In the world of computing, where graphical user interfaces (GUIs) have become the norm, the command line interface (CLI) continues to hold its ground as a powerful tool for performing tasks efficiently and effectively. By leveraging the command line, also known as the terminal or shell, users can tap into a realm of unparalleled control and flexibility. While GUIs provide visual representations of operations, the command line goes beyond superficial appearances, allowing users to harness the true potential of their machines.

The command line offers a direct and text-based interaction with the underlying operating system, empowering users to execute commands, manipulate files and directories, manage processes, and perform a wide range of tasks with remarkable speed and ease. By mastering the command line, individuals can unlock a multitude of possibilities for automating tasks, navigating complex systems, and working with large volumes of data more efficiently than ever before.

Unlike GUIs, which often require multiple clicks and navigation through menus, the command line offers a concise and straightforward syntax that can be executed by simply typing commands and pressing Enter. This streamlined approach not only saves time but also enables users to perform complex operations with a few simple keystrokes. Whether you are a system administrator, a developer, a data scientist, or an enthusiastic computer user, understanding the fundamentals of the command line is essential for maximizing your productivity and unleashing the full potential of your computing environment.

In this guide, we will delve into the realm of command line, equipping you with the knowledge and skills needed to harness the immense power and efficiency of the terminal. We will explore essential commands for navigating the file system, manipulating files and directories, managing processes, and configuring system settings. Along the way, we will uncover valuable tips, tricks, and shortcuts to enhance your command line prowess.

what is a shell

A shell is a command line interpreter or program that provides a user interface for interacting with an operating system. It acts as a bridge between the user and the operating system, allowing users to execute commands, run programs, and perform various tasks.

Shells provide a text-based interface where users can type commands and receive output or perform actions based on those commands. They interpret and execute the commands entered by the user, facilitating tasks such as file manipulation, process management, and system configuration.

There are different shells used in Linux, like bash, zsh, fish, ksh, and more. But don't worry, you can choose the shell you prefer for your Linux distribution. Many shells have similar syntax because they are based on bash.

what is the terminal

The terminal is a text-based interface that takes input from the user (commands) and provides them to the shell to execute the commands.

The terminal can be different and the prompt also, so don’t be surprised if you see another prompt like only the “$” sign or terminal theme.

Which shell is the default in my linux distro ?

to know which shell is the default in your linux distro, open a terminal and run this command

basename $SHELL

Here is a list of known linux shell:

1. Bash (Bourne Again SHell):

• Default shell on many Linux distributions.
• Extensive compatibility with POSIX standards and earlier shells.
• Offers features such as command line editing, history, job control, and shell scripting capabilities.
• Widely used and well-documented, with a large user and support community.

2. Zsh (Z Shell):

• Extends and improves upon features from other shells, including Bash and Ksh.
• Offers advanced tab completion, spelling correction, globbing options, and powerful scripting capabilities.
• Highly customizable with themes and plugins.
• Improved interactive usage experience, with advanced prompt customization options.

3. Ksh (Korn Shell):

• Combines features from the Bourne Shell (sh) and C Shell (csh).
• Provides advanced scripting capabilities, including associative arrays, advanced string manipulation, and built-in math operations.
• Not as widely used as Bash or Zsh, but popular on some Unix systems.
• Good choice for scripting complex tasks and for users familiar with the Bourne Shell syntax.

4. Fish (Friendly Interactive SHell):

• Designed to be user-friendly and intuitive, especially for new users.
• Offers features like syntax highlighting, autosuggestions, and a clean, modern design.
• Provides an extensive set of built-in functions and convenient defaults.
• Emphasizes ease of use and discoverability.

In my linux I use zsh as my default shell since it has a good syntax highlight and auto-suggestions.

Which shell should I use ?

The shell you should use depends on your taste, choose one from the list or try them all to know the best shell for you.

Use terminal to do tasks

when you open the terminal it opens in the home directory by default which is indicated with the sign “~”.

TEXT MANIPULATIONS

---

Print text to terminal

To print text to terminal we use “echo”

If you want to create a new line use \n:

Clear the terminal

you can remove all the previous command output by running

clear

FILES AND DIRECTORIES MANIPULATIONS

---

List files and directories

If you want to see files and directories existing in this directory you can run the command

ls

result:

But there are hidden files and directories which start with a point, to list them in addition to normal files use the command

ls -a

result:

as you can see there are other files which start with a point in their name

If you want to see more information like file permissions use the command ls with the argument  -l.

ls -l

result:

As you can see there is other information about the files.

What if we want to list hidden files and their information, we can combine two argument -a and -l like this:

ls -a -l

result:

By default the ls command list files of the working directory you can use another directory with:

ls directory_name

Print the content of a file

Use the command cat to print the content of the file to terminal like this:

Change the working directory

Use cd to jump to another directory

the working directory changed to Desktop

Show the current working directory path

Use pwd

Create and Remove a directory

Create a directory with mkdir and remove it with rmdir

Move, copy and rename a file

Use the command mv to move files and cp to copy them from a location to another one


The difference between cp and mv is that mv remove the file from its old location while cp keep the file in the two locations, you can copy or move multiple file by adding their name before the destination like this:

cp/mv file1 file2 directory3 destination

you can also copy or move the directory.

