/etc/systemd

The /etc/systemd directory is a critical part of the systemd init system on Linux. It contains configuration files and subdirectories that control the behavior of system and service units. Understanding the structure and purpose of this directory is essential for effective system management.

Overview of /etc/systemd

The /etc/systemd directory typically contains the following subdirectories:

• /etc/systemd/system/: Local unit files and custom unit configurations.

• /etc/systemd/user/: User-specific unit files and custom unit configurations.

/etc/systemd/system/

The /etc/systemd/system/ directory is where administrators place unit files to override the default ones provided by the system or create custom units. This directory takes precedence over the system's default unit files located in /lib/systemd/system/.

Common Directories and Files

• Unit Files: Files that define how services, sockets, targets, and other units are managed.

• multi-user.target.wants/: Directory containing symlinks to unit files that are wanted by the multi-user.target.

• graphical.target.wants/: Directory containing symlinks to unit files that are wanted by the graphical.target.

Creating and Managing Unit Files

Unit files in systemd describe the configuration and behavior of services and other system resources. They typically have the extension .service, .socket, .target, etc.

Structure of a Unit File

A typical .service unit file contains sections like [Unit], [Service], and [Install].

[Unit]
Description=My Custom Service
After=network.target

[Service]
ExecStart=/usr/bin/my-service
Restart=always
User=myuser

[Install]
WantedBy=multi-user.target

Enabling and Disabling Units

Enabling a unit means creating symlinks in the appropriate .wants directory so that the unit starts automatically at boot.

Enable a Unit

To enable a unit (e.g., my-service.service):

sudo systemctl enable my-service.service

Disable a Unit

To disable a unit:

sudo systemctl disable my-service.service

Reloading and Restarting Units

When changes are made to unit files, you need to reload the systemd configuration and potentially restart the affected service.

Reload systemd Configuration

To reload the systemd manager configuration:

sudo systemctl daemon-reload

Restart a Service

To restart a service:

sudo systemctl restart my-service.service

Viewing and Editing Unit Files

Viewing Unit Files

To view the content of a unit file:

systemctl cat my-service.service

Editing Unit Files

To edit a unit file (using a text editor like nano):

sudo nano /etc/systemd/system/my-service.service

After editing, reload the systemd configuration:

sudo systemctl daemon-reload

Overriding Default Unit Files

To override or extend the default unit files without modifying them directly, you can create drop-in files.

Creating Drop-In Files

To create a drop-in file for my-service.service:

sudo mkdir -p /etc/systemd/system/my-service.service.d
sudo nano /etc/systemd/system/my-service.service.d/override.conf

In the override.conf file, you can add or modify specific settings:

[Service]
Environment="MY_ENV_VAR=some_value"

Managing User-Specific Units

The /etc/systemd/user/ directory is used for user-specific unit files. These are typically managed by non-root users and allow user-level control over services.

Enabling User Units

To enable a user unit:

systemctl --user enable my-user-service.service

Starting User Units

To start a user unit:

systemctl --user start my-user-service.service

Conclusion

The /etc/systemd directory is crucial for managing systemd configurations on a Linux system. It allows administrators to customize, enable, disable, and override system services and other units. Understanding how to navigate and utilize this directory is essential for effective system administration with systemd.