Oracle Solaris Run Levels Explained

In this tutorial, we will learn about Run Levels in Oracle Solaris operating system which are being used by System Administrators and defines what services and resources are available to users.

Table of Contents

What are Run Levels

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. A run level is a state of init and the whole system that defines what system services are operating. Run levels are identified by numbers.

Oracle Solaris has eight run levels, 0 to 6 and s (or S). The default run level is specified in the /etc/inittab file as run level 3. Users can modify the preset runlevels or even create new ones if needed. Runlevel 4 is typically for user-defined runlevels.

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 stop the operating system and reboot to the state that is defined by the initdefault entry in the /etc/inittab file.

When to Use Run Levels

Booting a system into different run levels solves certain problems. For example, if a system fails to boot due to a damaged configuration file, refuses to allow the user to log in due to a corrupted /etc/passwd file or if you forget your password, you can solve these problems by booting into single-user mode.

If you want to reset the root user password in Solaris OS, You may check out our post “How to reset root password in Solaris 11“.

Determining a System’s Current Run Level

To determine a system’s current run level, use the who -r command.

For example

$ who -r

Output of who –r command	Description
run-level 3	Identifies the current run level
Mar 18 19:45	Identifies the date of last run level change
3	Also identifies the current run level
0	Identifies the number of times the system has been at this run level since the last reboot
S	Identifies the previous run level