Guidelines for Shutting Down a System

Keep the following in mind when you shut down a system:

Use either the shutdown or the init command to shut down a system. Both commands perform a clean system shutdown, which means that all system processes and services are terminated normally.
You need to be the root role to use the shutdown and init commands.
Both the shutdown and init commands take a run level as an argument.

The three most common run levels are as follows:
- Run level 3 – All system resources are available and users can log in. By default, booting a system brings it to run level 3, which is used for normal day-to-day operations. This run level is also known as the multiuser state with NFS resources shared.
- Run level 6 – Shuts down the system to run level 0, and then reboots the system to a multiuser level with SMB or NFS resources shared (or whatever run level is the default in the inittab file).
- Run level 0 – The operating system is shut down, and it is safe to turn off power. You need to bring a system to run level 0 whenever you move a system, or add or remove hardware.
Run levels are fully described in How Run Levels Work.

System Shutdown Commands

The shutdown and init commands are the primary commands that are used to shut down a system. Both commands perform a clean shutdown of the system. As such, all file system changes are written to disk, and all system services, processes, and the operating system are terminated normally.

Turning a system off and then on is not a clean shutdown because system services are terminated abruptly. However, sometimes these actions are needed in emergency situations.

The following table describes the various shutdown commands and provides recommendations for using them.

Table 3-2 Shutdown Commands

Command	Description	When to Use
`shutdown`	An executable that calls the `init` program to shut down the system. The system is brought to run level S by default.	Use this command to shut down servers that are operating at run level 3.
`init`	An executable that terminates all active processes and synchronizes the disks before changing run levels.	This command provides a faster system shutdown. The command is preferred for shutting down stand-alone systems when other users will not be affected.
`reboot`	An executable that synchronizes the disks and passes boot instructions to the `uadmin` system call. In turn, this system call stops the processor.	The `init` command is the preferred method.
`halt`, `poweroff`	An executable that synchronizes the disks and stops the processor.	Not recommended because it does not shut down all processes or unmount any remaining file systems. Stopping the services, without doing a clean shutdown, should only be done in an emergency or if most of the services are already stopped.

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