Learn Linux, 101
Control mounting and unmounting of filesystems
Getting to your data
This content is part # of # in the series: Learn Linux, 101
This content is part of the series:Learn Linux, 101
Stay tuned for additional content in this series.
In this tutorial, learn to:
- Mount and unmount filesystems manually
- Configure filesystem mounting on bootup
- Configure user-mountable, removable filesystems such as tape drives, external USB drives, floppies, and CDs or DVDs
This tutorial helps you prepare for Objective 104.3 in Topic 104 of the Linux Server Professional (LPIC-1) exam 101. The objective has a weight of 3.
The Linux filesystem is one big tree rooted at /, and yet we have filesystems on different devices and partitions. How do we resolve this apparent incongruity? The root (/) filesystem is mounted as part of the initialization process. Each of the other filesystems that you create is not usable by your Linux system until it is mounted at a mount point.
To get the most from the tutorials in this series, you should have a basic knowledge of Linux and a working Linux system on which you can practice the commands covered in this tutorial. Unless otherwise noted, the examples in this tutorial use CentOS 6, with a 2.6.32-504 kernel. Your results on other systems may differ. Sometimes different versions of a program will format output differently, so your results may not always look exactly like the listings and figures shown here.
You should also be familiar with the material in our tutorial, "Learn Linux 101: Create partitions and filesystems."
In the current set of mounted filesystems, a mount point is simply a directory where the filesystem on a device is grafted into the tree. Mounting is the process of making the filesystem on the device accessible. For example, you might mount filesystems on hard drive partitions as /boot, /tmp, or /home, and you might mount the filesystem on a floppy drive as /mnt/floppy and the filesystem on a CD-ROM as /media/cdrom1. As you can see, mount points may be in the root directory, or in a subdirectory farther down the tree.
Besides filesystems on partitions, floppy disks, and CDs, there are other types of filesystems. The tmpfs filesystem is a virtual memory filesystem. You can also mount filesystems from one system on another system using a networked filesystem such as NFS or AFS. You can even create a file in an existing filesystem and format that as a, possibly different, kind of filesystem and mount that too. This is often done with images of optical media, where you perhaps download an ISO CD or DVD image, then mount that file rather than burning it to real media. Swap space in a file rather than a dedicated swap partition is another example.
While the mount process actually mounts the filesystem on some device (or other resource), it is common to simply say that you "mount the device," which is understood to mean "mount the filesystem on the device."
Mounting and unmounting filesystems usually requires root authority. If you
are logged in as an ordinary user, you will either use
to switch to root or
sudo. In our examples, when the command
prompt ends with #, as in the listing below, you will need root
The basic form of the
mount command takes two parameters: the
device (or other resource) containing the filesystem to be mounted, and
the mount point. We'll mount our small FAT32 partition /dev/sda3 at the
mount point /dos. The mount point must exist before you mount anything
over it. If it does not, you will get an error and need to create the
mount point or use a different mount point. We illustrate these aspects of
basic mounting in Listing 1.
Listing 1. Mount error
[root@attic4-cent ~]# mount /dev/sda3 /dos mount: mount point /dos does not exist [root@attic4-cent ~]# mkdir /dos [root@attic4-cent ~]# mount /dev/sda3 /dos
When you mount a filesystem over an existing directory, the files on the filesystem you are mounting become the files and subdirectories of the mount point. If the mount point directory already contained files or subdirectories, they are not lost, but are no longer visible until the mounted filesystem is unmounted, at which point they become visible again. It is a good idea to avoid this problem by using only empty directories as mount points.
After mounting a filesystem, any files or directories created or copied to the mount point or any directory below it will be created on the mounted filesystem. So a file such as /dos/sampdir/file.txt will be created on the FAT32 filesystem that we mounted at /dos in our example.
mount command will automatically detect the type
of filesystem being mounted. Occasionally you may need to specify the
filesystem type explicitly using the
-t option as shown in
Listing 2. Mounting with explicit filesystem type
[root@attic4-cent ~]# mount -t vfat /dev/sda3 /dos
To see what filesystems are mounted, use the
with no parameters. Listing 3 shows
our example system. Note that you do not need root authority to simply
list mounted filesystems.
