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@ubuntu1404:/$ ls -l
total 92
drwxr-xr-x   2 root root  4096 sept. 27  2014 bin
drwxr-xr-x   3 root root  4096 sept. 27  2014 boot
drwxrwxr-x   2 root root  4096 sept. 27  2014 cdrom
drwxr-xr-x  15 root root  4120 sept. 25 15:17 dev
drwxr-xr-x 131 root root 12288 sept. 25 15:17 etc
drwxr-xr-x   3 root root  4096 sept. 27  2014 home
lrwxrwxrwx   1 root root    33 sept. 27  2014 initrd.img -> boot/initrd.img-3.13.0-32-generic
drwxr-xr-x  23 root root  4096 sept. 27  2014 lib
drwx------   2 root root 16384 sept. 27  2014 lost+found
drwxr-xr-x   3 root root  4096 sept. 28  2014 media
drwxr-xr-x   2 root root  4096 avril 11  2014 mnt
drwxr-xr-x   3 root root  4096 sept. 28  2014 opt
dr-xr-xr-x 102 root root     0 sept. 25 15:17 proc
drwx------   2 root root  4096 oct.  14  2014 root
drwxr-xr-x  20 root root   700 sept. 25 15:17 run
drwxr-xr-x   2 root root 12288 sept. 28  2014 sbin
drwxr-xr-x   2 root root  4096 juil. 22  2014 srv
dr-xr-xr-x  13 root root     0 sept. 25 15:17 sys
drwxrwxrwt   4 root root  4096 sept. 25 15:17 tmp
drwxr-xr-x  10 root root  4096 juil. 22  2014 usr
drwxr-xr-x  13 root root  4096 juil. 23  2014 var
lrwxrwxrwx   1 root root    30 sept. 27  2014 vmlinuz -> boot/vmlinuz-3.13.0-32-generic

trainee@ubuntu1604:/$ ls -l
total 100
drwxr-xr-x   2 root root  4096 mai    3 07:47 bin
drwxr-xr-x   3 root root  4096 mai    3 07:49 boot
drwxrwxr-x   2 root root  4096 mai    3 07:25 cdrom
drwxr-xr-x  19 root root  4180 août  18 12:39 dev
drwxr-xr-x 129 root root 12288 sept. 25 11:12 etc
drwxr-xr-x   3 root root  4096 mai    3 07:27 home
lrwxrwxrwx   1 root root    32 mai    3 07:31 initrd.img -> boot/initrd.img-4.4.0-21-generic
drwxr-xr-x  22 root root  4096 mai    3 07:47 lib
drwxr-xr-x   2 root root  4096 avril 21 00:07 lib64
drwx------   2 root root 16384 mai    3 07:17 lost+found
drwxr-xr-x   2 root root  4096 avril 21 00:07 media
drwxr-xr-x   2 root root  4096 avril 21 00:07 mnt
drwxr-xr-x   3 root root  4096 mai    3 08:14 opt
dr-xr-xr-x 114 root root     0 août  18 12:39 proc
drwx------   4 root root  4096 mai    3 08:33 root
drwxr-xr-x  24 root root   820 sept. 25 11:40 run
drwxr-xr-x   2 root root 12288 mai    3 07:51 sbin
drwxr-xr-x   2 root root  4096 avril 19 16:31 snap
drwxr-xr-x   2 root root  4096 avril 21 00:07 srv
dr-xr-xr-x  13 root root     0 août  18 12:38 sys
drwxrwxrwt   9 root root  4096 sept. 25 11:17 tmp
drwxr-xr-x  11 root root  4096 avril 21 00:13 usr
drwxr-xr-x  14 root root  4096 avril 21 00:19 var
lrwxrwxrwx   1 root root    29 mai    3 07:31 vmlinuz -> boot/vmlinuz-4.4.0-21-generic

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:

trainee@ubuntu1404:/$ mount
/dev/sda1 on / type ext4 (rw,errors=remount-ro)
proc on /proc type proc (rw,noexec,nosuid,nodev)
sysfs on /sys type sysfs (rw,noexec,nosuid,nodev)
none on /sys/fs/cgroup type tmpfs (rw)
none on /sys/fs/fuse/connections type fusectl (rw)
none on /sys/kernel/debug type debugfs (rw)
none on /sys/kernel/security type securityfs (rw)
udev on /dev type devtmpfs (rw,mode=0755)
devpts on /dev/pts type devpts (rw,noexec,nosuid,gid=5,mode=0620)
tmpfs on /run type tmpfs (rw,noexec,nosuid,size=10%,mode=0755)
none on /run/lock type tmpfs (rw,noexec,nosuid,nodev,size=5242880)
none on /run/shm type tmpfs (rw,nosuid,nodev)
none on /run/user type tmpfs (rw,noexec,nosuid,nodev,size=104857600,mode=0755)
none on /sys/fs/pstore type pstore (rw)
systemd on /sys/fs/cgroup/systemd type cgroup (rw,noexec,nosuid,nodev,none,name=systemd)

