RH12401 - File System

• RH12401 - File System
  • Module content
  • LAB #1 - Linux File Hierarchy System
  • 1.1 - File Types
  • 1.2 - The mount command
  • 1.3 - The umount command
  • 1.4 - The /etc/fstab file
• Mount Options
  • LAB #2 - Unix File System
  • 2.1 - Superblock
  • 2.2 - Inodes
  • 2.3 - Data blocks
  • 2.4 - Physical links
  • 2.5 - Symbolic links

The Linux file system is organised around a single tree with a starting point called the root, represented by the character /. Below this root are directories containing files and sub-directories. The organisation of directories conforms to a standard called the Linux File Hierarchy System.

[trainee@redhat9 ~]$ cd /
[trainee@redhat9 /]$ ls -l
total 28
dr-xr-xr-x. 2 root root 6 Aug 10 2021 afs
lrwxrwxrwx. 1 root root 7 Aug 10 2021 bin -> usr/bin
dr-xr-xr-x. 5 root root 4096 Sep 25 12:30 boot
drwxr-xr-x. 20 root root 3320 Sep 25 12:44 dev
drwxr-xr-x. 133 root root 8192 Sep 25 12:44 etc
drwxr-xr-x. 3 root root 21 Oct 19 2023 home
lrwxrwxrwx. 1 root root 7 Aug 10 2021 lib -> usr/lib
lrwxrwxrwx. 1 root root 9 Aug 10 2021 lib64 -> usr/lib64
drwxr-xr-x. 2 root root 6 Aug 10 2021 media
drwxr-xr-x. 2 root root 6 Aug 10 2021 mnt
drwxr-xr-x. 2 root root 6 Aug 10 2021 opt
dr-xr-xr-x. 242 root root 0 Sep 25 12:44 proc
dr-xr-x---. 4 root root 4096 Oct 19 2023 root
drwxr-xr-x. 44 root root 1160 Sep 25 12:44 run
lrwxrwxrwx. 1 root root 8 Aug 10 2021 sbin -> usr/sbin
drwxr-xr-x. 2 root root 6 Aug 10 2021 srv
dr-xr-xr-x. 13 root root 0 Sep 25 12:44 sys
drwxrwxrwt. 15 root root 4096 Sep 25 12:48 tmp
drwxr-xr-x. 12 root root 144 Oct 19 2023 usr
drwxr-xr-x. 20 root root 4096 Oct 19 2023 var

• /afs: Andrew File System (AFS) is a distributed file system that uses a set of trusted servers to present a consistent, location-transparent file namespace.
• /bin: is short for binary or binaries. It contains programs such as ls. Under RHEL 9 it is a symbolic link pointing to /usr/bin.
• /boot: contains the files needed to boot the system.
• /dev: contains the nodes used to access all types of hardware, such as /dev/fd0 for the floppy drive. The udev binary is responsible for dynamically creating and deleting nodes.
• /etc: contains configuration files such as passwd for passwords and fstab, which is the list of file systems to be mounted when the system boots.
• /home: contains the directories for each user except root.
• /lib: contains the common 32-bit libraries used by programs and modules. Under RHEL 9 this is a symbolic link pointing to /usr/lib.
• /lib64: contains the common 64-bit libraries used by programs and modules. Under RHEL 9 this is a symbolic link pointing to /usr/lib64.
• /media: contains directories for each mounted file system (accessible to the Linux system) such as floppy, cdrom etc.
• /mnt: contains directories for each file system temporarily mounted by root.
• /opt: contains optional applications.
• /proc: contains a virtual file system which extracts the information being processed from memory. The contents of the files are created dynamically during consultation. Only root can view all the information in the /proc directory.
• /root: the home of root, the system administrator.
• /run: replaces the /var/run directory.
• /sbin: contains binaries, i.e. programmes, for administering the local system. Under RHEL 9 this is a symbolic link pointing to /usr/sbin.
• /srv: contains data for the services hosted by the system, such as ftp, databases, web, etc.
• /sys: contains a virtual file system whose role is to describe the hardware for udev.
• /tmp: stores temporary files created by programs.
• /usr: contains user commands in /usr/bin, HOWTOs in /usr/share/doc, manuals in /usr/share/man and other major entries.
• /var: contains files of variable size.

