Init Systems

Overview

Init systems have evolved significantly:

# Historical progression:
# SysVinit (System V) - Traditional init system
# Upstart - Ubuntu replacement for SysVinit
# systemd - Modern init system (most Linux distributions)

# Check which init system is running
ps aux | grep -E "^\s*1\s" | grep -v grep
# Output shows: /sbin/init or /lib/systemd/systemd

# Alternative check
systemctl --version  # If systemd
# or
initctl --version    # If Upstart

SysVinit (Traditional)

Service Management

# Check if SysVinit is in use
ls -la /etc/init.d/

# List available services
ls /etc/init.d/

# Start service
sudo /etc/init.d/apache2 start
sudo service apache2 start

# Stop service
sudo /etc/init.d/apache2 stop
sudo service apache2 stop

# Restart service
sudo /etc/init.d/apache2 restart
sudo service apache2 restart

# Reload service (graceful)
sudo /etc/init.d/apache2 reload
sudo service apache2 reload

# Get service status
sudo /etc/init.d/apache2 status
sudo service apache2 status

Runlevels

# SysVinit uses runlevels 0-6:
# 0 - Halt
# 1 - Single user (maintenance mode)
# 2 - Multi-user, no networking (Debian default)
# 3 - Multi-user with networking (Red Hat default)
# 4 - Unused (customizable)
# 5 - Multi-user with GUI
# 6 - Reboot

# Check current runlevel
runlevel

# Set default runlevel (edit /etc/inittab)
# Look for line: id:3:initdefault:

# Change runlevel temporarily
sudo init 3  # Switch to runlevel 3
sudo init 5  # Switch to runlevel 5

# View /etc/inittab
cat /etc/inittab

Managing Services at Boot

# Update-rc.d (Debian/Ubuntu)
sudo update-rc.d apache2 enable   # Enable at boot
sudo update-rc.d apache2 disable  # Disable at boot
sudo update-rc.d apache2 remove   # Remove service links

# chkconfig (Red Hat/CentOS)
sudo chkconfig apache2 on         # Enable at boot
sudo chkconfig apache2 off        # Disable at boot
sudo chkconfig --list apache2     # List current status

# Check if service starts on boot
ls /etc/rc3.d/ | grep apache2
# S = start, K = kill (stop)
# Format: S99apache2 (start at position 99)

Systemd (Modern Init System)

Systemd is the default init system on most modern Linux distributions.

Service Management

# Check if systemd is in use
systemctl --version

# Start service
sudo systemctl start apache2
sudo systemctl start mysql

# Stop service
sudo systemctl stop apache2
sudo systemctl stop mysql

# Restart service
sudo systemctl restart apache2
sudo systemctl restart mysql

# Reload service (graceful, no downtime)
sudo systemctl reload apache2
sudo systemctl reload nginx

# Get service status
sudo systemctl status apache2
# Shows: running, stopped, failed, etc.

# View service details
sudo systemctl show apache2

Service Management at Boot

# Enable service to start at boot
sudo systemctl enable apache2

# Disable service from starting at boot
sudo systemctl disable apache2

# Check if service is enabled
sudo systemctl is-enabled apache2

# Mask service (prevent start even if enabled)
sudo systemctl mask apache2

# Unmask service
sudo systemctl unmask apache2

# Check enabled/disabled status
sudo systemctl list-unit-files --state=enabled
sudo systemctl list-unit-files --state=disabled

Targets (Runlevels equivalent)

# Systemd targets replace SysVinit runlevels:
# poweroff.target (runlevel 0)
# rescue.target (runlevel 1 - single user)
# multi-user.target (runlevel 3 - multi-user, no GUI)
# graphical.target (runlevel 5 - multi-user with GUI)
# reboot.target (runlevel 6)

# Check current target
systemctl get-default

# Set default target
sudo systemctl set-default multi-user.target
sudo systemctl set-default graphical.target

# Switch to target temporarily
sudo systemctl isolate multi-user.target
sudo systemctl isolate rescue.target

# List all available targets
systemctl list-unit-files --type=target

Unit Files

# Systemd unit files location:
# /lib/systemd/system/ - Package-provided units
# /etc/systemd/system/ - Administrator units
# ~/.config/systemd/user/ - User units

