init command in Linux with examples

init is parent of all Linux processes with PID or process ID of 1. It is the first process to start when a computer boots up and runs until the system shuts down. init stands for initialization. In simple words the role of init is to create processes from script stored in the file /etc/inittab which is a configuration file which is to be used by initialization system. It is the last step of the kernel boot sequence.

/etc/inittab Specifies the init command control file.

• init script initializes the service. So, it responsible for initializing the system.
• Init scripts are also called rc scripts (run command scripts)
• Init script is also used in UNIX.

Syntax

init [OPTIONS...] {COMMAND}

Run Levels

Run Levels is the state of init where a group of processes are defined to start at the startup of OS. The process spawned by init for each of this run levels are defined in the file /etc/inittab. Each runlevel has a certain number of services stopped or started. Conventionally seven runlevel exist numbers from zero to six.

Standard Run Levels for Red Hat Based Distribution:

Runlevel	Mode	Action
0	Halt	Shuts down system
1	Single-User Mode	Does not configure network interfaces, start daemons,  or allow non-root logins
2	Multi-User Mode	Does not configure network interfaces or start daemons.
3	Multi-User Mode with Networking	Starts the system normally.
4	Undefined	Not used/User-definable
5	X11	As runlevel 3 + display manager(X)
6	Reboot	Reboots the system

By default most of the LINUX based system boots to runlevel 3 or runlevel 5. In addition to the standard runlevels, users can modify the preset runlevels or even create new ones according to the requirement. Runlevels 2 and 4 are used for user defined runlevels and runlevel 0 and 6 are used for halting and rebooting the system.

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