Determining the filesystem for storage media

Linux can handle a wide variety of filesystems. However, in order to achieve problem-free use of a data storage device, it is a good idea to at least know what type of filesystem is on the device. Attempts to restore data on storage medium that have an unknown filesystem can be very difficult. Similar difficulties occur in situations in which you want to read a downloaded image of a DVD or an unknown data storage device and therefore have to incorporate it in the system. This can also be the case for new media, like recently purchased memory cards.

We take a closer look at the relevant out-of-the-box tools available for determining the correct type of filesystem. After that, we look at the partition and then the integration of the directory, which is made available via the network and the storage medium, into the system.

Mounting a filesystem is accomplished by a user with administrative rights on the mount command. Usually, this means as a root user. As part of the mount procedure, you should enter a mount point that tells the system where it should incorporate the filesystem in the local filesystem tree. If you do not enter a mount point, then the command pulls the prepared partitions or storage media into the filesystems table (/etc/fstab ) in order to find a directory that should serve as the mounting point.

There is a list of the filesystems that have already been incorporated in the /proc/mounts file. This file mirrors the kernel's information. The mounted filesystems table, /etc/mtab , was previously responsible for this task. Today, this table usually is a symbolic link to /proc/mounts .

The -t option in the mount command specifies the interpretation that the operating system applies to the type of the filesystem it will incorporate. Below, you can see an example of a call that mounts a partition in the /opt/usb-drive/ directory. This is the first partition of the second hard drive, and it has been formatted with ext3.

# mount -t ext3 /dev/sdb1 /opt/usb-drive

The types of filesystem that your system supports depends on the type of kernel components statically integrated or what has been loaded as a kernel module. In order to determine the correct type of filesystem, you can take the following approaches:

1. Try things out with mount until something works
2. Have the operating system recognize what type of filesystem sits on the storage media
3. Use a suitable tool for finding out by yourself what kind of filesystem it is.

The first approach is frustrating and can take a long time. The second approach is reliable and works well. The third approach is appropriate when you don't want to bother with the first approach, and the second approach hasn't worked for reasons you don't know but would like to uncover.

Auto-Detect

Automatic identification of the filesystem type is accomplished in mount by calling the program either without the -t or explicitly with -t auto . mount will try to identify the filesystem with the help of the libblkid [1] library. If the filesystem is successfully identified, the program will mount the media without acknowledging this directly by using so-called silent mode.

Otherwise, mount evaluates the entries in the /proc/filesystems file. Figure 1 shows the file as it appears on an Ubuntu system. The entries marked with the nodev keyword indicate that no corresponding filesystem is currently allocated to a block device [2].

Figure 1: A standard kernel's integrated filesystems under Ubuntu.

In most cases, mounting works like this, at least for common filesystems. These would include ext2/3/4, VFAT, and MS-DOS. Btrfs, XFS, and ZFS, on the other hand, are more difficult candidates for mounting. They need additional modules and potentially also more software packages. Once these needs are met, the corresponding versions of commands from fsck , mount , and mkfs also work with these systems.

The -v option causes mount to produce an additional status message once the process has been successfully completed. Listing 1 demonstrates this for a Fedora image, which mount recognizes as a CD/DVD (ISO 9660).

# mount -v Fedora-Xfce-Live-x86_64-24-1.2.iso /tmp/usb-platte/
mount: /dev/loop0 is write-protected, mounting read-only
mount: /dev/loop0 mounted on /tmp/usb-drive.

If the automatic detection function fails to identify the filesystem that is to be mounted, then you will be forced to switch over to the third approach described above and find out for yourself what type of filesystem you are dealing with. Fortunately, there are some on-board tools that work smoothly with this approach.

fdisk

You can use fdisk and its convenient ncurse version, cfdisk, to arrange the partitions on the storage medium including setting up new partitions, deleting old ones, and setting a bootable flag to make a partition boot-ready. Use

fdisk /dev/sda -l

in the terminal to output the first hard drive's entire partition table. In a similar fashion,

cfdisk /dev/sda

puts the information you have searched for up on the screen for you to see.

fdisk and cfdisk usually only display the type of partition, meaning you only see filesystems that are potentially supported. This is common with Linux, Solaris, BSD, Mac OS X and Windows. We are a bit luckier with the mounted ISO image in Figure 2, and we have learned some useful details. The second entry contains a FAT filesystem.

Figure 2: Filesystems in a mounted ISO image, above in fdisk and below in cfdisk.