'client role' active probing selective channel anytime ?

[sevesdion]

Greetings!

My hostAP works as client role now, some problems confuse me these days.

(in these case, hostAP acts as client role.)
1. How can i do active probe anytime?
ie. at command line, what command can i use for active probing.

2. Can i active probe optional channel?
ie. my favorite channel is 1,6, and  11. 
Each time, when i want to do probe, i only scan channel 1, 6 , and 11.
Furthermore, i can just scan channel 1, in order to save probing delay.
how can i do that?

Any help would be apprieciated! 
Thanks,
sevesdion. J.