What's the difference between chkconfig on vs chkconfig --add?

chkconfig --add adds a new service to the list of services managed by chkconfig. I believe --add is an implicit on.

chkconfig on sets the runlevel for the service.

That said, I tend to use chkconfig -add for adding new services to a system, but ntsysv to manage them (on RHEL-like systems).

In Redhat/CentOS, there is one line in the init script which looks like

# chkconfig: - 65 10

If you use --add when the first arg of above line is "-", this adds no start links, only the kill links. So for --add to work, you have to edit the init script and change to e.g.

# chkconfig: 345 65 10

But if chkconfig --add ... was executed with the "-" as first arg (which is mostly the case from rpm post script): Changing afterwards the first arg and reexecute chkconfig --add has no effect as long as the kill links are present. In this case --list shows off for all runlevel:

chkconfig --list saslauthd
saslauthd       0:off   1:off   2:off   3:off    4:off    5:off    6:off

To make --add work in this case, you first have to use --del, then the output of --list is:

chkconfig --list saslauthd
service saslauthd supports chkconfig, but is not referenced in any runlevel (run 'chkconfig --add saslauthd')

Now you can use --add and it has the desired effect.

Alternatively you can use "on" with the --level argument to avoid the need of editing the init script and to avoid first --del followed by --add.