EXPLAIN: What are GNU/Linux Runlevels?
When GNU/Linux starts, it doesn’t just power up and go. It actually runs through a series of runlevels until it gets to a user specified runlevel and then stops. The different runlevels are used to allow services to start and stop, and to generally control exactly how the box is booting. Most of this is transparent to the end user. Generally, you can just turn on a GNU/Linux box and walk away. It will run through the runlevels all by itself.
Depending on who you listen to there are 6, 8, or 10 runlevels available in GNU/Linux. However, only 0-6 are used consistently between distributions.
The two run levels that are mostly used are 3 and 5. When a terminal boots up to runlevel 3 it is running in text mode (the black screen with the dreaded login prompt). When a terminal boots up to runlevel 5, it is running in full graphical mode (with a GUI desktop like KDE, Gnome, or Fluxbox running).
The runlevels are:
Runlevel 0: Halt System - To shutdown the system
Runlevel 1: Single user mode
Runlevel 2: Basic multi user mode without NFS
Runlevel 3: Full multi user mode (text based)
Runlevel 4: unused
Runlevel 5: Multi user mode with Graphical User Interface
Runlevel 6: Reboot System
I always liked runlevel 6. When I reboot the system it doesn’t just shut down, it actually enters runlevel 6.
Neat!
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