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Chapter 4.6 - Jobs and Background Tasks

Learning Objectives​

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

  • Start background processes
  • Manage jobs with jobs, fg, bg
  • Use nohup for persistent processes
  • Understand screen and tmux
  • Manage processes after disconnection

Background Processes​

Starting with &​

# Background process
command &

# Example
sleep 60 &
tail -f /var/log/app.log &

# See the PID
echo $!  # PID of the last background process

Redirection​

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

# Ignore output
command > /dev/null 2>&1 &

# Example
long_script.sh > script.log 2>&1 &

Job Management​

jobs - View Jobs​

# List jobs
jobs

# With PIDs
jobs -l

# Also finished jobs
jobs -a

# Format:
# [1]  + running    sleep 60 &
# [2]  - running    tail -f log &
# Number, state, command

fg - Foreground​

# Last job to foreground
fg

# Specific job
fg %1
fg %2
fg %name

# With Ctrl+Z, the process is suspended
# Then fg brings it to the foreground

bg - Background​

# Last suspended job to background
bg

# Specific job
bg %1
bg %2

nohup - Ignore SIGHUP​

Problem​

When you disconnect, the shell sends SIGHUP to all its child processes, killing them.

Solution: nohup​

# Ignore SIGHUP
nohup command &

# With redirection
nohup command > output.log 2>&1 &

# The nohup.out file is automatically created
# if no redirection

Features:

  • Process continues after disconnection
  • Output in nohup.out (by default)
  • Can be redirected

screen - Persistent Sessions​

Installation​

sudo apt install screen  # Ubuntu/Debian
sudo yum install screen  # Red Hat/CentOS

Basic Usage​

# Start a session
screen
screen -S session_name  # With name

# Detach: Ctrl+A then D
# Reconnect: screen -r
# Reconnect named session: screen -r session_name

# List sessions
screen -ls

# Kill session
screen -X -S session_name quit

screen Commands (Ctrl+A)​

Ctrl+A then:
  d    # Detach
  c    # New window
  n    # Next window
  p    # Previous window
  "    # List windows
  ?    # Help

tmux - Modern Terminal Multiplexer​

Installation​

sudo apt install tmux  # Ubuntu/Debian
sudo yum install tmux  # Red Hat/CentOS

Basic Usage​

# Start session
tmux
tmux new -s session_name

# Detach: Ctrl+B then D
# Reconnect: tmux attach
# Reconnect session: tmux attach -t session_name

# List sessions
tmux ls

# Kill session
tmux kill-session -t session_name

tmux Commands (Ctrl+B)​

Ctrl+B then:
  d    # Detach
  c    # New window
  n    # Next window
  p    # Previous window
  %    # Vertical split
  "    # Horizontal split
  ?    # Help

Comparison​

ToolUsageAdvantages
&Simple backgroundSimple, quick
nohupPersistent processesContinues after disconnection
screenTerminal sessionsMultiple windows, detachable
tmuxModern terminalMore features than screen

Practical Examples​

Scenario 1: Long Script​

# Start in background with nohup
nohup long_script.sh > script.log 2>&1 &

# Disconnect
# The script continues

Scenario 2: Work Session​

# Start screen
screen -S work

# Run multiple commands
# Detach: Ctrl+A D

# Reconnect later
screen -r work

Scenario 3: Continuous Monitoring​

# In screen/tmux
watch -n 1 'ps aux --sort=-%cpu | head'

# Detach and reconnect when needed

Summary​

In this chapter, you learned:

Background: command & to start in background
Jobs: jobs, fg, bg to manage jobs
nohup: Ignore SIGHUP, persistent processes
screen: Detachable terminal sessions
tmux: Modern terminal multiplexer
Practice: Choose according to needs (simple vs sessions)

Next Steps​

Module 5: Users and Permissions
Module 6: Shell Scripting

Chapter created on: December 2024