root@ubuntu1604:~# mount
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
udev on /dev type devtmpfs (rw,nosuid,relatime,size=230832k,nr_inodes=57708,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
tmpfs on /run type tmpfs (rw,nosuid,noexec,relatime,size=50028k,mode=755)
/dev/sda1 on / type ext4 (rw,relatime,errors=remount-ro,data=ordered)
securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
tmpfs on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k)
tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,mode=755)
cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,release_agent=/lib/systemd/systemd-cgroups-agent,name=systemd,nsroot=/)
pstore on /sys/fs/pstore type pstore (rw,nosuid,nodev,noexec,relatime)
cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,relatime,hugetlb,nsroot=/)
cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory,nsroot=/)
cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,freezer,nsroot=/)
cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset,nsroot=/)
cgroup on /sys/fs/cgroup/pids type cgroup (rw,nosuid,nodev,noexec,relatime,pids,nsroot=/)
cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,blkio,nsroot=/)
cgroup on /sys/fs/cgroup/net_cls,net_prio type cgroup (rw,nosuid,nodev,noexec,relatime,net_cls,net_prio,nsroot=/)
cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,nosuid,nodev,noexec,relatime,perf_event,nsroot=/)
cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices,nsroot=/)
cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpu,cpuacct,nsroot=/)
systemd-1 on /proc/sys/fs/binfmt_misc type autofs (rw,relatime,fd=22,pgrp=1,timeout=0,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)
fusectl on /sys/fs/fuse/connections type fusectl (rw,relatime)
tmpfs on /run/user/1000 type tmpfs (rw,nosuid,nodev,relatime,size=50028k,mode=700,uid=1000,gid=1000)

The following switches can be used with the mount command:

root@ubuntu1604:~# mount --help

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

Mount a filesystem.

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
 -i, --internal-only     don't call the mount.<type> helpers
 -l, --show-labels       show also filesystem 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
 -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:

root@ubuntu1404:~# cat /etc/fstab
# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# / was on /dev/sda1 during installation
UUID=70eb8bc5-1759-433d-9797-9342a7b82cb2 /               ext4    errors=remount-ro 0       1
# swap was on /dev/sda5 during installation
UUID=85017f2f-081d-464e-ad83-52c3c895a113 none            swap    sw              0       0

root@ubuntu1604:~# cat /etc/fstab
# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# / was on /dev/sda1 during installation
UUID=c27fce7f-cc8a-4c6f-b19b-d929a4d570f2 /               ext4    errors=remount-ro 0       1
# swap was on /dev/sda5 during installation
UUID=68f67549-63f1-4833-b792-3566455bbe95 none            swap    sw              0       0

[root@centos5 ~]# cat /etc/fstab
LABEL=/                 /                       ext3    defaults        1 1
LABEL=/boot             /boot                   ext3    defaults        1 2
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
LABEL=SWAP-sda3         swap                    swap    defaults        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:

root@ubuntu1604:~# umount --help

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

Unmount filesystems.

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:

root@ubuntu1604:~# mount | grep ext
/dev/sda1 on / type ext4 (rw,relatime,errors=remount-ro,data=ordered)
root@ubuntu1604:~# dumpe2fs /dev/sda1 | grep -i superblock
dumpe2fs 1.42.13 (17-May-2015)
  Primary superblock at 0, Group descriptors at 1-1
  Backup superblock at 32768, Group descriptors at 32769-32769
  Backup superblock at 98304, Group descriptors at 98305-98305
  Backup superblock at 163840, Group descriptors at 163841-163841
  Backup superblock at 229376, Group descriptors at 229377-229377
  Backup superblock at 294912, Group descriptors at 294913-294913
  Backup superblock at 819200, Group descriptors at 819201-819201
  Backup superblock at 884736, Group descriptors at 884737-884737
  Backup superblock at 1605632, Group descriptors at 1605633-1605633

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

# e2fsck -f -b 32768 /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:

root@ubuntu1604:~# ls -ld /dev/console /dev/sda1 /etc /etc/passwd
crw-------   1 root root  5, 1 sept. 28 10:31 /dev/console
brw-rw----   1 root disk  8, 1 sept. 28 10:31 /dev/sda1
drwxr-xr-x 129 root root 12288 sept. 28 10:41 /etc
-rw-r--r--   1 root root  2296 mai    3 08:08 /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:

root@ubuntu1604:~# ls -ldi /dev/console /dev/sda1 /etc /etc/passwd
    14 crw-------   1 root root  5, 1 sept. 28 10:31 /dev/console
   376 brw-rw----   1 root disk  8, 1 sept. 28 10:31 /dev/sda1
390913 drwxr-xr-x 129 root root 12288 sept. 28 10:41 /etc
396002 -rw-r--r--   1 root root  2296 mai    3 08:08 /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.

root@ubuntu1604:~# cd /tmp; mkdir inode; cd inode; touch file1; ls -ali
total 8
521308 drwxr-xr-x  2 root root 4096 sept. 29 10:26 .
390918 drwxrwxrwt 10 root root 4096 sept. 29 10:26 ..
521340 -rw-r--r--  1 root root    0 sept. 29 10:26 file1

file1 shows an inode number of 521340 and a single name, indicated by the number 1 in the third column:

521340 -rw-r--r--  1 root root    0 sept. 29 10:26 file1

now create the hard link and check the result:

root@ubuntu1604:/tmp/inode# ln file1 file2
root@ubuntu1604:/tmp/inode# ls -ali
total 8
521308 drwxr-xr-x  2 root root 4096 sept. 29 10:27 .
390918 drwxrwxrwt 10 root root 4096 sept. 29 10:26 ..
521340 -rw-r--r--  2 root root    0 sept. 29 10:26 file1
521340 -rw-r--r--  2 root root    0 sept. 29 10:26 file2

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

521340 -rw-r--r--  2 root root    0 sept. 29 10:26 file1
521340 -rw-r--r--  2 root root    0 sept. 29 10:26 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.

root@ubuntu1604:/tmp/inode# ln -s file1 file3
root@ubuntu1604:/tmp/inode# ls -ali
total 8
521308 drwxr-xr-x  2 root root 4096 sept. 29 10:32 .
390918 drwxrwxrwt 10 root root 4096 sept. 29 10:26 ..
521340 -rw-r--r--  2 root root    0 sept. 29 10:26 file1
521340 -rw-r--r--  2 root root    0 sept. 29 10:26 file2
521342 lrwxrwxrwx  1 root root    5 sept. 29 10:32 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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