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mounting

(Written by Paul Cobbaut, https://github.com/paulcobbaut/, with contributions by: Alex M. Schapelle, https://github.com/zero-pytagoras/)

Once you've put a file system on a partition, you can mount it. Mounting a file system makes it available for use, usually as a directory. We say mounting a file system instead of mounting a partition because we will see later that we can also mount file systems that do not exist on partitions.

On all UNIX systems, every file and every directory is part of one big file tree. To access a file, you need to know the full path starting from the root directory. When adding a file system to your computer, you need to make it available somewhere in the file tree. The directory where you make a file system available is called a mount point.

mounting local file systems

mkdir

This example shows how to create a new mount point with mkdir.

student@linux:~$ sudo mkdir /mnt/project42

mount

When the mount point is created, and a file system is present on the partition, then mount can mount the file system on the mount point directory.

student@linux:~$ sudo mount -t ext2 /dev/sdb1 /mnt/project42/

Once mounted, the new file system is accessible to users.

/etc/filesystems

Actually the explicit -t ext2 option to set the file system is not always necessary. The mount command is able to automatically detect a lot of file systems.

When mounting a file system without specifying explicitly the file system, then mount will first probe /etc/filesystems. Mount will skip lines with the nodev directive.

student@linux:~$ cat /etc/filesystems 
ext3
ext2
nodev proc
nodev devpts
iso9660
vfat
hfs

/proc/filesystems

When /etc/filesystems does not exist, or ends with a single * on the last line, then mount will read /proc/filesystems.

student@linux:~$ cat /proc/filesystems | grep -v ^nodev
        ext3
        ext2
        ext4
        btrfs
        fuseblk
        squashfs
        vfat

umount

You can unmount a mounted file system using the umount command.

student@linux:~$ sudo umount /mnt/project42

displaying mounted file systems

To display all mounted file systems, issue the mount command. Or look at the files /proc/mounts and /etc/mtab.

mount

The simplest and most common way to view all mounts is by issuing the mount command without any arguments.

student@linux:~$ mount | grep /dev/sdb
/dev/sdb1 on /mnt/project42 type ext2 (rw)

The output of mount used to be simple enough, but these days, with all the pseudo filesystems under /sys, /run and the like, you have to pipe it through grep to find useful information through all the noise.

However, as it turns out, using mount to list mounted filesystems is deprecated. From the man page mount(8):

   Listing the mounts
       The listing mode is maintained for backward compatibility only.

       For more robust and customizable output use findmnt(8), especially in your scripts.

findmnt

The findmnt command is way more useful to show mounted filesystems. Even the default output looks better structured than mount and there are a lot of options and arguments to customize the output. Try the default output yourself, as it's quite verbose.

A few examples of useful options:

  • --real: Only show "real" filesystems (i.e. not pseudo filesystems):

    [vagrant@el ~]$ findmnt --real
TARGET       SOURCE    FSTYPE  OPTIONS
/            /dev/sda2 xfs     rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota
├─/mnt/part3 /dev/sdc5 xfs     rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota
├─/mnt/part4 /dev/sdc6 fuseblk rw,relatime,user_id=0,group_id=0,allow_other,blksize=4096
├─/mnt/part2 /dev/sdc2 ext4    rw,relatime,seclabel
└─/mnt/part1 /dev/sdc1 ext3    rw,relatime,seclabel

    Note the tree structure and aligned columns!

  • --fstab, -s: Show only filesystems listed in /etc/fstab:

    [vagrant@el ~]$ findmnt --fstab
TARGET     SOURCE                                      FSTYPE OPTIONS
/          UUID=303db791-9236-4ac4-a176-e2a033576d89   xfs    defaults
none       UUID=a4814ebe-c0b2-4819-8129-30f32b3e8772   swap   defaults
/mnt/part1 UUID="f89d27bf-94d3-40a4-9fca-2af9cbe94856" ext3   defaults
/mnt/part2 UUID="cb44e6c4-a49e-43e1-b9a8-ed5d272b8efb" ext4   defaults
/mnt/part3 UUID="6b306b44-23fe-4cef-8c3e-423188e5c088" xfs    defaults
/mnt/part4 UUID="2856B0906FE06278"                     ntfs   defaults
/vagrant   vagrant                                     vboxsf uid=1000,gid=1000,_netdev

  • --json, -J: Output in JSON format

  • --pairs, -P: Output in key-value pairs

    [vagrant@el ~]$ findmnt --pairs
TARGET="/proc" SOURCE="proc" FSTYPE="proc" OPTIONS="rw,nosuid,nodev,noexec,relatime"
TARGET="/sys" SOURCE="sysfs" FSTYPE="sysfs" OPTIONS="rw,nosuid,nodev,noexec,relatime,seclabel"
[...]

