📘 What are File Permissions and Ownership in Linux?
Every file and directory in Linux has:
1. Owner – The user who created the file.
2. Group – A set of users who can also access the file.
3. Permissions – Define what the owner, group, and others can do with the file.
📂 File Ownership
Each file or folder has 3 types of ownership:
Ownership Description
User (u) The owner of the file (creator).
Group (g) A group of users with shared access.
Others (o) All other users on the system.
Use this command to check:
ls -l filename
Example:
-rw-r--r-- 1 koushik devs 1340 May 2 file.txt
koushik = owner
devs = group
🔐 File Permissions
There are 3 types of permissions:
Symbol Permission What it Allows
r Read View file content
w Write Edit/modify the file
x Execute Run the file (for scripts or binaries)
Each permission applies to user, group, and others.
📊 Permission Format (Example Explained)
-rwxr-xr--
Breakdown:
Part Meaning
- File type (- = file, d = directory)
rwx User (owner) permissions: read, write, execute
r-x Group permissions: read, execute
r-- Others: read only
📘 chown – Full Detailed Explanation
🔑 Definition:
chown stands for "change ownership".
It is used to change the owner and/or group of a file or directory.
📚 Syntax:
chown [OPTIONS] NEW_OWNER[:NEW_GROUP] FILE
📌 Key Points:
NEW_OWNER → the user who will become the new owner.
NEW_GROUP → the group that will be associated (optional).
If only owner is specified: chown koushik file.txt
If owner and group are specified: chown koushik:devops file.txt
You need sudo/root privileges to run this command.
🧪 Examples:
1. ✅ Change file owner:
sudo chown ram file.txt
Now ram becomes the owner of file.txt.
2. ✅ Change file owner and group:
sudo chown ram:devops file.txt
Owner → ram, Group → devops
3. ✅ Change only group:
sudo chown :devops file.txt
Owner remains unchanged, group changes to devops.
4. ✅ Change ownership of a directory:
sudo chown koushik:devs mydir
5. ✅ Recursive change (apply to all inside folder):
sudo chown -R koushik:devs /var/www/
-R Recursively change ownership of directory and all files inside
-v Verbose output (shows what changed)
🔐 Why Use chown in Real Time?
To fix permission issues after copying files.
To ensure files created by a service or script belong to the correct user.
During DevOps deployments, you might change ownership to www-data, nginx, or jenkins users.
📌 For Interview:
"chown is a Linux command used to change the ownership of files or directories. We can assign a new user or
group, or both, and use -R to apply recursively for directories."
📘 What is chmod?
chmod stands for "change mode".
It is used to change the permissions (read, write, execute) of a file or directory for owner, group, and others.
📚 Syntax:
chmod [OPTIONS] PERMISSIONS FILE
🔑 File Permission Basics:
There are 3 types of permissions:
Symbol Meaning Value
r Read 4
w Write 2
x Execute 1
There are 3 levels of users:
User Symbol
Owner u
Group g
Others o
All a
🧪 Two Ways to Use chmod:
✅ 1. Symbolic Mode (using letters)
1. 🔼 Add Permissions (+)
chmod u+x file.txt # Add execute for user
chmod g+w file.txt # Add write for group
chmod o+r file.txt # Add read for others
chmod a+x file.txt # Add execute for all (user, group, others)
2. 🔽 Remove Permissions (-)
chmod u-w file.txt # Remove write from user
chmod g-x file.txt # Remove execute from group
chmod o-r file.txt # Remove read from others
chmod a-w file.txt # Remove write from all
3. 🟰 Set Exact Permissions (=)
This removes all existing permissions and sets only given ones:
chmod u=r file.txt # User can only read
chmod g=rw file.txt # Group can read and write only
chmod o= file.txt # Others have no permission
chmod a=r file.txt # Everyone can only read
✅ Combined Examples:
chmod ug+r file.txt # Add read for user and group
chmod u+x,g-w file.txt # Add execute to user, remove write from group
chmod u=rw,g=r,o= file.txt # Set multiple exact permissions
✅ 2. Numeric Mode (using numbers)
Each permission has a value:
r = 4, w = 2, x = 1
You add values for each user type:
chmod 755 file.sh
Means:
Owner → 7 → rwx → 4+2+1
Group → 5 → r-x → 4+0+1
Others → 5 → r-x
More examples:
chmod 777 file.txt # All full access
chmod 644 file.txt # Owner: rw-, Group: r--, Others: r--
chmod 700 file.sh # Only owner can read, write, execute
🔐 1. What is Authentication?
