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)

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⚙️ 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

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🧠 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)

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🛠️ 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

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📁 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.

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Background/Foreground Jobs

These are Linux/Unix commands for job control - managing processes (jobs) in your terminal. Let me explain each:

& (ampersand)

• Example: sleep 60 &
• Runs a command in the background immediately
• Returns a job ID and PID (process ID)
• You get your prompt back right away

$ sleep 300 &
[1] 12345  # [job ID] [process ID]

Ctrl+Z

• Keyboard shortcut (not a typed command)
• Suspends/pauses the currently running foreground job
• Puts it in a stopped state and returns you to the prompt
• Useful for pausing long-running commands temporarily

bg (background)

• Resumes a suspended/stopped job but runs it in the background
• Example: After Ctrl+Z, type bg
• You can also specify: bg %1 (resume job #1)

fg (foreground)

• Brings a background or suspended job to the foreground
• Example: fg %2 (bring job #2 to foreground)
• Without arguments, brings the current job (marked with +)

jobs

• Lists all jobs associated with your terminal session
• Shows job ID, status, and command

$ jobs
[1]-  Running    sleep 300 &
[2]+  Stopped    vim file.txt

Typical Workflow:

1. Start a long job in background:

   $ find / -name "*.txt" > output.txt &

2. Suspend a running job:

   • Start ping google.com (runs continuously)
   • Press Ctrl+Z → job suspends

3. Resume it in background:

   $ bg  # or bg %1

4. Check job status:

   $ jobs
   [1]+  Running    ping google.com &

5. Bring back to foreground when needed:

   $ fg %1

Job Status Indicators:

• Running - Active in background
• Stopped - Suspended (Ctrl+Z)
• Done - Completed
• Terminated - Killed