How Run Levels Work

A system's run level (also known as an init state) defines what services and resources are available to users. A system can be in only one run level at a time.

Oracle Solaris has eight run levels, which are described in the following table. The default run level is specified in the /etc/inittab file as run level 3.

Table 1-3 Oracle Solaris Run Levels

Run Level	Init State	Type	Purpose
0	Power-down state	Power-down	To shut down the operating system so that it is safe to turn off power to the system.
s or S	Single-user state	Single-user	To run as a single user with some file systems mounted and accessible.
1	Administrative state	Single-user	To access all available file systems. User logins are disabled.
2	Multiuser state	Multiuser	For normal operations. Multiple users can access the system and all file systems. All daemons are running except for the NFS server daemons.
3	Multiuser level with NFS resources shared	Multiuser	For normal operations with NFS resources shared. This is the default run level.
4	Alternative multiuser state	Multiuser	Not configured by default, but available for customer use.
5	Power-down state	Power-down	To shut down the operating system so that it is safe to turn off power to the system. If possible, automatically turns off power on systems that support this feature.
6	Reboot state	Reboot	To shut down the system to run level 0, and then reboot to a multiuser level with NFS resources shared (or whatever run level is the default in the `inittab` file).

In addition, the svcadm command can be used to change the run level of a system, by selecting a milestone at which to run. The following table shows which run level corresponds to each milestone.

Table 1-4 Run Levels and SMF Milestones

Run Level	SMF Milestone FMRI
S	`milestone/single-user:default`
2	`milestone/multi-user:default`
3	`milestone/multi-user-server:default`

What Happens When a System Is Booted to a Multiuser State (Run Level 3)

The init process is started and reads the properties defined in the svc:/system/environment:init SMF service to set any environment variables. By default, only the TIMEZONE variable is set.
Then, init reads the inittab file and does the following:
1. Executes any process entries that have sysinit in the action field so that any special initializations can take place before users log in to the system.
2. Passes the startup activities to svc.startd.
For a detailed description of how the init process uses the inittab file, see the init(1M) man page.

When to Use Run Levels or Milestones

In general, changing milestones or run levels is an uncommon procedure. If it is necessary, using the init command to change to a run level will change the milestone as well and is the appropriate command to use. The init command is also good for shutting down a system.

However, booting a system using the none milestone can be very useful for debugging startup problems. There is no equivalent run level to the none milestone. For more information, see How to Boot a System Without Starting Any Services.

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