Process

A process is a program in execution. Every command you run on the terminal spawns at least one process.

Key Identifiers:

• PID: Process ID – a unique number assigned to each process.
• PPID: Parent Process ID – ID of the process that spawned it.
• UID: User ID – the user that owns the process.
• TTY: Terminal associated (if any).

Types of Processes

Process States

Viewing Processes

ps – Process Snapshot

ps aux

Example output:

USER       PID %CPU %MEM COMMAND
root         1  0.0  0.1 /sbin/init
www-data  1092  0.5  1.2 nginx: worker process

top – Real-time Process Monitoring

top

Press q to quit. Other keys: k to kill, r to renice, P for CPU sort, M for memory sort.

htop – Enhanced Interactive Process Viewer

htop

• Color-coded
• Mouse support
• Tree view (F5)
• Filter by user (u)

pgrep – Search for Processes by Name

pgrep sshd

Use -l to list with names:

pgrep -l nginx

pidof – Get PID of a Running Program

pidof sshd

pstree – Show Process Hierarchy

pstree -p

Shows nested parent-child relationship.

Managing Processes

kill – Terminate a Process

kill -SIGTERM <PID>     # Graceful stop
kill -9 <PID>           # Forceful kill

Common signals:

• SIGTERM (15): Request termination
• SIGKILL (9): Force termination
• SIGSTOP (19): Pause process
• SIGCONT (18): Resume process

killall – Kill by Name

killall firefox

nice – Launch Process with Priority

nice -n 10 myscript.sh

Values range from -20 (highest) to 19 (lowest).

renice – Change Priority of a Running Process

renice -n 5 -p 1234

Background & Foreground Jobs

🧾 Metadata of a Process (Example)

ps -p 1234 -o pid,ppid,uid,%cpu,%mem,cmd,start,etime

Shows PID, parent, user, CPU%, memory%, command, start time, elapsed time.

Example Combined Workflow

# Start a long process in the background
sleep 500 &
 
# Check it’s running
jobs
ps aux | grep sleep
 
# Bring it to the foreground
fg
 
# Suspend it
Ctrl+Z
 
# Resume in background
bg
 
# Kill it
kill %1

Key Takeaways

• Every running program is a process with a unique PID
• Use ps, top, htop, pgrep, pidof to inspect
• Use kill, killall, nice, renice to manage
• Processes can be foreground, background, or daemons