To attach a certain file system, use the mount
command in the following form:
$
mount
[option…] device directory
The device can be identified by:
a full path to a block device: for example,
/dev/sda3
a universally unique identifier [
UUID]: for example,
UUID=34795a28-ca6d-4fd8-a347-73671d0c19cb
a volume label: for example,
LABEL=home
Note that while a file system is mounted, the original content of the directory is not accessible.
Linux does not prevent a user from mounting a file system to a directory with a file system already attached to it. To determine whether a particular directory serves as a mount point, run the findmnt
utility with the directory as its argument and verify the exit code:
findmnt
directory;echo
$?
If no file system is attached to the directory, the given command returns 1
.
When you run
the mount
command without all required information, that is without the device name, the target directory, or the file system type, the mount
reads the contents of the /etc/fstab
file to check if the given file system is listed. The /etc/fstab
file contains a list of device names and the directories in which the selected file systems are set to be mounted as well as the file system type and mount options. Therefore, when mounting a file system that is specified in /etc/fstab
, you can choose one
of the following options:
mount
[option…] directorymount
[option…] device
To determine the UUID and—if the device uses it—the label of a particular device, use the blkid
command in the following form:
blkid
device
For example, to display information about /dev/sda3
:
#
blkid /dev/sda3
/dev/sda3: LABEL="home" UUID="34795a28-ca6d-4fd8-a347-73671d0c19cb" TYPE="ext3"
19.2.1. Specifying the File System Type
In most cases, mount
detects the file system automatically. However, there are certain file systems,
such as NFS
[Network File System] or CIFS
[Common Internet File System], that are not recognized, and need to be specified manually. To specify the file system type, use the mount
command in the following form:
$
mount -t type device directory
Table 19.1. Common File System Types
ext2
| The ext2 file system.
|
ext3
| The ext3 file system.
|
ext4
| The ext4 file system.
|
btrfs
| The btrfs file system.
|
xfs
| The xfs file system.
|
iso9660
| The ISO 9660 file system. It is commonly used by optical media, typically CDs.
|
nfs
| The NFS file system. It is commonly used to access files over the network.
|
nfs4
| The NFSv4 file system. It is commonly used to access files over the network.
|
udf
| The UDF file system. It is commonly used by optical media, typically DVDs.
|
vfat
| The FAT file system. It is commonly used on machines that are running the Windows operating system, and on certain digital media such as USB flash drives or floppy disks.
|
Example 19.2. Mounting a USB Flash Drive
Older USB flash drives often use the FAT file system. Assuming that such drive uses the /dev/sdc1
device and that the /media/flashdisk/
directory exists, mount it to this directory by typing the following at a shell prompt as root
:
~]# mount -t vfat /dev/sdc1 /media/flashdisk
19.2.2. Specifying the Mount Options
To specify additional mount options, use the command in the following form:
mount
-o
options device directory
When supplying multiple options, do not insert a space after a comma, or mount
interprets incorrectly the values following spaces as additional parameters.
Table 19.2. Common Mount Options
async
| Allows the asynchronous input/output operations on the file system. |
auto
| Allows the file system to be mounted automatically using the mount -a command.
|
defaults
| Provides an alias for async,auto,dev,exec,nouser,rw,suid .
|
exec
| Allows the execution of binary files on the particular file system. |
loop
| Mounts an image as a loop device. |
noauto
| Default behavior disallows the automatic mount of the file system using the mount -a command.
|
noexec
| Disallows the execution of binary files on the particular file system. |
nouser
| Disallows an ordinary user [that is, other than root ] to mount and unmount the file system.
|
remount
| Remounts the file system in case it is already mounted. |
ro
| Mounts the file system for reading only. |
rw
| Mounts the file system for both reading and writing. |
user
| Allows an ordinary user [that is, other than root ] to mount and unmount the file system.
|
Example 19.3. Mounting an ISO Image
An ISO image [or a disk image in general] can be mounted by using the loop device. Assuming that the ISO image of the Fedora 14 installation disc is present in the current working directory and that the /media/cdrom/
directory exists, mount the image to this directory by running the following command:
#
mount -o ro,loop Fedora-14-x86_64-Live-Desktop.iso /media/cdrom
Note that ISO 9660 is by design a read-only file system.