The mv command is used also to change a filename with

mv oldfilename newfilename

Create a file

The touch command is used to create an empty file

Find a file

find command is used to search for a file in the specified directory in active mode which mean that it open and read directory to find files, to search for a file with the exact name use

find /path -name "exactname"

if you are not sure about the name of the file you can use the * sign, the asterisk symbol (*) is known as a wildcard or a globbing character. It is used for pattern matching and represents any number of characters (including zero characters) in a filename or a string.

find /path -name "*ctnam*"

or

find /path -name "*ctnam*.txt"

this will output only files that end with .txt and have the string “ctnam” in their names.

Locate is a command used to search for files in passive mode, it has a database to search in. You can use the locate command like this:

locate expression

you can also use the asterisk symbol like in find command.

locate is faster than find since it uses a database of existing files but if the file is newly created and not indexed by locate use find to search all the system.

Dealing with large files

It’s time consuming to open and manipulate large files, but you can use the tail command to make it faster

1. watching changes in a file: you can use tail to watch live changes made to the file

tail -f file.txt

2. print the last 10 lines of a file

tail -n 10 file

3. print the file content starting from line 10

tail -n +10 file

Sorting a file

sorting a file mean to arrange his lines alphabetically or numerically

To sort in ascending use

sort filename.txt

To sort in descending use

sort -r filename.txt

Search for expression in a file

1. simple grep:

Sometimes, you have a long file and you want to search for one line or more that contain an expression, you can use the grep command to search in a file like this:

grep expression file

example:

2. grep with regex:

regex or regular expressions are like codes and patterns that can find a specific string in a text following rules. For example you can find all words that start with “a” and end with “b”.

• for example to search for a word that start with “z” and end with “w” use

grep -E "\bz\w*w\b" file.txt

The \bz indicates that “z” is the first character in the word.

The \w indicate any single character, when adding * mean any number of characters.

The w\b indicates that “w is the last character in the word.

Note: when using regular expressions with grep use the -E argument.

Print a file without duplicate lines

The uniq command can print a file without printing duplicates lines multiple times you can use it like this:

uniq file.txt

Find the difference between two files

let’s say you have two files and you can’t find the difference between them, you can use diff

diff file1.txt file2.txt

Change the owner of a file

as you learned in chapter “user and group management” files have owner and you can change the owner of a file with chown

chown new_owner file

if the permission is denied use sudo, sudo run a command as the root user

sudo command

so use

sudo chown new_user file

to change the owner of all files inside a directory use

sudo chown -R new_user directory

Change the group of a file

as you learned in chapter “user and group management” files have group and you can change the group of a file with chgrp

sudo chgrp group file

or with chown

sudo chown :group file

to change the group of all files inside a directory use

sudo chgrp -R group directory

or

sudo chown -R :group directory

you can change the owner and group in the same command with chown:

sudo chown owner:group file

Change the mode

as you learned in chapter file permissions, files have permissions for owner, group and others. To edit these permissions we use the command chmod permissions file

permissions are set like this u=owner_permission,g=group_permission,o=others_permission

you can set the permissions in two presentations, octal and symbolic. Symbolic provide more human readable way, “w” present write permission, “r” present read permission, and “x” present execute permission, so to set permission that give read, write, execute access to owner and read, execute to group, and read to others in symbolic presentation use

chmod u=rwx,g=rx,o=r file

in octal presentation we use number:

• 4 stands for "read",
• 2 stands for "write",
• 1 stands for "execute",

so the permission read, write, execute is 4+2+1 and it’s 7

and read, execute is 4+1 which is 5

and read is 4, so you can use

chmod 754 file

the first number is for owner, the second is for group and the third is for others

Calculate the size of a directory or more

To see the size of a directory use du directory  

But it print the size in non human readable format, to print it in human readable format use the -h argument

SYSTEM

---

• general command:

1. Get disk usage information, with →use df
2. show all process →use ps ax

Processes in Linux are running instances of programs or tasks that are executed by the operating system. Each process has its own unique process identifier (PID) and consumes system resources, such as CPU time, memory, and input/output.

3. display dynamic real time information about running process like task manager → use top

Stop running process

To stop a running process using its PID you can use

kill PID

you can also kill all process with a specific name for example if you have multiple process with the name i3wm you can use

killall i3wm

you must use the complete name else it won’t kill the process

Pass a command to background

When you pass a command to the background you won’t see any output and you can execute more commands in the terminal session, but the command is still running in the background.