  • --df, -D: imitate the output of df(1)

    [vagrant@el ~]$ findmnt  --df
SOURCE    FSTYPE     SIZE  USED  AVAIL USE% TARGET
devtmpfs  devtmpfs     4M     0     4M   0% /dev
tmpfs     tmpfs    384.5M     0 384.5M   0% /dev/shm
tmpfs     tmpfs    153.8M  4.4M 149.4M   3% /run
/dev/sda2 xfs       61.9G  1.8G    60G   3% /
/dev/sdc5 xfs        4.9G 67.5M   4.9G   1% /mnt/part3
/dev/sdc6 fuseblk      5G 26.2M     5G   1% /mnt/part4
/dev/sdc2 ext4       5.8G   24K   5.5G   0% /mnt/part2
/dev/sdc1 ext3       3.9G   96K   3.7G   0% /mnt/part1
tmpfs     tmpfs     76.9M     0  76.9M   0% /run/user/1000

Mount info in /proc

The kernel provides the info through the /proc pseudo file system (i.e., /proc does not exist as a file on any hard disk but provides a file-like interface to information about the kernel).

Several files contain relevant information:

  • /proc/self/mounts: a list of all mounted file systems, the content is like the output of mount. For backwards compatibility, /etc/mtab and /proc/mounts are symlinks to this file.
  • /proc/self/mountinfo: a more detailed list of mounted file systems
  • /proc/self/mountstats: statistics about mounted file systems
student@linux:~$ cat /proc/mounts | grep /dev/sdb
    /dev/sdb1 /mnt/project42 ext2 rw 0 0

However, findmnt is a better way to get and present this information.

df

A more user friendly way to look at mounted file systems is df. The df (diskfree) command has the added benefit of showing you the free space on each mounted disk. Like a lot of Linux commands, df supports the -h switch to make the output more human readable.

student@linux:~$ df
Filesystem         1K-blocks      Used Available Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
                    11707972   6366996   4746240  58% /
/dev/sda1             101086      9300     86567  10% /boot
none                  127988         0    127988   0% /dev/shm
/dev/sdb1             108865      1550    101694   2% /mnt/project42
student@linux:~$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
                       12G  6.1G  4.6G  58% /
/dev/sda1              99M  9.1M   85M  10% /boot
none                  125M     0  125M   0% /dev/shm
/dev/sdb1             107M  1.6M  100M   2% /mnt/project42

df -h

In the df -h example below you can see the size, free space, used gigabytes and percentage and mount point of a partition.

student@linux:~$ df -h | egrep -e "(sdb2|File)"
Filesystem            Size Used Avail Use% Mounted on
/dev/sdb2              92G  83G  8.6G  91% /media/sdb2

du

The du command can summarize disk usage for files and directories. By using du on a mount point you effectively get the disk space used on a file system.

While du can go display each subdirectory recursively, the -s option will give you a total summary for the parent directory. This option is often used together with -h. This means du -sh on a mount point gives the total amount used by the file system in that partition.

student@linux:~$ du -sh /boot /srv/wolf
6.2M    /boot
1.1T    /srv/wolf

from start to finish

Below is a screenshot that show a summary roadmap starting with detection of the hardware (/dev/sdb) up until mounting on /mnt.

student@linux:~$ sudo dmesg | grep '\[sdb\]'
sd 3:0:0:0: [sdb] 150994944 512-byte logical blocks: (77.3 GB/72.0 GiB)
sd 3:0:0:0: [sdb] Write Protect is off
sd 3:0:0:0: [sdb] Mode Sense: 00 3a 00 00
sd 3:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
sd 3:0:0:0: [sdb] Attached SCSI disk

student@linux:~$ sudo parted /dev/sdb

(parted) mklabel msdos
(parted) mkpart primary ext4 1 77000
(parted) print
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sdb: 77.3GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End     Size    Type     File system  Flags
    1      1049kB  77.0GB  77.0GB  primary

