When to Shut Down a System

To turn off system power due to anticipated power outage 

Run level 0, where it is safe to turn off power 

Chapter 10, Shutting Down a System (Tasks)

To change kernel parameters in the /etc/system file

Run level 6 (reboot the system) 

Chapter 10, Shutting Down a System (Tasks)

To perform file system maintenance, such as backing up or restoring system data 

Run level S (single-user level) 

Chapter 10, Shutting Down a System (Tasks)

To repair a system configuration file such as /etc/system

See When to Boot a System

N/A 

To add or remove hardware from the system 

Reconfiguration boot (also to turn off power when adding or removing hardware) 

Reconfiguration boot (shut down and turn off power when adding or removing devices, if the devices are not hot-pluggable)  

Adding a Peripheral Device to a System in System Administration Guide: Devices and File Systems

To repair an important system file that is causing system boot failure 

See When to Boot a System

N/A 

To boot the kernel debugger (kmdb) to track down a system problem

Run level 0, if possible 

Chapter 10, Shutting Down a System (Tasks)

To recover from a hung system and force a crash dump 

See When to Boot a System

N/A 

Reboot the system by using the kernel debugger (kmdb), if the debugger can't be loaded at runtime.

Run level 6 (reboot the system) 

For SPARC based systems:

SPARC: How to Boot the System With the Kernel Debugger (kmdb)

For x86 based systems:

,x86: How to Boot a System With the Kernel Debugger in the GRUB Boot Environment (kmdb)