To pass command to the background you can append “&” in the end of the command

Show command running in background

To see all commands running in the background use the command jobs

Resume suspended job in the background

When a command is running you can suspend it by pressing Ctrl+z and it will be displayed when running jobs 

you can resume the command by using bg [JOB_SPEC] to resume it in the background. Where JOB_SPEC can be one of the following:

• %n - where n is the job number
• %abc - refers to a job started by a command beginning with abc
• %?abc - refers to a job started by a command containing abc
• %- - specifies the previous job

Resume a job in the foreground (terminal)

What if you want to resume the job in the foreground and not in the background, you can use fg [JOB_SPEC] instead of bg [JOB_SPEC]

Create an alias

Sometimes, you  run a command with a set of arguments multiple times and you are tired of repeating this long command, so you can create an alias.

An alias is one command that runs another command with a set of argument, it’s like a shortcut for a long command.

To create an alias you can use the command alias name="cmd –arg1 –arg2". For example to create an alias for the command ls with the argument -l and -a to show more informations you can run the command:

alias lss="ls -a -l"

This command create an alias named lss that run the command ls -a -l

To remove an alias you can use the command

unalias aliasName

example

unalias lss

Locate the executable file associated with a command

When you run a command you actually run an executable file associated with it except shell built-in command that may doesn’t have an executable file, sometimes they are included in the shell executable, to find the file you can use the command which command_name

To show all paths to the executable file of a command use the -a argument.

Make a command run even if you close the session

Sometimes, you need to keep a command running even if you close the session. For example when closing the terminal you close the session, or when you connected to another computer remotely and then get disconnected, the session will be closed and there are commands that you don’t want them to stop running so you can use the nohup command  and you can run it also in the background with nohup command & .The output of the command will be written in the file nohup.out

Print username of the user currently logged in to terminal

Just use whoami

Display users logged in to the system

just use who

Switch to another user in terminal

Use su another_username

Change the password of a user

To change the password of a user you can use the passwd command, but you must be the root or use sudo or have the user's old password.

When running from the root or using sudo, you don’t need the user’s password:

When running from normal users without sudo, you can only change the password of current user and you need the old password:

Show commands history

You can use the history command to achieve this purpose. This show commands with numbers

You can use the syntax !<command number> to repeat the command

HARDWARE AND SYSTEM

---

Uname

Run the uname command to print kernel name, add:

•  -a, --all                    print all information, in the following order,except omit -p and -i if unknown.
•   -s, --kernel-name            print the kernel name
•   -n, --nodename               print the network node hostname
•   -r, --kernel-release         print the kernel release
•   -v, --kernel-version         print the kernel version
•   -m, --machine                print the machine hardware name
•   -p, --processor              print the processor type (non-portable)
•   -i, --hardware-platform  print the hardware platform (non-portable)
•   -o, --operating-system   print the operating system

Neofetch

Neofetch is a command-line tool for displaying system and hardware informations, it’s not a part of the shell but it’s included in almost all linux distro, or you can install it from https://github.com/dylanaraps/neofetch/wiki/Installation

NOTE: almost all commands have a help option and a manual, you can access the help by adding -h or –help that depend on command developer, to access the manual on how to use the command use man command

Piping

Sometimes, you want to use the output of a command as the input for other command, and this is called piping

For example if i have a file named animal.txt and i want to print its content but it’s a large file, you can pip the output of cat to the command less like this, so it print few lines and when you enter it add some few other lines too:

cat animals.txt | less

Another example, if you are in a directory that has a lot of files and you want to search for file containing the string “dog”, you just run:

ls | grep "dog"

Output redirection

Sometimes, you need to save the output of a command to a file, this is called output redirection

echo dog > animals.txt #create or rewrite the file dir.txt to the string "dog"

echo dog >> animals.txt #create or append to the file dir.txt the string "dog"

Running multiple commands in one line

You can run multiple commands separated by “;” in one time like this:

you can run whatever number of commands you want.

Terminal shortcut

Terminal shortcuts can be customized but most terminals come with default shortcuts.

Navigation Shortcuts:

• Ctrl + A: Move the cursor to the beginning of the line.
• Ctrl + E: Move the cursor to the end of the line.
• Ctrl + B or Left Arrow: Move the cursor back one character.
• Ctrl + F or Right Arrow: Move the cursor forward one character.
• Ctrl + P or Up Arrow: Retrieve the previous command from history.
• Ctrl + N or Down Arrow: Retrieve the next command from history.

Text Manipulation Shortcuts:

• Ctrl + D: Delete the character at the current cursor position.
• Ctrl + K: Delete the text from the current cursor position to the end of the line.
• Ctrl + U: Delete the text from the current cursor position to the beginning of the line.
• Ctrl + W: Delete the word preceding the current cursor position.
• Ctrl + Y: Paste (yank) the previously deleted text.

Command History Shortcuts:

• Ctrl + R: Search backward through the command history for a specific command.
• Ctrl + G: Escape from the history search mode initiated by Ctrl + R.
• !!: Repeat the last command.
• !xyz: Execute the most recent command starting with "xyz" from the command history.

Terminal Control Shortcuts:

• Ctrl + L: Clear the terminal screen.
• Ctrl + C: Interrupt (cancel) the currently running command.
• Ctrl + Z: Suspend (pause) the currently running command and put it in the background.
• Ctrl + D: Exit the current shell or close the terminal session.