In this guide, you’ll learn how to inspect, configure, and control services on a Linux system using systemd —the modern init system. We cover unit files, service management commands, and best practices for maintaining your server’s services.
Table of Contents
Introduction to systemd Unit File Types Service Unit Structure Inspecting the SSH Daemon Service Editing and Reverting a Unit Monitoring Service Status Managing Service Lifecycle Enabling and Disabling at Boot Masking and Unmasking Services Listing All Service Units Further Reading
Introduction to systemd When Linux boots, systemd orchestrates service startup in parallel while respecting dependencies. If a critical service fails, systemd can restart it automatically, ensuring high availability. All behavior is defined by unit files —plain-text configurations that tell systemd how to manage resources.
Unit File Types Unit files end in .service, .socket, .device, .timer, and more. Here’s a quick overview:
Unit Type Description service Defines how to start, stop, and manage a service socket Configures socket activation target Groups units and handles synchronization timer Schedules tasks similar to cron
Service Unit Structure Service units reside in /etc/systemd/system/ or /usr/lib/systemd/system/. They have three main sections:
[Unit] Description =Human-readable description After =network.target [Service] Type =simple ExecStart =/usr/bin/myapp Restart =on-failure [Install] WantedBy =multi-user.target
Key directives:
ExecStart : Command to launch the serviceExecReload : How to reload without a full restartRestart : Policy (no, on-failure, always)
Consult the manual for all options:
Inspecting the SSH Daemon Service Most servers run the OpenSSH daemon (sshd) as a systemd service. View its complete unit file:
$ systemctl cat sshd.service # /usr/lib/systemd/system/sshd.service [Unit] Description = OpenSSH server daemon Documentation = man:sshd ( 8 ) man:sshd_config(5 ) After = network.target sshd-keygen.target Wants = sshd-keygen.target [Service] Type = notify EnvironmentFile = /etc/crypto-policies/back-ends/opensshserver.config EnvironmentFile = -/etc/sysconfig/sshd ExecStart = /usr/sbin/sshd -D $OPTIONS $CRYPTO_POLICY ExecReload = /bin/kill -HUP $MAINPID KillMode = process Restart = on-failure RestartSec = 42s [Install] WantedBy = multi-user.target
Editing and Reverting a Unit To fully edit the sshd.service file:
$ sudo systemctl edit --full sshd.service
To discard changes and restore the vendor-provided unit:
$ sudo systemctl revert sshd.service
Monitoring Service Status Check detailed status and recent logs:
$ sudo systemctl status sshd.service
Sample output:
● sshd.service - OpenSSH server daemon Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled) Active: active (running) since Wed 2021-12-08 16:48:53 CST; 15min ago Docs: man:sshd(8) man:sshd_config(5) Main PID: 1031 (sshd) Tasks: 1 (limit: 23555) Memory: 2.1M CGroup: /system.slice/sshd.service └─1031 /usr/sbin/sshd -D [email protected] ,chacha20-poly1305 Dec 08 16:48:53 LFCS-CentOS systemd[1]: Starting OpenSSH server daemon... Dec 08 16:48:53 LFCS-CentOS sshd[1031]: Server listening on 0.0.0.0 port 22. Dec 08 16:48:53 LFCS-CentOS systemd[1]: Started OpenSSH server daemon.
Press Q to exit.
Managing Service Lifecycle Use systemctl for day-to-day operations:
Operation Command Description Start sudo systemctl start sshd.serviceLaunch the service immediately Stop sudo systemctl stop sshd.serviceStop the service Restart sudo systemctl restart sshd.serviceStop and then start the service Reload sudo systemctl reload sshd.serviceReload configuration without full restart Reload or Restart sudo systemctl reload-or-restart sshd.serviceTry reload, else restart
Enabling and Disabling at Boot Control automatic startup:
# Enable at boot $ sudo systemctl enable sshd.service # Disable at boot $ sudo systemctl disable sshd.service # Check status $ sudo systemctl is-enabled sshd.service
Combine with --now to apply immediately:
# Enable and start now $ sudo systemctl enable --now sshd.service # Disable and stop now $ sudo systemctl disable --now sshd.service
Disabling and stopping sshd.service will prevent all SSH logins.
Masking and Unmasking Services Prevent any activation—manual or dependency-driven—by masking:
$ sudo systemctl mask atd.service
Reverse masking:
$ sudo systemctl unmask atd.service
Attempting to start a masked service results in:
Failed to enable unit: Unit file /etc/systemd/system/atd.service is masked. Failed to start atd.service: Unit atd.service is masked.
Listing All Service Units View every service, regardless of state:
$ systemctl list-units --type=service --all
Further Reading 
