File Hierarchy System

The Linux filesystem hierarchy starts with the root represented by a / character. Under the root can be found other directories containing task specific files. The hierarchy conforms to a standard called the Linux File Hierarchy System.

trainee@SLES11SP1:/> ls -l
total 101
drwxr-xr-x  2 root root  4096 25 sept. 15:48 bin
drwxr-xr-x  4 root root  1024 25 sept. 17:08 boot
drwxr-xr-x 16 root root  4020 25 sept. 17:08 dev
drwxr-xr-x 88 root root 12288 25 sept. 17:14 etc
drwxr-xr-x  3 root root  4096 25 sept. 15:55 home
drwxr-xr-x 13 root root  4096 25 sept. 15:49 lib
drwxr-xr-x  8 root root 12288 25 sept. 15:48 lib64
drwx------  2 root root 16384 25 sept. 15:43 lost+found
drwxr-xr-x  2 root root  4096  5 mai    2010 media
drwxr-xr-x  2 root root  4096  5 mai    2010 mnt
drwxr-xr-x  2 root root  4096  5 mai    2010 opt
dr-xr-xr-x 88 root root     0 25 sept. 17:07 proc
drwx------  6 root root  4096 25 sept. 17:14 root
drwxr-xr-x  3 root root 12288 25 sept. 15:50 sbin
drwxr-xr-x  2 root root  4096  5 mai    2010 selinux
drwxr-xr-x  4 root root  4096 25 sept. 15:43 srv
drwxr-xr-x 12 root root     0 25 sept. 17:07 sys
drwxrwxrwt  4 root root  4096 25 sept. 17:22 tmp
drwxr-xr-x 13 root root  4096 25 sept. 15:43 usr
drwxr-xr-x 15 root root  4096 25 sept. 15:44 var

trainee@SLES12SP1:/> ls -l
total 0
drwxr-xr-x   1 root root 1810 20 sept. 13:33 bin
drwxr-xr-x   1 root root 1096 21 sept. 04:19 boot
drwxr-xr-x  16 root root 3620 21 sept. 04:18 dev
drwxr-xr-x   1 root root 4746 21 sept. 04:18 etc
drwxr-xr-x   1 root root   14 20 sept. 13:34 home
drwxr-xr-x   1 root root 2906 20 sept. 13:32 lib
drwxr-xr-x   1 root root 4998 20 sept. 13:31 lib64
drwxr-xr-x   1 root root    0 21 sept.  2014 mnt
drwxr-xr-x   1 root root    0 21 sept.  2014 opt
dr-xr-xr-x 100 root root    0 20 sept. 13:47 proc
drwx------   1 root root  112 20 sept. 14:00 root
drwxr-xr-x  25 root root  640 21 sept. 04:18 run
drwxr-xr-x   1 root root 5044 20 sept. 13:33 sbin
drwxr-xr-x   1 root root    0 21 sept.  2014 selinux
drwxr-xr-x   1 root root   12 20 sept. 13:29 srv
dr-xr-xr-x  12 root root    0 20 sept. 13:47 sys
drwxrwxrwt   1 root root  102 21 sept. 04:18 tmp
drwxr-xr-x   1 root root  130 20 sept. 13:29 usr
drwxr-xr-x   1 root root  108 20 sept. 13:33 var

The three major file types under Linux are :

• Ordinary files,

• Directories,

• Special files or Devices.

Note that :

• Ordinary files can be anything from text files to binaries.
• The length of a file name is limited to 225 characters, including the file extension.
• Linux is case sensitive.
• If a file name starts with a dot (.), it is a hidden file.

In order to be able to use external file systems, such as a CDRom or DVDRom, Linux needs to be informed of their availability. This is accomplished by using the mount command:

# mount /dev/<special_file> /mnt/<directory_name> [Enter]

where /dev/<special_file> is the file system to mount and /mnt/<directory_name> is the target directory where the mounted file system will be available to the system. The directory /mnt/<directory_name> must exist prior to using the mount command.