Listing 3. Displaying mounted filesystems
[ian@attic4-cent ~]$ mount /dev/sda11 on / type ext3 (rw) proc on /proc type proc (rw) sysfs on /sys type sysfs (rw) devpts on /dev/pts type devpts (rw,gid=5,mode=620) tmpfs on /dev/shm type tmpfs (rw,rootcontext="system_u:object_r:tmpfs_t:s0") /dev/sda1 on /grubfile type ext3 (rw) /dev/sdb2 on /home/ian/data type ext4 (rw) /dev/sdb3 on /home/ian/research type ext3 (rw) /dev/sdc1 on /home/ian/pictures type ext4 (rw) none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw) /dev/sda3 on /dos type vfat (ro) /dev/sr0 on /media/KNOPPIX type iso9660 (ro,nosuid,nodev,uhelper=udisks,uid=1000, gid=1000,iocharset=utf8,mode=0400,dmode=0500)
You can also view similar information by displaying /proc/mounts or /etc/mtab, both of which contain information about mounted filesystems.
mount command has several options that override the
default behavior. For example, you can mount a filesystem read-only by
-o ro. If the filesystem is already mounted, add
remount as shown in Listing 4.
Listing 4. Remounting read-only
[root@attic4-cent ~]# mount -o remount,ro /dos
- Use commas to separate multiple options, such as
- When remounting an already mounted filesystem, it suffices to specify either the mount point or the device name. It is not necessary to specify both.
- You cannot mount a read-only filesystem as read-write. Media that cannot be modified, such as CD-ROM discs, will automatically be mounted read-only.
- To remount a writable device read-write, specify
Remount commands will not complete successfully if any process has open files or directories in the filesystem you are remounting. See Unmounting filesystems below for additional information.
Labels, UUIDs, and links
In UNIX and early Linux systems, the /dev directory usually contained
entries for all the devices that might ever be attached to a system. Any
device that was used was always located in the same place in the /dev
tree, so using names such as /dev/sda6 was natural. With the advent of
hot-plugging of devices such as USB or Firewire (IEEE 1394) attached
devices, a given device might appear in one USB port today, and that same
device might be plugged into a different USB port tomorrow. In this
environment, you might want to always mount your USB stick at
/media/myusbstick, regardless of which USB port you plug it in to. In the
tutorial for topic 102, "Learn
Linux, 101: Boot managers," you learned about using labels and
UUIDs (Universally Unique IDs) instead of device names to identify
partitions. If the filesystem on the partition supports either, you can
use these with the
mount command too. Use the
blkid command to find out the UUID and label (if present)
associated with a device.
Listing 5 shows how to use
blkid to find the label and UUID for our root partition and
then how to create two additional mount points and mount the root
partition at these two additional points. This example is for
illustration. You would not normally do this in a production environment.
Listing 5. Mount using labels or UUIDs
[root@attic4-cent ~]# blkid /dev/sda11 /dev/sda11: UUID="2f60a3b4-ef6c-4d4c-9ef4-50d7f75124a2" TYPE="ext3" LABEL="CentOS 6" [root@attic4-cent ~]# mkdir /mnt/sda11label [root@attic4-cent ~]# mkdir /mnt/sda11uuid [root@attic4-cent ~]# mount LABEL="CentOS 6" /mnt/sda11label [root@attic4-cent ~]# mount UUID="2f60a3b4-ef6c-4d4c-9ef4-50d7f75124a2" /mnt/sda11uuid
With the advent of udev, you will find additional symbolic links in the /dev directory for devices such as hard drives. Listing 6 shows the links to /dev/sda6 on my CentOS 6 system.
Listing 6. Symbolic links to /dev/sda6
[root@attic4-cent ~]# find /dev -lname "*sda11"/dev/root /dev/disk/by-label/CentOS\x206 /dev/disk/by-uuid/2f60a3b4-ef6c-4d4c-9ef4-50d7f75124a2 /dev/disk/by-id/wwn-0x50014ee056628af6-part11 /dev/disk/by-id/scsi-SATA_WDC_WD6401AALS-_WD-WMASY6347052-part11 /dev/disk/by-id/ata-WDC_WD6401AALS-00L3B2_WD-WMASY6347052-part11 /dev/disk/by-path/pci-0000:00:11.0-scsi-0:0:0:0-part11 /dev/block/8:11 /dev/.udev/watch/113 /dev/.udev/links/disk\x2fby-label\x2fCentOS\x5cx206/b8:11 /dev/.udev/links/root/b8:11 /dev/.udev/links/disk\x2fby-id\x2fwwn-0x50014ee056628af6-part11/b8:11 /dev/.udev/links/disk\x2fby-uuid\x2f2f60a3b4-ef6c-4d4c-9ef4-50d7f75124a2/b8:11 /dev/.udev/links/disk\x2fby-path\x2fpci-0000:00:11.0-scsi-0:0:0:0-part11/b8:11 /dev/.udev/links/disk\x2fby-id\x2fscsi-SATA_WDC_WD6401AALS-_WD-WMASY6347052-part11/b8:11 /dev/.udev/links/disk\x2fby-id\x2fata-WDC_WD6401AALS-00L3B2_WD-WMASY6347052-part11/b8:11
You can also use a symbolic link as another way of specifying the device name when mounting a device.
Boot time and fstab
In the tutorial for topic 102, "Learn
Linux, 101: Boot managers," you learned how to use the
root= parameter in both GRUB and LILO to tell the boot loader
what filesystem should be mounted as root. For GRUB2, this is the
set root statement. Once the root filesystem is
mounted, the initialization process runs
mount with the
-a option to automatically mount a set of filesystems. The
set is specified in the file /etc/fstab.
Listing 7 shows /etc/fstab for a sample CentOS 6 system. In this example, most hard drive partitions are identified by UUID. I have added examples of how to mount /dev/sda3 at /dos as we did earlier, and also how to use a label to mount a labeled partition at /mnt/fedora22.
Listing 7. Example CentOS 6 fstab
[root@attic4-cent ~]# cat /etc/fstab # # /etc/fstab # Created by anaconda on Wed Jul 8 09:34:46 2015 # # 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 # UUID=2f60a3b4-ef6c-4d4c-9ef4-50d7f75124a2 / ext3 defaults 1 1 UUID=3c3de27e-779a-44d5-ad7a-61c5fd03d9e7 /grubfile ext3 defaults 1 2 UUID=158d605e-2591-4749-bf59-5e92e1b1c01d 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 UUID=4c962b67-c646-467f-96fb-cbbd6de40140 /home/ian/data ext4 defaults 1 2 UUID=0998d33c-3398-463d-b0e3-7c13ca0c675f /home/ian/research ext3 defaults 1 2 UUID=e3be4658-b79b-470d-82fe-bb434bcdcc2f /home/ian/pictures ext4 defaults 1 2 LABEL="FEDORA22" /mnt/fedora22 ext4 defaults 1 2 /dev/sda3 /dos vfat defaults 0 0
Lines starting with a # character are comments. Remaining lines contain six fields. Because the fields are positional, they must all be specified.
- file system
- This may be a device name such as /dev/sda1, or a label (LABEL=) or UUID (UUID=). For the root filesystem of our CentOS 6 example, it could be /dev/sda11, LABEL="CentOS 6", or UUID=2f60a3b4-ef6c-4d4c-9ef4-50d7f75124a2. Using a label or UUID makes your system more robust when devices are added or removed.