Authentication is the process of verifying identity. In Linux or cloud systems (like AWS), when we connect to a
server (using SSH), the system checks whether we are allowed or not.
🔑 2. Types of SSH Authentication
Type Description
Password-based You enter a password to access the server
Key-based (SSH Key) You use a cryptographic key pair (private/public) to log in
🔐 Password-based Authentication
✅ How it works:
You run ssh user@server
Server asks: “What is your password?”
You type the user password.
If correct → access granted.
✅ Advantages:
Simple and quick to set up
No need to generate SSH keys
❌ Disadvantages:
Less secure (passwords can be guessed, stolen, or brute-forced)
Not scalable for many users or servers
Cannot be automated easily
📄 Location:
/etc/ssh/sshd_config
✅ This is the SSH daemon (server) configuration file — it controls how SSH behaves on the server side.
🔧 1. Enable or Disable Password Authentication
✅ To Allow Password Login:
PasswordAuthentication yes
❌ To Disable Password Login (for key-based only):
PasswordAuthentication no
🔒 Recommended for secure environments using only SSH keys.
sshd -t (for testing)
Restart the SSH service:
sudo systemctl restart sshd
Or:
sudo service ssh restart
✅ You can now log in using passwords!
🔑 Key-based Authentication
✅ How it works
✅ Step-by-Step: Where to Paste the Public Key
🔑 Public key file example:
If you generated a key pair using:
ssh-keygen -f mykey
You will get:
mykey → private key (keep secure)
mykey.pub → public key (you will paste this)
User should send his public key to admin to give access.
📍 Where to paste the .pub key?
✅ Paste the contents of your mykey.pub into the following file on the remote server:
/home/username/.ssh/authorized_keys
🔧 Steps:
1. Connect to the remote server (if you still have password access):
ssh username@remote-server
2. Create the .ssh directory (if not already exists):
mkdir -p .ssh/authorized_keys
chmod 700 .ssh
3. Paste your public key into authorized_keys:
nano .ssh/authorized_keys
# OR
vi .ssh/authorized_keys
📌 Paste the entire contents of mykey.pub into the file.
4. Set proper permissions:
chmod 600 .ssh/authorized_keys
Sure, Koushik! To enable password-based SSH login, follow these steps:
✅ Step-by-Step: Change PasswordAuthentication no to yes
1. Open the SSH configuration file:
sudo nano /etc/ssh/sshd_config
Or use vi if you prefer:
sudo vi /etc/ssh/sshd_config
2. Find this line:
PasswordAuthentication no
🔁 Change it to:
PasswordAuthentication yes
👉 If the line is commented out (starts with #), remove the # too.
sshd -t (for testing)
Restart the SSH service:
sudo systemctl restart sshd
Or:
sudo service ssh restart
✅ Then, from your local machine:
Use your private key to log in:
ssh -i mykey username@remote-server
🔒 Important Points:
File Purpose Permission
~/.ssh/ SSH config dir 700
~/.ssh/authorized_keys Stores allowed public keys 600
🔧 Bonus: Copy key automatically (if ssh-copy-id available):
ssh-copy-id -i ~/.ssh/mykey.pub username@remote-server
✅ Advantages:
More secure than passwords
Can disable password login completely
Better for automation (scripts, DevOps tools, CI/CD)
Works with Git, AWS EC2, Ansible, etc.
❌ Disadvantages:
Need to manage key files
Private key must be protected carefully
A bit technical for beginners
Summary Table:
Feature Password Auth Key-Based Auth
Setup Simplicity Easy Slightly complex
Security Weaker Stronger
Automation Friendly ❌ No ✅ Yes
Multi-Server Usage Tedious Scalable
Risk Password leak Key theft if unprotected
SSH Command ssh user@ip ssh -i key user@ip
🧠 Interview Tip:
"Password-based authentication uses a user password to log in, but is less secure and harder to scale. Key-
based authentication uses a private/public key pair, offering better security, automation support, and is
commonly used in DevOps and cloud environments."
