How to Check Running Processes in Linux (ps, top, kill Explained)

This guide is not just about commands.

It is about understanding how Linux processes actually work — so you can debug real production issues with confidence.

By the end of this article, you should be able to:

• Identify problematic processes
• Understand why they behave that way
• Control or terminate them safely
• Debug stuck or broken processes in real servers

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Understanding Linux Processes (From Zero to Expert)

Every running program in Linux is a process.

But internally, a process is created using:

• fork() → duplicates parent process
• exec() → loads a new program into memory

This is why every process has:

• PID (Process ID)
• PPID (Parent Process ID)

You can see this relationship:

ps -ef --forest

This hierarchy matters in debugging because:

• Killing parent may kill children
• Orphaned processes can behave unexpectedly

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Process States (Critical for Real Debugging)

Linux processes exist in different states:

State	Meaning
R	Running
S	Sleeping
D	Uninterruptible sleep (I/O wait ⚠️)
Z	Zombie
T	Stopped

⚠️ D State (Most Important)

If a process is in D state, it is waiting on disk or I/O.

👉 You CANNOT kill it even with kill -9

This usually indicates:

• Disk issues
• NFS/network storage problems
• Kernel-level blocking

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Using ps (Deep Understanding)

Basic:

ps aux

Expert usage:

ps -eo pid,ppid,cmd,%mem,%cpu,state --sort=-%cpu

This gives:

• Sorted CPU usage
• Process state
• Parent-child relationships

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Using top (Real-Time Monitoring)

top

Key advanced usage:

• Press P → sort by CPU
• Press M → sort by memory
• Press 1 → show per-core CPU

What Experts Look For

• High CPU but low load → normal
• High load but low CPU → blocked processes (⚠️ investigate D state)

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/proc Filesystem (Expert-Level Insight)

Every process has a directory:

/proc/PID/

Example:

cat /proc/1234/status

This reveals:

• Memory usage
• State
• Threads

👉 This is how Linux internally tracks processes

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Killing Processes (Correct Way)

Step 1: Graceful kill

kill PID

Step 2: Force kill (last resort)

kill -9 PID

Step 3: Kill by name

pkill nginx

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Signals (Beyond Basics)

Signal	Purpose
SIGTERM (15)	Graceful shutdown
SIGKILL (9)	Force kill
SIGSTOP	Pause process
SIGCONT	Resume process
SIGHUP	Reload config

👉 Example (pause process):

kill -STOP PID

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Controlling CPU Usage (nice & renice)

Set priority:

nice -n 10 command

Change running process:

renice 10 -p PID

Lower value = higher priority

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Real Debugging Tools (What Experts Actually Use)

strace (system call tracing)

strace -p PID

Shows what the process is doing internally.

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lsof (open files)

lsof -p PID

Useful for:

• Checking file locks
• Debugging stuck services

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Real Server Scenarios (Production-Level)

Scenario 1: High Load but CPU is Low

• Check top
• Look for D state processes

👉 Likely disk or I/O issue

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Scenario 2: Zombie Processes

ps aux | grep Z

Solution:

• Kill parent process
• Or restart service

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Scenario 3: PHP-FPM Exhaustion

Symptoms:

• High process count
• Slow website

Check:

ps aux | grep php-fpm

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Scenario 4: MySQL Hanging Queries

• High load
• Low CPU

Investigate:

lsof -p MYSQL_PID

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Common Mistakes

Overusing kill -9

This can:

• Corrupt data
• Break services

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Killing SSH process

You will disconnect yourself.

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Ignoring process states

State tells you the real problem — not just CPU usage.

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When This Matters in Production

Process management is critical when:

• Server becomes unresponsive
• Load spikes unexpectedly
• Applications hang
• Resource exhaustion occurs

On production infrastructure like:

• VPS servers
• Dedicated servers
• Cloud environments

Improper handling can lead to downtime.

👉 Explore infrastructure options:

• Offshore Streaming Servers
• Offshore Dedicated Servers
• Offshore Cloud Servers

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Related Linux Guides

• How to Check CPU Usage in Linux (top, htop, uptime Explained)
• How to Check Memory Usage in Linux (free, vmstat, htop Explained)
• How to Check Server Load in Linux (Load Average Explained)
• How to Check Server Disk Usage in Linux (df, du, and ncdu Explained)

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Final Takeaway

A beginner uses ps and top.

An expert understands:

• Why a process is stuck
• What state it is in
• What system resource is blocking it

That difference is what keeps production systems stable.