There are three main types of file on the Linux system:

• ordinary files
• directories
• special files (or Devices).

Ordinary files are text files, tables or executables.

The length of the file name is limited to 255 characters.

There is a distinction between upper and lower case.

If a file name begins with ., the file becomes hidden.

In order for Linux to be informed of the presence of a file system, this system must be mounted. To mount a file system, use the mount command:

# mount /dev/<special_file> /mnt/<target_directory>

where /dev/<special_file> is the device to be mounted and /mnt/<target_directory> is the directory that will be used as a ‘window’ to view the contents of the file system. This directory must exist before attempting to mount the file system.

If the mount command is used without options, the system returns a list of all the file systems currently mounted:

[trainee@redhat9 /]$ su -
Password: fenestros
[root@redhat9 ~]# mount
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime,seclabel)
devtmpfs on /dev type devtmpfs (rw,nosuid,seclabel,size=4096k,nr_inodes=976019,mode=755,inode64)
securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev,seclabel,inode64)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,seclabel,gid=5,mode=620,ptmxmode=000)
tmpfs on /run type tmpfs (rw,nosuid,nodev,seclabel,size=1573912k,nr_inodes=819200,mode=755,inode64)
cgroup2 on /sys/fs/cgroup type cgroup2 (rw,nosuid,nodev,noexec,relatime,seclabel,nsdelegate,memory_recursiveprot)
pstore on /sys/fs/pstore type pstore (rw,nosuid,nodev,noexec,relatime,seclabel)
bpf on /sys/fs/bpf type bpf (rw,nosuid,nodev,noexec,relatime,mode=700)
/dev/mapper/rhel-root on / type xfs (rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota)
selinuxfs on /sys/fs/selinux type selinuxfs (rw,nosuid,noexec,relatime)
systemd-1 on /proc/sys/fs/binfmt_misc type autofs (rw,relatime,fd=29,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=12983)
tracefs on /sys/kernel/tracing type tracefs (rw,nosuid,nodev,noexec,relatime,seclabel)
debugfs on /sys/kernel/debug type debugfs (rw,nosuid,nodev,noexec,relatime,seclabel)
mqueue on /dev/mqueue type mqueue (rw,nosuid,nodev,noexec,relatime,seclabel)
hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,seclabel,pagesize=2M)
none on /run/credentials/systemd-tmpfiles-setup-dev.service type ramfs (ro,nosuid,nodev,noexec,relatime,seclabel,mode=700)
fusectl on /sys/fs/fuse/connections type fusectl (rw,nosuid,nodev,noexec,relatime)
configfs on /sys/kernel/config type configfs (rw,nosuid,nodev,noexec,relatime)
none on /run/credentials/systemd-sysctl.service type ramfs (ro,nosuid,nodev,noexec,relatime,seclabel,mode=700)
/dev/sda1 on /boot type xfs (rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota)
none on /run/credentials/systemd-tmpfiles-setup.service type ramfs (ro,nosuid,nodev,noexec,relatime,seclabel,mode=700)
tmpfs on /run/user/42 type tmpfs (rw,nosuid,nodev,relatime,seclabel,size=786956k,nr_inodes=196739,mode=700,uid=42,gid=42,inode64)
tmpfs on /run/user/1000 type tmpfs (rw,nosuid,nodev,relatime,seclabel,size=786956k,nr_inodes=196739,mode=700,uid=1000,gid=1000,inode64)

This information is stored in the /etc/mtab file:

[root@redhat9 ~]# cat /etc/mtab
proc /proc proc rw,nosuid,nodev,noexec,relatime 0 0
sysfs /sys sysfs rw,seclabel,nosuid,nodev,noexec,relatime 0 0
devtmpfs /dev devtmpfs rw,seclabel,nosuid,size=4096k,nr_inodes=976019,mode=755,inode64 0 0
securityfs /sys/kernel/security securityfs rw,nosuid,nodev,noexec,relatime 0 0
tmpfs /dev/shm tmpfs rw,seclabel,nosuid,nodev,inode64 0 0
devpts /dev/pts devpts rw,seclabel,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
tmpfs /run tmpfs rw,seclabel,nosuid,nodev,size=1573912k,nr_inodes=819200,mode=755,inode64 0 0
cgroup2 /sys/fs/cgroup cgroup2 rw,seclabel,nosuid,nodev,noexec,relatime,nsdelegate,memory_recursiveprot 0 0
pstore /sys/fs/pstore pstore rw,seclabel,nosuid,nodev,noexec,relatime 0 0
bpf /sys/fs/bpf bpf rw,nosuid,nodev,noexec,relatime,mode=700 0 0
/dev/mapper/rhel-root / xfs rw,seclabel,relatime,attr2,inode64,logbufs=8,logbsize=32k,noquota 0 0
selinuxfs /sys/fs/selinux selinuxfs rw,nosuid,noexec,relatime 0 0
systemd-1 /proc/sys/fs/binfmt_misc autofs rw,relatime,fd=29,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=12983 0 0
tracefs /sys/kernel/tracing tracefs rw,seclabel,nosuid,nodev,noexec,relatime 0 0
debugfs /sys/kernel/debug debugfs rw,seclabel,nosuid,nodev,noexec,relatime 0 0
mqueue /dev/mqueue mqueue rw,seclabel,nosuid,nodev,noexec,relatime 0 0
hugetlbfs /dev/hugepages hugetlbfs rw,seclabel,relatime,pagesize=2M 0 0
none /run/credentials/systemd-tmpfiles-setup-dev.service ramfs ro,seclabel,nosuid,nodev,noexec,relatime,mode=700 0 0
fusectl /sys/fs/fuse/connections fusectl rw,nosuid,nodev,noexec,relatime 0 0
configfs /sys/kernel/config configfs rw,nosuid,nodev,noexec,relatime 0 0
none /run/credentials/systemd-sysctl.service ramfs ro,seclabel,nosuid,nodev,noexec,relatime,mode=700 0 0
/dev/sda1 /boot xfs rw,seclabel,relatime,attr2,inode64,logbufs=8,logbsize=32k,noquota 0 0
none /run/credentials/systemd-tmpfiles-setup.service ramfs ro,seclabel,nosuid,nodev,noexec,relatime,mode=700 0 0
tmpfs /run/user/42 tmpfs rw,seclabel,nosuid,nodev,relatime,size=786956k,nr_inodes=196739,mode=700,uid=42,gid=42,inode64 0 0
tmpfs /run/user/1000 tmpfs rw,seclabel,nosuid,nodev,relatime,size=786956k,nr_inodes=196739,mode=700,uid=1000,gid=1000,inode64 0 0

To unmount a file system, use the umount command:

# umount /mnt/<target_directory>

or

# umount /dev/cdrom

====1.4 - The /etc/fstab file

If the mount command is used with the -a option, all the file systems listed in a special file called /etc/fstab will be mounted at the same time:

[root@redhat9 ~]# cat /etc/fstab====

#
# /etc/fstab
# Created by anaconda on Thu Oct 19 16:05:58 2023
#
# Accessible filesystems, by reference, are maintained under ‘/dev/disk/’.
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info.
#
# After editing this file, run ‘systemctl daemon-reload’ to update systemd
# units generated from this file.
#
/dev/mapper/rhel-root / xfs defaults 0 0
UUID=6f6c5bb9-30be-4734-bc23-03fed8541616 /boot xfs defaults 0 0
/dev/mapper/rhel-swap none swap defaults 0 0

Each line in this file contains 6 fields:

The UUID ( Universally Unique Identifier ) is a 128-bit string. UUIDs are created automatically and at random when the filesystem is created on the partition. They can be modified by the administrator.

The most important mount options are :

Each partition on a Unix system can host one of the following structures:

• superbloc
• inode
• data block
• indirection blocks

The superblock contains:

• the size of the blocks
• the size of the file system
• the number of mounts performed for this file system
• a pointer to the root of the file system
• pointers to the list of free inodes
• pointers to the list of free data blocks

Each file is represented by an inode. The inode contains :

• the type of file, i.e. -, d, l, b, c, p, s
• access rights, for example rwx rw- r–
• the number of physical links, i.e. the number of names
• the creator's UID or the UID assigned by the chown command if there has been a modification
• the GID of the creator process or the GID assigned by the chgrp command
• the size of the file in bytes
• the date on which the inode was last modified, i.e. ctime.
• the date on which the file was last modified, i.e. mtime.
• the date of the last access, i.e. atime.
• the addresses which point to the data blocks in the file.

