9.3. Run levels

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. Some system administrators use run levels to define which subsystems are working, e.g., whether X is running, whether the network is operational, and so on. Others have all subsystems always running or start and stop them individually, without changing run levels, since run levels are too coarse for controlling their systems. You need to decide for yourself, but it might be easiest to follow the way your Linux distribution does things.

The following table defines how most Linux Distributions define the different run levels. However, run-levels 2 through 5 can be modified to suit your own tastes.

Table 9-1. Run level numbers

0Halt the system.
1Single-user mode (for special administration).
2Local Multiuser with Networking but without network service (like NFS)
3Full Multiuser with Networking
4Not Used
5Full Multiuser with Networking and X Windows(GUI)
6Reboot.

Services that get started at a certain runtime are determined by the contents of the various rcN.d directories. Most distributions locate these directories either at /etc/init.d/rcN.d or /etc/rcN.d. (Replace the N with the run-level number.)

In each run-level you will find a series of if links pointing to start-up scripts located in /etc/init.d. The names of these links all start as either K or S, followed by a number. If the name of the link starts with an S, then that indicates the service will be started when you go into that run level. If the name of the link starts with a K, the service will be killed (if running).

The number following the K or S indicates the order the scripts will be run. Here is a sample of what an /etc/init.d/rc3.d may look like.
# ls -l /etc/init.d/rc3.d lrwxrwxrwx 1 root root 10 2004-11-29 22:09 K12nfsboot -> ../nfsboot lrwxrwxrwx 1 root root 6 2005-03-29 13:42 K15xdm -> ../xdm lrwxrwxrwx 1 root root 9 2004-11-29 22:08 S01pcmcia -> ../pcmcia lrwxrwxrwx 1 root root 9 2004-11-29 22:06 S01random -> ../random lrwxrwxrwx 1 root root 11 2005-03-01 11:56 S02firewall -> ../firewall lrwxrwxrwx 1 root root 10 2004-11-29 22:34 S05network -> ../network lrwxrwxrwx 1 root root 9 2004-11-29 22:07 S06syslog -> ../syslog lrwxrwxrwx 1 root root 10 2004-11-29 22:09 S08portmap -> ../portmap lrwxrwxrwx 1 root root 9 2004-11-29 22:07 S08resmgr -> ../resmgr lrwxrwxrwx 1 root root 6 2004-11-29 22:09 S10nfs -> ../nfs lrwxrwxrwx 1 root root 12 2004-11-29 22:40 S12alsasound -> ../alsasound lrwxrwxrwx 1 root root 8 2004-11-29 22:09 S12fbset -> ../fbset lrwxrwxrwx 1 root root 7 2004-11-29 22:10 S12sshd -> ../sshd lrwxrwxrwx 1 root root 8 2005-02-01 09:24 S12xntpd -> ../xntpd lrwxrwxrwx 1 root root 7 2004-12-02 20:34 S13cups -> ../cups lrwxrwxrwx 1 root root 6 2004-11-29 22:09 S13kbd -> ../kbd lrwxrwxrwx 1 root root 13 2004-11-29 22:10 S13powersaved -> ../powersaved lrwxrwxrwx 1 root root 9 2004-11-29 22:09 S14hwscan -> ../hwscan lrwxrwxrwx 1 root root 7 2004-11-29 22:10 S14nscd -> ../nscd lrwxrwxrwx 1 root root 10 2004-11-29 22:10 S14postfix -> ../postfix lrwxrwxrwx 1 root root 6 2005-02-04 13:27 S14smb -> ../smb lrwxrwxrwx 1 root root 7 2004-11-29 22:10 S15cron -> ../cron lrwxrwxrwx 1 root root 8 2004-12-22 20:35 S15smbfs -> ../smbfs 

How run levels start are configured in /etc/inittab by lines like the following:
l2:2:wait:/etc/init.d/rc 2
The first field is an arbitrary label, the second one means that this applies for run level 2. The third field means that init should run the command in the fourth field once, when the run level is entered, and that init should wait for it to complete. The /etc/init.d/rc command runs whatever commands are necessary to start and stop services to enter run level 2.

The command in the fourth field does all the hard work of setting up a run level. It starts services that aren't already running, and stops services that shouldn't be running in the new run level any more. Exactly what the command is, and how run levels are configured, depends on the Linux distribution.

When init starts, it looks for a line in /etc/inittab that specifies the default run level:
id:2:initdefault:
You can ask init to go to a non-default run level at startup by giving the kernel a command line argument of single or emergency. Kernel command line arguments can be given via LILO, for example. This allows you to choose the single user mode (run level 1).

While the system is running, the telinit command can change the run level. When the run level is changed, init runs the relevant command from /etc/inittab.