Overview¶

Linux provides commands to monitor processes, manage system resources, and control services. These are essential for:

System monitoring
Performance tuning
Troubleshooting
Cybersecurity and incident response

Process Monitoring Commands¶

top — Real-Time Process Viewer¶

top

Explanation

Displays running processes in real time
Shows CPU usage, memory usage, and system load

Key Information Displayed¶

PID (Process ID)
User
CPU and memory usage
Running time

htop — Interactive Process Viewer¶

htop

Explanation

Advanced version of top
User-friendly interface
Allows scrolling, searching, and killing processes interactively

free — Memory Usage¶

free -h

Explanation

Displays total, used, and free memory
-h → human-readable format (MB/GB)

Example Output¶

              total   used   free   shared   buff/cache   available
Mem:           8.0G   2.5G   3.0G     200M        2.5G        5.0G

ps — Process Snapshot¶

ps aux

Explanation

Displays a snapshot of all running processes

Breakdown¶

a → all users
u → detailed format
x → include background processes

kill — Terminate Process¶

kill PID

Example¶

kill 1234

Explanation

Sends a signal to terminate a process
Default signal is SIGTERM (graceful stop)

Force Kill Process¶

kill -9 PID

Explanation

-9 → SIGKILL (forcefully terminates process)
Use only if normal kill fails

Systemd — Service Management¶

systemctl — Control Services¶

systemctl <action> servicename

Common Actions¶

Action	Description
start	Start service
stop	Stop service
status	Show status
restart	Restart service
reload	Reload configuration

Example¶

systemctl start tor

Check Service Status¶

systemctl status ssh

Services at Startup¶

Check if Service is Enabled¶

systemctl is-enabled servicename

Enable Service at Startup¶

systemctl enable servicename

Disable Service at Startup¶

systemctl disable servicename

Reload Systemd Configuration¶

systemctl daemon-reload

Explanation

Reloads service configuration files
Required after modifying service files

System V — Legacy Service Management¶

service Command¶

service servicename <action>

Example¶

service apache2 start

Available Actions¶

Action	Description
start	Start service
stop	Stop service
status	Check status
restart	Restart service

Practical Examples¶

Monitor System Processes¶

top
htop

Check Memory Usage¶

free -h

Find and Kill Process¶

ps aux | grep apache
kill 1234

Manage Services¶

systemctl start nginx
systemctl status nginx
systemctl enable nginx

Important Notes¶

htop may need installation (sudo apt install htop)
Always verify PID before killing a process
Prefer systemctl over service in modern systems
Use kill -9 only when necessary

Summary Table¶

Command	Purpose
`top`	Real-time process monitoring
`htop`	Interactive process viewer
`free -h`	Memory usage
`ps aux`	Process snapshot
`kill`	Terminate process
`systemctl`	Manage services
`service`	Legacy service control

Conclusion¶

Process and service management commands are essential for:

Monitoring system performance
Managing applications and services
Troubleshooting issues

Mastering these tools provides complete control over Linux system operations and is crucial for system administrators and cybersecurity professionals.