Essential Linux Commands for DevOps Engineers: A Beginner’s Guide

Linux is a powerful operating system, and learning its basic commands can help you manage files, users, directories, and software easily. In this blog, we’ll explore the most commonly used Linux commands in a simple and beginner-friendly way.

🗂️ How to Create a File in Linux

There are different ways to create files in Linux:

1. Using the cat command

The cat command is used to create files, view content, and combine multiple files into one.

You can create a single file.

azureuser@myvm:~$ cat > file1
Hi
I am Pankaj
azureuser@myvm:~$ cat file1
Hi
I am Pankaj

Press ‘ctrl+d’ to save the content of the file1.

You can merge (concatenate) the content of multiple files into one.

azureuser@myvm:~$ ls
file1  file2
//Merging the content of file1 and file2 and redirecting the output to file3
azureuser@myvm:~$ cat file1 file2 > file3
azureuser@myvm:~$ cat file3
Hi
I am Pankaj
file2 content

You can copy content from one file to another.

azureuser@myvm:~$ cat file2
file2 content
//Override the data of file1 to file2
azureuser@myvm:~$ cat file1 > file2
azureuser@myvm:~$ cat file2
Hi
I am Pankaj

You can append/add content to an existing file

azureuser@myvm:~$ cat file1
Hi
I am Pankaj
azureuser@myvm:~$ cat >> file1
adding new data
azureuser@myvm:~$ cat file1
Hi
I am Pankaj
adding new data

You can use tac to view the content from bottom to top.

azureuser@myvm:~$ tac file1
adding new data
I am Pankaj
Hi

2. Using the touch command

The touch command is used to create empty files and update timestamps.

You can create one or more empty files.

azureuser@myvm:~$ touch tfile1 tfile2
azureuser@myvm:~$ ls
file1  file2  file3  tfile1  tfile2
// Since tfile1 and tfile2 are empty, the cat command produces no output
azureuser@myvm:~$ cat tfile1

You can update the access and modify time of a file.

Time Stamps in Linux

stat command
- stat command displays detailed information about the file file1, such as its size, permissions, owner, group, last access time, last modification time, and more

// to check the time stamp of a file
azureuser@myvm:~$ stat file1
  File: file1
  Size: 32              Blocks: 8          IO Block: 4096   regular file
Device: 801h/2049d      Inode: 258086      Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/azureuser)   Gid: ( 1000/azureuser)
Access: 2025-07-26 16:25:12.300405360 +0000
Modify: 2025-07-26 16:15:09.179520655 +0000
Change: 2025-07-26 16:25:12.300405360 +0000
 Birth: 2025-07-26 16:05:14.899247475 +0000

Access time: Last time the file was opened.
Example: touch -a file1

Note:

The touch -a file1 command updates only the access time of the file file1, without changing its content or the modification time.

For example, before running the command, the stat output was shown above:

Access: 2025-07-26 16:25:12.300405360 +0000

After executing touch -a file1, the access time is updated to the current time.

Important:
The change time (ctime) is also updated because it reflects changes to the file's metadata, such as access or modification times—even if the file content remains unchanged.

azureuser@myvm:~$ touch -a file1
azureuser@myvm:~$ stat file1
  File: file1
  Size: 32              Blocks: 8          IO Block: 4096   regular file
Device: 801h/2049d      Inode: 258086      Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/azureuser)   Gid: ( 1000/azureuser)
Access: 2025-07-26 16:33:27.708122788 +0000
Modify: 2025-07-26 16:15:09.179520655 +0000
Change: 2025-07-26 16:33:27.708122788 +0000
 Birth: 2025-07-26 16:05:14.899247475 +0000

Modify time: Last time the file content was changed.
Example: touch -m file1

Note: Only modify time changed

azureuser@myvm:~$ touch -m file1
azureuser@myvm:~$ stat file1
  File: file1
  Size: 32              Blocks: 8          IO Block: 4096   regular file
Device: 801h/2049d      Inode: 258086      Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/azureuser)   Gid: ( 1000/azureuser)
Access: 2025-07-26 16:33:27.708122788 +0000
Modify: 2025-07-26 16:42:37.350568502 +0000
Change: 2025-07-26 16:42:37.350568502 +0000
 Birth: 2025-07-26 16:05:14.899247475 +0000

Change time: Last time the file’s metadata (like size or format) was changed.
Note: When a file is modified/Access, the change time is updated automatically.