In the case where the mount command is used without options, the current mounted file systems are shown:

SLES11SP1:~ # mount
/dev/sda2 on / type ext3 (rw,acl,user_xattr)
proc on /proc type proc (rw)
sysfs on /sys type sysfs (rw)
debugfs on /sys/kernel/debug type debugfs (rw)
udev on /dev type tmpfs (rw,mode=0755)
tmpfs on /dev/shm type tmpfs (rw,mode=1777)
devpts on /dev/pts type devpts (rw,mode=0620,gid=5)
/dev/sda1 on /boot type ext3 (rw,acl,user_xattr)
fusectl on /sys/fs/fuse/connections type fusectl (rw)
securityfs on /sys/kernel/security type securityfs (rw)

SLES12SP1:~ # mount
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
devtmpfs on /dev type devtmpfs (rw,nosuid,size=1931968k,nr_inodes=482992,mode=755)
securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
tmpfs on /run type tmpfs (rw,nosuid,nodev,mode=755)
tmpfs on /sys/fs/cgroup type tmpfs (rw,nosuid,nodev,noexec,mode=755)
cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,release_agent=/usr/lib/systemd/systemd-cgroups-agent,name=systemd)
pstore on /sys/fs/pstore type pstore (rw,nosuid,nodev,noexec,relatime)
cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset)
cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpuacct,cpu)
cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)
cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,freezer)
cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,blkio)
cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,nosuid,nodev,noexec,relatime,perf_event)
cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,relatime,hugetlb)
/dev/sda2 on / type btrfs (rw,relatime,space_cache,subvolid=259,subvol=/@/.snapshots/1/snapshot)
systemd-1 on /proc/sys/fs/binfmt_misc type autofs (rw,relatime,fd=31,pgrp=1,timeout=300,minproto=5,maxproto=5,direct)
mqueue on /dev/mqueue type mqueue (rw,relatime)
debugfs on /sys/kernel/debug type debugfs (rw,relatime)
hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime)
/dev/sda2 on /.snapshots type btrfs (rw,relatime,space_cache,subvolid=258,subvol=/@/.snapshots)
/dev/sda2 on /var/lib/mailman type btrfs (rw,relatime,space_cache,subvolid=269,subvol=/@/var/lib/mailman)
/dev/sda2 on /var/lib/mariadb type btrfs (rw,relatime,space_cache,subvolid=270,subvol=/@/var/lib/mariadb)
/dev/sda2 on /var/log type btrfs (rw,relatime,space_cache,subvolid=274,subvol=/@/var/log)
/dev/sda2 on /tmp type btrfs (rw,relatime,space_cache,subvolid=265,subvol=/@/tmp)
/dev/sda2 on /var/spool type btrfs (rw,relatime,space_cache,subvolid=276,subvol=/@/var/spool)
/dev/sda2 on /var/lib/named type btrfs (rw,relatime,space_cache,subvolid=272,subvol=/@/var/lib/named)
/dev/sda2 on /srv type btrfs (rw,relatime,space_cache,subvolid=264,subvol=/@/srv)
/dev/sda2 on /usr/local type btrfs (rw,relatime,space_cache,subvolid=266,subvol=/@/usr/local)
/dev/sda2 on /var/opt type btrfs (rw,relatime,space_cache,subvolid=275,subvol=/@/var/opt)
/dev/sda2 on /var/lib/pgsql type btrfs (rw,relatime,space_cache,subvolid=273,subvol=/@/var/lib/pgsql)
/dev/sda2 on /opt type btrfs (rw,relatime,space_cache,subvolid=263,subvol=/@/opt)
/dev/sda2 on /var/tmp type btrfs (rw,relatime,space_cache,subvolid=277,subvol=/@/var/tmp)
/dev/sda2 on /var/lib/mysql type btrfs (rw,relatime,space_cache,subvolid=271,subvol=/@/var/lib/mysql)
/dev/sda2 on /var/lib/libvirt/images type btrfs (rw,relatime,space_cache,subvolid=268,subvol=/@/var/lib/libvirt/images)
/dev/sda2 on /var/crash type btrfs (rw,relatime,space_cache,subvolid=267,subvol=/@/var/crash)
/dev/sda2 on /home type btrfs (rw,relatime,space_cache,subvolid=262,subvol=/@/home)
/dev/sda2 on /boot/grub2/x86_64-efi type btrfs (rw,relatime,space_cache,subvolid=261,subvol=/@/boot/grub2/x86_64-efi)
/dev/sda2 on /boot/grub2/i386-pc type btrfs (rw,relatime,space_cache,subvolid=260,subvol=/@/boot/grub2/i386-pc)

