The /etc/fstab file is one of the most important files in a Linux-based system, since it stores static information about filesystems, their mountpoints and mount options. In this tutorial we will learn to know its structure in details, and the syntax we can use to specify each entry in the file.

In this tutorial you will learn:
  • How to use the fstab file to provide static filesystem information
  • How the fstab file is structured
  • What is the purpose of each entry field in the file

Software Requirements and Conventions Used

Software Requirements and Linux Command Line Conventions
Category Requirements, Conventions or Software Version Used
System Distribution-independent
Software No specific software is needed to follow this tutorial
Other Familiarity with basic concepts like 'mountpoint', and 'filesystem'
Conventions # - requires given linux commands to be executed with root privileges either directly as a root user or by use of sudo command
$ - requires given linux commands to be executed as a regular non-privileged user

The role of fstab

The first thing we must know about the fstab file is that is meant to be only read by programs and never written except by the system administrator. Each line in the file describes a filesystem, and contain fields used to provide information about its mountpoint, the options which should be used when mounting it etc. Each field can be separated by another either by spaces or tabs.  Let's analyze each field and its role in an entry.


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Fstab fields

Each entry line in the fstab file contains six fields, each one of them describes a specific information about a filesystem.

First field - The block device

The first field in each fstab entry holds information about the local or remote block device which should be mounted. The most typical way to reference a block device is by using its node inside the /dev directory, so for example to reference the first partition of the sda block device we use /dev/sda1 as value.

Alternative ways to reference a block device is by using its LABEL or UUID (Universal Unique IDentifier). The latter is the absolutely preferred method, since it guarantees to univocally reference a filesystem, as its name states. On GPT partitioned disks it's also possible to reference a filesystem by using PARTUUID or PARTLABEL.

To obtain information about filesystems we can run the lsblk command, eventually with the -o option to specify the fields we want to retrieve, or by using the -fs one, which is the equivalent of using -o and provide NAME,FSTYPE,LABEL,UUID,MOUNTPOINT as arguments. By default the program will display information about all existing filesystems. To avoid this behavior, a filesystem reference must be passed as an argument:



$ lsblk -d -fs /dev/sdb1
NAME FSTYPE LABEL UUID                                 FSAVAIL FSUSE%
MOUNTPOINT
sdb1 ext4         80b496fa-ce2d-4dcf-9afc-bcaa731a67f1   13.3G     1%
/mnt/example

In the example above we used also the -d option for lsblk, short for --nodeps, to hide filesystems structure trees from the output.  Now that we gathered information about a filesystem we can create an entry in fstab for it. In the first field of the entry, to reference the /dev/sdb1we will use its UUID:

UUID=80b496fa-ce2d-4dcf-9afc-bcaa731a67f1

Second field - The mountpoint

In each fstab entry, the second field specifies the mountpoint for the filesystem: what directory in the system should be used to access its content. This should be always provided except if the block device we are referencing it's used as swap. In that case "none" should be used. Suppose we want to mount our filesystem to "/mnt/example"; we would write:

UUID=80b496fa-ce2d-4dcf-9afc-bcaa731a67f1 /mnt/example

Third field - The filesystem type

The third field of an fstab entry specifies the type of filesystem in use on the raw block device or partition. The filesystem must be among the ones supported by the operating system like, for example ext4, xfs etc. In case of a remote filesystem we can use, for example cifs as the value of this field if the filesystem is shared via samba or nfs if it is shared via the Network File System. In the case of our example, we know the sdb1 device is formatted with the ext4 filesystem, therefore our fstab entry becomes:

UUID=80b496fa-ce2d-4dcf-9afc-bcaa731a67f1 /mnt/example ext4

Fourth field - Mount options

The fourth field of each entry in the fstab file is used to provide a list of options to be used when mounting the filesystem. To use the default set of mount options we specify default as a value. Default options are:

  • rw (read-write);
  • suid (respect setuid and setgid bits);
  • dev (interpret characters and block devices on the filesystem);
  • exec (allow executing binaries and scripts);
  • auto (mount the filesystem when the -a option of the mount command is used);
  • nouser(make the filesystem not mountable by a standard user);
  • async (perform I/O operations on the filesystem asynchronously).

To see the list of the available options we can consult the mount manual:

$ man mount

At this point, our entry becomes:

UUID=80b496fa-ce2d-4dcf-9afc-bcaa731a67f1 /mnt/example ext4    defaults

Fifth field - Should the filesystem be dumped ?

The fifth field in each entry can be either 0 or 1. The value is used by the dump backup program (if installed) to know what filesystem should be dumped. Typically our entry becomes:

UUID=80b496fa-ce2d-4dcf-9afc-bcaa731a67f1 /mnt/example ext4    defaults         0


Sixth field - Fsck order

The sixth field is used to establish the order by which another utility, fsck, should check filesystems on boot. The value of  1 must always be used for the root filesystem; for all the others we can use 2. If this value is not provided it defaults to 0, and the filesystem will not be checked. With this last field our example entry is finally complete:

UUID=80b496fa-ce2d-4dcf-9afc-bcaa731a67f1 /mnt/example ext4    defaults   0      2

Conclusions

In this tutorial we learned how /etc/fstab, one of the most important files in a linux-based operating system, is structured. We learned that is contains static information about filesystems and we saw that each entry in the file is composed by six fields, each one with a specific purpose we examined.

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