Example: In the above case, the change time (ctime) was also updated after running touch -a file1 or touch -m file1.
The updated change time is:
Change: 2025-07-26 16:42:37.350568502 +0000

  azureuser@myvm:~$ stat file1
    File: file1
    Size: 32              Blocks: 8          IO Block: 4096   regular file
  Device: 801h/2049d      Inode: 258086      Links: 1
  Access: (0664/-rw-rw-r--)  Uid: ( 1000/azureuser)   Gid: ( 1000/azureuser)
  Access: 2025-07-26 16:33:27.708122788 +0000
  Modify: 2025-07-26 16:42:37.350568502 +0000
  Change: 2025-07-26 16:42:37.350568502 +0000
   Birth: 2025-07-26 16:05:14.899247475 +0000

3.✍️ Text Editors in Linux

a.vi Editor

A standard editor used for programming and plain text.
Press i to start writing.
Press Esc and type:
- :w to save
- :wq or :x to save and quit
- :q to quit
- :q! to quit without saving

azureuser@myvm:~$ vi vifile.txt
azureuser@myvm:~$ cat vifile.txt
vi editor example

b.nano Editor

A simple editor that doesn’t require pressing i to start editing.
Press ctrl+x and type:
- y to save
- press ‘enter’ to exit out of the editor

azureuser@myvm:~$ nano nanofile.txt
azureuser@myvm:~$ cat nanofile.txt
nano editor example

4.📢 Using the echo Command

To create a file with content: echo "Welcome" > echofile

azureuser@myvm:~$ echo "Welcome" > echofile
azureuser@myvm:~$ cat echofile
Welcome

To add content to an existing file: echo "Namaste" >> echofile

azureuser@myvm:~$ echo "Namaste" >> echofile
azureuser@myvm:~$ cat echofile
Welcome
Namaste

To empty a file: echo > echofile
- This command clears the contents of the file named echofile. It does not produce any output on the terminal. If the file doesn't exist, it will be created as an empty file.

azureuser@myvm:~$ echo > echofile
azureuser@myvm:~$ cat echofile

📁 Directory and File Management

Lists files and directories in long format (with permissions, owner, size, and date):ls -l

azureuser@myvm:~$ ls -l
total 24
-rw-rw-r-- 1 azureuser azureuser  1 Jul 26 16:57 echofile
-rw-rw-r-- 1 azureuser azureuser 32 Jul 26 16:42 file1
-rw-rw-r-- 1 azureuser azureuser 16 Jul 26 16:11 file2

Lists all files, including hidden ones (those starting with .), in long format: ls -la

azureuser@myvm:~$ ls -la
total 64
drwxr-x--- 5 azureuser azureuser 4096 Jul 26 16:56 .
drwxr-xr-x 3 root      root      4096 Jul 26 16:02 ..
-rw------- 1 azureuser azureuser  263 Jul 26 16:35 .bash_history
-rw-r--r-- 1 azureuser azureuser  220 Jan  6  2022 .bash_logout
drwx------ 2 azureuser azureuser 4096 Jul 26 16:02 .ssh
-rw-r--r-- 1 azureuser azureuser    0 Jul 26 16:03 .sudo_as_admin_successful
-rw------- 1 azureuser azureuser  988 Jul 26 16:51 .viminfo
-rw-rw-r-- 1 azureuser azureuser    1 Jul 26 16:57 echofile
-rw-rw-r-- 1 azureuser azureuser   32 Jul 26 16:42 file1
-rw-rw-r-- 1 azureuser azureuser   16 Jul 26 16:11 file2
-rw-rw-r-- 1 azureuser azureuser   30 Jul 26 16:09 file3

Displays the list of previously executed commands: history

azureuser@myvm:~$ history
    1  clear
    2  touch -a file1
    3  stat file1
    4  touch -m file1
    5  stat file1

Creates a new directory named : mkdir dir1

azureuser@myvm:~$ mkdir dir1
azureuser@myvm:~$ ls
dir1

Creates nested directories in one command: mkdir -p dir6/dir7/dir8
- p → parent directories

azureuser@myvm:~$ ls
dir1  dir6
azureuser@myvm:~$ cd dir6
azureuser@myvm:~/dir6$ ls
dir7

Changes the current directory to dir1: cd dir1

azureuser@myvm:~$ cd dir1
azureuser@myvm:~/dir1$

Moves up/ one step back from one directory level: cd ..

azureuser@myvm:~$ ls
dir1  dir6 
azureuser@myvm:~$ cd dir1
azureuser@myvm:~/dir1$ cd ..
azureuser@myvm:~$

Prints the current working directory path: pwd

azureuser@myvm:~$ pwd
/home/azureuser

Moves up(steps back) three directory levels: cd ../../..
- Current directory: /home/azureuser
- After moving up three levels: / (root directory)

azureuser@myvm:~$ pwd
/home/azureuser
azureuser@myvm:~$ cd ../../..
// moved to root directory
azureuser@myvm:/$ pwd
/

Jumps directly to the specified nested path: cd /home/azureuser/dir1

azureuser@myvm:/$ cd /home/azureuser/dir1
azureuser@myvm:~/dir1$

Creates multiple directories at once: mkdir dir8 dir9 dir10

azureuser@myvm:~$ mkdir dir8 dir9 dir10
azureuser@myvm:~$ ls
dir1  dir10  dir2  dir3  dir6  dir8  dir9  echofile  file1  file2  file3  nanofile.txt  tfile1  tfile2  vifile.txt

Creates a hidden file named : touch .file
- The touch .file command creates a hidden file named .file. In Linux, any file or directory that starts with a dot (.) is considered hidden and won’t appear in a normal ls listing.
  
