Basic Linux Commands

15 min readJan 14, 2025

What is Linux ?

Linux is an open-source operating system that serves as the backbone for many different devices, from personal computers and servers to smartphones and embedded systems. It functions similarly to other operating systems like Windows and macOS, providing the essential interface between hardware and software so that applications can run efficiently.

Linux Architecture

— — — — — — — — — — — — — — — — — — — — — — — — -
| User Applications |
— — — — — — — — — — — — — — — — — — — — — — — — -
| System Libraries |
— — — — — — — — — — — — — — — — — — — — — — — — -
| Shell |
— — — — — — — — — — — — — — — — — — — — — — — — -
| Linux Kernel |
| — Process Management — File Systems |
| — Memory Management — Networking |
| — Device Drivers — System Calls |
— — — — — — — — — — — — — — — — — — — — — — — — -
| Hardware |
— — — — — — — — — — — — — — — — — — — — — — — — -

Hardware Layer

This is the physical layer, which includes:

CPU (Central Processing Unit)
Memory (RAM)
Storage devices
Peripherals (keyboards, monitors, printers, etc.)

The Linux operating system interacts with these hardware components via the kernel.

Kernel Layer

The kernel is the core of the Linux operating system. It acts as a bridge between hardware and software. The kernel has several responsibilities:

Hardware Management: Communicates with and controls hardware devices.
Process Management: Manages processes by allocating CPU time and memory.
Memory Management: Controls the use of RAM and manages virtual memory.
Device Drivers: Allows the kernel to interact with specific hardware components.
File System Management: Manages how data is stored and retrieved.
Networking: Handles data exchange between devices and networks.

The kernel operates in kernel space, which is isolated from the user and application processes for security and stability.

Shell Layer

The shell is an interface between the user and the kernel. It allows users to execute commands and interact with the operating system. There are two types of shells:

Command-Line Shells: Examples include Bash, Zsh, and Fish, where users input text-based commands.
Graphical Shells: Used in desktop environments like GNOME or KDE, offering a graphical user interface (GUI).

User Space

This is where applications and user processes run. It is divided into two main categories:

1. System Libraries

Provide reusable functions and tools that applications can use to perform specific tasks (e.g., accessing files, communicating with the network).
The most common library in Linux is the GNU C Library (glibc).

2. User Applications

Programs or software that users interact with, such as web browsers, text editors, and media players.

Applications communicate with the kernel indirectly via system calls provided by system libraries.

General Knowlege Before We starts

What is the Command Line here ?

When we talk about the command line, we are actually referring to the Shell. The shell is a program that receives commands typed on the keyboard and passes them to the operating system (OS) for execution. One popular shell program is Bash, which stands for “Bourne Again Shell.” It is an enhanced version of the original Unix Shell program, which was written by Steve Bourne.

What Terminal Emulators ?

In Linux, terminal emulators are software applications that provide a command-line interface (CLI) for interacting with the operating system. They emulate the functionalities of traditional hardware terminals and allow users to execute shell commands, run scripts, and manage the system. Terminal emulators are essential for system administration, development, and various technical tasks in Linux.

Common terminal emulators in Linux : GNOME, Konsole, xterm,xterm

First Steps

Launch the terminal emulator ! Once you open it up you will see some thing like this.

[user1@linuxbox ~ ]$

This is what we call a shell prompt. Now the terminal imulator is ready for take input.

If it is prompt with # sign rather than $ that means you have higher “supper user” privilages.

[user1@linuxbox~ ]#

In here

`User1`

Represents the username of the person logged into the system.
In this case, User1 is the current user.

`@`

Separates the username (User1) from the hostname (linuxbox).
It indicates the format: username@hostname.

`linuxbox`

Refers to the hostname or name of the computer.
In this case, linuxbox is the name given to the system by the user or administrator.

`~`

Represents the current working directory.
The ~ symbol is shorthand for the user's home directory (e.g., /home/me).
If the user navigates to a different directory, this part of the prompt will display the full path (e.g., /usr/bin).

`$`

Indicates the type of user currently operating in the shell:
A dollar sign ($) means the user is a regular user with limited permissions.
If the user were a root user (administrator), the prompt would display a hash symbol (#) instead.

Navigation

Understand the filesystem tree

The Linux filesystem is organized as a hierarchical tree structure, with the root directory (/) at the base. All files and directories stem from this root directory.

The first directory in the filesystem is called the root directory.The root directory contains files and subdirectories and so on.

note : Unlike the windows ,which has a seperate filesystem tree for each storage device ,Unix systems like Linux always have a single filesystem tree,regarrdless of how many drives or storage divices are attached to the computer.

File Hyrachy in windows

File Sytem in Linux

Current working directory — pwd

pwd refers to the directory in the file system where the user is currently operating. It is the location where commands will be executed, files will be searched, or new files and directories will be created by default.

Listing the contents in the directory -ls

ls is used to list the contents of a directory. When you use the ls command without any options, it displays a list of files and directories present in the current working directory.Using cd helps you move around the file system quickly and efficiently, making it easier to find and manage files and directories.

