The primary role of an init system is to start and stop essential processes. Linux has seen three major init implementations: System V, Upstart, and systemd. This lesson focuses on the most traditional version, System V init, often referred to as SysV or simply systemv (pronounced 'System Five').
To determine if your system uses the systemv implementation, check for an /etc/inittab file. If this file exists, you are most likely running a SysV-based distribution.
How System V Manages Processes
The systemv init process starts and stops services sequentially. For instance, if a service named foo-b depends on foo-a, foo-b cannot start until foo-a is fully running. The initv system achieves this using shell scripts. These scripts, located in specific directories, handle the starting and stopping of services. While you can write custom scripts, most systems rely on the pre-packaged ones that manage essential OS services.
Advantages and Disadvantages
The main advantage of this sequential approach is its simplicity and predictability. Troubleshooting dependencies is straightforward because you know foo-a always starts before foo-b. However, the major drawback of the init v model is performance, as services are typically started one at a time, leading to slower boot times compared to modern parallel systems.
Understanding Runlevels in System V
In a systemv environment, the machine's state is defined by runlevels, numbered from 0 to 6. These modes can vary slightly between Linux distributions, but they generally follow this standard convention:
- 0: Shutdown
- 1: Single User Mode
- 2: Multiuser mode without networking
- 3: Multiuser mode with networking
- 4: Unused
- 5: Multiuser mode with networking and GUI
- 6: Reboot
Init Scripts and Directories
When your system boots, it checks its configured runlevel and executes the corresponding scripts. These scripts are typically found in directories like /etc/rc.d/rc[runlevel].d/ or within a central /etc/init.d directory. Scripts beginning with S (Start) are executed during startup, while those beginning with K (Kill) are run during shutdown. The numbers following S or K dictate the execution order.
For example:
pete@icebox:/etc/rc.d/rc0.d$ ls
K10updates K80openvpnIn this example, switching to runlevel 0 (shutdown) will first run the script to kill the updates service, followed by the script for OpenVPN. You can find the default runlevel for your machine in the /etc/inittab file, where you can also change it.
It is important to note that System V has been largely superseded by systemd in most modern Linux distributions. However, you may still encounter the concept of runlevels in newer init systems, as they often provide a compatibility layer to support legacy services that rely on systemv init scripts.