Graphically, this organisation can be represented as follows:

To understand this better, type the following command:

[root@redhat9 ~]# ls -ld /dev/console /dev/sda1 /etc /etc/passwd
crw--w----. 1 root tty 5, 1 Sep 25 12:44 /dev/console
brw-rw----. 1 root disk 8, 1 Sep 25 12:44 /dev/sda1
drwxr-xr-x. 133 root root 8192 Sep 25 12:44 /etc
-rw-r--r--. 1 root root 2109 Oct 19 2023 /etc/passwd

The first character of each line can be one of the following:

• - - a file
• d - a directory
• l - a symbolic link
• b - a block device
• c - a character device
• p - a named pipe for inter-process communication
• s - a socket in a network context

To view the inode number, use the -i option:

[root@redhat9 ~]# ls -ldi /dev/console /dev/sda1 /etc /etc/passwd
12 crw--w----. 1 root tty 5, 1 Sep 25 12:44 /dev/console
273 brw-rw----. 1 root disk 8, 1 Sep 25 12:44 /dev/sda1
67154049 drwxr-xr-x. 133 root root 8192 Sep 25 12:44 /etc
68914044 -rw-r--r--. 1 root root 2109 Oct 19 2023 /etc/passwd

Data is stored in data blocks. In the case of a directory, the data block contains a table which references the inodes and file names in the directory. This table is called a catalogue table.

The name of a file is not stored in the inode but in a catalogue table. This feature allows us to give two different names to the same file. To add a new name to a file, a physical link must be created.

A physical or hard link is created using the following command:

• ln file_name additional_name

To illustrate this point, type the following command line:

[root@redhat9 ~]# cd /tmp; mkdir inode; cd inode; touch file1; ls -ali
total 4
33625083 drwxr-xr-x. 2 root root 22 Sep 25 13:08 .
33554561 drwxrwxrwt. 16 root root 4096 Sep 25 13:08 .
33625154 -rw-r--r--. 1 root root 0 Sep 25 13:08 file1

Note the inode number of the file file1. Also note that the number in the third field of the file1 line has the value 1 :

33625154 -rw-r--r--. 1 root root 0 Sep 25 13:08 file1

Now create a physical link :

[root@redhat9 inode]# ln file1 file2
[root@redhat9 inode]# ls -ali
total 4
33625083 drwxr-xr-x. 2 root root 38 Sep 25 13:09 .
33554561 drwxrwxrwt. 16 root root 4096 Sep 25 13:08 .
33625154 -rw-r--r--. 2 root root 0 Sep 25 13:08 file1
33625154 -rw-r--r--. 2 root root 0 Sep 25 13:08 file2

Note the following two lines:

33625154 -rw-r–r–. 2 root root 0 Sep 25 13:08 file1
33625154 -rw-r–r–. 2 root root 0 Sep 25 13:08 file2

Both files, file1 and file2, are referenced by the same inode. The number of links is therefore increased by 1 (the number in the third field).

A symbolic link is a shortcut to another file or directory. A symbolic link is created using the following command:

• ln -s filename shortcut_name

To illustrate this point, type the following command line:

[root@redhat9 inode]# ln -s file1 file3
[root@redhat9 inode]# ls -ali
total 4
33625083 drwxr-xr-x. 2 root root 54 Sep 25 13:10 .
33554561 drwxrwxrwt. 16 root root 4096 Sep 25 13:08 .
33625154 -rw-r--r--. 2 root root 0 Sep 25 13:08 file1
33625154 -rw-r--r--. 2 root root 0 Sep 25 13:08 file2
33625156 lrwxrwxrwx. 1 root root 8 Sep 25 13:10 file3 -> file1

Note that the symbolic link is referenced by another inode. The symbolic link points to file1.

Copyright © 2024 Hugh Norris.