  To view hidden files, use: ls -la

azureuser@myvm:~$ touch .file
azureuser@myvm:~$ ls -la
total 5
4 drwxrwxr-x 2 azureuser azureuser 4096 Jul 26 17:07 dir1
4 drwxrwxr-x 2 azureuser azureuser 4096 Jul 26 17:20 dir10
-rw-rw-r--  1 azureuser azureuser    0 Jul 26 17:21 .file

Creates a hidden directory named : mkdir .dir11
- Hidden files in Linux (those starting with a dot .) are not primarily for security, but they help prevent accidental modification or deletion, especially by inexperienced users.
  
  For example, some hidden files are critical for system configuration. If a user unintentionally deletes them, it could lead to system instability or even corruption.

azureuser@myvm:~$ mkdir .dir11
total 6
drwxrwxr-x  2 azureuser azureuser 4096 Jul 26 17:26 .dir11

📤 Copy, Move, and Delete Files

Copy a file: cp file1 file10
- cp → copy
- file1 → source file
- file10 → destination file

azureuser@myvm:~$ cat file2
Hi
I am Pankaj
azureuser@myvm:~$ touch file10
azureuser@myvm:~$ ls
 file10  file2
azureuser@myvm:~$ cat file10
//no output
azureuser@myvm:~$ cp file2 file10
azureuser@myvm:~$ cat file10
Hi
I am Pankaj

Move a file: mv file1 dir1
- it is just like cut and paste in windows
- mv → move
- file → source file
- dir1→ destination file

azureuser@myvm:~$ mv file1 dir1
azureuser@myvm:~$ cd dir1
azureuser@myvm:~/dir1$ ls
file1

Rename a file: mv file2 newfile
- There is no dedicated command to rename a file in Linux.
  Instead, we use the mv (move) command to move the current file to a new name or location, which effectively renames it.

azureuser@myvm:~$ ls
dir10 file10  file2
azureuser@myvm:~$ mv file2 newfile
azureuser@myvm:~$ ls
dir10 file10 newfile

Remove an empty directory: rmdir dir2

azureuser@myvm:~$ ls
dir1 dir2
azureuser@myvm:~$ rmdir dir2
azureuser@myvm:~$ ls
dir1

Remove nested empty directories: rmdir -p dir6/dir7/dir8
- rmdir → remove directory
- p → parent
- Nested directory → dir6/dir7/dir8

azureuser@myvm:~$ ls
dir3  dir6
azureuser@myvm:~/dir6$ ls
dir7
azureuser@myvm:~/dir6$ cd dir7
azureuser@myvm:~/dir6/dir7$ ls
dir8
azureuser@myvm:~/dir6/dir7$ cd dir8
azureuser@myvm:~/dir6/dir7/dir8$ 

//Removing Nested directory
azureuser@myvm:~$ rmdir -p dir6/dir7/dir8
azureuser@myvm:~$ ls
dir3

Remove with verbose output: rmdir -pv dirTest/dirX/dirY
- This command removes empty directories(last directory) and shows messages for each one it deletes.
  - -p means: remove parent directories too, if they are empty.
  - -v means: show a message for each directory removed.
```
  azureuser@myvm:~$ mkdir -p dirTest/dirX/dirY
  azureuser@myvm:~$ ls
  dirTest 
  azureuser@myvm:~$ rmdir -pv dirTest/dirX/dirY
  rmdir: removing directory, 'dirTest/dirX/dirY'
  rmdir: removing directory, 'dirTest/dirX'
  rmdir: removing directory, 'dirTest'
```
  - ⚠️ The last directory (dirY) must be empty, otherwise the command will fail.
    - Example: Let’s create the same folder structure and add a file inside dirY

            azureuser@myvm:~$ mkdir -p dirTest/dirX/dirY
            azureuser@myvm:~$ cd dirTest/dirX/dirY
            azureuser@myvm:~/dirTest/dirX/dirY$ touch file1
            azureuser@myvm:~/dirTest/dirX/dirY$ ls
            file1
            azureuser@myvm:~/dirTest/dirX/dirY$ cd ..//..//..
            azureuser@myvm:~$ ls
            dirTest 
            azureuser@myvm:~$ rmdir -pv dirTest/dirX/dirY
            rmdir: removing directory, 'dirTest/dirX/dirY'
            rmdir: failed to remove 'dirTest/dirX/dirY': Directory not empty

Remove non-empty directories: rm -rf dir1
- rm → remove
- rf→ remove forcefully

azureuser@myvm:~$ ls
dir1  dir10  dir3  dir8  dir9  dirTest 
azureuser@myvm:~/dir1$ ls
file1  test
azureuser@myvm:~/dir1$ cd ..
azureuser@myvm:~$ rm -rf dir1
azureuser@myvm:~$ ls
dir10  dir3  dir8  dir9  dirTest

Remove empty directory: rm -r dir1
- rm→ remove
- r→ recursive – it tells rm to delete the directory and everything inside it, including subdirectories and files.
- You only need to specify the original directory or file name, such as dirTest to move the nested directory

azureuser@myvm:~$ ls
dir10  dir3  dir8  dir9  dirTest  
azureuser@myvm:~$ cd dirTest
azureuser@myvm:~/dirTest$ ls
dirX
azureuser@myvm:~/dirTest$ cd dirX
azureuser@myvm:~/dirTest/dirX$ ls
dirY
azureuser@myvm:~/dirTest/dirX$ cd dirY
azureuser@myvm:~/dirTest/dirX/dirY$ ls
file1
azureuser@myvm:~/dirTest/dirX/dirY$ cd ..//..//..
azureuser@myvm:~$ rm -r dirTest
azureuser@myvm:~$ ls
10  dir10  dir3  dir8  dir9

🧹 Clearing the Console

To clear the terminal screen and remove all previous commands from view, use:

azureuser@myvm:~$ clear

📄 Viewing File Content in Different Ways

To check the first page of a file: less file
- file → filename (eg : file2, file3 etc)
- Press q to exit the view.

azureuser@myvm:~$ ls
dir10  dir3  dir8  dir9  echofile  file 
azureuser@myvm:~$ less file
1
2
2
3
3
4
5
6
7
8
file

To check the first 10 lines of a file: head file
- file → filename

azureuser@myvm:~$ head file
1
2
2
3
3
4
5
6
7
8

To check the last 10 lines of a file : tail file
- file → filename

azureuser@myvm:~$ tail file
r
u
u
n
n
n
ng
g
k
last page content

To view a file page by page (useful for large files): more file
- file → filename
- press ‘Enter’ to view next page and ‘q’ to move out of the file

azureuser@myvm:~$ more file
1
2
2
3
3
4
5
--More--(45%)

To sort the contents of a file in alphabetical order: sort file
- file → filename

azureuser@myvm:~$ cat file2
a
v
g
b
z
t
azureuser@myvm:~$ sort file2
a
b
g
t
v
z

To filter a specific word or alphabet from a file: grep "man" file2
- Example:
  - Word to find: man
  - File to search in: file2

azureuser@myvm:~$ cat file2
a
v
g
b
z
t
man
cse
book
azureuser@myvm:~$ grep "man" file2
man

🌐 Network and System Information

Check IP and network: ifconfig

azureuser@myvm:~$ ifconfig
Command 'ifconfig' not found, but can be installed with:
sudo apt install net-tools
azureuser@myvm:~$ sudo apt install net-tools
azureuser@myvm:~$ ifconfig
enP43463s1: flags=6211<UP,BROADCAST,RUNNING,SLAVE,MULTICAST>  mtu 1500
        inet6 fe80::222:48ff:fe22:8615  prefixlen 64  scopeid 0x20<link>
        ether 00:22:48:22:86:15  txqueuelen 1000  (Ethernet)
        RX packets 56300  bytes 76705466 (76.7 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 55300  bytes 17330159 (17.3 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

Check only IP: hostname -i

azureuser@myvm:~$ hostname -i
10.0.0.4

Check OS version: cat /etc/os-release or cat /etc/os-rel*

azureuser@myvm:~$ cat /etc/os-release
PRETTY_NAME="Ubuntu 22.04.5 LTS"
NAME="Ubuntu"
VERSION_ID="22.04"
VERSION="22.04.5 LTS (Jammy Jellyfish)"
VERSION_CODENAME=jammy
ID=ubuntu
ID_LIKE=debian
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
UBUNTU_CODENAME=jammy

📦 Software and Package Management

Note: I have used Apache2 as an example. Please feel free to replace it with the software of your choice.

In Ubuntu:

Install software: sudo apt install apache2 -y

azureuser@myvm:~$ sudo apt install apache2 -y

Check if a software is installed: which apache2
- which apache2 checks if the apache2 binary is in your system's PATH.
- If Apache2 is installed and in the PATH, it will return the path to the executable (e.g., /usr/sbin/apache2).
- If it's not installed, it will return nothing.

azureuser@myvm:~$ which apache2
/usr/sbin/apache2

Update packages: sudo apt update

azureuser@myvm:~$ sudo apt update
Hit:1 http://azure.archive.ubuntu.com/ubuntu jammy InRelease
Get:2 http://azure.archive.ubuntu.com/ubuntu jammy-updates InRelease [128 kB]
Hit:3 http://azure.archive.ubuntu.com/ubuntu jammy-backports InRelease
Get:4 http://azure.archive.ubuntu.com/ubuntu jammy-security InRelease [129 kB]
Fetched 257 kB in 0s (517 kB/s)
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
All packages are up to date.

Start service: sudo systemctl start apache2

azureuser@myvm:~$ sudo systemctl start apache2

Check service status: sudo systemctl status apache2
- If the output shows Active: active (running) since Mon 2025-07-28 08:43:47 UTC; 5min ago, it means the Apache2 service is currently running on your system.

azureuser@myvm:~$ sudo systemctl status apache2
● apache2.service - The Apache HTTP Server
     Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
     Active: active (running) since Mon 2025-07-28 08:43:47 UTC; 5min ago
       Docs: https://httpd.apache.org/docs/2.4/
   Main PID: 1598 (apache2)
      Tasks: 55 (limit: 9462)
     Memory: 5.4M
        CPU: 40ms
     CGroup: /system.slice/apache2.service
             ├─1598 /usr/sbin/apache2 -k start
             ├─1600 /usr/sbin/apache2 -k start
             └─1601 /usr/sbin/apache2 -k start

Jul 28 08:43:47 myvm systemd[1]: Starting The Apache HTTP Server...
Jul 28 08:43:47 myvm systemd[1]: Started The Apache HTTP Server.

