Linux Process Troubleshooting

🎯 Objective

Learn how to monitor, analyze, and troubleshoot running processes in Linux using system tools to identify performance issues, stuck processes, or resource overuse.

# 📘 What is a Process in Linux?

A process is a running instance of a program.

When a command or application runs in Linux, the system creates a process with its own:

Example:

Running:

firefox

creates a Firefox process in the system.

# 🧠 Process States in Linux

# 🔍 Viewing Running Processes

Using ps

ps aux

Explanation:

Example output fields:

View process tree

ps -ef --forest

Shows parent-child relationship between processes.

# 📊 Real-Time Process Monitoring

Using top

top

Shows:

Important keys inside top:

Using htop

Install:

sudo yum install htop -y

Run:

htop

Features:

# 🔎 Finding a Specific Process

Using grep

Example:

ps aux | grep nginx

Finds Nginx processes.

Using pgrep

pgrep nginx

Shows only process IDs.

# 🧾 Process Priority

Each process has a priority value called nice value.

Run command with priority

nice -n 10 command

Change priority of running process

renice5 -p 1234

Where 1234 = PID

# 🛑 Killing a Process

Sometimes processes hang or consume excessive resources.

Kill by PID

kill 1234

Force kill process

kill -9 1234

Signal 9 = SIGKILL

Kill by process name

pkill nginx

or

killall nginx

# 📈 Monitoring CPU Usage

Check CPU usage:

top

or

mpstat

# 📊 Monitoring Memory Usage

Check memory:

free -h

Example output:

# 📁 Check Disk Usage

Sometimes processes fail due to disk full issues.

Check disk:

df -h

# 🔍 Identify High Resource Processes

Sort by CPU usage:

ps aux --sort=-%cpu

Sort by memory:

ps aux --sort=-%mem

# 🧪 Example Troubleshooting Scenario

Problem

Server is slow.

Step 1: Check CPU usage

top

Find process using high CPU.

Step 2: Identify process

Example:

PID 2345 java

Step 3: Investigate process

ps -p 2345 -f

Step 4: Restart or kill process

kill 2345

or

kill -9 2345

# 📜 Viewing Open Files by Process

Use:

lsof -p PID

Example:

lsof -p 1234

Shows files opened by that process.

# 🌐 Viewing Process Network Connections

ss -tunap

Shows processes using network ports.

# 📑 Viewing System Logs

If process crashes:

journalctl -xe

or

journalctl -u service-name

Example:

journalctl -u nginx

# ⚠ Common Process Issues

High CPU usage

Check:

top

Fix:

Restart or kill process.

Zombie processes

Check:

ps aux | grep Z

Fix:

Restart parent process.

Memory leak

Symptoms:

Check:

top

Restart service if needed.

# 🧰 Useful Process Commands

# ✅ Key Learning Points