Day 3 Linux: System levels Commands with Practical Examples

Command	Description	Use Cases
uname	Displays system information (OS, kernel, architecture).	Check OS details before installing software on cloud servers.
uptime	Shows system uptime and load averages.	Monitor server health and performance in cloud environments.
date	Displays or sets the system date and time.	Timestamp logs, automate cron jobs, and synchronize cloud instances.
who	Lists currently logged-in users.	Monitor active users on production servers for security.
whoami	Shows the current logged-in user.	Verify user privileges before executing critical commands.
id	Displays user ID (UID) and group ID (GID).	Check user permissions in DevOps environments.
sudo	Executes commands as a superuser.	Perform administrative tasks like installing software.
shutdown	Shuts down the system.	Schedule controlled server shutdowns in cloud environments.
reboot	Reboots the system.	Restart cloud instances after updates or maintenance.
apt	Package manager for Debian-based systems.	Install and update software in Ubuntu, Debian cloud instances.
yum	Package manager for RHEL-based systems.	Manage packages in RHEL, CentOS cloud servers.
dnf	Modern package manager for RHEL/Fedora.	Install software on Fedora-based cloud environments.
pacman	Package manager for Arch Linux.	Deploy lightweight servers using Arch Linux in cloud setups.
portage	Source-based package management in Gentoo.	Optimize and compile software in Gentoo-based cloud deployments.

1. uname – Get System Information

Usage: Displays system information (OS, kernel version, architecture).
How It Works:

• Retrieves system details from /proc/sys/kernel and uname system call.

Examples:

uname            # Display OS name
uname -r        # Show kernel version
uname -a        # Show all system information

Output:

Linux ip-172-31-24-100 5.11.0-41-generic #45-Ubuntu SMP Thu Jan 6 17:35:33 UTC 2023 x86_64 GNU/Linux

📌 Case Study: A DevOps engineer checks the OS and kernel version before updating a cloud server.

2. uptime – Show System Uptime

Usage: Displays system uptime and load averages.
How It Works:

• Retrieves uptime and load information from /proc/uptime and /proc/loadavg.

Example:

uptime

Output:

12:05:30 up 10 days,  3:42,  2 users,  load average: 0.12, 0.14, 0.09

📌 Case Study: A cloud engineer checks uptime before scheduling maintenance on an AWS EC2 instance.

3. date – Display or Set System Date and Time

Usage: Shows or sets the system date and time.
How It Works:

• Reads and modifies /etc/localtime and system clock.

Examples:

date           # Show current date and time
date +%Y-%m-%d # Display only date in YYYY-MM-DD format
date -s "2025-01-18 12:00:00"  # Set system date/time (requires sudo)

📌 Case Study: A DevOps engineer schedules automated backups and uses date to timestamp log files.

4. who – Show Logged-in Users

Usage: Displays currently logged-in users.
How It Works:

• Fetches user login data from /var/run/utmp.

Example:

who

Output:

devops  pts/0  2025-01-18 10:05  (192.168.1.100)

📌 Case Study: A system admin monitors remote users accessing a production server.

5. whoami – Display Current User

Usage: Prints the currently logged-in username.
How It Works:

• Reads the user ID from the environment.

Example:

whoami

Output:

devops

📌 Case Study: A developer checks their user privileges before executing a command.

6. id – Display User ID (UID) and Group ID (GID)

Usage: Shows the user ID, group ID, and group memberships.
How It Works:

• Retrieves user and group info from /etc/passwd and /etc/group.

Example:

id devops

Output:

uid=1001(devops) gid=1001(devops) groups=1001(devops),27(sudo)

📌 Case Study: A DevOps engineer checks if a user has sudo privileges.

7. sudo – Execute Commands as Superuser

Usage: Runs commands as a superuser or another user.
How It Works:

• Reads /etc/sudoers to check user permissions.

Examples:

sudo apt update    # Run package update as root
sudo -u devops ls  # Execute command as another user

📌 Case Study: A system admin uses sudo to update cloud instances securely.

8. shutdown – Power Off or Restart System

Usage: Shuts down or reboots the system.
How It Works:

• Sends SIGTERM to all processes and powers off the system.

Examples:

sudo shutdown -h now  # Shutdown immediately
sudo shutdown -r +5   # Reboot in 5 minutes

📌 Case Study: A cloud admin schedules a safe shutdown of EC2 instances during maintenance.

9. reboot – Restart System

Usage: Reboots the system immediately.
How It Works:

• Executes a system call to reboot.

Example:

sudo reboot

📌 Case Study: A DevOps engineer restarts a Kubernetes node after applying updates.

10. apt – Package Manager for Debian-based Systems

Usage: Manages software packages in Ubuntu/Debian.
How It Works:

• Uses /etc/apt/sources.list to fetch package data.

Examples:

sudo apt update      # Update package lists
sudo apt install nginx  # Install Nginx
sudo apt remove apache2  # Remove Apache

📌 Case Study: A DevOps engineer installs monitoring tools on AWS Ubuntu instances.

11. yum – Package Manager for RHEL/CentOS

Usage: Manages software packages in RHEL/CentOS.
How It Works:

• Uses /etc/yum.repos.d/ for repositories.

Examples:

sudo yum update      # Update system packages
sudo yum install httpd  # Install Apache
sudo yum remove nginx  # Remove Nginx

📌 Case Study: A cloud engineer sets up web servers using yum in AWS RHEL-based AMIs.