Stop service: sudo systemctl stop apache2
- If the Apache service stops manually, you need to restart it using the command: sudo systemctl restart apache2
- If the output shows Active: inactive (dead) since Mon 2025-07-28 08:51:57 UTC; 7s ago, it means the Apache2 service is currently not running on your system.

azureuser@myvm:~$ sudo systemctl stop apache2
azureuser@myvm:~$ sudo systemctl status apache2
○ apache2.service - The Apache HTTP Server
     Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
     Active: inactive (dead) since Mon 2025-07-28 08:51:57 UTC; 7s ago
       Docs: https://httpd.apache.org/docs/2.4/
    Process: 2248 ExecStop=/usr/sbin/apachectl graceful-stop (code=exited, status=0/SUCCESS)
   Main PID: 1598 (code=exited, status=0/SUCCESS)
        CPU: 62ms

Jul 28 08:43:47 myvm systemd[1]: Starting The Apache HTTP Server...
Jul 28 08:43:47 myvm systemd[1]: Started The Apache HTTP Server.
Jul 28 08:51:57 myvm systemd[1]: Stopping The Apache HTTP Server...
Jul 28 08:51:57 myvm systemd[1]: apache2.service: Deactivated successfully.
Jul 28 08:51:57 myvm systemd[1]: Stopped The Apache HTTP Server.

Restart service after manual stop: sudo systemctl restart apache2

azureuser@myvm:~$ sudo systemctl restart apache2
// check the status
azureuser@myvm:~$ sudo systemctl status apache2
● apache2.service - The Apache HTTP Server
     Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
     Active: active (running) since Mon 2025-07-28 08:59:03 UTC; 5s ago
       Docs: https://httpd.apache.org/docs/2.4/
    Process: 4231 ExecStart=/usr/sbin/apachectl start (code=exited, status=0/SUCCESS)
   Main PID: 4235 (apache2)
      Tasks: 55 (limit: 9462)
     Memory: 5.2M
        CPU: 24ms
     CGroup: /system.slice/apache2.service
             ├─4235 /usr/sbin/apache2 -k start
             ├─4236 /usr/sbin/apache2 -k start
             └─4237 /usr/sbin/apache2 -k start

Jul 28 08:59:03 myvm systemd[1]: Starting The Apache HTTP Server...
Jul 28 08:59:03 myvm systemd[1]: Started The Apache HTTP Server.

Enable service on boot: sudo systemctl enable apache2

azureuser@myvm:~$ sudo systemctl enable apache2
Synchronizing state of apache2.service with SysV service script with /lib/systemd/systemd-sysv-install.
Executing: /lib/systemd/systemd-sysv-install enable apache2

Disable service on boot: sudo systemctl disable apache2

sudo systemctl disable apache2
Synchronizing state of apache2.service with SysV service script with /lib/systemd/systemd-sysv-install.
Executing: /lib/systemd/systemd-sysv-install disable apache2
Removed /etc/systemd/system/multi-user.target.wants/apache2.service.

Remove software: sudo apt remove apache2

azureuser@myvm:~$ sudo apt remove apache2
azureuser@myvm:~$ sudo systemctl status apache2
○ apache2.service
     Loaded: masked (Reason: Unit apache2.service is masked.)
     Active: inactive (dead)

List installed packages: apt list --installed

azureuser@myvm:~$ apt list --installed
Listing... Done
adduser/jammy,now 3.118ubuntu5 all [installed,automatic]
apache2-bin/jammy-updates,jammy-security,now 2.4.52-1ubuntu4.15 amd64 [installed,auto-removable]
apache2-data/jammy-updates,jammy-security,now 2.4.52-1ubuntu4.15 all [installed,auto-removable]
apache2-utils/jammy-updates,jammy-security,now 2.4.52-1ubuntu4.15 amd64 [installed,auto-removable]
apparmor/jammy-updates,jammy-security,now 3.0.4-2ubuntu2.4 amd64 [installed,automatic]
------- many more

Question -1: What is the difference between start and enable when managing the any software( eg: apache2 ) service

🔹 start – Start the Service Now

Command: sudo systemctl start apache2
What it does: Immediately starts the Apache2 service.
Effect: Apache2 begins running right now, but it won’t start automatically after a reboot unless you also enable it.

🔹 enable – Start the Service on Boot

Command: sudo systemctl enable apache2
What it does: Configures Apache2 to start automatically every time the system boots.
Effect: Doesn’t start the service immediately, just sets it to start on future reboots

Question -2: What is the difference between stop and disable when managing the any software( eg: apache2 ) service

🔴 stop Apache2

Command: sudo systemctl stop apache2
Effect: Immediately stops the Apache2 service.
Temporary: Apache2 will start again after a reboot if it was enabled.
Use case: You want to temporarily stop the web server without changing its startup behavior.

