System V Init refers to the way the start up and shutdown of an
operating system (
mostly linux and such right now. correct me if i'm wrong) and it's services (
daemons) is handled. The flexibility and control are obtained by the use of
runlevels - different sets of
configurations.
The runlevel config is typically specified in
/etc/rc.d/rc0.d/ (
/etc/rc.d/rc1.d/, /etc/rc.d/rc2.d/ etc) directory where rc0.d is replaced by any runlevel number (ranging from 0-6). The files in these
directories have the following format:
lrwxrwxrwx 1 root root 15 Jan 9 2000 K45named -> ../init.d/named
lrwxrwxrwx 1 root root 16 Jan 9 2000 S55routed -> ../init.d/routed
Every entry is in fact, a
symbolic link to a respective
start up script.
Each file beginning with "S" will envoke the corresponding script with a "start" command (eg: ../init.d/routed start from the previous example). On the other hand "K" entries will send the
kill command ( or stop).
- To remove a service a from runlevel, remove a corresponding symlink; or alternatively rename the symlink to something else (like making S55routed into s55routed - thanks to say.).
- To add a service to a runlevel, add a symlink to an appropriate start up script.
- To change the current runlevel, use init run_level_number
- To change the default boottime runlevel, look in /etc/inittab
List of Runlevels
from Red Hat 7 /etc/inittab
0 -
halt (Do NOT set initdefault to this)
1 -
Single user mode
2 -
Multiuser, without
NFS (The same as 3, if you do not have networking)
3 -
Full multiuser mode
4 - unused
5 -
X11
6 -
reboot (Do NOT set initdefault to this)
Note 1: Credits go to melodrame
Note 2: Based on penguinmagazine.com