System Startup, Boot Targets, and Runlevels

Understanding the Linux boot process and system initialization is crucial for system administration, troubleshooting, and optimization. From traditional SysV init to modern systemd, this guide covers the complete startup ecosystem, including boot targets, runlevels, and service management for efficient system control.

Init Systems Comparison: systemd vs SysV init

Aspect	systemd	SysV init
Architecture	Parallel, event-driven	Sequential, script-based
Boot Speed	Faster (parallel startup)	Slower (sequential)
Configuration	Unit files (.service, .target)	Init scripts in /etc/init.d/
Dependency Management	Automatic, declarative	Manual, script-based
Logging	journald (binary logs)	Text files in /var/log/
Status	Modern standard	Legacy, deprecated

Quick Reference:
• systemd - Modern init system (most distributions)
• SysV init - Traditional init system (legacy systems)
• systemctl - Control systemd and services
• journalctl - View system logs
• runlevel - Check current runlevel (SysV)
• init - Change runlevels (SysV)
• Boot targets replace runlevels in systemd

Linux Boot Process Workflow

Complete System Startup Sequence

BIOS/UEFI

→

Bootloader

→

Kernel

→

Init

→

Runlevel/Target

Traditional SysV Runlevels

Runlevel	Description	Purpose	Directory
0	Halt	System shutdown	`/etc/rc0.d/`
1	Single User	Maintenance mode	`/etc/rc1.d/`
2	Multi-user	Without networking	`/etc/rc2.d/`
3	Multi-user	With networking	`/etc/rc3.d/`
4	User-defined	Custom purposes	`/etc/rc4.d/`
5	Graphical	With display manager	`/etc/rc5.d/`
6	Reboot	System restart	`/etc/rc6.d/`

systemd Boot Targets

Boot Target	Equivalent Runlevel	Description	Purpose
poweroff.target	0	System power off	Shutdown system
rescue.target	1	Rescue mode	Single user maintenance
multi-user.target	3	Multi-user console	Text mode with networking
graphical.target	5	Graphical interface	Full desktop environment
reboot.target	6	System reboot	Restart system

Essential systemd Commands

⚙️

systemctl

Control the systemd system and service manager.

systemctl [options] command [unit...]

Service Management:

systemctl start service - Start service
systemctl stop service - Stop service
systemctl restart service - Restart service
systemctl reload service - Reload configuration
systemctl status service - Check status

Enable/Disable:

systemctl enable service - Start at boot
systemctl disable service - Don't start at boot
systemctl is-enabled service - Check enabled

📊

journalctl

Query and display messages from the journal.

journalctl [options]

Common Usage:

journalctl -u service - Service logs
journalctl -f - Follow logs
journalctl --since "1 hour ago"
journalctl -p err - Error messages only
journalctl -b - Current boot logs

Filtering:

--since and --until for time
-p for priority level
-u for specific unit
_PID= for process ID

🎯

Boot Target Management

Control system boot targets and default targets.

systemctl [isolate|get-default|set-default] target

Target Operations:

systemctl get-default - Current default
systemctl set-default target - Set default
systemctl isolate target - Switch to target
systemctl list-unit-files --type=target

Examples:

# Set graphical as default
sudo systemctl set-default graphical.target

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

# Check current target
systemctl get-default

Boot Process Stages

Stage	Component	Purpose	Key Files
1. Firmware	BIOS/UEFI	Hardware initialization	Firmware settings
2. Bootloader	GRUB2	Kernel loading	`/boot/grub/grub.cfg`
3. Kernel	Linux Kernel	System initialization	`/boot/vmlinuz*`
4. Init	systemd/init	User space startup	`/sbin/init`
5. Runlevel/Target	Services	Service management	Unit files, init scripts

Traditional Init System Commands

🔄

init

Change system runlevel (SysV init).

init runlevel | telinit runlevel

Common Runlevels:

init 0 - Shutdown
init 1 - Single user
init 3 - Multi-user text
init 5 - Graphical
init 6 - Reboot

Usage:

# Switch to runlevel 3
sudo init 3

# Check current runlevel
runlevel
N 3

📝

service

Run a System V init script (SysV init).

service SCRIPT COMMAND

Common Commands:

service ssh start
service ssh stop
service ssh restart
service ssh status

Init Script Locations:

/etc/init.d/ - Main directory
/etc/rc*.d/ - Runlevel links

🔗

chkconfig

Update and query runlevel information (SysV init).

chkconfig [--list] [--level levels] service

Common Usage:

chkconfig --list - List all services
chkconfig --level 35 ssh on
chkconfig ssh off - Disable service

Alternative:

update-rc.d - Debian alternative

Practical System Management Examples

Real-World System Startup Scenarios

# 1. systemd Service Management
# Check service status
systemctl status ssh
systemctl status nginx
systemctl status mysql

# Start/stop services
sudo systemctl start apache2
sudo systemctl stop mysql
sudo systemctl restart nginx

# Enable/disable services at boot
sudo systemctl enable ssh
sudo systemctl disable telnet
sudo systemctl is-enabled apache2

# 2. Boot Target Management
# Check current default target
systemctl get-default

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

# Switch to graphical target temporarily
sudo systemctl isolate graphical.target

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

# 3. Journal Management
# View system logs
journalctl
journalctl -f  # Follow logs

# View service-specific logs
journalctl -u nginx
journalctl -u ssh --since "1 hour ago"

# View error messages
journalctl -p err -b
journalctl --since "2025-12-03 09:00:00" --until "2025-12-03 10:00:00"

# 4. System Boot Analysis
# View boot time performance
systemd-analyze
systemd-analyze blame
systemd-analyze critical-chain

