The contents of my wpa_supplicant.conf is similar to this:
ctrl_interface=/var/run/wpa_supplicant
ctrl_interface_group=staff
update_config=1
network={
ssid="WPA-TKIP"
psk="hp5PwSRfJm40MevSJVLhGryiDQFDjIBcx"
proto=WPA
key_mgmt=WPA-PSK
pairwise=TKIP
disabled=1
id_str=""
}
network={
ssid="WPA-CCMP"
psk="YL0q1EfQPppNBHVBvhVNEMEaInNhXL"
proto=RSN
key_mgmt=WPA-PSK
pairwise=CCMP
priority=10
disabled=0
id_str="default"
}
network={
ssid="FREE"
key_mgmt=NONE
disabled=1
id_str=""
}
network={
key_mgmt=NONE
disabled=1
id_str="any"
}
Just set the disabled parameter to 0 of the particular network that you want to use.
You can use wpa_cli to manage this file. I haven't tried it though, I had only used wpa_gui which is the gui version on my debian setup.