UNIX/Linux Tip #4

Using the Linux loop Device

This tip (or more appropriately tips) cover the Linux loop device. The loop device is a kernel module that can be used to mount a file as a file system. This is most commonly used to mount ISO image files, but it can be used for other files as you will soon see.

• Is the Loop Device Loaded into the Kernel?
• Loading the Loop Device Kernel Module
• Mount an ISO image on Linux
• Creating a File System within a File
• A Final Note

Is the Loop Device Loaded into the Kernel?

Since the later tips rely on the fact that the loop device is actually loaded into the kernel, let's explore how we can determine whether or not it is actually loaded. One obvious indicator that the module is not loaded is the lack of any loop device special files in /dev, which would become evident if you tried to mount an ISO file to a mount point.

For example:

1. The mount command fails.

   # mount -o loop RHEL52-DVD.iso /mnt/image
   mount: could not find any device /dev/loop#
   

2. There are no /dev/loop* device special files.

   # ls /dev/loop*
   ls: /dev/loop*: No such file or directory
   

3. There is no loop device kernel module loaded.

   # lsmod | grep loop
   
   

Loading the Loop Device Kernel Module

In the previous example we illustrated what would occur if the loop device was not mounted. Well, in order to continue this tip we need to load the loop device. How is this accomplished?

1. Load the kernel module.

   # modprobe loop
   
   

2. Verify that it is loaded.

   Is the kernel module loaded?

   # lsmod | grep loop
   loop                   19145  0
   

   Are the /dev/loop* devices created?

   # ls /dev/loop*
   /dev/loop0  /dev/loop2  /dev/loop4  /dev/loop6
   /dev/loop1  /dev/loop3  /dev/loop5  /dev/loop7
   

Mount an ISO image on Linux

Once you have downloaded an ISO image to your Linux machine, you can mount it as a loop device. This will give you access to the files in the ISO without you first having to burn it to a CD or DVD.

For example if you would like to mount FILENAME.iso on /mnt/isoimage you would run the following commands:

1. Create the mount point (if it does not already exist).

   # mkdir /mnt/isoimage
   

2. Mount the file.

   # mount -o loop -t iso9660 FILENAME.iso /mnt/isoimage
   

Creating a File System within a File

In addition, you can create file systems within a file!

1. Create a zero-filled file for use in this example.

   # dd if=/dev/zero of=/tmp/loopfs.img bs=1024 count=10000
   10000+0 records in
   10000+0 records out
   

   # file /tmp/loopfs.img
   /tmp/loopfs.img: data
   

2. Using the losetup(8) command, associate our new file with a loop device.

   # losetup /dev/loop0 /tmp/loopfs.img
   
   

   # file -s /dev/loop0
   /dev/loop0: data
   

3. Create an Ext3 file system on the device.

   # mkfs -t ext3 /dev/loop0
   
   mke2fs 1.35 (28-Feb-2004)
   Filesystem label=
   OS type: Linux
   Block size=1024 (log=0)
   Fragment size=1024 (log=0)
   2512 inodes, 10000 blocks
   500 blocks (5.00%) reserved for the super user
   First data block=1
   Maximum filesystem blocks=10485760
   2 block groups
   8192 blocks per group, 8192 fragments per group
   1256 inodes per group
   Superblock backups stored on blocks: 
          8193
          
   Writing inode tables: done 
   Creating journal (1024 blocks): done
   Writing superblocks and filesystem accounting information: done
   
   This filesystem will be automatically checked every 29 mounts or
   180 days, whichever comes first.  Use tune2fs -c or -i to override.
   

4. Create a mount point, and mount the new file system.

   # mkdir /mnt/image && mount /dev/loop0 /mnt/image
   

   # mount | grep loop0
   /dev/loop0 on /mnt/image type ext3 (rw)
   

5. Unmount the file system and detach the loop device, then mount the file system with the ?loop? mount option.

   # umount /mnt/image && losetup -d /dev/loop0
   

   # mount -o loop /tmp/loopfs.img /mnt/image
   

   # mount | grep loopfs
   /tmp/loopfs.img on /mnt/image type ext3 (rw,loop=/dev/loop0)
   

6. Create a file on our new file system.

   # cd /mnt/image
   

   # touch myNewFile
   

   # ls -l
   total 22
   drwxr-xr-x  3 root root  1024 Dec 24 02:01 .
   drwxr-xr-x  5 root root  4096 Dec 24 01:41 ..
   drwx------  2 root root 12288 Dec 24 02:00 lost+found
   -rw-r--r--  1 root root     0 Dec 24 02:01 myNewFile
   

A Final Note

We hope that you've found this tip useful. If you have any questions, comments, or suggestions for future tips, please contact us.