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Lab 4.2 - Process Control

Objectives​

By the end of this lab, you will be able to:

  • Start processes
  • Stop processes (kill, killall, pkill)
  • Suspend and resume processes
  • Manage background jobs
  • Understand Linux signals

Prerequisites​

  • Ubuntu 24.04 installed
  • Terminal open
  • Normal user access

Exercise 1: Starting Processes​

Step 1: Foreground Processes​

# Start a simple process
sleep 60

# Start an interactive process
nano test.txt
# (Ctrl+X to exit)

Step 2: Background Processes​

# Start in background with &
sleep 300 &

# See the PID
echo $!

# See the jobs
jobs

# See background processes
ps aux | grep sleep

Step 3: nohup​

# Start a process that continues after logout
nohup sleep 600 > sleep.log 2>&1 &

# Verify
ps aux | grep sleep
cat sleep.log

Exercise 2: Linux Signals​

Step 1: Understanding Signals​

# See all signals
kill -l

# Common signals:
# SIGTERM (15): Graceful termination
# SIGKILL (9):  Forced termination (cannot be ignored)
# SIGSTOP (19): Suspend
# SIGCONT (18): Resume
# SIGHUP (1):   Hang up (restart)

Step 2: Sending Signals​

# Start a process
sleep 300 &
PID=$!

# Send SIGTERM (graceful termination)
kill $PID

# Or with the number
kill -15 $PID

# Verify
ps -p $PID

Exercise 3: kill, killall, pkill​

Step 1: kill​

# Start multiple processes
sleep 100 &
PID1=$!
sleep 200 &
PID2=$!

# Kill a specific process
kill $PID1

# Kill multiple processes
kill $PID1 $PID2

# Kill with force (SIGKILL)
kill -9 $PID1

Step 2: killall​

# Start multiple instances
sleep 100 &
sleep 200 &
sleep 300 &

# Kill all "sleep" processes
killall sleep

# With confirmation
killall -i sleep

# Verify
ps aux | grep sleep

Step 3: pkill​

# Start processes
sleep 100 &
sleep 200 &

# Kill by name (pattern)
pkill sleep

# Kill by user
pkill -u $USER sleep

# See what will be killed (dry-run)
pkill -l sleep

Exercise 4: Suspend and Resume​

Step 1: Suspend a Process​

# Start a process
sleep 300 &

# Suspend with kill
kill -STOP $!

# Or with Ctrl+Z (if in foreground)
# sleep 300
# [Press Ctrl+Z]

# See the state
ps aux | grep sleep
# Note the "T" (stopped)

Step 2: Resume a Process​

# Resume in background
kill -CONT $PID

# Resume in foreground
fg

# Resume in background
bg

Exercise 5: Job Management​

Step 1: Job Commands​

# Start multiple jobs
sleep 100 &
sleep 200 &
sleep 300 &

# See the jobs
jobs

# See with PID
jobs -l

# Bring a job to the foreground
fg %1  # Job number 1

# Move to background
bg %1

# Kill a job
kill %1

Step 2: Script with Jobs​

# Create a script
cat > test-jobs.sh << 'EOF'
#!/bin/bash
sleep 10 &
JOB1=$!
sleep 20 &
JOB2=$!

echo "Jobs started: $JOB1, $JOB2"
jobs

# Wait for completion
wait $JOB1
echo "Job 1 completed"
wait $JOB2
echo "Job 2 completed"
EOF

chmod +x test-jobs.sh
./test-jobs.sh

Exercise 6: Zombie Processes​

Step 1: Create a Zombie (for testing)​

# Script that creates a zombie
cat > zombie.sh << 'EOF'
#!/bin/bash
# Fork a child process that dies immediately
# Parent does not wait()
(
    sleep 1
    exit
) &
# Parent does not wait, child becomes zombie
sleep 10
EOF

chmod +x zombie.sh
./zombie.sh &

# See the zombies
ps aux | grep -w Z

Step 2: Clean Up Zombies​

# Zombies are automatically cleaned by init (PID 1)
# But can be forced by killing the parent
ps aux | grep zombie
# Find the parent PID and kill it

Exercise 7: Complete Practice​

Step 1: Real Scenario​

# Start a test web server
python3 -m http.server 8000 &
SERVER_PID=$!

# Verify it's running
ps -p $SERVER_PID
curl http://localhost:8000

# Stop gracefully
kill $SERVER_PID

# Verify
ps -p $SERVER_PID

Step 2: Management Script​

# Script to manage a process
cat > manage-process.sh << 'EOF'
#!/bin/bash
PROCESS="python3"
ACTION=$1

case $ACTION in
    start)
        python3 -m http.server 8000 > /dev/null 2>&1 &
        echo "Server started: $!"
        ;;
    stop)
        pkill -f "python3 -m http.server"
        echo "Server stopped"
        ;;
    status)
        if pgrep -f "python3 -m http.server" > /dev/null; then
            echo "Server active"
            ps aux | grep "python3 -m http.server" | grep -v grep
        else
            echo "Server inactive"
        fi
        ;;
    restart)
        $0 stop
        sleep 1
        $0 start
        ;;
    *)
        echo "Usage: $0 {start|stop|status|restart}"
        exit 1
        ;;
esac
EOF

chmod +x manage-process.sh

# Use
./manage-process.sh start
./manage-process.sh status
./manage-process.sh stop

Verification​

Checklist​

Verify that you can:

  • Start processes (foreground and background)
  • Send signals (kill, killall, pkill)
  • Suspend and resume processes
  • Manage jobs (jobs, fg, bg)
  • Understand different signals
  • Manage processes practically

Reflection Questions​

  1. What is the difference between SIGTERM and SIGKILL?

    • SIGTERM: Graceful termination (can be intercepted)
    • SIGKILL: Forced termination (cannot be ignored)

  2. Why use nohup?

    • Allows the process to continue after terminal logout
    • Useful for long-running tasks

  3. What is a zombie process?

    • Process that has terminated but whose parent has not waited
    • Automatically cleaned by init

Next Steps​

  • Lab 4.3: Priorities and Nice Values
  • Lab 4.4: Jobs and Background Tasks
  • Chapter 4.2: Process Visualization

Lab created on: December 2024 - Tested on Ubuntu 24.04