- mount point
- This is the mount point we discussed in Mounting filesystems above. For swap space, this should be the value 'none' or 'swap'. On older systems you will usually find the value 'none'.
- Specifies the type of filesystem. CD/DVD drives will often support
either ISO9660 or UDF filesystems, so you may specify multiple
possibilities in a comma-separated list if you specify such a drive in
/etc/fstab. If you want
mountto automatically determine the type, specify
auto. For example, you might see lines like the following on some older systems for a CD or DVD and a floppy disk.
/dev/scd0 /media/cdrom0 udf,iso9660 user,noauto,exec,utf8 0 0 /dev/fd0 /media/floppy0 auto rw,user,noauto,exec,utf8 0 0
- Specifies the mount options. Specify
defaultsif you want default mount options. Some options you will want to know about are:
rospecify whether the filesystem should be mounted read-write or read-only.
noautospecifies that this filesystem should not be automatically mounted at boot time or whenever
mount -ais issued. In our example, this is done for the removable drives.
userspecifies that a non-root user is permitted to mount and unmount the filesystem. This is especially useful for removable media. In older systems, this option is specified in /etc/fstab rather than on the
mountcommand. With newer systems, it may be specified in udev rules that are located in rules files within /lib/udev/rules.d or /etc/udev/rules.d. The options for the DVD drive on my CentOS 6 system come from udev rules, and that is why there is no entry in /etc/fstab for an optical drive.
noexecspecify whether or not to allow execution of files from the mounted filesystem. User-mounted filesystems default to
execis specified after
noatimewill disable recording of access times. Not using access times may improve performance.
- Specifies whether the
dumpcommand should consider this ext2 or ext3 filesystem for backups. A value of 0 tells
dumpto ignore this filesystem.
- Non-zero values of pass specify the order of checking filesystems at boot time, as discussed in our tutorial "Learn Linux, 101: Maintain the integrity of filesystems."
When you mount a filesystems that is listed in /etc/fstab, you can give either the device name or the mount point when mounting the filesystem. You do not need to give both.
On some systems, for example SUSE 11.2, you may find that the
fstab generated at install time uses symbolic links to the
device. So, you may see
than /dev/sda6 for the file system value.
to Listing 6 for additional possibilities
Consult the man pages for
udev for additional information, including options not
All mounted filesystems are usually unmounted automatically by the system when it is rebooted or shut down. When a filesystem is unmounted, any cached filesystem data in memory is flushed to the device.
You may also unmount filesystems manually. Indeed, you should do this when removing writable media such as diskettes or USB drives or memory keys.
unmount command to unmount the filesystem, specifying
either the device name or mount point as an argument. This
listing shows how to unmount /dos, then remount it and unmount again using
the device name.
Listing 8. Unmounting filesystems
[root@attic4-cent ~]# umount /dos [root@attic4-cent ~]# mount /dev/sda3 /dos [root@attic4-cent ~]# umount /dev/sda3
After a filesystem is unmounted, any files in the directory used for the mount point are visible again.
If you attempt to unmount a filesystem while a process has open files on
that filesystem, you will see an error message. Before unmounting a
filesystem, you should check that there are no processes running that have
open files on the filesystem. Use the
fusercommand to determine what files are open or what process
has open files. You may need the
-w option on
lsof to avoid warning messages related to the Gnome Virtual
File system (gvfs). Check the man pages to learn about additional mount
lsof. If you are checking a whole device, you can
specify the device name or the mount point. You may also check whether an
individual file is in use or not.