🚫 disable Apache2

Command: sudo systemctl disable apache2
Effect: Prevents Apache2 from starting automatically at boot.
Does not stop the currently running service.
Use case: You don’t want Apache2 to start automatically when the system reboots.

👤 User and Group Management

Check current user: whoami

azureuser@myvm:~$ whoami
azureuser

Show a message: echo "hello" | sudo wall
- Scenario: Suppose you're working on your VM and your friend is also logged into the same VM. To show a message to your friend, you can use the echo command along with a method to target their terminal session.

azureuser@myvm:~$ echo "hello" | sudo wall

Broadcast message from root@myvm (pts/1) (Mon Jul 28 09:57:57 2025):

hello

Broadcast message from root@myvm (pts/1) (Mon Jul 28 09:57:57 2025):

hello

Create a user: useradd pankaj
- logged-in as root user (cmd: sudo su)

root@myvm:/home/azureuser# useradd pankaj

Set password: passwd pankaj
- logged-in as root user (cmd: sudo su)

root@myvm:/home/azureuser# passwd pankaj
New password:
Retype new password:
passwd: password updated successfully

To view the created or existing users on a Linux system: cat /etc/passwd

root@myvm:/home/azureuser# cat /etc/passwd
azureuser:x:1000:1000:Ubuntu:/home/azureuser:/bin/bash
lxd:x:999:100::/var/snap/lxd/common/lxd:/bin/false
pankaj:x:1001:1001::/home/pankaj:/bin/bash

Create a group: groupadd cse
- logged-in as root user (cmd: sudo su)

root@myvm:/home/azureuser# groupadd cse

To view the group or existing group on a Linux system: cat /etc/group
- Note: This file contains all the groups on the system, and by default, each user is also automatically associated with at least one group listed here

root@myvm:/home/azureuser# cat /etc/group
azureuser:x:1000:
ssl-cert:x:123:
pankaj:x:1001:
cse:x:1002:

Add user to group: gpasswd -a pankaj cse
- logged-in as root user (cmd: sudo su)
- a→ add user

root@myvm:/home/azureuser# gpasswd -a pankaj cse
Adding user pankaj to group cse
// let's validate user
root@myvm:/home/azureuser# cat /etc/group
cse:x:1002:pankaj

Delete a user: sudo userdel username
- Add sudo at the beginning of the command if you are not logged in as the root user
- the user to be delected: arup

root@myvm:/home/azureuser# userdel arup

Delete a group: sudo userdel username
- Add sudo at the beginning of the command if you are not logged in as the root user
- the group to be deleted: ece

root@myvm:/home/azureuser# groupdel ece

Add multiple users to a group: gpasswd -M ajay,sameer,vijay cse
- -M: stands for member user
- Note: The user should be created before being added to a group
- I have logged-in as root user.
- Output: cse:x:1002:ram,peter,vijay

root@myvm:/home/azureuser# gpasswd -M ram,peter,vijay cse
pankaj:x:1001:
cse:x:1002:ram,peter,vijay
ram:x:1003:
peter:x:1004:
vijay:x:1005:

🔗 Links and Backups

Soft Link:

Create a soft link (just like window’s shortcut): ln -s file1 softlinkfile1

A soft link is like a shortcut or pointer to another file.
ln → the command to create links.
-s → tells ln to create a symbolic (soft) link instead of a hard link.

azureuser@myvm:~$ ln -s file1 softlinkfile1

It stores the path to the original file.
Check if a file has a soft link: ls -l

azureuser@myvm:~$ ls -l
total 2
-rw-rw-r-- 1 azureuser azureuser   22 Jul 28 10:30 file1
lrwxrwxrwx 1 azureuser azureuser    5 Jul 28 11:57 softlinkfile1 -> file1

If the original file is deleted, the soft link becomes broken.

azureuser@myvm:~$ rm file1
azureuser@myvm:~$ cat softlinkfile1
cat: softlinkfile1: No such file or directory

✅ Use case:
- Useful when you want to reference a file from multiple locations
- Common in backups or shared configurations.

Hard link:

Create a hard link (for backup purpose): ln file2 backupfile2
- A hard link is a direct reference to the data on disk.
- Both the original file and the hard link share the same inode.
- If you update ‘file2’, then ‘backupfile2’ will also be updated — and the same happens if you modify ‘backupfile2’ — only if they are linked using a hard link.

    azureuser@myvm:~$ ln file2 backupfile2
    azureuser@myvm:~$ ls -l
    total 28
    -rw-rw-r-- 2 azureuser azureuser   26 Jul 28 09:38 backupfile2
    -rw-rw-r-- 2 azureuser azureuser   26 Jul 28 09:38 file2

If the original file is deleted, the hard link still works — the data remains.

    azureuser@myvm:~$ rm file2
    azureuser@myvm:~$ ls
    backupfile2  file3 
    azureuser@myvm:~$ cat backupfile2
    a
    v
    g
    b
    z
    t
    man
    cse

✅ Use case:
- Useful for backups where you want multiple names pointing to the same data.
- Saves space because no duplicate data is created.

