Linux Services Management Guide (Focused & Practical)

Linux Services Management Guide (Focused & Practical)
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This guide helps you manage and troubleshoot Linux services using systemd and legacy tools, with a real-world example (CUPS service on port 631).

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GENERAL SERVICE MANAGEMENT
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List all services (enabled, disabled, static):
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sudo systemctl list-unit-files --type=service --all

List active (running) services:
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sudo systemctl list-units --type=service

Check detailed status of a specific service:
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sudo systemctl status [service_name]

Start a service:
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sudo systemctl start [service_name]

Stop a service:
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sudo systemctl stop [service_name]

Restart a service:
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sudo systemctl restart [service_name]

Reload service configuration:
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sudo systemctl reload [service_name]

Enable a service to start at boot:
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sudo systemctl enable [service_name]

Disable a service from starting at boot:
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sudo systemctl disable [service_name]

Mask a service (blocks manual & automatic starts):
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sudo systemctl mask [service_name]

Unmask a service:
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sudo systemctl unmask [service_name]

List failed services:
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systemctl --failed

Show service dependencies:
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systemctl list-dependencies [service_name]

Check service startup time impact:
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systemd-analyze blame

Legacy service commands (SysVinit compatible):
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sudo service [service_name] start
sudo service [service_name] stop
sudo service [service_name] restart
sudo service [service_name] status

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PORT-RELATED SERVICE TROUBLESHOOTING
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Find which process is using a specific port:
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sudo fuser -v [port]/tcp

Check which ports are open:
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sudo netstat -tulnp
# OR
ss -tuln

Scan for a specific port:
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sudo nmap -p [port] localhost

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DISABLING & BLOCKING A SERVICE (CUPS Example)
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1. Stop the service:
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sudo service cups stop

2. Disable at boot (SysV method):
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sudo update-rc.d -f cupsys remove

3. Disable with systemd:
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sudo systemctl disable cups

4. Mask the service (to fully block it):
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sudo systemctl mask cups

5. Reboot and confirm it's closed:
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sudo reboot
sudo nmap -p 631 localhost

If port 631 is STILL active after reboot:

6. Repeat post-boot cleanup:
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sudo systemctl unmask cups.service
sudo service cups start
sudo service cups status
sudo service cups stop
sudo systemctl disable cups
sudo systemctl mask cups
sudo nmap localhost
sudo reboot
sudo nmap localhost

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REMINDERS
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- Masked services are completely blocked from starting.
- Use `fuser` or `netstat` to identify what is keeping a port open.
- After disabling services, always reboot or reload configurations to apply changes.
- Avoid disabling essential system services unless you're certain.

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