All About systemd: Start, Stop, and Manage Services

systemd has become the de facto initialization system and service manager on most modern Linux distributions. It offers parallel startup, on-demand service activation, snapshotting, and sophisticated dependency resolution. This article explores every aspect of starting, stopping, enabling, and managing services under systemd.

1. Introduction to systemd

PID 1: systemd takes the first process slot (PID 1) and remains active until shutdown.
Unit Types: service, socket, target, timer, etc.
Dependency Management: Handles After=, Requires=, Wants=.
Parallelization: Starts multiple services simultaneously.

For official documentation, visit systemd Wiki.

2. Basic Commands with `systemctl`

Action	Command
Start a service now	`systemctl start foo.service`
Stop a service now	`systemctl stop foo.service`
Enable on boot	`systemctl enable foo.service`
Disable on boot	`systemctl disable foo.service`
View status	`systemctl status foo.service`
View logs	`journalctl -u foo.service`

3. Managing Units and Dependencies

Units are configuration files that define how services start and stop. They reside in:

/etc/systemd/system/ (local overrides)
/run/systemd/system/ (runtime units)
/lib/systemd/system/ (package-supplied)

Key directives:

Description= Human-readable name
ExecStart= Program to run
After= Ordering dependency
Requires= Hard dependency
Wants= Soft dependency

3.1 Overriding Units

Create a drop-in file under /etc/systemd/system/foo.service.d/override.conf:

ltServicegt
  Restart=always
lt/Servicegt

Then reload: systemctl daemon-reload and restart service.

4. Targets and Boot Sequence

Targets are groupings of units. Examples:

multi-user.target: Normal non-graphical multi-user mode.
graphical.target: Multi-user GUI.
rescue.target: Single-user emergency mode.

Switch runlevels:

systemctl isolate rescue.target

5. Timers and Socket Activation

Timers can replace cron. Define foo.timer alongside foo.service:

[Timer]
OnCalendar=-- 03:00:00
[Install]
WantedBy=timers.target

Socket activation allows starting services on-demand when a socket is accessed, e.g., sshd.socket.

6. Logging and Troubleshooting

View logs for all units: journalctl -xe
Limit logs: journalctl -u foo.service --since 2024-01-01
Check for failed units: systemctl --failed

Enable persistent logs by creating /var/log/journal directory and restarting systemd-journald.

7. Advanced Management

Snapshotting: systemctl snapshot and later systemctl isolate [snapshot].
Resource Control: Integrates cgroups via directives like MemoryLimit=, CPUQuota=.
Masking: Prevent a service from being started:
systemctl mask foo.service
Preset: Distribution-prescribed enabled/disabled state:
systemctl preset foo.service

8. Use Cases and Examples

Example: Auto-restart a critical database:

ltUnitgt
  Description=MyDB Service
  After=network.target
lt/Unitgt
ltServicegt
  ExecStart=/usr/local/bin/mydb
  Restart=on-failure
  RestartSec=5
lt/Servicegt
ltInstallgt
  WantedBy=multi-user.target
lt/Installgt

9. Best Practices

Never edit package-supplied units use overrides.
Group related units under a custom .target.
Keep unit files minimal delegate complex logic to scripts.
Use systemctl daemon-reload after every change.

10. VPN Services and systemd

Integrating VPNs with systemd ensures secure networking on boot. Below are popular solutions:

NordVPN: Official Linux app can be run as a service.
ExpressVPN: Provides a systemd unit for auto-connect.
Surfshark: Offers surf-proxy.service for automatic startup.
Private Internet Access: Uses pia-client.service with systemd.

Example unit for OpenVPN:

ltUnitgt
  Description=OpenVPN tunnel for private network
  After=network.target
lt/Unitgt
ltServicegt
  ExecStart=/usr/sbin/openvpn --config /etc/openvpn/client.conf
  Restart=always
  RestartSec=10
lt/Servicegt
ltInstallgt
  WantedBy=multi-user.target
lt/Installgt

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All About systemd: Start, Stop, and Manage Services