Table of Contents
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 exists 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.
This example shows how to create a new mount point with mkdir.
root@RHELv4u2:~# mkdir /home/project42
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.
root@RHELv4u2:~# mount -t ext2 /dev/sdb1 /home/project42/
Once mounted, the new file system is accessible to users.
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.
paul@RHELv4u4:~$ cat /etc/filesystems ext3 ext2 nodev proc nodev devpts iso9660 vfat hfs
To display all mounted file systems, issue the mount command. Or look at the files /proc/mounts and /etc/mtab.
The simplest and most common way to view all mounts is by issuing the mount command without any arguments.
root@RHELv4u2:~# mount | grep /dev/sdb /dev/sdb1 on /home/project42 type ext2 (rw)
The kernel provides the info in /proc/mounts in file form, but /proc/mounts does not exist as a file on any hard disk. Looking at /proc/mounts is looking at information that comes directly from the kernel.
root@RHELv4u2:~# cat /proc/mounts | grep /dev/sdb /dev/sdb1 /home/project42 ext2 rw 0 0
The /etc/mtab file is not updated by the kernel, but is maintained by the mount command. Do not edit /etc/mtab manually.
root@RHELv4u2:~# cat /etc/mtab | grep /dev/sdb /dev/sdb1 /home/project42 ext2 rw 0 0
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.
root@RHELv4u2:~# 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% /home/project42 root@RHELv4u2:~# 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% /home/project42
In the df -h example below you can see the size, free space, used gigabytes and percentage and mount point of a partition.
root@laika:~# df -h | egrep -e "(sdb2|File)" Filesystem Size Used Avail Use% Mounted on /dev/sdb2 92G 83G 8.6G 91% /media/sdb2
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.
root@debian6~# du -sh /boot /srv/wolf 6.2M /boot 1.1T /srv/wolf
Below is a screenshot that show a summary roadmap starting with detection of the hardware (/dev/sdb) up until mounting on /mnt.
[root@centos65 ~]# 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 [root@centos65 ~]# 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 [root@centos65 ~]# mkfs.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 ) ... [root@centos65 ~]# mount /dev/sdb1 /mnt [root@centos65 ~]# mount | grep mnt /dev/sdb1 on /mnt type ext4 (rw) [root@centos65 ~]# df -h | grep mnt /dev/sdb1 71G 180M 67G 1% /mnt [root@centos65 ~]# du -sh /mnt 20K /mnt [root@centos65 ~]# umount /mnt
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.
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.
root@RHELv4u2:~# 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 /home/project42 ext2 defaults 0 0
File systems can be secured with several mount options. Here are some examples.
The ro option will mount a file system as read only, preventing anyone from writing.
root@rhel53 ~# mount -t ext2 -o ro /dev/hdb1 /home/project42 root@rhel53 ~# touch /home/project42/testwrite touch: cannot touch `/home/project42/testwrite': Read-only file system
The noexec option will prevent the execution of binaries and scripts on the mounted file system.
root@rhel53 ~# mount -t ext2 -o noexec /dev/hdb1 /home/project42 root@rhel53 ~# cp /bin/cat /home/project42 root@rhel53 ~# /home/project42/cat /etc/hosts -bash: /home/project42/cat: Permission denied root@rhel53 ~# echo echo hello > /home/project42/helloscript root@rhel53 ~# chmod +x /home/project42/helloscript root@rhel53 ~# /home/project42/helloscript -bash: /home/project42/helloscript: Permission denied
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.
root@rhel53 ~# mount -o nosuid /dev/hdb1 /home/project42 root@rhel53 ~# cp /bin/sleep /home/project42/ root@rhel53 ~# chmod 4555 /home/project42/sleep root@rhel53 ~# ls -l /home/project42/sleep -r-sr-xr-x 1 root root 19564 Jun 24 17:57 /home/project42/sleep
But users cannot exploit the setuid feature.
root@rhel53 ~# su - paul [paul@rhel53 ~]$ /home/project42/sleep 500 & [1] 2876 [paul@rhel53 ~]$ ps -f 2876 UID PID PPID C STIME TTY STAT TIME CMD paul 2876 2853 0 17:58 pts/0 S 0:00 /home/project42/sleep 500 [paul@rhel53 ~]$
To prevent cluttering permissions with acl's, use the noacl option.
root@rhel53 ~# mount -o noacl /dev/hdb1 /home/project42
More mount options can be found in the manual page of mount.
The Samba team (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.
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.
[root@centos65 ~]# mount -t cifs -o user=paul //10.0.0.42/data2 /home/data2 Password: [root@centos65 ~]# mount | grep cifs //10.0.0.42/data2 on /home/data2 type cifs (rw)
The above requires yum install cifs-client.
Unix servers often use nfs (aka the network file system) to share directories over the network. Setting up an nfs server is discussed later. 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 (/home/data) must already exist.
[root@centos65 ~]# mount -t nfs server42:/srv/data /home/data [root@centos65 ~]#
If this server42 has ip-address 10.0.0.42 then you can also write:
[root@centos65 ~]# mount -t nfs 10.0.0.42:/srv/data /home/data [root@centos65 ~]# mount | grep data 10.0.0.42:/srv/data on /home/data type nfs (rw,vers=4,addr=10.0.0.42,clienta\ ddr=10.0.0.33)
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)
1. Mount the small 200MB partition on /home/project22.
2. Mount the big 400MB primary partition on /mnt, the copy some files to it (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) Describe the difference between these commands: find, locate, updatedb, makewhatis, whereis, apropos, which and type.
8. (optional) Perform a file system check on the partition mounted at /srv/nfs/salesnumbers.
1. Mount the small 200MB partition on /home/project22.
mkdir /home/project22 mount /dev/sdc1 /home/project22
2. Mount the big 400MB primary partition on /mnt, the copy some files to it (everything in /etc). Then umount, and mount the file system as read only on /srv/nfs/salesnumbers. Where are the files you copied ?
mount /dev/sdb1 /mnt cp -r /etc /mnt ls -l /mnt
umount /mnt ls -l /mnt
mkdir -p /srv/nfs/salesnumbers mount /dev/sdb1 /srv/nfs/salesnumbers
You see the files in /srv/nfs/salenumbers now...
But physically they are on ext3 on partition /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 /home/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) Describe the difference between these commands: find, locate, updatedb, makewhatis, whereis, apropos, which and type.
man find man locate ...
8. (optional) Perform a file system check on the partition mounted at /srv/nfs/salesnumbers.
# umount /srv/nfs/salesnumbers (optional but recommended) # fsck /dev/sdb1