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The Linux operating system calls a running program a process. A process can run in the foreground, displaying output on a display, or it can run in background, behind the scenes. The kernel controls how the Linux system manages all the processes running on the system.

The kernel creates the first process, called the init process, to start all other processes on the system. When the kernel starts, it loads the init process into virtual memory. As the kernel starts each additional process, it gives it a unique area in virtual memory to store the data and code that the process uses.

There are a few different types of init process implementations available in Linux, but these days, the two most popular are as follows:

 SysVinit—The SysVinit (SysV) initialization method was the original method used by Linux and was based on the Unix System V initialization method. Though it is not used by many Linux distributions these days, you still may find it around in older Linux distributions.

 Systemd—The systemd initialization method was created in 2010 and has become the most popular initialization and process management system used by Linux distributions.

The SysVinit initialization method used a concept called runlevels to determine what processes to start. The runlevel defines the state of the running Linux system and what processes should run in each state. Table 1.1 shows the different runlevels associated with the SysVinit initialization method.

TABLE 1.1: The SysVinit Runlevels

RUNLEVEL	DESCRIPTION
0	Shuts down the system
1	Single‐user mode used for system maintenance
2	Multiuser mode without networking services enabled
3	Multiuser mode with networking services enabled
4	Custom
5	Multiuser mode with GUI available
6	Reboots the system

The /etc/inittab file defines the default runlevel for a system. The processes that start for specific runlevels are defined in subdirectories of the /etc/rc.d directory. You can view the current runlevel at any time using the runlevel command, as shown here:

$ runlevel N 5 $

The systemd initialization method became popular because it has the ability to start processes based on different events such as these:

 When the system boots

 When a particular hardware device is connected

 When a service is started

 When a network connection is established

 When a timer has expired

The systemd method determines what processes to run by linking events to unit files. Each unit file defines the programs to start when the specified event occurs. The systemctl program allows you to start, stop, and list the unit files currently running on the system.

The systemd method groups unit files together into targets. A target defines a specific running state of the Linux system, similar to the SysVinit runlevel concept. At system startup, the default.target unit defines all the unit files to start. You can view the current default target using the systemctl command.

$ systemctl get-default graphical.target $

The graphical.target target defines the processes to start when a multiuser graphical environment is running, similar to the old SysVinit runlevel 5.