# Plot boot process
systemd-analyze plot > boot.svg

# 5. Emergency and Rescue Modes
# Boot into rescue mode (systemd)
sudo systemctl rescue

# Boot into emergency mode
sudo systemctl emergency

# From GRUB, add to kernel line:
# systemd.unit=rescue.target
# systemd.unit=emergency.target

# 6. Traditional Init Commands
# Check current runlevel
runlevel

# Change runlevel
sudo init 3
sudo telinit 5

# Manage services (SysV)
sudo service networking restart
sudo service apache2 status

# Configure service startup
sudo chkconfig --list
sudo chkconfig --level 35 httpd on
sudo update-rc.d apache2 enable

# 7. Custom Service Creation
# Create a simple systemd service
sudo nano /etc/systemd/system/myservice.service

# Example service file:
[Unit]
Description=My Custom Service
After=network.target

[Service]
Type=simple
ExecStart=/usr/local/bin/myscript.sh
Restart=on-failure
User=myuser

[Install]
WantedBy=multi-user.target

# Reload and enable
sudo systemctl daemon-reload
sudo systemctl enable myservice
sudo systemctl start myservice

# 8. Troubleshooting Boot Issues
# Check failed services
systemctl --failed

# Reset failed state
systemctl reset-failed

# Check service dependencies
systemctl list-dependencies nginx

# View service environment
systemctl show nginx

# 9. Runlevel to Target Mapping
# Traditional to systemd conversion:
# init 0 → systemctl poweroff
# init 1 → systemctl rescue
# init 3 → systemctl isolate multi-user.target
# init 5 → systemctl isolate graphical.target
# init 6 → systemctl reboot

# 10. Advanced systemd Features
# Create a user service
systemctl --user start myapp
systemctl --user enable myapp

# Manage systemd timers
systemctl list-timers
systemctl status daily-backup.timer

# Socket activation
systemctl status sockets-enabled

Common Use Cases

System Administration

Server Management: systemctl enable|start|stop nginx
Boot Optimization: systemd-analyze blame to find slow services
Troubleshooting: journalctl -u service --since "1 hour ago"
Service Dependencies: systemctl list-dependencies service

Development & Deployment

Custom Services: Create systemd unit files for applications
Service Monitoring: Use systemctl status for health checks
Log Analysis: journalctl for application debugging
Boot Testing: Test services in different targets

Maintenance & Recovery

Emergency Access: systemctl rescue for single-user mode
Service Recovery: systemctl reset-failed to clear errors
Boot Repair: Use GRUB to boot different targets
Performance Tuning: Disable unnecessary services at boot

Advanced System Management

🔧

Custom Unit Files

Create and manage custom systemd services.

/etc/systemd/system/service.service

Key Sections:

[Unit] - Metadata and dependencies
[Service] - Service configuration
[Install] - Installation info

Example:

[Unit]
Description=My App
After=network.target

[Service]
Type=simple
ExecStart=/usr/local/bin/myapp
Restart=always

[Install]
WantedBy=multi-user.target

⏱️

systemd Timers

Schedule tasks using systemd instead of cron.

.timer files with .service files

Benefits:

Integrated logging with journald
Systemd dependency management
Better error handling
Calendar events and monotonic timing

Example:

# /etc/systemd/system/daily-backup.timer
[Unit]
Description=Daily Backup Timer

[Timer]
OnCalendar=daily
Persistent=true

[Install]
WantedBy=timers.target

🛡️

Security Features

systemd security and resource control.

Service security directives

Security Options:

PrivateTmp=true - Private /tmp
NoNewPrivileges=true - No privilege escalation
ProtectSystem=strict - Filesystem protection
ReadWritePaths=/path - Allow write access

Resource Control:

MemoryLimit=500M
CPUQuota=50%

Important Considerations:
• systemd is not available on all systems - know your init system
• Changing default targets/runlevels affects system behavior
• Emergency and rescue modes require physical/console access
• Custom unit files require proper permissions and reloading
• Journal logs are not persistent by default on some systems
• Service dependencies can cause boot failures if misconfigured
• Always test service changes in non-production first
• Understand the difference between systemctl reload and systemctl restart

Pro Tips:
• Use systemd-analyze to optimize boot times
• Create custom targets for specific use cases
• Use journalctl --vacuum-size= to manage log size
• Enable persistent journaling with Storage=persistent
• Use systemctl mask to completely disable services
• Create drop-in files with systemctl edit service
• Monitor failed services with systemctl --failed
• Use hostnamectl and localectl for system info

Key Takeaways

Mastering system startup processes, boot targets, and runlevels is essential for effective Linux system administration. The transition from traditional SysV init to modern systemd has brought significant improvements in boot speed, service management, and system reliability. Understanding both systems ensures you can work effectively across different Linux distributions and versions. Remember that proper service configuration, dependency management, and logging are crucial for maintaining stable and performant systems. Whether you're managing simple services or complex multi-target environments, these foundational skills enable you to control system behavior from boot to shutdown with precision and confidence.

Next Step: Explore advanced systemd features like socket activation, namespaces, and resource control for building secure, efficient, and scalable services.