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Chapter 4.3 - Process Control

Learning Objectives​

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

  • Start processes
  • Stop and kill processes
  • Suspend and resume
  • Manage background processes
  • Understand signals

Starting a Process​

Command Line​

# Foreground process
command

# Background process
command &

# Example
sleep 60 &

nohup - Ignore SIGHUP​

# Continue after logout
nohup command &

# Redirect output
nohup command > output.log 2>&1 &

Stopping a Process​

kill - Send a Signal​

# TERM signal (graceful termination)
kill PID
kill -TERM PID
kill -15 PID

# KILL signal (forced)
kill -9 PID
kill -KILL PID

# By name
killall process_name
killall -9 process_name  # Force

# By pattern
pkill pattern
pkill -9 pattern

Common Signals​

SignalNumberAction
SIGHUP1Hang up (reload config)
SIGINT2Interrupt (Ctrl+C)
SIGQUIT3Quit (core dump)
SIGTERM15Terminate (graceful)
SIGKILL9Kill (forced, cannot be ignored)
SIGSTOP19Stop (suspend)
SIGCONT18Continue (resume)

Suspending and Resuming​

Suspend​

# Send SIGSTOP
kill -STOP PID
kill -19 PID

# Or with Ctrl+Z in the terminal
# (suspends the foreground process)

Resume​

# Send SIGCONT
kill -CONT PID
kill -18 PID

# Foreground
fg

# Background
bg

Background Processes​

jobs - View Jobs​

# View jobs
jobs

# With PIDs
jobs -l

# Also completed jobs
jobs -a

fg and bg​

# Bring to foreground
fg
fg %1        # Job number 1
fg %name     # Job by name

# Send to background
bg
bg %1

disown - Detach a Job​

# Detach from terminal
disown %1
disown -h %1  # Do not send SIGHUP

Practical Examples​

Scenario 1: Unresponsive Process​

# 1. Find the PID
ps aux | grep process

# 2. Try graceful
kill PID

# 3. Wait a few seconds
sleep 5

# 4. If still there, force
kill -9 PID

Scenario 2: Background Process​

# Start in background
long_script.sh &

# View jobs
jobs

# Bring to foreground
fg %1

# Suspend (Ctrl+Z)
# Resume in background
bg %1

Scenario 3: Process Continues After Logout​

# With nohup
nohup long_task.sh > output.log 2>&1 &

# Or with screen/tmux
screen -S session
# Start process
# Detach: Ctrl+A then D
# Reattach: screen -r session

Signals and Their Usage​

SIGTERM (15) - Graceful Termination​

kill -TERM PID
# The process can:
# - Save data
# - Close files
# - Clean up resources

SIGKILL (9) - Forced Termination​

kill -9 PID
# The process:
# - Cannot be ignored
# - Is killed immediately
# - No cleanup possible

Use only if SIGTERM does not work!

SIGHUP (1) - Reload Configuration​

# Often used to reload config
kill -HUP PID
# Example: reload nginx
kill -HUP $(pidof nginx)

Summary​

In this chapter, you learned:

Start: command, command &, nohup
Stop: kill, killall, pkill with signals
Signals: TERM (graceful), KILL (forced), HUP (reload)
Suspend: SIGSTOP, Ctrl+Z
Resume: SIGCONT, fg, bg
Background: jobs, fg, bg, disown

Next Steps​

Chapter 4.4: Priorities and Nice Values
Chapter 4.5: Processes and Resources

Chapter created on: December 2024