🆚 Summary: Soft Link vs Hard Link

Example: Imagine you have a room in your house where you keep your bike.

A hard link is like giving your friend a duplicate key to that room.
- Both of you can open the same room.
- Even if you lose your key, your friend can still access the bike.
A soft link is like writing a note that says:
“My bike is in Room 101.”
- If the room is moved or deleted, the note becomes useless

🟢 Hard link = Another key to the same room (file content)
🟡 Soft link = A shortcut pointing to the room (file path)

What Does an Inode Store?
- In Linux, an inode (index node) is a data structure used by the file system to store information about a file or directory except its name and actual data.
- Each file or directory has an inode that contains:
  - File type (regular file, directory, etc.)
  - Permissions (read, write, execute)
  - Owner and group
  - File size
  - Timestamps (created, modified, accessed)
  - Number of hard links
  - Pointers to data blocks (where the actual file content is stored
Remove file content: > file1

//viewing the content of file3
azureuser@myvm:~$ cat file3
content of file

//removing the content of the file3
azureuser@myvm:~$ >file3
azureuser@myvm:~$ cat file3

🗜️ Archiving and Compression

Archive files: tar -cvf dirX.tar dirX
- tar is an archiving tool used to combine multiple files or directories into one.
- -cvf stands for:
  - c: create a new archive
  - v: verbose mode (shows the progress in the terminal)
    - Example:
  - f: specifies the filename of the archive
- dirX.tar is the name of the archive file that will be created.
- dirX is the name of the directory (or file) you want to archive.

azureuser@myvm:~$ tar -cvf dirX.tar dirX
dirX/
dirX/dirY/
dirX/dirY/dirZ/
dirX/dirY/dirZ/file3

Note: The .tar file is just an archive, not compressed by default. It is treated as a regular file by the system, not as a directory.

Example: -rw-rw-r-- 1 azureuser azureuser 10240 Jul 28 13:01 dirX.tar

azureuser@myvm:~$ ls -l
total 2
drwxrwxr-x 3 azureuser azureuser  4096 Jul 28 12:58 dirX
-rw-rw-r-- 1 azureuser azureuser 10240 Jul 28 13:01 dirX.tar

Compress a file: gzip dirX.tar
- Reduce file size
- gzip compresses the file dirX.tar and replaces it with a new file named dirX.tar.gz.
- The original dirX.tar file is removed by default after compression

azureuser@myvm:~$ gzip dirX.tar
azureuser@myvm:~$ ls -l
total 2
drwxrwxr-x 3 azureuser azureuser 4096 Jul 28 12:58 dirX
-rw-rw-r-- 1 azureuser azureuser  197 Jul 28 13:01 dirX.tar.gz

Unzip a file: gunzip dirX.tar.gz
- This command will decompress the dirX.tar.gz file and leave you with dirx.tar, which is still an archive
- The original dirX.tar.gz file is removed by default after decompression

azureuser@myvm:~$ gunzip dirX.tar.gz
azureuser@myvm:~$ ls -l
total 2
drwxrwxr-x 3 azureuser azureuser  4096 Jul 28 12:58 dirX
-rw-rw-r-- 1 azureuser azureuser 10240 Jul 28 13:01 dirX.tar

Extract a tar file: tar -xvf dirX.tar
- To extract the contents of a .tar archive
- x: extract the archive
- v: verbose mode (shows progress)
- f: specifies the archive file name

This command will extract all files and directories from dirx.tar into the current directory.

azureuser@myvm:~$ tar -xvf dirX.tar
dirX/
dirX/dirY/
dirX/dirY/dirZ/
dirX/dirY/dirZ/file3

Download a file from the web: wget https://www.pdfdrive.to/dl/a-programmers-guide-to-the-mind
- wget is a command-line utility used to download files from the web.
- https://www.pdfdrive.to/dl/a-programmers-guide-to-the-mind is the actual URL of the file to download.
- The file will be saved in your current working directory

azureuser@myvm:~$ wget https://www.pdfdrive.to/dl/a-programmers-guide-to-the-mind
azureuser@myvm:~$ ls
 a-programmers-guide-to-the-mind

🔐 File Permissions in Linux

Linux is a multi-user operating system, and managing file access is crucial for security and collaboration. In this post, we’ll explore how to control file and directory permissions using commands like chmod, chown, and chgrp.

Access Modes:

🔍 Example of File Permission Format

In Linux, file ownership and permissions are divided into three parts:

User (Owner) – The first set of rw- means the owner of the file can read and write it.
Group – The second set of rw- means users in the same group can also read and write the file.
Others – The last set of r-- means all other users can only read the file.
- → It's a file (not a directory)
Note: The first azureuser is the owner, and the second azureuser is the group.

