Linux System Fundamentals Lesson 2: Understanding Linux File System

Sadia Bashir

Contents

Linux File Types

Linux File System Hierarchy Default Directories

Understanding Linux Configuration files

Common Linux File Extensions

• Hidden Files

• Environment Variables

• Symbolic Links

Linux File Types

On a Linux system, almost everything is a file, and if it is not a file, then it is a process

Directories : files that are lists of other files Special file : mechanism used for input and output. /dev directory holds

special files Links : way to make file or directory visible in multiple parts of the operating

systems Sockets : special type of files that provide inter-process networking such as

TCP/IP Pipes : special type of files that allow inter-process communication such as

“netlinks” that are used for inter-process communication between a process running in user-space and a process running in the kernel-space

Linux File System Hierarchy

Linux File System Hierarchy - Default Directories

/bin stands for binary -> executables Home for several commands to be executed either by root or normal users Some common commands found in this folder are:

cat, ls, pwd, cp, mv, rm, mkdir, date, nano, grep, egrep, touch, chmod, chown etc.

/sbin: System binaries OR super binaries Similar to bin but contains commands run by UNIX - OS specific commands

System-level settings such as: disk and network management. Ifconfig, lvm etc.

Commands are executed only as/by root users

/root: home directory of the “system admin” or “root” user

Linux File System Format (contd….)

/etc: Contains most important system-wide configuration files “configuration file is a local file used to control operations of a program” these are static files and not executable binary files

Configuration tables (crontab, fstab) Configuration files (shadow, group, passwd, hosts, resolv.conf, network/interfaces) Running configuration files: force a service to start/stop for a user to use specific environment

Bash.bashrc, rc.local, nanorc, wgetrc

*.d convention: a directory holding a bunch of configuration fragments (init.d, profile.d, sysctl.d etc.) Deamon related configuration files

/opt: Contains all third-party packages and installations

/media: Mount point for removable media – CD-RoM, USB etc.

Linux File System Format (contd….)

/boot: holds files used in booting the operating system Contents: linux kernel files, boot loader files, file to loading kernel, and configuration

files containing linux kernel configuration settings vmlinuz, initrd, grub.conf etc.

/dev: holds device files representing hardware devices/peripheral components on the

system (denoted by “yellow” color-code) Pseudo devices:

/dev/null, /dev/zero: accepts and discards input, produces no output

Tty 0 - 63: denote terminals /dev/loop: 0 – 7: pseudo devices used for mounting virtual CD(ISO) files /dev/had OR sda OR /dev/disk : devices to represent partitions or hard drives

Linux File System Format (contd….)

/lib: Library essentials for binaries in /bin and /sbin - shared libraries needed by the programs on

the root filesystem Libraries required to boot the system and run the command in root Libraries essential for basic system functionality

Modules: Loadable kernel modules, especially those needed to boot the system after disasters.

/usr (UNIX System Resources): holds sharable read-only data - contains all commands, libraries, man pages, games and static

files for normal operation bin - Almost all user commands – man, who, whoami etc. sbin – Extended set of system admin commands not needed on the root filesystem lib - Unchanging data files for programs and subsystems local - The place for locally installed software and other third party programs man - Manual pages doc - Documentation

Linux File System Format (contd….)

/var: Contains variable data

Logging files, backups, state info for an application, lock files, logins/logouts

var/run: contains process identification files PIDs of system services and other information about the system that is valid until the system is next booted

/var/run/utmp: contains info about currently logged in users

/proc: It is a virtual filesystem that exists in the kernels imagination which is memory

provides process and kernel information, runtime system information

Linux File System Format (contd….)

/mnt: Generic mount point used to point external file system such as CD-ROM or

Digital Camera

after mounting, file’ll be accessible under mount point

additional mount points can be created here

/tmp: Temporary files (also /var/tmp) often not preserved between system reboots

Mounting:In Linux, partitions or devices are typically not visible in the directory tree unless they are mounted, it means they are integrated into the file system at a specific location in the directory tree. A normal user can access data on a partition or a device after it is mounted.