19.2.3. Sharing Mounts
Occasionally, certain system administration tasks require access to the same file system from more than one place in the directory tree [for example, when preparing a chroot environment]. This is possible, and Linux allows you to mount the same file system to as many directories as necessary. Additionally, the
mount
command implements the --bind
option that provides a means for duplicating certain mounts. Its usage is as follows:
$
mount --bind old_directory new_directory
Although this command allows a user to access the file system from both places, it does not apply on the file systems that are mounted within the original directory. To include these mounts as well, use the following command:
$
mount --rbind old_directory new_directory
Additionally, to provide as much flexibility as possible, Red Hat Enterprise Linux 7 implements the functionality known as shared subtrees. This feature allows the use of the following four mount types:
Shared Mount
A shared mount allows the creation of an exact replica of a given mount point. When a mount point is marked as a shared mount, any mount within the original mount point is reflected in it, and vice versa. To change the type of a mount point to a shared mount, type the following at a shell prompt:
$
mount --make-shared mount_point
Alternatively, to change the mount type for the selected mount point and all mount points under it:
Slave Mount$
mount --make-rshared mount_point
A slave mount allows the creation of a limited duplicate of a given mount point. When a mount point is marked as a slave mount, any mount within the original mount point is reflected in it, but no mount within a slave mount is reflected in its original. To change the type of a mount point to a slave mount, type the following at a shell prompt:
mount
--make-slave
mount_point
Alternatively, it is possible to change the mount type for the selected mount point and all mount points under it by typing:
mount
--make-rslave
mount_point
Example 19.5. Creating a Slave Mount Point
This
example shows how to get the content of the /media/
directory to appear in /mnt/
as well, but without any mounts in the /mnt/
directory to be reflected in /media/
. As root
, first mark the /media/
directory as shared:
~]#mount --bind /media /media
~]#mount --make-shared /media
Then create its duplicate in /mnt/
, but mark it as "slave":
~]#mount --bind /media /mnt
~]#mount --make-slave /mnt
Now verify that a mount within /media/
also appears in /mnt/
. For example, if the CD-ROM drive contains non-empty media and the /media/cdrom/
directory exists, run the following
commands:
~]#mount /dev/cdrom /media/cdrom
~]#ls /media/cdrom
EFI GPL isolinux LiveOS ~]#ls /mnt/cdrom
EFI GPL isolinux LiveOS
Also verify that file systems mounted in the /mnt/
directory are not reflected in /media/
. For instance, if a non-empty USB flash drive that uses the /dev/sdc1
device is plugged in and the /mnt/flashdisk/
directory is present, type:
~]#Private Mountmount /dev/sdc1 /mnt/flashdisk
~]#ls /media/flashdisk
~]#ls /mnt/flashdisk
en-US publican.cfg
A private mount is the default type of mount, and unlike a shared or slave mount, it does not receive or forward any propagation events. To explicitly mark a mount point as a private mount, type the following at a shell prompt:
mount
--make-private
mount_point
Alternatively, it is possible to change the mount type for the selected mount point and all mount points under it:
mount
--make-rprivate
mount_point
Example 19.6. Creating a Private Mount Point
Taking into account the scenario in
Example 19.4, “Creating a Shared Mount Point”, assume that a shared mount point has been previously created by using the following commands as root
:
~]#mount --bind /media /media
~]#mount --make-shared /media
~]#mount --bind /media /mnt
To mark the /mnt/
directory as private, type:
~]# mount --make-private /mnt
It is now possible to verify that none
of the mounts within /media/
appears in /mnt/
. For example, if the CD-ROM drives contains non-empty media and the /media/cdrom/
directory exists, run the following commands:
~]#mount /dev/cdrom /media/cdrom
~]#ls /media/cdrom
EFI GPL isolinux LiveOS ~]#ls /mnt/cdrom
~]#
It is also possible to verify that file systems mounted in the /mnt/