-rw-rw-r-- 1 azureuser azureuser     0 Jul 28 13:53 file1

🛠️ How to Set Permissions Using chmod

Example:

Old Permission of file : rw- rw- r--

New Permission of the file to be provided : rwx r-x r--

for owner: rwx
for group: r-x
for others: r--

Note: We can assign permissions in three ways. Please find the details below:

Method 1: Numeric Mode

Each permission is represented by a number:

rwx = 4 + 2 + 1 = 7

r-x = 4 + 0 + 1 = 5

r-- = 4 + 0 + 0 = 4

To apply these permissions to a file

azureuser@myvm:~$ chmod 754 file1
azureuser@myvm:~$ ls -l
total 1
-rwxr-xr-- 1 azureuser azureuser     0 Jul 28 13:53 file1

Method 2: Symbolic Mode

u → user (owner)

g → group

o → others

+ → add permission

- → remove permission

= → set exact permission

u=rwx → user gets read, write, and execute

g=rx→ group gets read and execute

o=r → others get read

azureuser@myvm:~$ chmod u=rwx,g=rx,o=r file1

Method 3: Assigning Specific Permissions

u+rwx → user gets read, write, and execute

g+rx → group gets read and execute

o+r → others get read

azureuser@myvm:~$ chmod u+rwx,g+rx,o+r file1

🔄 How to Change Ownership

Before assigning ownership, make sure the file or directory exists and that you have the necessary permissions to change its owner

Change Owner: sudo chown pankaj file1

before assigning owner: azureuser

        azureuser@myvm:~$ ls -l
        total 1
        -r--rwx-wx 1 azureuser azureuser     0 Jul 28 13:53 file1

after assigning owner: pankaj

  azureuser@myvm:~$ sudo chown pankaj file1
  azureuser@myvm:~$ ls -l
  total 1
  -r--rwx-wx 1 pankaj    azureuser     0 Jul 28 13:53 file1

Change Group:

before assigning group: azureuser

  azureuser@myvm:~$ ls -l
  total 1
  -r--rwx-wx 1 pankaj    azureuser     0 Jul 28 13:53 file1

before assigning group: cse

  azureuser@myvm:~$ sudo chgrp cse file1
  azureuser@myvm:~$ ls -l
  total 1
  -r--rwx-wx 1 pankaj    cse           0 Jul 28 13:53 file1

Remove all permissions:

chmod 000 file1
- result will show: ----------
- This removes all permissions (read, write, and execute) for user, group, and others.
- Note: No one, including the file owner, can access this file unless they are the root user. This is because only the root user can override file permission restrictions

    azureuser@myvm:~$ sudo chmod 000 file1
    azureuser@myvm:~$ ls -l
    total 1
    ---------- 1 pankaj    cse           0 Jul 28 13:53 file1
    azureuser@myvm:~$ cat file1
    cat: file1: Permission denied

🔁 Miscellaneous

Restart system: sudo reboot
```
  azureuser@myvm:~$ sudo reboot
```

🌳View directory structure:

Sometimes, you want to see the entire folder structure of a directory in a visual format. The tree command is perfect for this!

Step 1: Install tree : If it's not already installed, you can install it using:
```
  azureuser@myvm:~$ sudo apt install tree
```

Step 2: Use tree to View Directory Structure

  azureuser@myvm:~$ tree
  .
  ├── backupfile2
  ├── dir1
  ├── dir2
  ├── dir3
  ├── dir4
  │   └── dir5
  │       └── dir6
  ├── dirX.tar
  6 directories, 2 files

to see specific directory

  azureuser@myvm:~$ tree dir4
  dir4
  └── dir5
      └── dir6

🃏Using Wildcards in Linux

Wildcards are special characters that help match filenames or patterns. One common use is with the cat command to read files that follow a naming pattern.

Example: View OS Release Info : cat /etc/os-rel*
- * is a wildcard that matches any number of characters. It can be used to match filenames that start with specific characters, such as the first three characters rel.
  
  It will match files like:
  - rel
  - rel1.txt
  - rel_report.doc
  - rel123data.csv
- This command reads all files starting with os-rel in the /etc directory.
- Typically, it displays the content of /etc/os-release, which contains information about your Linux distribution.

    azureuser@myvm:~$ cat /etc/os-rel*
    PRETTY_NAME="Ubuntu 22.04.5 LTS"
    NAME="Ubuntu"
    VERSION_ID="22.04"
    VERSION="22.04.5 LTS (Jammy Jellyfish)"
    VERSION_CODENAME=jammy
    ID=ubuntu
    ID_LIKE=debian
    HOME_URL="https://www.ubuntu.com/"
    SUPPORT_URL="https://help.ubuntu.com/"
    BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
    PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
    UBUNTU_CODENAME=jammy

🧠 Final Thoughts

This guide covered the most essential Linux commands every DevOps engineer or beginner should know. From creating files and managing directories to handling users, permissions, and packages, these commands form the foundation of working efficiently in a Linux environment. Whether you're just starting out or brushing up your skills, mastering these commands will help you navigate and manage Linux systems with confidence.

- - -

- Written by Pankaj Roy | DevOps & Cloud Enthusiast

Essential Linux Commands: From File Creation to Permissions

Table of contents