Managing System Services with Systemctl: An In-Depth Guide

If you manage Linux servers, understanding systemd is a must-have skill nowadays. Systemd has been widely adopted as the default init system and service manager in most mainstream distros like RHEL, CentOS, Ubuntu, Debian and opensuse. It starts services in parallel at boot time for faster load times and offers unified control of system components via systemctl.

In this comprehensive guide, let‘s deep dive into essential systemd concepts and how to manage system services using systemctl commands. Even if you have used SysV init in the past, systemd represents a completely new way of thinking about service management in Linux.

Why Systemd? A Better Init System

Traditionally Linux used a System V (SysV) style init system which booted all services serially. This slowed down boot times as servers added more services. SysV init shell scripts also made service management messy with no unified control.

Systemd was created specifically to overcome issues with SysV init by parallelizing service startups to optimize boot time. It also centralizes management by having unified unit files for each component rather than individual scripts spread across folders. Some key improvements over legacy init systems:

Speed: Boots services simultaneously to reduce target system runtime.

Dependency handling: Service units can specify dependencies on other units intelligently.

Resource control: Integrated Linux control groups (cgroups) for tuning service resource usage.

Unified management: Centralized unit files using .service, .mount, .socket and other extensions.

Core Systemd Concepts

Before diving into systemctl usage, let‘s understand core systemd concepts. Key unit types include:

Unit Description File suffix
Service units Manage daemon services .service
Target units Logical groupings of services .target
Mount units Control filesystem mounts .mount
Socket units IPC socket activation .socket

These units incorporate information like metadata, dependencies, environment variables and execution instructions for systemd.

Targets are pre-defined states in system boot process. For example = operational system with network, logins etc. enabled.

An Overview of Systemctl Commands

systemctl is used for managing services and other unit types in systemd. Common systemctl commands include:

Managing services life-cycle: start, stop, restart, reload

Runtime status: status, is-active

On-boot behavior: enable, disable

View system state: list-units, list-unit-files

Now let‘s explore essential systemctl usage in-depth…

Stopping and Starting Services

The basic systemctl command structure for service units is:

sudo systemctl [action] [unit name]  

To start a service:

sudo systemctl start apache2

Stop a currently running service:

sudo systemctl stop apache2  

Restart a service:

sudo systemctl restart apache2

This will first stop it before starting it again.

To reload a service configuration without interrupting service:

sudo systemctl reload apache2

For network service changes to apply on RHEL/CentOS 7 and older, you need to restart the network service itself:

sudo systemctl restart network

Enabling Services at Boot

To start a service automatically at system bootup:

sudo systemctl enable apache2

This creates a symlink from unit file at /etc/systemd/system to the /etc/systemd/system/ folder.

To prevent a service from starting automatically:

sudo systemctl disable apache2 

Checking Status Information

View service status, process logs and other runtime information:

sudo systemctl status apache2

Verify if a service is actively running:

sudo systemctl is-active apache2  

Check whether a service is enabled or disabled from auto-starting on boot:

sudo systemctl is-enabled apache2

This displays the underlying enable symlink for the service.

Viewing Systemd Units

To list all active systemd units running on the system:

sudo systemctl list-units

Filter to only display loaded service units:

sudo systemctl list-units --type=service

To view all installed unit files whether running or not:

sudo systemctl list-unit-files

Managing Targets and Dependencies

System boot process relies on targets which are groupings of service units and other dependencies.

Check the default target set:

sudo systemctl get-default 

List all targets:

sudo systemctl list-units --type=target

Set the multi-user target:

sudo systemctl set-default  

This means all services for a multi-user production system will be started.

Units can also specify dependencies on other services for runtime ordering using Requires= and After= directives in unit files.

Debugging Systemd Services

Troubleshooting services first requires checking status:

sudo systemctl status apache2

Then query logs using journalctl which consolidates all systemd unit logs:

sudo journalctl -u apache2.service --since today

Boots are profiled using systemd-analyze:

systemd-analyze blame 

This outputs the time taken by each service to start and stop during the boot process.

For more detailed profiling, create plots using systemd-analyze plot > output.svg.


Learning systemctl is essential for controlling services, mounts, sockets and other components via systemd. Compared to SysV init scripts, it offers standardized and unified management of all core Linux functionality. With practice, system administrators can quickly get proficient at handling services from the command line using systemctl.