[CentOS] Semi-OT: configuring mongodb for sharding

Anyone know about this? Googling, all I can find is mongodb's 3.x manual,
nothing for the 2.4 we get from epel.

What I need to do, CentOS 6.6, is start it as a service, not a user, and
have it do sharding. I see examples of how to start it as a user... but I
can't find if there's a syntax for /etc/mongodb.conf to tell it that, and
I don't want to have to edit /etc/init.d/mongod....

Clues for the poor?

       mark