directory are not reflected in /media/
. For instance, if a non-empty USB flash drive that uses the /dev/sdc1
device is plugged in and the /mnt/flashdisk/
directory is present, type:
~]#Unbindable Mountmount /dev/sdc1 /mnt/flashdisk
~]#ls /media/flashdisk
~]#ls /mnt/flashdisk
en-US publican.cfg
In order to prevent a given mount point from being duplicated whatsoever, an unbindable mount is used. To change the type of a mount point to an unbindable mount, type the following at a shell prompt:
mount
--make-unbindable
mount_point
Alternatively, it is possible to change the mount type for the selected mount point and all mount points under it:
mount
--make-runbindable
mount_point
Example 19.7. Creating an Unbindable Mount Point
To prevent the /media/
directory from being shared, as root
:
#
mount --bind /media /media
#
mount --make-unbindable /media
This way, any subsequent attempt to make a duplicate of this mount fails with an error:
#
mount --bind /media /mnt
mount: wrong fs type, bad option, bad superblock on /media, missing codepage or helper program, or other error In some cases useful info is found in syslog - try dmesg | tail or so
19.2.4. Moving a Mount Point
To change the directory in which a file system is mounted, use the following command:
#
mount --move old_directory new_directory
Example 19.8. Moving an Existing NFS Mount Point
An NFS storage contains user directories and is already mounted in /mnt/userdirs/
. As root
, move this mount point to /home
by using the following command:
#
mount --move /mnt/userdirs /home
To verify the mount point has been moved, list the content of both directories:
#
ls /mnt/userdirs
#
ls /home
jill joe
19.2.5. Setting Read-only Permissions for root
Sometimes, you need to mount the root file system with read-only permissions. Example use cases include enhancing security or ensuring data integrity after an unexpected system power-off.
19.2.5.1. Configuring root
to Mount with
Read-only Permissions on Boot
In the
/etc/sysconfig/readonly-root
file, changeREADONLY
toyes
:# Set to 'yes' to mount the file systems as read-only. READONLY=yes [output truncated]
Change
defaults
toro
in the root entry [/
] in the/etc/fstab
file:/dev/mapper/luks-c376919e... / ext4 ro,x-systemd.device-timeout=0 1 1
Add
ro
to theGRUB_CMDLINE_LINUX
directive in the/etc/default/grub
file and ensure that it does not containrw
:GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet ro"
Recreate the GRUB2 configuration file:
#
grub2-mkconfig -o /boot/grub2/grub.cfg
If you need to add files and directories to be mounted with write permissions in the
tmpfs
file system, create a text file in the/etc/rwtab.d/
directory and put the configuration there. For example, to mount/etc/example/file
with write permissions, add this line to the/etc/rwtab.d/example
file:files /etc/example/file
Changes made to files and directories in
tmpfs
do not persist across boots.Reboot the system.
19.2.5.2. Remounting root
Instantly
If root [/
] was mounted with read-only permissions on system boot, you can remount it with write permissions:
#
mount -o remount,rw /
This can be particularly useful when /
is incorrectly mounted with read-only permissions.
To remount /
with read-only permissions again, run:
#
mount -o remount,ro /
19.2.5.3. Files and Directories That Retain Write Permissions
For the system
to function properly, some files and directories need to retain write permissions. With root in read-only mode, they are mounted in RAM in the tmpfs
temporary file system. The default set of such files and directories is read from the /etc/rwtab
file, which contains:
dirs /var/cache/man dirs /var/gdm [output truncated] empty /tmp empty /var/cache/foomatic [output truncated] files /etc/adjtime files /etc/ntp.conf [output truncated]
Entries in the /etc/rwtab
file follow this format:
how the file or directory is copied to tmpfs path to the file or directory
A file or directory can be copied to tmpfs
in the following three ways:
empty path
: An empty path is copied totmpfs
. Example:empty /tmp
dirs path
: A directory tree is copied totmpfs
, empty. Example:dirs /var/run
files path
: A file or a directory tree is copied totmpfs
intact. Example:files /etc/resolv.conf
The same format applies when adding custom paths to /etc/rwtab.d/
.