop device. Create the loop device with the following command.
# losetup /dev/loop1 /loopfile
If you see no errors, the regular file
/loopfileis now associated with the loop device
3. Confirm creation of the loop device
# losetup /dev/loop1 /dev/loop1: :214 (/loopfile)
Creating a Linux Filesystem With the Loop Device
You can now create a normal Linux filesystem with this loop device.
1. Create an ext4 filesystem using
# mkfs -t ext4 -v /dev/loop1 mke2fs 1.45.3 (14-Jul-2019) fs_types for mke2fs.conf resolution: 'ext4', 'small' Discarding device blocks: done Filesystem label= OS type: Linux Block size=4096 (log=2) Fragment size=4096 (log=2) Stride=0 blocks, Stripe width=0 blocks 12800 inodes, 12800 blocks 640 blocks (5.00%) reserved for the super user First data block=0 Maximum filesystem blocks=14680064 1 block group 32768 blocks per group, 32768 fragments per group 12800 inodes per group Allocating group tables: done Writing inode tables: done Creating journal (1024 blocks): done Writing superblocks and filesystem accounting information: done
2. Create a mount point for the filesystem.
# mkdir /mnt/loopfs
3. Mount the newly created filesystem.
# mount -t ext4 /dev/loop1 /mnt/loopfs
This command mounts the loop device as a normal Linux ext4 filesystem, on which normal filesystem operations can be performed.
4. Check disk usage of the file system.
# df -h /dev/loop1 Filesystem Size Used Avail Use% Mounted on /dev/loop1 45M 48K 41M 1% /mnt/loopfs
tune2fs to see the filesystem settings.
# tune2fs -l /dev/loop1 tune2fs 1.45.3 (14-Jul-2019) Filesystem volume name: <none> Last mounted on: <not available> Filesystem UUID: b1b13d6e-c544-45dd-a549-5846371fbde6 Filesystem magic number: 0xEF53 Filesystem revision #: 1 (dynamic) Filesystem features: has_journal ext_attr resize_inode dir_index filetype needs_recovery extent 64bit flex_bg sparse_super large_file huge_file dir_nlink extra_isize metadata_csum Filesystem flags: signed_directory_hash Default mount options: user_xattr acl Filesystem state: clean Errors behavior: Continue Filesystem OS type: Linux Inode count: 12800 Block count: 12800 Reserved block count: 640 Free blocks: 11360 Free inodes: 12789 First block: 0 Block size: 4096 Fragment size: 4096 Group descriptor size: 64 Reserved GDT blocks: 6 Blocks per group: 32768 Fragments per group: 32768 Inodes per group: 12800 Inode blocks per group: 400 Flex block group size: 16 Filesystem created: Sun Mar 19 08:56:47 2023 Last mount time: Sun Mar 19 09:00:52 2023 Last write time: Sun Mar 19 09:00:52 2023 Mount count: 1 Maximum mount count: -1 Last checked: Sun Mar 19 08:56:47 2023 Check interval: 0 (<none>) Lifetime writes: 37 kB Reserved blocks uid: 0 (user root) Reserved blocks gid: 0 (group root) First inode: 11 Inode size: 128 Journal inode: 8 Default directory hash: half_md4 Directory Hash Seed: e489fd33-4003-4235-9347-144c7a5d4d73 Journal backup: inode blocks Checksum type: crc32c Checksum: 0x3b8c797a
6. To unmount the filesystem and delete the loop device, run the following commands.
# umount /mnt/loopfs/ # losetup -d /dev/loop1
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