# Find a service unit file
systemctl cat apache2
# Shows full path and contents

# Edit service unit file
sudo systemctl edit apache2
# Opens in default editor (creates override in /etc/systemd/system/)

# Reload unit file changes
sudo systemctl daemon-reload
sudo systemctl restart apache2

# Example unit file structure:
# [Unit]
# Description=Apache HTTP Server
# After=network.target
#
# [Service]
# Type=notify
# ExecStart=/usr/sbin/apache2ctl start
# ExecStop=/usr/sbin/apache2ctl stop
# Restart=on-failure
#
# [Install]
# WantedBy=multi-user.target

Monitoring and Logs

# View systemd journal logs
journalctl

# View logs for specific service
sudo journalctl -u apache2

# View last N lines
sudo journalctl -u apache2 -n 50

# Follow logs in real-time (like tail -f)
sudo journalctl -u apache2 -f

# Show logs since last boot
sudo journalctl -b

# Show logs from specific time range
sudo journalctl --since "2024-01-15 10:00:00" --until "2024-01-15 11:00:00"

# Filter by priority level
sudo journalctl -p err    # Errors only
sudo journalctl -p warning  # Warnings and above

# Show available services and their status
systemctl list-units --type=service
systemctl list-units --type=service --state=running
systemctl list-units --type=service --state=failed

# Check for failed units
systemctl --failed

Timers (Cron Replacement)

# Systemd can replace cron for scheduled tasks

# List all timers
systemctl list-timers

# Create a timer unit in /etc/systemd/system/
# Example: backup.timer

# View timer status
sudo systemctl status backup.timer

# Enable timer
sudo systemctl enable backup.timer

# Start timer
sudo systemctl start backup.timer

# Check timer details
sudo systemctl cat backup.timer

Comparison Table

Operation	SysVinit	Upstart	Systemd
Start service	`service nginx start`	`start nginx`	`systemctl start nginx`
Stop service	`service nginx stop`	`stop nginx`	`systemctl stop nginx`
Restart service	`service nginx restart`	`restart nginx`	`systemctl restart nginx`
Enable at boot	`chkconfig nginx on`	`initctl nginx enable`	`systemctl enable nginx`
Disable at boot	`chkconfig nginx off`	`initctl nginx disable`	`systemctl disable nginx`
Check status	`service nginx status`	`status nginx`	`systemctl status nginx`
View logs	`tail /var/log/syslog`	`tail /var/log/syslog`	`journalctl -u nginx`
Manage runlevels	`init 3`	N/A	`systemctl isolate multi-user.target`

Common Troubleshooting

# Service fails to start
sudo systemctl status apache2
# Check "Active" line for error details

# View detailed error messages
sudo journalctl -u apache2 -n 100

# Service stuck in failed state
sudo systemctl reset-failed apache2

# Check if service file has syntax errors
sudo systemd-analyze verify apache2.service

# Debug service startup
sudo systemctl start apache2 -v
# Or check journal: journalctl -u apache2 -e

# Port already in use
sudo lsof -i :80
# Kill process if needed: sudo kill -9 <PID>

# Permission denied errors
# Check service unit file for User= directive
sudo systemctl cat apache2 | grep User=

# Service dependency issues
# View unit dependencies
sudo systemctl list-dependencies apache2

# Deadlock or circular dependencies
sudo systemd-analyze
# Shows dependency graph

Best Practices

Use systemctl instead of service commands (systemd standard)
Enable services explicitly: systemctl enable service-name
Monitor service status regularly: systemctl list-units --failed
Check logs immediately after service restart: journalctl -u service-name -f
Use systemctl reload instead of restart when possible (no service interruption)
Keep service unit files in /etc/systemd/system/ for modifications
Use systemctl daemon-reload after editing unit files
Set up automated restarts for critical services
Monitor journal size: journalctl --disk-usage
Archive old logs: sudo journalctl --vacuum=30d (keep 30 days)

Resources

Last updated: 2026-03-30