[stextbox id='black' image='null'] Important : Note that with SLES 11, the default filesystem is ext3whereas with SLES 12, the default filesystem is btrfs. Please see the unit Managing Disks, Swap Space and Filesystems for further coursework concerning ext3 and btrfs filesystems. [/stextbox]

The following switches can be used with the mount command:

SLES12SP1:~ # mount --help

Usage:
 mount [-lhV]
 mount -a [options]
 mount [options] [--source] <source> | [--target] <directory>
 mount [options] <source> <directory>
 mount <operation> <mountpoint> [<target>]

Options:
 -a, --all               mount all filesystems mentioned in fstab
 -c, --no-canonicalize   don't canonicalize paths
 -f, --fake              dry run; skip the mount(2) syscall
 -F, --fork              fork off for each device (use with -a)
 -T, --fstab <path>      alternative file to /etc/fstab
 -h, --help              display this help text and exit
 -i, --internal-only     don't call the mount.<type> helpers
 -l, --show-labels       lists all mounts with LABELs
 -n, --no-mtab           don't write to /etc/mtab
 -o, --options <list>    comma-separated list of mount options
 -O, --test-opts <list>  limit the set of filesystems (use with -a)
 -r, --read-only         mount the filesystem read-only (same as -o ro)
 -t, --types <list>      limit the set of filesystem types
     --source <src>      explicitly specifies source (path, label, uuid)
     --target <target>   explicitly specifies mountpoint
 -v, --verbose           say what is being done
 -V, --version           display version information and exit
 -w, --rw, --read-write  mount the filesystem read-write (default)

 -h, --help     display this help and exit
 -V, --version  output version information and exit

Source:
 -L, --label <label>     synonym for LABEL=<label>
 -U, --uuid <uuid>       synonym for UUID=<uuid>
 LABEL=<label>           specifies device by filesystem label
 UUID=<uuid>             specifies device by filesystem UUID
 PARTLABEL=<label>       specifies device by partition label
 PARTUUID=<uuid>         specifies device by partition UUID
 <device>                specifies device by path
 <directory>             mountpoint for bind mounts (see --bind/rbind)
 <file>                  regular file for loopdev setup

Operations:
 -B, --bind              mount a subtree somewhere else (same as -o bind)
 -M, --move              move a subtree to some other place
 -R, --rbind             mount a subtree and all submounts somewhere else
 --make-shared           mark a subtree as shared
 --make-slave            mark a subtree as slave
 --make-private          mark a subtree as private
 --make-unbindable       mark a subtree as unbindable
 --make-rshared          recursively mark a whole subtree as shared
 --make-rslave           recursively mark a whole subtree as slave
 --make-rprivate         recursively mark a whole subtree as private
 --make-runbindable      recursively mark a whole subtree as unbindable

For more details see mount(8).

In the case where the mount command is used with the -a option, all mount points specified in the /etc/fstab file are mounted:

SLES11SP1:~ # cat /etc/fstab 
/dev/disk/by-id/ata-VBOX_HARDDISK_VB62af9a29-d9a982d5-part3 swap                 swap       defaults              0 0
/dev/disk/by-id/ata-VBOX_HARDDISK_VB62af9a29-d9a982d5-part2 /                    ext3       acl,user_xattr        1 1
/dev/disk/by-id/ata-VBOX_HARDDISK_VB62af9a29-d9a982d5-part1 /boot                ext3       acl,user_xattr        1 2
proc                 /proc                proc       defaults              0 0
sysfs                /sys                 sysfs      noauto                0 0
debugfs              /sys/kernel/debug    debugfs    noauto                0 0
usbfs                /proc/bus/usb        usbfs      noauto                0 0
devpts               /dev/pts             devpts     mode=0620,gid=5       0 0