Understanding Linux configuration files System Administration

/etc/group : Contains the valid group names and the users included in the specified groups

/etc/passwd : Holds some user account info including passwords

/etc/shadow : Secure user account information

/etc/shell : Holds the list of possible "shells" available to the system

/etc/crontab : Lists commands and times to run them for the cron deamon.

Networking /etc/networks : Lists names and addresses of our own and other networks, used by the route

command.

/etc/resolv.conf : Tells the kernel which name server should be queried when a program asks to "resolve" an IP Address

/etc/network/interfaces : describes the network interfaces available on your system and how to activate them

/etc/host.conf : Specifies how host names are resolved

/etc/hosts : List hosts for name lookup use that are locally required

Understanding Linux configuration files (contd….)

User Configuration Files /.bashrc : Initialization script executed whenever the bash shell is started

/.bash_history : a file containing up to 1000 of the most recent commands available for recall using the up and down arrow keys

/.bash_logout : script that is run automatically by the bash shell when the user logs out of the system

/.bash_profile : It is the initialization script which is run by the bash shell upon user login to setup user’s customized environment such as variables and aliases. Upon login, if bash fails to find .bash_profile file in the user’s home directory, it looks for .bash_login, If there is no .bash_loginfile, it then looks for a .profile file.

Note: most user programs define two configuration files: the first one at a "system" level, located in /etc/; and the other one, "private" to the user, that can be found in his or her home directory.

Common Linux File Extensions .au : an audio file

.bin : binary file meant to be executed

.bz2 : a file compressed using bzip2

.conf : a configuration file

.deb : a Debian Package

.html/.htm : an HTML file

.iso : an image (copy) of a CD-ROM or DVD

.lock : a lock file that prevents the use of another file

.log : a system or program’s log file

.pid : Some programs write their process ID into a file with this extention

.sh : a shell script

.so : a Shared Object, which is a shared library

.src : a source code file

.tar.bz2, .tar.gz : a compressed file per File Compression

.txt : a plain text file

.zip : extension for files in ZIP format, a popular file compression format

Hidden (dot) Files

Hidden files have names that begin with a period; hence, they have been given the nickname of dot files.

Hidden files found in the user’s home directory, are dependent upon several factors, such as; applications installed on the system, the utilities that are in use and, the command shell that is being used. The home directory contains the bash related scripts as the default shell for Linux is the bash shell.

many of the configuration files are hidden

Examples: .bashrc, .bash_history, .profile, .login, .logout etc.

Environment Variables What are Environment variables?

Dynamic values which affect the processes or programs on a computer Can be created, edited, saved and deleted Gives information about the system behavior Environment variables can change the way a software/programs behaves. variables are case-sensitive and are created in upper case Examples: PATH, USER, UID, SHELL, HOME

List ENV Variables: env, printenv, set

Accessing Environment Variables: echo $VARIABLE

Defining New Environment Variables : VARIABLE_NAME= variable_value

Deleting Variables : unset VARIABLE_NAME

Setting Environment Variables: VARIABLE=«some value» : sets variable only for current shell export PATH=${PATH}:/home/Eva/bin OR export PATH=${PATH}:${HOME}/bin export –p : list all exported variables /etc/environment : variables are added in /etc/environment to set them permanently, and system wide (all users, all

processes)

set it for current shell and all processes started from current shell

Symbolic Links

shortcuts or references to the actual file or directory

Symbolic links are used to link libraries and make sure files are in consistent places without moving or copying the original file

Links are used to “store” multiple copies of the same file in different places but still reference to one file

Can reference a file/folder on a different hard disk/volume

Link remains if the original file is deleted

Link will NOT reference the file anymore if it is moved

Creating a Symbolic link : ln -s /path/to/original/ /path/to/linkName

What happens if a link is edited? Any modifications to the linked file will be made on the original file

What happens if a link is deleted? If a link is deleted, the original file will be unchanged and it will still exist

What happens if the original file is deleted but not the link? The link will remain but will point to a file that does not exist. This is called an orphaned or dangling link.