AIX Runlevels

In AIX runlevels define the system’s mode of operation—determining which services and processes are active at any given time. Managed by the init process through the /etc/inittab file, runlevels control system startup, shutdown, and maintenance states.

The default runlevel in AIX is 2, representing the standard multi-user mode with networking and most services enabled. This differs from Linux, where runlevels 3 or 5 are often used for full operation.

The AIX Runlevel Catalog
Unlike some Linux distributions where you might bounce between level 3 (text) and level 5 (GUI), AIX keeps it streamlined.

Runlevel	State	Description
0	Halt	Shuts down the operating system and prepares the hardware for power-off.
1	Single-User	Maintenance mode. Only the root user is logged in, and most network services are disabled.
2	Multi-User	The Default. Full operational mode with networking, NFS, and all standard enterprise services.
3–9	User-Defined	Custom states defined by the administrator for specific application environments.
S / s	Single-User	Similar to level 1, often used interchangeably during maintenance tasks.
M / m	Maintenance	A specific state used for system recovery and diagnostic tasks.

You can check the current runlevel using:

# who -r

and change it manually with:

# init <runlevel>

(For example, init 1 for maintenance or init 0 to shut down.)

AIX also allows administrators to define custom runlevels for specific tasks by adding entries in /etc/inittab, such as starting dedicated applications or services.

In essence, AIX runlevels provide a flexible framework for managing system states—ensuring controlled transitions between maintenance, normal, and customized operating modes.


AIX vs. Linux:
If you are coming from a Linux background, the most important thing to remember is that AIX does not use Runlevel 5 for a GUI. In AIX, if a Graphical User Interface (like CDE or GNOME) is installed, it is simply a service started within Runlevel 2.