(parted) quit
student@linux:~$ sudomkfs.ext4 /dev/sdb1
mke2fs 1.41.12 (17-May-2010)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
4702208 inodes, 18798592 blocks
939929 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=4294967296
574 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
( output truncated )
...
student@linux:~$ sudomount /dev/sdb1 /mnt
student@linux:~$ sudomount | grep mnt
/dev/sdb1 on /mnt type ext4 (rw)
student@linux:~$ sudodf -h | grep mnt
/dev/sdb1              71G  180M   67G   1% /mnt
student@linux:~$ sudodu -sh /mnt
20K     /mnt
student@linux:~$ sudoumount /mnt

permanent mounts

Until now, we performed all mounts manually. This works nice, until the next reboot. Luckily there is a way to tell your computer to automatically mount certain file systems during boot.

/etc/fstab

The file system table located in /etc/fstab contains a list of file systems, with an option to automtically mount each of them at boot time.

Below is a sample /etc/fstab file.

student@linux:~$ cat /etc/fstab 
/dev/VolGroup00/LogVol00 /                ext3    defaults        1 1
LABEL=/boot             /boot             ext3    defaults        1 2
none                    /dev/pts          devpts  gid=5,mode=620  0 0
none                    /dev/shm          tmpfs   defaults        0 0
none                    /proc             proc    defaults        0 0
none                    /sys              sysfs   defaults        0 0
/dev/VolGroup00/LogVol01 swap             swap    defaults        0 0

By adding the following line, we can automate the mounting of a file system.

/dev/sdb1                /mnt/project42      ext2    defaults    0 0

mount /mountpoint

Adding an entry to /etc/fstab has the added advantage that you can simplify the mount command. The command in the screenshot below forces mount to look for the partition info in /etc/fstab.

student@linux:~$ sudo mount /mnt/project42

securing mounts

File systems can be secured with several mount options. Here are some examples.

ro

The ro option will mount a file system as read only, preventing anyone from writing.

student@linux:~$ sudo mount -t ext2 -o ro /dev/hdb1 /mnt/project42
student@linux:~$ sudo touch /mnt/project42/testwrite
touch: cannot touch `/mnt/project42/testwrite': Read-only file system

noexec

The noexec option will prevent the execution of binaries and scripts on the mounted file system.

student@linux:~$ sudo mount -t ext2 -o noexec /dev/hdb1 /mnt/project42
student@linux:~$ sudo cp /bin/cat /mnt/project42
student@linux:~$ sudo /mnt/project42/cat /etc/hosts
-bash: /mnt/project42/cat: Permission denied
student@linux:~$ sudo echo echo hello > /mnt/project42/helloscript
student@linux:~$ sudo chmod +x /mnt/project42/helloscript 
student@linux:~$ sudo /mnt/project42/helloscript 
-bash: /mnt/project42/helloscript: Permission denied

nosuid

The nosuid option will ignore setuid bit set binaries on the mounted file system.

Note that you can still set the setuid bit on files.

student@linux:~$ sudo mount -o nosuid /dev/hdb1 /mnt/project42
student@linux:~$ sudo cp /bin/sleep /mnt/project42/
student@linux:~$ sudo chmod 4555 /mnt/project42/sleep 
student@linux:~$ sudo ls -l /mnt/project42/sleep 
-r-sr-xr-x 1 root root 19564 Jun 24 17:57 /mnt/project42/sleep

But users cannot exploit the setuid feature.

student@linux:~$ su - paul
[paul@linux ~]$ /mnt/project42/sleep 500 &
[1] 2876
[paul@linux ~]$ ps -f 2876
UID        PID  PPID  C STIME TTY      STAT   TIME CMD
paul      2876  2853  0 17:58 pts/0    S      0:00 /mnt/project42/sleep 500

noacl

To prevent cluttering permissions with acl's, use the noacl option.

student@linux:~$ sudo mount -o noacl /dev/hdb1 /mnt/project42

More mount options can be found in the manual page of mount(8).

mounting remote file systems

smb/cifs

The Samba team (https://samba.org/) has a Unix/Linux service that is compatible with the SMB/CIFS protocol. This protocol is mainly used by networked Microsoft Windows computers for sharing files or printers (a.k.a. Network Neighbourhood).

