/etc/fstab is a key file for file systems in any Linux Unix system. Learn fields, formats within /etc/fstab file. Understand the meaning of each field and how it can be set.

/etc/fstab is one of the key files in running a Linux or UNIX system. File system mounting can be controlled using this file. This is one of the files being used at boot to validate and mount file systems on the machine.  This file is human-readable and can be edited with a text editor like vi.

This file contains 6 parameters per row. Each row represents one file system details. They are as below :

1. Volume
2. Mount point
3. File system type
4. Options
5. Dump
6. Pass

Let’s see one by one –

1. Volume

This is a disk or logical volume which is the source to be mounted on the mount point specified in the second field. See the below example of fstab from the Linux system.

# cat /etc/fstab


# /etc/fstab
# Created by anaconda on Thu Dec  5 15:47:52 2013
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
/dev/mapper/vg_00-lv_root /                       ext4    defaults        1 1
UUID=f2918ad9-f5ce-485d-81ae-e874f57f6f57 /boot                   ext4    defaults        1 2
/dev/mapper/vg_00-lv_home /home                   ext4    defaults        1 2
/dev/mapper/vg_00-lv_tmp /tmp                    ext4    defaults        1 2
/dev/mapper/vg_00-lv_usr /usr                    ext4    defaults        1 2
/dev/mapper/vg_00-lv_var /var                    ext4    defaults        1 2
/dev/mapper/vg_00-lv_swap swap                    swap    defaults        0 0
tmpfs                   /dev/shm                tmpfs   defaults        0 0
devpts                  /dev/pts                devpts  gid=5,mode=620  0 0
sysfs                   /sys                    sysfs   defaults        0 0
proc                    /proc                   proc    defaults        0 0
/dev/sdb                /app                    ext3    defaults        1 2
10.10.2.3:/my_share     /tmp/nfs_share          nfs      defaults       0 0

In the above example, you can see volume is specified by UUID or logical volume name or disk name or IP:/directory.

/boot entry is specified by UUID. UUID is a universally unique ID assigned to each disk when it’s formatted in the system. The disk can be identified by UUID or disk name in the kernel.  Since its unique number, it’s ideal to use UUID in fstab for important file systems!

/var, /tmp, etc entries are defined using volume as a logical volume name. They are logical volumes part of the volume group vg00. See LVM legends to get familiarize with naming conventions.

/dev/shm is defined by tmpfs volume. Its a temporary file system volume created and identified by the kernel on the root disk. devpts, sysfsare part of such system-defined file systems.

Second Last entry,  you can see disk sdb is also defined as a volume for /app entry.

Lastly, the NFS share is mounted on /tmp/nfs_share directory. There IP address of the NFS server and its exported share volume name combination is defined as a volume.

This is the first argument to be supplied in mount command while mounting any filesystem.

Normally HPUX uses LVM as a partition manager hence only logical volumes are found as a volume entry in fstab. See below the example of fstab from the HPUX system.

$ cat /etc/fstab

# System /etc/fstab file.  Static information about the file systems
# See fstab(4) and sam(1M) for further details on configuring devices.
/dev/vg00/lvol3 / vxfs delaylog 0 1
/dev/vg00/lvol1 /stand vxfs tranflush 0 1
/dev/vg00/lvol4 /home vxfs delaylog 0 2
/dev/vg00/lvol5 /opt vxfs delaylog 0 2
/dev/vg00/lvol6 /tmp vxfs delaylog 0 2
/dev/vg00/lvol7 /usr vxfs delaylog 0 2
/dev/vg00/lvol8 /var vxfs delaylog 0 2
/dev/vg00/lvol10 /var/adm/sw vxfs delaylog 0 2
/dev/vg00/lvol11 /admin vxfs delaylog 0 2
10.10.2.3:/my_share /tmp/nfs_share nfs defaults 0 0

2. Mount point

Its second field in an entry of fstab. This is the name of the directory on which volume should be mounted. It should always be an absolute path (i.e. starts with/and has all directory hierarchy till last expected directory) in this field.

Directories like /var, /boot, /tmp, /stand, /usr, /home, /proc, /sys are (and should be) reserved for system mount points. In HPUX even logical volume numbers of root VG are reserved for system mount points like lvol1 should always be /stand. 2 for swap, 3 for root, etc.

This is the second argument to be supplied to mount command when mounting any file system.

3. File system type

This is FS type to be considered while mounting the given volume on the specified mount point. Different file system types have different functions and advantages to offer. You need to specify the same FS type which was used at the time of formatting respective volume. ext3, ext4 (Linux FS), vxfs (veritas FS), NFS (Network FS), swap (SWAP FS) are a few types.

This can be supplied to mount command with -F option.

4. Options

Those are file system options that will enhance the user experience of the mount point. They also impact on the performance of the file system and impact in recovery in case of failures. Value defaults in the above example instructs mount command to use parameters defined inbuilt. They can be seen in the man page :

defaults
              Use default options: rw, suid, dev, exec, auto, nouser, async, and relatime.

All available options can be summarized as below :

These options can be supplied to mount command using -o.

5. Dump

This is an old fashioned backup option in case the server goes down. If this is set to 1 then FS dump will happen when the system goes down due to some issue. Setting this 0 will nullify this option.

6. Pass

This tells kernel about file system check priority or sequence. fsck is a facility that checks the file system for its consistency. During boot, if fsck is invoked then it looks for this file. If set to 0, fsck will be skipped for that mount point. If set to 1 then that mount points will be first in sequence to be fscked.