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Understanding /etc/fstab

Here's an example of the contents of /etc/fstab:
/dev/hda2 	/ 	ext2 	defaults 	1 1
/dev/hdb1 	/home 	ext2 	defaults 	1 2
/dev/cdrom 	/media/cdrom 	auto 	ro,noauto,user,exec 	0 0
/dev/fd0 	/media/floppy 	auto 	rw,noauto,user,sync 	0 0
proc 	/proc 	proc 	defaults 	0 0
/dev/hda1 	swap 	swap 	pri=42 	0 0

The first and second columns should be pretty straightforward. They mean that /dev/hda2 will be mounted to / and /dev/hdb1 to /home. This is done automatically when your Linux system boots up.

The third column in /etc/fstab specifies the filesystem type of the device or partition.

The fourth column in fstab lists all the mount options for the device or partition. This is also the most confusing column in the fstab file, but knowing, what some of the most common options mean, saves you from a big headache. Yes, there are many options available, but I'll take a look at the most widely used ones only. For more information, check out the man page of mount.

auto and noauto With the auto option, the device will be mounted automatically (at bootup, just like I told you a bit earlier, or when you issue the mount -a command). auto is the default option. If you don't want the device to be mounted automatically, use the noauto option in /etc/fstab. With noauto, the device can be mounted only explicitly.

user and nouser These are very useful options. The user option allows normal users to mount the device, whereas nouser lets only the root to mount the device. nouser is the default, which is a major cause of headache for new Linux users. If you're not able to mount your cdrom, floppy, Windoze partition, or something else as a normal user, add the user option into /etc/fstab.

exec and noexec exec lets you execute binaries that are on that partition, whereas noexec doesn't let you do that. noexec might be useful for a partition that conatins binaries you don't want to execute on your system, or that can't even be executed on your system. This might be the case of a Windows partition.

exec is the default option, which is a good thing. Imagine what would happen if you accidentally used the noexec option with your Linux root partition...

ro Mount the filesystem read-only.

rw Mount the filesystem read-write. Again, using this option might cure the headache of many new Linux users who are tearing their hair off because they can't write to their floppies, Windows partitions, or something else.

sync and async How the input and output to the filesystem should be done. sync means it's done synchronously. If you look at the example fstab, you'll notice that this is the option used with the floppy. In plain English, this means that when you, for example, copy a file to the floppy, the changes are physically written to the floppy at the same time you issue the copy command.

However, if you have the async option in /etc/fstab, input and output is done asynchronously. Now when you copy a file to the floppy, the changes may be physically written to it long time after issuing the command. This isn't bad, and may sometimes be favorable, but can cause some nasty accidents: if you just remove the floppy without unmounting it first, the copied file may not physically exist on the floppy yet!

async is the default. However, it may be wise to use sync with the floppy, especially if you're used to the way it's done in Windoze and have a tendency to remove floppies before unmounting them first.

defaults Uses the default options that are rw, suid, dev, exec, auto, nouser, and async.

5th and 6th columns Well, dump is a backup utility and fsck is a filesystem check utility. I won't discuss them in great length here (they would both need their own tuXfile), but I'll mention them, because otherwise you'd spend the rest of the day wondering what on God's green Earth do these things mean.

The 5th column in /etc/fstab is the dump option. Dump checks it and uses the number to decide if a filesystem should be backed up. If it's zero, dump will ignore that filesystem. If you take a look at the example fstab, you'll notice that the 5th column is zero in most cases.

The 6th column is a fsck option. fsck looks at the number in the 6th column to determine in which order the filesystems should be checked. If it's zero, fsck won't check the filesystem.