Connecting to a Samba server (or to a Microsoft computer) is also done with the mount command.

This example shows how to connect to the 10.0.0.42 server, to a share named data2.

student@linux:~$ sudo mount -t cifs -o user=paul //10.0.0.42/data2 /mnt/data2
Password: 
student@linux:~$ sudo mount | grep cifs
//10.0.0.42/data2 on /mnt/data2 type cifs (rw)

The above requires the package cifs-utils (on both Debian and Enterprise Linux based distributions).

nfs

Unix servers often use nfs (aka the network file system) to share directories over the network. Setting up an nfs server is not discussed here. Connecting as a client to an nfs server is done with mount, and is very similar to connecting to local storage.

This command shows how to connect to the nfs server named server42, which is sharing the directory /srv/data. The mount point at the end of the command (/mnt/data) must already exist.

student@linux:~$ mount -t nfs server42:/srv/data /mnt/data

If this server42 has ip-address 10.0.0.42 then you can also write:

student@linux:~$ sudo mount -t nfs 10.0.0.42:/srv/data /mnt/data
student@linux:~$ mount | grep data
10.0.0.42:/srv/data on /mnt/data type nfs (rw,vers=4,addr=10.0.0.42,clienta\
ddr=10.0.0.33)

nfs specific mount options

  • bg: If mount fails, retry in background.
  • fg: (default) If mount fails, retry in foreground.
  • soft: Stop trying to mount after X attempts.
  • hard: (default) Continue trying to mount.

The soft+bg options combined guarantee the fastest client boot if there are NFS problems.

retrans=X Try X times to connect (over udp).
tcp Force tcp (default and supported)
udp Force udp (unsupported)

practice: mounting file systems

  1. Mount the small 200MB partition on /mnt/project22.

  2. Mount the big 400MB primary partition on /mnt/salesnumbers, then copy some files to it (e.g. everything in /etc). Then umount, and mount the file system as read only on /srv/nfs/salesnumbers. Where are the files you copied ?

  3. Verify your work with fdisk, df and mount. Also look in /etc/mtab and /proc/mounts.

  4. Make both mounts permanent, test that it works.

  5. What happens when you mount a file system on a directory that contains some files ?

  6. What happens when you mount two file systems on the same mount point?

  7. (optional) Perform a file system check on the partition mounted at /srv/nfs/salesnumbers.

solution: mounting file systems

  1. Mount the small 200MB partition on /mnt/project22.

    mkdir /mnt/project22
mount /dev/sdc1 /mnt/project22

  2. Mount the big 400MB primary partition on /mnt/salesnumbers, then copy some files to it (e.g. everything in /etc). Then umount, and mount the file system as read only on /srv/nfs/salesnumbers. Where are the files you copied ?

    sudo mount /dev/sdb1 /mnt
sudo cp -r /etc /mnt
ls -l /mnt

sudo umount /mnt
ls -l /mnt

sudo mkdir -p /srv/nfs/salesnumbers
sudo mount /dev/sdb1 /srv/nfs/salesnumbers

    You see the files in /srv/nfs/salenumbers now. But physically they are on the ext3 filesystem /dev/sdb1.

  3. Verify your work with fdisk, df and mount. Also look in /etc/mtab and /proc/mounts.

    fdisk -l
df -h
mount

    All three the above commands should show your mounted partitions.

    grep project22 /etc/mtab
grep project22 /proc/mounts

  4. Make both mounts permanent, test that it works.

    add the following lines to /etc/fstab

    /dev/sdc1 /mnt/project22         auto  defaults  0 0
/dev/sdb1 /srv/nfs/salesnumbers  auto  defaults  0 0

  5. What happens when you mount a file system on a directory that contains some files ?

    The files are hidden until umount.

  6. What happens when you mount two file systems on the same mount point?

    Only the last mounted fs is visible.

  7. (optional) Perform a file system check on the partition mounted at /srv/nfs/salesnumbers

    umount /srv/nfs/salesnumbers
fsck /dev/sdb1