Process Management

📌 1. What is a Process?

A process is a running instance of a program. Each process has:

• A unique PID (Process ID)

• A parent process

• Assigned system resources (CPU, memory)

• A state (running, sleeping, stopped, zombie)

---

⚙️ 2. Key Process Commands

🔍 Viewing Processes

Command	Description
ps aux	Shows all processes
top	Live, real-time process view
htop	Enhanced version of top with UI
pstree	Visual tree of processes

🔫 Controlling Processes

Command	Description
kill PID	Sends signal to terminate a process
kill -9 PID	Force kills a process (SIGKILL)
killall name	Kill all processes by name
nice / renice	Adjust process priority

---

🧠 3. Daemon Processes

• Daemons run in the background, detached from the terminal.

• Usually started at boot by the init system (e.g., systemd).

• Examples: sshd, nginx, crond

Characteristics:

• No controlling terminal

• Long-running and persistent

• Usually ends with a d (e.g., sshd)

---

🛠️ 4. Managing Services (Daemons) with systemd

Use systemctl for managing systemd services:

Command	Purpose
systemctl status nginx	Check status of nginx
systemctl start nginx	Start nginx
systemctl stop nginx	Stop nginx
systemctl restart nginx	Restart nginx
systemctl enable nginx	Enable on boot
systemctl disable nginx	Disable on boot
journalctl -u nginx	View logs for nginx

---

📁 5. Process States (Common)

• R – Running

• S – Sleeping

• Z – Zombie

• T – Stopped

• D – Waiting (uninterruptible sleep)

Use ps -eo pid,stat,cmd to see process states.

---