Systemd is a system and service manager for Linux operating systems. It initializes the system, manages system processes, and handles service management tasks. Systemd replaces the traditional System V init system and offers parallel service startup, on-demand starting of daemons, and more robust service management capabilities.

Understanding Systemd Components

1. Unit Files: Systemd uses unit files to define services, sockets, devices, and other system resources. These files are typically located in /etc/systemd/system/ or /lib/systemd/system/.

2. Targets: Targets are a grouping of unit files that define a specific state of the system, similar to runlevels in System V init.

3. Journal: Systemd includes a logging system called the journal, which captures logs from the kernel, initrd, services, and other sources.

Examples: Managing Services with Systemd

1. Starting and Stopping Services

   To start a service, use the systemctl start command followed by the service name:

   sudo systemctl start apache2

   To stop a service, use the systemctl stop command:

   sudo systemctl stop apache2

2. Enabling and Disabling Services

   To enable a service to start at boot, use the systemctl enable command:

   sudo systemctl enable apache2

   To disable a service from starting at boot, use the systemctl disable command:

   sudo systemctl disable apache2

3. Checking the Status of a Service

   To check the status of a service, use the systemctl status command:

   systemctl status apache2

   This command provides detailed information about the service, including whether it is active, any errors, and recent log entries.

4. Viewing Logs

   To view logs for a specific service, use the journalctl command:

   journalctl -u apache2

   This command displays logs related to the Apache service. You can use various options with journalctl to filter logs by time, priority, and other criteria.

5. Reloading and Restarting Services

   To reload a service's configuration without stopping it, use the systemctl reload command:

   sudo systemctl reload apache2

   To restart a service, which stops and then starts it again, use the systemctl restart command:

   sudo systemctl restart apache2

6. Creating a Custom Service

   To create a custom service, you need to create a unit file. For example, to create a service for a simple script, follow these steps:

   • Create a script, e.g., /usr/local/bin/myscript.sh:

     #!/bin/bash
     echo "Hello, World!" >> /var/log/myscript.log

   • Make the script executable:

     sudo chmod +x /usr/local/bin/myscript.sh

   • Create a unit file at /etc/systemd/system/myscript.service:

     [Unit]
     Description=My Custom Script
     
     [Service]
     ExecStart=/usr/local/bin/myscript.sh
     
     [Install]
     WantedBy=multi-user.target

   • Enable and start the service:

     sudo systemctl enable myscript.service
     sudo systemctl start myscript.service