init

init [runlevel]

init is the parent of all processes on the system, responsible for starting and stopping services at different runlevels. It is the traditional System V init daemon that manages the system state.Runlevels define different system states, from single-user maintenance mode to full graphical desktop. Changing runlevels causes init to start or stop services as defined in /etc/rc.d or /etc/init.d scripts.On modern systemd-based systems, init may be a compatibility wrapper that translates runlevel commands to systemd targets.

0

Halt the system

Single user mode (maintenance)

Multi-user mode without NFS (Debian default multi-user with GUI)

Multi-user mode with networking, text console only

Unused/custom (available for user-defined purposes)

Multi-user mode with networking and graphical desktop (Red Hat default GUI)

Reboot the system

Behavior varies between distributions. On systemd-based systems, init is a symlink to systemd and runlevel commands are translated to systemd targets (e.g., runlevel 3 maps to multi-user.target, runlevel 5 to graphical.target). Direct runlevel changes can disrupt running services. Use systemctl on modern systems.

System V init originated in AT&T UNIX System V in 1983. It was the standard init system on most Linux distributions until systemd began replacing it around 2011.

systemctl(1), telinit(8), runlevel(8), halt(8), reboot(8), shutdown(8), service(8)