Linux 118: Systemd Deep Dive – Managing Services and Boot Processes

Linux 118: Systemd Deep Dive – Managing Services and Boot Processes

Systemd is the modern init system used by most major Linux distributions. It manages everything from service startup to logging, device management, and system states. Understanding systemd is essential for effective system administration.

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1. What is systemd?

Systemd is a suite of system components designed to manage:

• System boot and shutdown
• Service supervision
• Device and mount point handling
• Logging via journal

It replaces older init systems like SysVinit and Upstart.

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2. systemctl – The Core Command

The systemctl command is used to interact with systemd.

Start, Stop, Enable Services

sudo systemctl start nginx
sudo systemctl stop nginx
sudo systemctl restart nginx

Enable/Disable on Boot

sudo systemctl enable nginx
sudo systemctl disable nginx

Check Status

systemctl status nginx

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3. Listing Services and Targets

List All Services

systemctl list-units --type=service

List Enabled Services

systemctl list-unit-files --state=enabled

Check Target (Runlevel Equivalent)

systemctl get-default

Change target:

sudo systemctl set-default multi-user.target

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4. Understanding Unit Files

Unit files define how systemd manages resources like services, mounts, devices, and sockets.

Types include:

• .service – services
• .target – groups of units
• .socket – network sockets
• .timer – scheduled tasks

Example unit file: /etc/systemd/system/myapp.service

[Unit]
Description=My Custom App

[Service]
ExecStart=/usr/bin/myapp
Restart=on-failure

[Install]
WantedBy=multi-user.target

Enable and start it:

sudo systemctl enable myapp
sudo systemctl start myapp

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5. Journald – Viewing Logs

Systemd uses journald for centralized logging.

View Recent Logs

journalctl

Filter by Service

journalctl -u nginx

Follow Logs in Real Time

journalctl -f

Limit to Boot Session

journalctl -b

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6. Debugging Boot Issues

Analyze Boot Time

systemd-analyze

View Boot Processes by Time

systemd-analyze blame

Graph Boot Process

systemd-analyze plot > boot.svg

Open the file to see a visual representation of boot performance.

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7. Scheduling Tasks with systemd Timers

Timers can replace cron for more advanced scheduling.

Create a Timer:

myjob.timer

[Unit]
Description=Run My Job

[Timer]
OnCalendar=*-*-* 12:00:00
Unit=myjob.service

[Install]
WantedBy=timers.target

Create myjob.service as a companion unit.

Enable and start:

sudo systemctl enable --now myjob.timer

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8. Managing System States

Reboot and Shutdown

sudo systemctl reboot
sudo systemctl poweroff

Suspend and Hibernate

sudo systemctl suspend
sudo systemctl hibernate

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9. Creating Rescue and Emergency Modes

• Rescue mode: Minimal services and root shell
• Emergency mode: Only root filesystem and shell

Enter:

sudo systemctl rescue
sudo systemctl emergency

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10. Conclusion

Systemd is more than just a service manager — it's the backbone of modern Linux systems. Mastering its components helps you control services, debug boot problems, manage logs, and automate tasks effectively.

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Next Steps:

• Learn how to write custom .service and .timer units
• Explore systemd-resolved and systemd-networkd for networking
• Practice troubleshooting with systemd-analyze and journalctl

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Coming next: Linux 119: Using Containers with Podman and Docker