fstab
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The fstab (/etc/fstab) (or file systems table) file is commonly found on Unix systems as part of the system configuration. The fstab file typically lists all available disks and disk partitions, and indicates how they are to be initialized or otherwise integrated into the overall system's file system.
The fstab file is most commonly used by the mount command, which reads the fstab file to determine which options should be used when mounting the specified device.
Traditionally, the fstab was only read by programs, and not written. However, some administration tools can automatically build and edit fstab, or act as graphical editors for it, such as the Kfstab graphical configuration utility available for KDE. Modern systems use udev to handle hot swapping devices instead of rewriting fstab file on the fly. It is the duty of the system administrator to properly create and maintain this file.
The file may have other names on a given Unix variant; for example, it is /etc/vfstab on Solaris.
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[edit] Example
The following is an example of an fstab file on a Red Hat Linux system:
# device name mount point fs-type options dump-freq pass-num LABEL=/ / ext3 defaults 1 1 /dev/hda6 swap swap defaults 0 0 none /dev/pts devpts gid=5,mode=620 0 0 none /proc proc defaults 0 0 none /dev/shm tmpfs defaults 0 0 # Removable media /dev/cdrom /mount/cdrom udf,iso9660 noauto,owner,kudzu,ro 0 0 /dev/fd0 /mount/floppy auto noauto,owner,kudzu 0 0 # NTFS Windows XP partition /dev/hda1 /mnt/WinXP ntfs-3g quiet,defaults,locale=en_US.utf8,umask=0 0 0 # Partition shared by Windows and Linux /dev/hda7 /mnt/shared vfat umask=000 0 0 # mounting tmpfs tmpfs /mnt/tmpfschk tmpfs size=100m 0 0
The columns are as follows:
- The device name or other means of locating the partition or data source.
- The mount point, where the data is to be attached to the filesystem.
- The filesystem type, or the algorithm used to interpret the filesystem.
- Options, including if the filesystem should be mounted at boot. (kudzu is an option specific to Red Hat and Fedora Core.)
- dump-freq adjusts the archiving schedule for the partition (used by dump).
- pass-num indicates the order in which the fsck utility will scan the partitions for errors when the computer powers on. 0 = none, 1 = first, 2 = next (and all others in order)
A value of zero in either of the last 2 columns disables the corresponding feature.
[edit] Options common to all filesystems
As the filesystems in /etc/fstab will eventually be mounted using mount(8) it isn't surprising that the options field simply contains a comma-separated list of options which will be passed directly to mount when it tries to mount the filesystem.
The options common to all filesystems are:
atime / noatime- By default Linux records when files are last accessed, modified and created. This behaviour can be controlled with the noatime option, which will prevent this information from being recorded. The advantage might be a performance increase, especially when files are accessed and modified often.
auto / noauto- With the auto option, the device will be mounted automatically at bootup or when the mount -a command is issued. 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 only mounted explicitly.
dev / nodev- Interpret/do not interpret block special devices on the filesystem.
exec / 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 contains no binaries, like /var, or contains binaries you don't want to execute on your system, or that can't even be executed on your system. Last might be the case of a Windows partition.
ro- Mount read-only.
rw- Mount the filesystem read-write. Again, using this option might alleviate confusion on the part of new Linux users who are frustrated because they can't write to their floppies, Windows partitions, or other media.
sync / 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.
suid / nosuid- Permit/Block the operation of suid, and sgid bits.
user / users / nouseruserpermits any user to mount the filesystem. This automatically implies noexec, nosuid, nodev unless overridden. Ifnouseris specified, only root can mount the filesystem. Ifusersis specified, every user in group users will be able to unmount the volume.owner(This is Linux specific)- Permit the owner of device to mount.
defaults- Use default settings. Default settings are defined per file system at the file system level. For ext3 file systems these can be set with the tune2fs command. The normal default for Ext3 file systems is equivalent to
rw,suid,dev,exec,auto,nouser,async(no acl support). Modern Red Hat based systems set acl support as default on the root file system but not on user created Ext3 file systems. Some file systems such as XFS enable acls by default. Default file system mount attributes can be over ridden in /etc/fstab.
[edit] Filesystem specific options
There are many options for the specific filesystems supported by mount. Listed below are some of the more commonly used. For the full list check out the man page for mount.
[edit] ext2
check={none, normal, strict}- Sets the fsck checking level.
debug- Print debugging info on each remount .
sb=n- n is the block which should be used as the superblock for the fs.
[edit] (k)ubuntu default
relatime- As of kernel 2.6.20, if this flag is set, access times are only updated if they are earlier than the modification time. This change allows utilities to see if the current version of a file has been read, but still cuts down significantly on atime updates. Using relatime can still confuse tools which want to ask questions like "has this file been accessed in the last week?"
[edit] fat
check={r[elaxed], n[ormal], s[trict]}- Not the same as ext2, but rather deals with allowed filenames. See mount(8).
conv={b[inary], t[ext], a[uto]}- Performs DOS<->UNIX text file conversions automatically. See mount(8).
uid=n, gid=n- Sets the user identifier, uid, and group identifier, gid, for all files on the filesystem.
umask=nnn, dmask=nnn, fmask=nnn- Sets the user file creation mode mask, umask, the same for directories only, dmask and for files only, fmask.
[edit] iso9660
norock- Disables Rock Ridge extensions.
More detailed information about the fstab file can be found in the man page about it.[1]
[edit] Mounting all filesystems
mount -a
This command will mount all (not-yet-mounted) filesystems mentioned in fstab and is used in system script startup during booting.