SLES12SP1:~ # cat /etc/fstab
UUID=db743358-c2d6-47f6-97d7-e7a9c650f0c5 swap swap defaults 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b / btrfs defaults 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /boot/grub2/i386-pc btrfs subvol=@/boot/grub2/i386-pc 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /boot/grub2/x86_64-efi btrfs subvol=@/boot/grub2/x86_64-efi 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /home btrfs subvol=@/home 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /opt btrfs subvol=@/opt 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /srv btrfs subvol=@/srv 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /tmp btrfs subvol=@/tmp 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /usr/local btrfs subvol=@/usr/local 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /var/crash btrfs subvol=@/var/crash 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /var/lib/libvirt/images btrfs subvol=@/var/lib/libvirt/images 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /var/lib/mailman btrfs subvol=@/var/lib/mailman 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /var/lib/mariadb btrfs subvol=@/var/lib/mariadb 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /var/lib/mysql btrfs subvol=@/var/lib/mysql 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /var/lib/named btrfs subvol=@/var/lib/named 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /var/lib/pgsql btrfs subvol=@/var/lib/pgsql 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /var/log btrfs subvol=@/var/log 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /var/opt btrfs subvol=@/var/opt 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /var/spool btrfs subvol=@/var/spool 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /var/tmp btrfs subvol=@/var/tmp 0 0
UUID=6b7e374a-ae42-4f93-b6aa-d288dfbbb74b /.snapshots btrfs subvol=@/.snapshots 0 0

Each line in /etc/fstab has 6 fields :

The UUID ( Universally Unique Identifier ) is a randomly generated 128 bit string that is automatically generated by the system when a filesystem is created on the partition.

The most important mount point options are as follows:

To unmount a file system, you need to use the umount command. For example:

# umount /mnt/target_directory [Entrée]

The following switches can be used with the umount command:

SLES12SP1:~ # umount --help

Usage:
 umount [-hV]
 umount -a [options]
 umount [options] <source> | <directory>

Options:
 -a, --all               unmount all filesystems
 -A, --all-targets       unmount all mountpoints for the given device in the
                           current namespace
 -c, --no-canonicalize   don't canonicalize paths
 -d, --detach-loop       if mounted loop device, also free this loop device
     --fake              dry run; skip the umount(2) syscall
 -f, --force             force unmount (in case of an unreachable NFS system)
 -i, --internal-only     don't call the umount.<type> helpers
 -n, --no-mtab           don't write to /etc/mtab
 -l, --lazy              detach the filesystem now, clean up things later
 -O, --test-opts <list>  limit the set of filesystems (use with -a)
 -R, --recursive         recursively unmount a target with all its children
 -r, --read-only         in case unmounting fails, try to remount read-only
 -t, --types <list>      limit the set of filesystem types
 -v, --verbose           say what is being done

 -h, --help     display this help and exit
 -V, --version  output version information and exit

For more details see umount(8).

Each file system contains the following :

• superblock
• inodes
• data blocks

The superblock contains :

• the block size,
• the size of the file system,
• the number of mounts for the file system,
• a pointer to the root of the file system,
• pointers to the free inodes,
• pointers to free data blocks.

Linux maintains multiple redundant copies of the superblock in every file system.

For example, to view the primary and backup superblock locations on ext filesystems, use the following command:

SLES11SP1:~ # mount | grep ext
/dev/sda2 on / type ext3 (rw,acl,user_xattr)
/dev/sda1 on /boot type ext3 (rw,acl,user_xattr)
SLES11SP1:~ # dumpe2fs /dev/sda1 | grep -i superblock
dumpe2fs 1.41.9 (22-Aug-2009)
  Primary superblock at 1, Group descriptors at 2-2
  Backup superblock at 8193, Group descriptors at 8194-8194
  Backup superblock at 24577, Group descriptors at 24578-24578
  Backup superblock at 40961, Group descriptors at 40962-40962
  Backup superblock at 57345, Group descriptors at 57346-57346
  Backup superblock at 73729, Group descriptors at 73730-73730

To repair an ext file system using a backup superblock use the following command :

# e2fsck -f -b 8193 /dev/sda1 [Enter]

Each file is represented by an inode. An inode contains the following information:

• the file type : -, d, l, b, c, p, s,
• file permissions, for example : rwx rw- r–,
• the number of hard links,
• the UID of the file creator or the current UID attributed by the chown command,
• the GID of the creating process or the current GID attributed by the chgrp command,
• the file size in bytes,
• the date of the last modification of the file's inode content : ctime,
• the date of the last modification of the file contents : mtime,
• the date of the last access : atime,
• allocation addresses that point to the data blocks used by the file.

