🔄 Process Management

What is a Process?

A process is a running instance of a program:

• Has a unique PID (Process ID)
• Runs as a specific user
• Uses CPU, memory, and other resources
• Can spawn child processes

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Viewing Processes

ps — Process Snapshot

ps                          # Your current terminal's processes
ps aux                      # All processes, detailed
ps -ef                      # All processes, different format
ps aux | grep firefox       # Find specific process

Understanding ps aux Output

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.1 169584 11148 ?        Ss   10:00   0:01 /sbin/init
alice     1234  2.0  3.5 3458920 286844 ?      Sl   10:05   1:20 /usr/bin/firefox

• USER: Who owns the process
• PID: Process ID
• %CPU / %MEM: Resource usage
• STAT: State (S=sleeping, R=running, Z=zombie)
• COMMAND: What’s running

Process States

• R: Running
• S: Sleeping (waiting for something)
• D: Uninterruptible sleep (usually I/O)
• Z: Zombie (finished but not cleaned up)
• T: Stopped

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top — Live Process Monitor

top                         # Interactive process viewer

top Keyboard Controls

| Key | Action | |-----|--------| | q | Quit | | k | Kill a process (enter PID) | | M | Sort by memory | | P | Sort by CPU | | u | Filter by user | | h | Help |

htop — Better Alternative

sudo apt install htop
htop

More colorful and user-friendly!

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Killing Processes

kill — Send Signal to Process

kill 1234                   # Send SIGTERM (gentle stop)
kill -9 1234                # Send SIGKILL (force kill)
kill -STOP 1234             # Pause process
kill -CONT 1234             # Resume process

Common Signals

| Signal | Number | Effect | |--------|--------|--------| | SIGTERM | 15 | Ask nicely to terminate | | SIGKILL | 9 | Force kill (cannot be caught) | | SIGSTOP | 19 | Pause | | SIGCONT | 18 | Resume | | SIGHUP | 1 | Hangup (often: reload config) |

killall — Kill by Name

killall firefox             # Kill all Firefox processes
killall -9 firefox          # Force kill all Firefox

pkill — Kill by Pattern

pkill -f "python script.py" # Kill process matching pattern

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Background & Foreground

Running in Background

command &                   # Start in background
./long_script.sh &          # Run script in background

Job Control

jobs                        # List background jobs
fg                          # Bring last job to foreground
fg %1                       # Bring job #1 to foreground
bg                          # Resume stopped job in background

Ctrl Shortcuts

• Ctrl + C: Kill foreground process
• Ctrl + Z: Suspend (pause) foreground process
• Ctrl + D: End input (EOF)

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pgrep — Find Process IDs

pgrep firefox               # PIDs of Firefox processes
pgrep -u alice              # All processes by alice
pgrep -l ssh                # PIDs and names

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nice & renice — Priority

Lower nice = higher priority (range: -20 to 19)

nice -n 10 ./cpu_heavy.sh   # Start with lower priority
sudo renice -n -5 1234      # Increase priority of running process

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Try It!

Practice process management in the terminal:

Exercises:

1. View all processes: ps aux
2. Find a specific process: ps aux | grep bash
3. See your PID: echo $$
4. Check process tree: pstree

Try it in the terminal

A sandboxed shell with this lesson’s commands. Type help to see what’s available.