The CHDK will be loaded on the SD card. I wish to start the camera through terminal commands , the camera should boot up into CHDK and run the script that has been loaded into the SD card.
You cannot power on the camera over USB, when the camera is off, it cannot receive USB commands.
I few cameras including the Powershot N, N Facebook edition and some other very old cameras will power on when power is applied to the USB connector. To use this to trigger power on from another device would require software control of the USB ports +5V line. This is not a standard feature, but should be possible with some DIY hardware.
For other cameras, you would need to do something with the physical power button. A common approach is to physically hold down the power button, and then use an externally controlled power supply to power on the camera. Most (if not all) cameras will run normally while the power button is held down. Too power off, you should shut down from script and then turn off the external power.
You can configure a CHDK script to auto run at boot in the script menu. If you have USB connection, you can also run scripts from the host using the CHDK PTP extension:
http://chdk.wikia.com/wiki/PTP_Extension