Day 2 Linux: Advance Commands with Practical Examples

Command	Why We Use It?	Where We Use It?	Usage in Cloud & DevOps
ssh	Securely login to a remote Linux system.	Remote server access, troubleshooting, deployment.	Used in AWS EC2, GCP VMs, Azure VMs for secure access to cloud instances.
df -h	Check disk space usage in human-readable format.	Monitoring system storage, detecting low disk space.	Used in Kubernetes, Docker, EC2 instances to monitor disk usage.
du	Display directory size usage.	Checking large files consuming disk space.	Used in log management, persistent storage in containers, cloud storage analysis.
ps	Show currently running processes.	Identifying high CPU/memory-consuming processes.	Used in AWS, Kubernetes, EC2 instances to monitor application performance.
top	Real-time monitoring of CPU, memory, processes.	Troubleshooting high resource utilization.	Used in containers, cloud instances, on-premises servers for monitoring.
fuser	Identify processes using a file or port.	Checking which process is locking a file or using a port.	Used in troubleshooting blocked ports in DevOps pipelines and CI/CD runners.
kill	Stop a running process.	Killing unresponsive applications.	Used in AWS, Docker, Kubernetes nodes to terminate problematic processes.
nohup	Run processes in the background even after logout.	Running long scripts, jobs, or servers persistently.	Used in background execution of deployments, cron jobs, and monitoring scripts.
free -h	Show available and used memory (RAM).	Checking system memory usage.	Used in AWS, Azure, Kubernetes, EC2 to monitor memory before scaling.
vmstat	Monitor CPU, memory, disk, and system performance.	Detecting performance bottlenecks.	Used in performance tuning, debugging high CPU/memory usage in cloud environments.

1. SSH (Secure Shell) – Remote Login

Used For: Secure remote login to a Linux system.

How It Works:

• Uses the SSH protocol to establish a secure encrypted connection between the client and the remote server.
• Can use password authentication or SSH key-based authentication for enhanced security.

Usage & Examples:

Password-Based Authentication

ssh username@remote-server-ip

Example:

ssh devops@192.168.1.100

Key-Based Authentication (More Secure)

ssh -i /path/to/private_key username@remote-server-ip

Example:

ssh -i ~/.ssh/id_rsa devops@192.168.1.100

📌 Best Practice: Use SSH keys instead of passwords for security.

2. df -h – Check Disk Space Usage

Used For: Displaying available and used disk space in human-readable format.
How It Works:

• Fetches data from the /proc filesystem to display mounted partitions and their usage.

Usage & Example:

df -h

Output:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       50G   25G   25G  50% /
tmpfs           8G    0     8G    0% /dev/shm

📌 Best Practice: Run df -hT to also display the filesystem type.

3. du – Check Directory Size

Used For: Displaying the size of files and directories.
How It Works:

• Recursively calculates the space occupied by directories and files.

Usage & Example:

du -sh /var/logs

Output:

2.4G    /var/logs

📌 Best Practice: Use du -h --max-depth=1 /home/ to get sizes of immediate subdirectories only.

4. ps – View Running Processes

Used For: Displaying currently running processes.
How It Works:

• Retrieves process information from /proc and displays process ID (PID), CPU usage, and command execution details.

Usage & Example:

ps aux | grep nginx

Output:

root       1221  0.0  0.3  94800  3772 ?        Ss   10:10   0:00 nginx: master process

📌 Best Practice: Use ps -ef for a full listing with parent-child process hierarchy.

5. top – Monitor System Performance

Used For: Viewing real-time CPU, memory, and process usage.
How It Works:

• Continuously updates system performance statistics and active processes.

Usage:

top

📌 Best Practice: Use htop for a more user-friendly version.

6. fuser – Identify Processes Using a File or Port

Used For: Finding processes that are using a specific file, directory, or port.
How It Works:

• Retrieves process information from the kernel and /proc.

Usage:

fuser -v /var/log/syslog

📌 Best Practice: Use fuser -k <port> to kill processes using a port.

7. kill – Terminate a Process

Used For: Stopping a running process.
How It Works:

• Sends signals to processes (default is SIGTERM which allows cleanup before exiting).

Usage:

kill 1234   # Terminate process with PID 1234
kill -9 5678  # Force kill process with PID 5678

📌 Best Practice: Use kill -15 <PID> first to allow graceful shutdown.

8. nohup – Run Commands in the Background

Used For: Running commands in the background even after logout.
How It Works:

• Redirects output to nohup.out and ignores SIGHUP signals.

Usage:

nohup python script.py &

📌 Best Practice: Use nohup command & disown to completely detach from the shell.

9. free -h – Check Memory Usage

Used For: Displaying free and used memory (RAM).
How It Works:

• Reads memory usage stats from /proc/meminfo.

Usage & Example:

free -h

Output:

             total        used        free      shared  buff/cache   available
Mem:           16G         5G         8G         1G         3G         9G

📌 Best Practice: Use free -m to display in megabytes.

10. vmstat – System Performance Monitoring

Used For: Monitoring CPU, memory, swap, I/O, and system performance.
How It Works:

• Collects statistics from /proc/stat and /proc/meminfo.

Usage & Example:

vmstat 1 5

📌 Best Practice: Use vmstat -d to see disk statistics.

DevOps & Cloud Use Cases

• Infrastructure Monitoring: df -h, du, free -h, top, and vmstat for resource utilization in cloud VMs & Kubernetes nodes.
• Process Management: ps, fuser, kill, nohup for managing services and applications in production.
• Remote Access: ssh for secure remote login to AWS EC2, GCP, and Azure instances.
• Automation & Troubleshooting: nohup for running long processes during deployment and automation scripts.