Change the current working directory — cd

cd is used to navigate through the file system by changing the current working directory to a specified path.

Absoulute path : A file or directory from the root directory (/). It starts from the root directory and includes all the directories in between.

Relative path : A file or directory location relative to the current working directory. It does not start from the root but uses the current directory as the reference point.

What are those dots really doing ? ..

cd .. is used to move up one level in the directory hierarchy, taking you to the parent directory of the current working directory.

In here

cd — stands for Change Directory.
.. refers to the parent directory, which is one level above the current directory.

Example :

Most commonly used commands you should know

1. Create folder — mkdir

mkdir <space> <Folder Name>

Checking available files and folders in current location

image 10

Creating the folder named “Prabha” and then checking the location just to verify whether the folder has been created.

2. Create text file

2.1 touch command :

Will create empty text file.The touch command is used only to create an empty file; it doesn't open the file for editing. After running the touch filename.txt command, you don't need to exit because it does not leave you in an interactive editing mode. The command finishes immediately after execution.

touch <textfilename.txt>

To edit the textfile you have created you can use cat, nano or vim commands to edit the content of the text file and you can save and exit via givving relevant commands accordingly.

example: edit the created text file using touch command I have used nano<textfilename.txt>. Then I have edit my text file.So to save I pressed Ctrl+O to save and Ctrl+X to close and exit the file.

2.2 echo command

The echo command in Linux is used to display a line of text or a string in the terminal.

2.2.1 Display a Message or Text — echo “text you want to display”
Display Messge or a Text

echo “<Text you needed to display in the terminal>”

image 14

2.2.2 Write Text to a File — echo “text you want to save to a text file” > textfilename.txt

Write Text to a File — echo “This is line 1” > echofile.txt

image 15

2.2.3 Append to a file — echo “text you want to append to a text file” >> textfilename.txt

Append to a File — echo “This is line 2” >> echofile.txt

Echo command is far more than that and will explain the usage in upcoming topic.

2.3. cat command

Following cat command will allow you to write text to the givven text file.

cat > filename.txt

Then you can start writing text in to the file.

After you finished you can simply press Ctrl + D to exit.

View and ammend the text file as below.

3. Copy files or directories

3.1 Copy a file to another directory:

cp <copeing file name> <path where you need to locate the file copy>

cp document.txt /home/user/documents/

3.2 Copy a directory and its contents:

cp -r <From Directory> <to Directory>

3.3 Copy and rename a file:

cp <file name> <path/<new name for rename>

Extra:

Important Options

-i: Interactive mode; prompts before overwriting.
-r: Recursive copy for directories.
-a: Archive mode; preserves file attributes and directory structure.
-u: Copy only when the source file is newer than the destination file or if the file doesn’t exist at the destination.
-v: Verbose mode; shows detailed output during the copy process.

4. Move or rename files or directories

4.1 Move a File to Another Directory:

mv <file1.txt>< path to destination>

4.2 Move Multiple Files :

mv <file1.txt><file2.txt> <path to destination>

4.3: Options for moving

Prompt Before Overwriting: Use the -i (interactive) flag to prompt before overwriting:

mv -i file1.txt /path/to/destination/

Force Overwrite Without Prompt: Use the -f (force) flag to overwrite without prompting:

mv -f file1.txt /path/to/destination/

image 26

Verbose Output: Use the -v (verbose) flag to display detailed output:

mv -v file1.txt /path/to/destination/

5. Remove files or directories

rm <filename>

Before you remove a file or directory just good to know below options.

Forceful deletion: Be cautious when using rm -f or rm -rf as it bypasses prompts and can delete important files or directories.
Recursive deletion: The -r flag is required to remove directories and their contents.
Safety tip: Always double-check the command before pressing Enter, especially when using wildcards like *.

Examples:

image 30

6. Change file permissions

Understanding the file permssion

rwxr-xr —

Permissions are represented as:

r: Read
w: Write
x: Execute
- : No permission
First character: File type (- for files, d for directories).
Next three characters: Permissions for the owner (user).
Next three characters: Permissions for the group.
Last three characters: Permissions for others (everyone else).

6.1 View permision

ls -l <file name>

image 31

6.2 Add Permission — chmod u+x <filename>

6.3 Add Excactlly Permission — chmod o=r <filename>

6.4 Remove Permission -chmod g-w <filename>

6.5 Give All Permissions to Everyone: chmod a+rwx <filename>

6.6 Changing Permissions Recursively

chmod -R 755 <path to directory>

6.7 Using Numeric Mode

In numeric mode, permissions are represented as a three-digit number (octal):

Read = 4
Write = 2
Execute = 1

Add these values to calculate permissions:

rwx = 4 + 2 + 1 = 7
rw- = 4 + 2 + 0 = 6
r-- = 4 + 0 + 0 = 4

Format: chmod [owner][group][others] <filename>

example :

a) Set permission to everyone -chmod 777 <filename>

b) Set Owner to Read/Write, Group to Read, Others to No Access —

chmod 640 <filename>

7. Change file ownership — chown

7.1 Verfying ownership

ls -l <filename>

7.2 Change the Owner of a File

chown newuser file.txt

7.3 Change Both Owner and Group:

chown newuser:newgroup file.txt

7.4 Change Only the Group:

Use a colon without specifying a user:

To change only group ownership without using chown, you can use the chgrp command:chgrp newgroup file.txt

chown :newgroup file.txt

7.5 Options for `chown`

Change Ownership Verbosely: Use the -v option to display changes:

chown -v alice file.txt

Preserve Permissions While Changing Ownership: The -c option reports changes without verbose output for every file:

chown -c alice:developers file.txt

Suppress Errors: Use the --silent or -f option to suppress error messages:

chown -f alice file.txt

7.6 Permissions Required

You must have root privileges or be the file owner to use chown. Use sudo if necessary

sudo chown alice file.txt

Practical examples

Example 01: Set Owner to bob and Group to staff -

chown bob:staff report.txt

Example 02: Set Owner for All Files in a Directory —

chown -R bob /home/bob/documents

Example 03:Set Group for a File

chown :engineers script.sh

8. Add new Users and Remove Users

sudo adduser <newuser>

sudo deluser <username>

8. Display disk space usage

df [options] [file_or_directory]

8.1 Display Disk Usage for All Filesystems -df

8.2 Display in Human-Readable Format: Use the -h option to show sizes in human-readable units (KB, MB, GB):

8.3 Check Disk Usage for a Specific Directory:

image 40

8.4 Display Information in Inodes: Use the -i option to show inode usage instead of disk space:

8.5 Include Filesystems of Specific Types: Use the -t option to filter by filesystem type (e.g., ext4):

image 42

8.6 Exclude Filesystems of Specific Types: Use the -x option to exclude certain filesystem types (e.g., tmpfs):

image 43

8.7 Show Filesystem Type: df -T

8.8 Command for Specific File/Folder Usage -du

8.8.1 Basic disk usage

8.8.2 Basic disk usage in Human readable format

8.8.3 Summerise total disk usage

image 47

9. Display information about active processes

9.1 ps Command

9.1.1 List all processes — ps -e

9.1.2 Detailed process information — ps -ef

9.2 top Command

9.3 htop Command

10. Terminate a process

10.1 Using kill Command

steps :

Find the PID of the Process: Use ps, top, or pgrep to identify the process ID (PID)
send the kill command

10.1.1 Kill by User

10.1.2 Kill by Port

image 55

image 56

10.2 Using pkill Command

10.3 Using killall Command

Example 01 : Kill a process by name- pkill -9 apache2

Example 02 : Force kill a process using its PID — kill -9 9876

Example 03 : Kill all instances of a program- killall firefox

Example 04 :Kill processes owned by a specific user — pkill -u john

11. Display network connections, routing tables, interface statistics, masquerade connections, and multicast memberships

11.1 The `netstat` command provides comprehensive network-related information.net stat command

Step 01: Install netstat if not available with sudo install netstat

Step 02: netstat command

Display Active Network Connections — netstat
Show Routing Tables- netstat -r
Display Interface Statistics- netstat -i
Show Masquerade Connections- netstat -M
Display Multicast Group Memberships — netstat -g

11.2 Using ss Command

Display Active Network Connections — ss
Show Listening Ports -ss -l
View All Sockets (TCP and UDP)- ss -tua
Display Process Information- ss -p

11.3 Using ip Command

Display Network Interfaces — ip link show
Show IP Addresses- ip addr show
Display Routing Table- ip route show
View Multicast Addresses- ip maddr show

11.4 Using the ifconfig Command

provides information about network interfaces.
If ifconfig is unavailable, install net-tools as shown above.

11.5 nmcli Command

Display Network Status — nmcli general status
Show Active Connections- nmcli connection show

12. Test network connectivity

ping — ping <hostname_or_IP>
traceroute — traceroute <hostname_or_IP>
curl- curl <URL>
wget- wget <URL>
telnet- telnet <hostname_or_IP> <port>
naslookup- nslookup <hostname>
ss- ss -tua

13. Securely connect to another machine over the network

First you have to ensure whether ssh installed in both local and remote machine.

Real-World Examples:

Access a remote server: ssh admin@server.example.com to log into the server securely.
Copy files to a remote machine: scp report.pdf

http://user@192.168.1.5/home/user/docs/

Create an SSH key for password-less login: Use ssh-keygen to generate a key, then ssh-copy-id to add it to the remote server.
Transfer large directories: Use rsync for efficient and secure directory synchronization.

14. Manage system services

15. View system logs

16. Monitor system processes

Resources : “The Linux Command Line” by Williams E.Shotts Jr, ChatGPT

Special thanks to my VU lecturer, Jimmy Mathew, for providing insightful resources and guidence, and to HelloMonday career coach, Katie Peterson, for her encouragement throughout the Digital Jobs Program.