A runlevel is one of the modes that a Unix -based operating system will run in. Each runlevel has a certain number of services stopped or started, giving the user control over the behavior of the machine. There are seven runlevels ,0-6.
After the Linux kernel has booted, the /sbin/init program reads the /etc/inittab file to determine the behavior for each runlevel.
Unless the user specifies another value as a kernel boot parameter, the system will attempt to enter (start) the default runlevel.
| ID | Name | Description |
|---|---|---|
| 0 | Halt | Shuts down the system. |
| 1 | Single-user mode | Mode for administrative tasks. |
| 2 | Multi-user mode | Does not configure network interfaces and does not export networks services. |
| 3 | Multi-user mode with networking | Starts the system normally. |
| 4 | Not used/user-definable For special purposes. | |
| 5 | Start the system normally with appropriate display manager (with GUI) | Same as runlevel 3 + display manager. |
| 6 | Reboot | Reboots the system. |