For example:

Execute the following command:

SLES12SP1:~ # ls -ld /dev/console /dev/sda1 /etc /etc/passwd
crw------- 1 root root 5, 1 Sep 28 10:37 /dev/console
brw-rw---- 1 root disk 8, 1 Sep 28 10:37 /dev/sda1
drwxr-xr-x 1 root root 4746 Sep 28 10:38 /etc
-rw-r--r-- 1 root root 1335 Sep 20 13:34 /etc/passwd

The first character of each line indicates the file type:

• - - an ordinary file,
• d - a directory,
• l - a symbolic link,
• b - a bloc type peripheral,
• c - a character type peripheral,
• p - a named pipe for communication between processes,
• s - a network socket.

To see the inode numbers, execute the previous command with, in addition, the -i option:

SLES12SP1:~ # ls -ldi /dev/console /dev/sda1 /etc /etc/passwd
 4306 crw------- 1 root root 5, 1 Sep 28 10:37 /dev/console
 6871 brw-rw---- 1 root disk 8, 1 Sep 28 10:37 /dev/sda1
  257 drwxr-xr-x 1 root root 4746 Sep 28 12:02 /etc
58930 -rw-r--r-- 1 root root 1335 Sep 20 13:34 /etc/passwd

File data is stored in data blocks. In the case of a directory, the data block contains a table referencing the inodes and the names of the contents of the directory.

The name of the file is stored in the parent directory's data block and not in the inode. This means that a file can be referenced by one or more different names. To add a name to a data block, you need to create what is called a hard link.

A hard link is created by using the ln command.

SLES12SP1:~ # cd /tmp; mkdir inode; cd inode; touch file1; ls -ali
total 0
442 drwxr-xr-x 1 root root  10 Sep 28 12:23 .
256 drwxrwxrwt 1 root root 112 Sep 28 12:23 ..
443 -rw-r--r-- 1 root root   0 Sep 28 12:23 file1

file1 shows an inode number of 443 and a single name, indicated by the number 1 in the third column:

443 -rw-r--r-- 1 root root   0 Sep 28 12:23 file1

now create the hard link and check the result:

SLES12SP1:/tmp/inode # ln file1 file2
SLES12SP1:/tmp/inode # ls -ali
total 0
442 drwxr-xr-x 1 root root  20 Sep 28 12:24 .
256 drwxrwxrwt 1 root root 112 Sep 28 12:23 ..
443 -rw-r--r-- 2 root root   0 Sep 28 12:23 file1
443 -rw-r--r-- 2 root root   0 Sep 28 12:23 file2

Now you can see two lines, one for file1 and a second for file2:

443 -rw-r--r-- 2 root root   0 Sep 28 12:23 file1
443 -rw-r--r-- 2 root root   0 Sep 28 12:23 file2

file1 and file2 are referenced by the same inode. As a result the number of names has been increased to two in the thrid column.

[stextbox id='black' image='null'] Important - Hard links cannot be created across file system boundaries. A hard link can only be created if the source file exists. [/stextbox]

A soft link is a shortcut to a file or directory. A soft link is created using the same ln command with the -s option.

SLES12SP1:/tmp/inode # ln -s file1 file3
SLES12SP1:/tmp/inode # ls -ali
total 4
442 drwxr-xr-x 1 root root  30 Sep 28 12:26 .
256 drwxrwxrwt 1 root root 112 Sep 28 12:23 ..
443 -rw-r--r-- 2 root root   0 Sep 28 12:23 file1
443 -rw-r--r-- 2 root root   0 Sep 28 12:23 file2
444 lrwxrwxrwx 1 root root   5 Sep 28 12:26 file3 -> file1

Note here that the soft link is referenced by a separate inode.

[stextbox id='black' image='null'] Important - A soft link can be created across file system boundaries and can be created even when the source file does not exist. [/stextbox]

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