To illustrate these commands, I created a copy of /etc/fstab on /dos and a
small script to read lines from stdin and print them to stdout with a 10
second pause between each line. Listing 9 shows the error message from
umount when files are in use and the use of
fuser to check for open files on /dos, or the underlying
Listing 9. Checking for open files
[root@attic4-cent ~]# umount /dos umount: /dos: device is busy. (In some cases useful info about processes that use the device is found by lsof(8) or fuser(1)) [root@attic4-cent ~]# lsof -w /dos COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME slowread. 28960 ian 0r REG 8,3 1207 2 /dos/fstab sleep 28972 ian 0r REG 8,3 1207 2 /dos/fstab [root@attic4-cent ~]# lsof -w /dev/sda3 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME slowread. 28960 ian 0r REG 8,3 1207 2 /dos/fstab sleep 28978 ian 0r REG 8,3 1207 2 /dos/fstab [root@attic4-cent ~]# lsof -w /dos/fstab COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME slowread. 28960 ian 0r REG 8,3 1207 2 /dos/fstab sleep 28989 ian 0r REG 8,3 1207 2 /dos/fstab [root@attic4-cent ~]# fuser -m /dos /dos: 28960 29001 [root@attic4-cent ~]# fuser -m /dev/sda3 /dev/sda3: 28960 29001
At this point you can either wait until the filesystem is no longer busy,
or you can do a lazy unmount, by specifying the
option. A lazy unmount detaches the filesystem from the filesystem tree
immediately, and cleans the references to the filesystem when it is no
We mentioned some issues with removable devices such as USB or Firewire
(IEEE 1394) attached devices. It is inconvenient to switch to root access
every time you need to mount or unmount such a device. The same goes for
CD, DVD, and floppy drives, where you need to unmount the device to change
media. In the discussion of
fstab above, we mentioned the
user option, which allows ordinary users to mount and unmount
Note that the filesystem types for the optical drive are specified as
udf,iso9660, while the filesystem type for the floppy is
auto. For the optical drive, the mount process
will check first for a udf filesystem (common on DVD) and then for an
iso9660 filesystem (common on CD). For the floppy drive, the mount process
will probe for a filesystem type. You can create or edit /etc/filesystems
to change the order in which the filesystems will be probed.
Note: You should always unmount removable drives or media before disconnecting the drive or attempting to remove the media. Failure to do so may result in loss of data that has not yet been written to the device.
If you run a graphical desktop such as Nautilus, you will usually find
options that allow removable devices and media to be automatically
mounted. For example, if I insert a Knoppix DVD into the DVD drive of my
system, I might see a mount entry such as shown in Listing 10. The presence of 'uid=1000' shows that the
user with id 1000 can unmount this disc. The
id command shows
the uid for user ian is 1000, so ian can unmount this disc.
Listing 10. Desktop mounting of DVD
[ian@attic4-cent ~]$ mount | grep sr0 /dev/sr0 on /media/KNOPPIX type iso9660 (ro,nosuid,nodev,uhelper=udisks,uid=1000, gid=1000,iocharset=utf8,mode=0400,dmode=0500) [ian@attic4-cent ~]$ id ian uid=1000(ian) gid=1000(ian) groups=1000(ian)
You may also use the
eject command to eject removable media
when the drive supports the operation as most CD and DVD drives do. If you
have not unmounted the device first, then
eject will both
unmount and eject the disc.
You may have noticed in the discussion of
fstab above that
swap space does not have a mount point. The boot process usually enables
swap space defined in /etc/fstab unless the
noauto option is
specified. To manually control swap space on a running system — for
example, if you added a new swap partition—use the
swapoff commands. See the man pages
You can view the currently enabled swap devices with
cat /proc/swaps or with
swapon -s as shown
in Listing 11.
Listing 11. Displaying swap space
[ian@attic4-cent ~]$ swapon -s Filename Type Size Used Priority /dev/sda2 partition 10241432 8 -1 [ian@attic4-cent ~]$ cat /proc/swaps Filename Type Size Used Priority /dev/sda2 partition 10241432 8 -1
This completes your introduction to device mounting and unmounting on Linux.
- developerWorks roadmap for LPIC-1
- Linux Professional Institute
- LPIC-1: Linux Server Professional Certification program
- LPIC-1 exam 101 objectives
- LPIC-1 exam 102 objectives
- Linux Documentation Project