🐧 Linux Commands – DevOps / SRE Quick Reference

A practical and concise reference guide for commonly used Linux commands with real-world DevOps and SRE use cases.

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📌 1️⃣ lsof – List Open Files

🔹 Overview

lsof stands for List Open Files.

In Linux, everything is treated as a file:

• Regular files
• Directories
• Network sockets
• Devices
• Pipes

lsof helps identify which process is using which file or network resource.

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🔹 Syntax

lsof [options]

🔹 Common Use Cases

✅ 1. Find which process is using a specific port

lsof -i :8080

Use Case:

Debug “Port already in use” errors

Identify service bound to a specific port

✅ 2. List files opened by a specific process (PID)

lsof -p <PID>

Use Case:

Debug memory/file descriptor leaks

Analyze application resource usage

✅ 3. Identify which process is using a file

lsof /var/log/app.log

Use Case:

Unable to delete a file

Log rotation troubleshooting

File lock investigation

✅ 4. List all network connections

lsof -i

🎯 Interview Explanation

lsof is used to identify open files and network sockets by processes. It helps troubleshoot port conflicts, file locks, and resource exhaustion issues.

##📌 2️⃣ netstat – Network Statistics 🔹 Overview

netstat is used to monitor:

Active network connections

Listening ports

Routing tables

Interface statistics

Protocol statistics

⚠️ Note: netstat is deprecated in many modern Linux distributions. Use ss instead.

🔹 Syntax

netstat [options]

🔹 Common Use Cases

✅ 1. Check listening ports

netstat -tuln

Options:

-t → TCP

-u → UDP

-l → Listening

-n → Numeric output (no DNS resolution)

Use Case:

Verify service is running

Confirm application binding to correct port

✅ 2. Identify process using a port

netstat -tulnp

(-p shows PID and program name)

Use Case:

Find which service is occupying a port

✅ 3. View active connections

netstat -an

Use Case:

Check ESTABLISHED connections

Investigate suspicious traffic

Analyze connection states (TIME_WAIT, CLOSE_WAIT)

✅ 4. Show routing table

netstat -r

Use Case:

Troubleshoot network routing issues

📌 Modern Alternative – ss
ss -tulnp
Why prefer ss?

Faster

More detailed output

Better performance on production systems

Replaces netstat in modern Linux distributions

🚀 Production Troubleshooting Scenarios Scenario Command Port already in use lsof -i :PORT Application not reachable netstat -tuln High number of connections netstat -an Cannot delete file lsof filename Verify service binding ss -tulnp