What is an Operating System (OS)?

An Operating System (OS) is system software that manages hardware resources and provides essential services for applications. It acts as an interface between the user and the computer, enabling seamless interaction. Examples include Windows, Linux, and macOS.

Difference Between Client OS and Server OS

What is Linux for DevOps?

Linux for DevOps refers to using Linux-based systems to automate infrastructure management, deployment, and monitoring in DevOps workflows. It provides stability, flexibility, and powerful tools for CI/CD, cloud computing, and server management.

Architecture of Linux :

• Hardware Layer

  • The physical components like CPU, RAM, disk, and network devices.

• Kernel (Core of Linux)

  • Manages CPU, memory, file systems, and device drivers.

  • Handles process scheduling, security, and hardware communication.

• Shell & Utilities

  • The command-line interface (CLI) that allows users to interact with the system.

  • Examples: Bash, Zsh, Fish.

• User Applications

  • Programs like browsers, text editors, and development tools that run on Linux.

Basic Linux Commands for DevOps in Daily Work

As a DevOps engineer, Linux is essential for managing servers, automation, and deployments. Here are some must-know Linux commands used in daily operations:

1. User & Access Management

• Check current user → whoami

• Switch user → su - username

• Create a new user → useradd -m username

• Add user to sudo group → usermod -aG sudo username

• Set or change password → passwd username

2. File & Directory Management

• List files → ls -l

• Create a directory → mkdir my_folder

• Navigate into a directory → cd my_folder

• Create a file → touch file.txt

• Copy a file → cp file.txt /destination/

• Move/Rename a file → mv oldname.txt newname.txt

• Delete a file → rm file.txt

• Delete a directory → rm -r folder_name

3. Permissions & Ownership

• Check file permissions → ls -l file.txt

• Change file permissions → chmod 750 file.txt

• Change file owner → chown user:group file.txt

4. Disk & Storage Management

• Check disk space usage → df -h

• Check folder size → du -sh /folder/

• Find large files → find / -type f -size +100M

5. Networking Commands

• Check network configuration → ip a or ifconfig

• Check active network connections → netstat -tulnp

• Ping a server → ping google.com

• Check open ports → ss -tuln

• Download a file → wget URL

6. Package Management

• Debian-based (Ubuntu, Debian):

  • apt update && apt upgrade -y (Update system)

  • apt install package-name (Install package)

  • apt remove package-name (Remove package)

Understanding sudo in Linux

What is sudo?

sudo (Superuser Do) is a command that allows a user to execute administrative (root) commands without logging in as root. It ensures security by limiting full system access to authorized users.

Why is sudo Important for DevOps?

• Prevents direct root login, reducing security risks.

• Allows controlled privilege escalation for specific tasks.

• Tracks command history via logs (/var/log/auth.log).

• Essential for automation scripts requiring elevated permissions.

Basic Usage of sudo

• Run a command with root privileges:

    bashCopyEditsudo apt update
  

• Switch to root user:

    bashCopyEditsudo su
  

• Run a command as another user:

    bashCopyEditsudo -u username command
  

• Edit system files (e.g., hosts file):

    bashCopyEditsudo nano /etc/hosts
  

Managing sudo Access

1. Check if a user has sudo access:

    bashCopyEditsudo -l
   

2. Add a user to the sudo group (Debian-based systems):

    bashCopyEditsudo usermod -aG sudo username
   

3. Add a user to the wheel group (RHEL-based systems):

    bashCopyEditsudo usermod -aG wheel username
   

4. Modify sudo permissions (/etc/sudoers):
   Open the file safely using:

    bashCopyEditsudo visudo
   

   Add a rule for a specific user:

    sqlCopyEditusername ALL=(ALL) NOPASSWD:ALL
   

   (Allows the user to run sudo commands without a password.)

Common sudo Errors & Fixes

🔹 "User is not in the sudoers file"
➡ Solution: Add the user to the sudo group using usermod -aG sudo username.

🔹 "Permission denied" while editing system files
➡ Solution: Use sudo before nano or vim, e.g., sudo nano /etc/hosts.

🔹 Accidentally removed sudo access for all users
➡ Solution: Boot into recovery mode and manually add a user to the sudoers file.

Users and Groups in Linux

Understanding Users & Groups in Linux

Linux is a multi-user operating system, meaning multiple users can work on the same system with different privileges.

• User: An account that interacts with the system. Each user has a unique UID (User ID).

• Group: A collection of users that share permissions. Each group has a GID (Group ID).

How to Create a User and Group in Linux?

1. Create a User (devops_user)

To create a new user:

bashCopyEditsudo useradd -m devops_user

• -m → Creates a home directory (/home/devops_user).

2. Create a Group (devops_team)

bashCopyEditsudo groupadd devops_team

This creates a new group named devops_team.

3. Add the User to the Group

bashCopyEditsudo usermod -aG devops_team devops_user

• -aG → Appends the user to the group.

4. Set a Password for the User

bashCopyEditsudo passwd devops_user

You will be prompted to enter a new password.

Granting sudo Access to the User

To allow devops_user to run commands as a superuser:

bashCopyEditsudo usermod -aG sudo devops_user   # Debian/Ubuntu  
sudo usermod -aG wheel devops_user  # RHEL/CentOS

Alternatively, you can edit the sudoers file:

bashCopyEditsudo visudo

Add the following line:

bashCopyEditdevops_user ALL=(ALL) NOPASSWD:ALL

This grants password-less sudo access to devops_user.

Restricting SSH Login for Certain Users

To prevent specific users from logging in via SSH, modify the SSH configuration:

1. Edit the SSH configuration file:

    bashCopyEditsudo nano /etc/ssh/sshd_config
   

2. Add the following lines at the end:

    bashCopyEditDenyUsers user1 user2
    AllowUsers devops_user
   

   (Replace user1 and user2 with actual usernames to restrict.)

3. Restart the SSH service to apply changes:

    bashCopyEditsudo systemctl restart sshd
   

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File Permissions in Linux

Understanding Linux File Permissions

In Linux, file permissions determine who can read, write, or execute a file or directory. Every file and directory has three types of users and three types of permissions:

1. User Categories:

• Owner → The user who created the file.

• Group → A set of users who share permissions.

• Others → Anyone else on the system.

2. Permission Types:

Viewing File Permissions

To check permissions of a file:

bashCopyEditls -l filename

Example output:

csharpCopyEdit-rwxr--r--  1 user group 1234 Feb 8 12:30 script.sh

Explanation:

• -rwxr--r-- → File type and permissions.

  • - → Regular file (or d for a directory).

  • rwx → Owner can read, write, execute.

  • r-- → Group can only read.

  • r-- → Others can only read.

Changing File Permissions (chmod)

1. Symbolic Method

Modify permissions using chmod:

• Give execute permission to the owner:

    bashCopyEditchmod u+x filename
  

• Remove write permission for others:

    bashCopyEditchmod o-w filename
  

• Grant read & write permissions to the group:

    bashCopyEditchmod g+rw filename
  

2. Numeric (Octal) Method

Permissions can also be changed using numbers:

bashCopyEditchmod 754 filename

• 7 (Owner) → rwx (4+2+1 = 7)

• 5 (Group) → r-x (4+0+1 = 5)

• 4 (Others) → r-- (4+0+0 = 4)

Linux File Permission Truth Table

This table summarizes the numeric values and their permissions: