###How to enable a service
As an example, to enable Apache web server in Debian, do the following -
update-rc.d apache2 defaults
... this will enable the Apache web server to start in the default run levels of 2,3,4 and 5. Of course, you can do it explicitly by giving the run levels instead of the defaults keyword as follows:
update-rc.d apache2 start 20 2 3 4 5 . stop 80 0 1 6 .
The above command modifies the sym-links in the respective /etc/rcX.d directories to start or stop the service in the destined runlevels. Here X stands for a value of 0 to 6 depending on the runlevel. One thing to note here is the dot (.) which is used to terminate the set which is important. Also 20 and 80 are the sequence codes which decides in what order of precedence the scripts in the /etc/init.d/ directory should be started or stopped.
To enable the service only in runlevel 5, you do this instead -
update-rc.d apache2 start 20 5 . stop 80 0 1 2 3 4 6 .
###How to disable a service
To disable the service in all the run levels, you execute the command:
update-rc.d -f apache2 remove
Here -f option which stands for force is mandatory.