To give a user sudo (admin) access using the /etc/sudoers file, you can add them under the wheel group or
directly as a sudo user.
Here’s how to do it properly and safely.
✅ Option 1: Add the User to the wheel Group (Recommended on RHEL/CentOS/Fedora)
Step 1: Add the user to the wheel group
sudo usermod -aG wheel username
Step 2: Make sure this line is enabled in /etc/sudoers:
Open the file using visudo (safe editor for sudoers):
sudo visudo
Make sure this line is uncommented:
%wheel ALL=(ALL) ALL
✅ Now any user in the wheel group can use sudo!
✅ Option 2: Add the User Directly to /etc/sudoers (Not recommended, but possible)
Open with visudo:
sudo visudo
Then add a line at the end:
username ALL=(ALL) ALL
This gives the user username full sudo privileges.
⚠️Important Notes:
Always use visudo to edit /etc/sudoers. It checks for syntax errors before saving.
Wrong syntax can lock you out of root/sudo access!
wheel group is the safest and standard way on RHEL-based systems.
✅ What is Package Management?
Package Management in Linux is the process of installing, upgrading, configuring, and removing software
packages (applications, libraries, tools) on your system.
🔧 What is a Package?
A package is a compressed archive file that contains:
The application or tool itself
Metadata (version, dependencies, etc.)
Scripts to install and configure it
🎯 Why is Package Management important?
As a DevOps engineer, package management helps you:
Automate software installation (e.g., via Ansible, Dockerfiles)
Ensure version control and consistency across servers
Resolve dependencies automatically
🧰 Types of Package Managers:
1. Debian-based (Ubuntu, Kali, etc.)
.deb files
Tools:
o apt (Advanced Package Tool)
o dpkg
2. Red Hat-based (CentOS, RHEL, Fedora, Amazon Linux)
.rpm files
Tools:
o yum (Yellowdog Updater, Modified)
o dnf (New version of yum)
o rpm
💡 Most Common Commands:
🔸 APT (Debian/Ubuntu):
Task Command
Install a package sudo apt install <package>
Update package list sudo apt update
Upgrade packages sudo apt upgrade
Remove a package sudo apt remove <package>
Search package apt search <name>
Show package info apt show <name>
🔹 YUM / DNF (RHEL/CentOS):
Task Command
Install a package sudo yum install <package> or sudo dnf install <package>
Update all packages sudo yum update
Remove a package sudo yum remove <package>
List all packages yum list installed
Search package yum search <name>
Package info yum info <name>
Available packages yum list available
✅ Process of Installing a Package in Linux
Let’s break it down into clear steps:
🔹 Step 1: Know Your Linux Distribution
Check if your system is:
Debian-based (like Ubuntu, Kali, Linux Mint)
RHEL-based (like CentOS, Red Hat, Fedora, Amazon Linux)
Command to check:
cat /etc/os-release
🔹 Step 2: Update Your Package Repository
Before installing, always update the local list of available packages.
For Debian-based:
sudo apt update
For RHEL-based:
sudo yum update # OR
sudo dnf update
🔹 Step 3: Search for the Package (Optional)
To make sure the package exists in the repository.
APT:
apt search <package-name>
YUM:
yum search <package-name>
🔹 Step 4: Install the Package
For Debian/Ubuntu (APT):
sudo apt install <package-name>
For RHEL/CentOS (YUM or DNF):
sudo yum install <package-name>
# or
sudo dnf install <package-name>
Example:
sudo apt install nginx
sudo yum install git
🔹 Step 5: Verify Installation
You can check if the package was installed:
<package-name> --version
Or check with:
which <package-name>
Example:
git --version
which nginx
🔹 Step 6: (Optional) Enable and Start the Service
If it’s a service like nginx, mysql, etc.:
sudo systemctl enable nginx
sudo systemctl start nginx
🔹 Step 7: (Optional) Check Status
sudo systemctl status nginx
🔐 Note:
Some packages may need dependencies – package managers automatically install them.
Use sudo for admin rights.
Always update before installation to avoid broken packages.
What is /etc/yum.repos.d/?
It is a directory where all the YUM repository configuration files (.repo files) are stored.
Each .repo file tells YUM where to find and download packages (URLs or mirrors).
📁 Path:
/etc/yum.repos.d/
Inside this folder, you'll see files like:
CentOS-Base.repo
epel.repo
remi.repo
Each file contains information about one or more repositories.
What is Service Management in Linux?
In Linux, services are background processes (also called daemons) that start during boot and run continuously—
for example:
🔸 nginx, httpd, sshd, mysql, etc.
Service management refers to starting, stopping, enabling, disabling, and checking the status of these services.
🔧 Tools Used:
Modern Linux distros (like RHEL 7+/CentOS 7+/Ubuntu 16+ etc.) use:
systemctl (Systemd service manager)
✅ 1. sudo systemctl start <service-name>
Starts the service temporarily (until next reboot).
🧪 Example:
sudo systemctl start nginx
✅ 2. sudo systemctl stop <service-name>
Stops the running service.
🧪 Example:
sudo systemctl stop nginx
✅ 3. sudo systemctl restart <service-name>
Stops and starts the service again.
Use when you make changes in configuration files.
🧪 Example:
sudo systemctl restart sshd
✅ 4. sudo systemctl reload <service-name>
Reloads the configuration without fully restarting the service.
(Not supported by all services.)
🧪 Example:
sudo systemctl reload apache2
✅ 5. sudo systemctl status <service-name>
Shows current status: running, stopped, failed, etc.
🧪 Example:
sudo systemctl status nginx
✅ 6. sudo systemctl enable <service-name>
Automatically starts the service at boot time.
🧪 Example:
sudo systemctl enable mysql
✅ 7. sudo systemctl disable <service-name>
Prevents the service from starting at boot.
🧪 Example:
sudo systemctl disable mysql
✅ 8. sudo systemctl is-enabled <service-name>
Checks if the service will start at boot.
🧪 Example:
sudo systemctl is-enabled sshd
✅ 9. systemctl list-units --type=service
Lists all currently loaded services and their states.
✅ 10. journalctl -u <service-name>
Shows logs/output of the service.
🧪 Example:
journalctl -u nginx
✅ df Command in Linux – Disk Filesystem Usage (Detailed & Simple)
The df (disk free) command is used to check disk space usage of file systems in Linux.
📌 Definition:
The df command reports the amount of used and available disk space on Linux file systems, including mount
points.
🧪 Basic Syntax:
df [OPTION]... [FILE]...
🔍 Sample Output:
$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda1 50G 20G 28G 42% /
tmpfs 2.0G 0 2.0G 0% /dev/shm
🔧 Important Columns Explained:
Column Meaning
Filesystem The name of the mounted file system (e.g., /dev/sda1)
Size Total size of the file system
Used How much space is used
Avail How much space is available
Use% Percentage of space used
Mounted on Where the file system is mounted (directory path)
Most Used Options:
Option Description
-h Human-readable format (e.g., GB, MB)
-T Shows the type of file system
-a Includes pseudo, duplicate, and inaccessible file systems
-i Displays inode usage instead of block usage
--total Adds a total at the end
🧪 Examples:
df -h # Human readable sizes
df -T # Show filesystem type (ext4, xfs, etc.)
df -i # Show inode usage
df --total # Total usage summary
✅ du Command in Linux — Full & Simple Explanation
The du (disk usage) command is used to check the size of directories and files in Linux.
📌 Definition:
The du command estimates the space used by files and directories on the disk.
🧪 Basic Syntax:
du [OPTIONS] [FILE or DIRECTORY]
🔍 Simple Example:
du -h /home/koushik
This shows the disk usage of /home/koushik in human-readable format (KB, MB, GB).
🔧 Important Options:
Option Description
-h Human-readable sizes (KB, MB, GB)
-s Summary: Shows only the total size of each argument
-a Shows size of each file and directory
Option Description
-d N Shows depth level up to N subdirectory levels
-c Adds a grand total at the end
--max-depth=N Limit report to N levels of subdirectories
📦 Examples:
du -h # Size of current directory and its subdirectories
du -sh * # Size of each file/folder in current directory
du -ah # Show size of all files and directories
du -sh /var/log # Total size of /var/log directory
du -h --max-depth=1 # Only one level depth shown
du -hc /etc /var/log # Show sizes and a grand total
🧠 Key Differences Between df and du:
Feature df du
Reports Free and used space on disk Space used by files/directories
Source Filesystem level File and directory level
Real-Time May show slight delay Checks actual file data
