This first video is a recording of Country Joe and the Fish singing Feel-Like-I'm-Fixing-To-Die-Rag. I couldn't find a video of the entire band performing this wonderful song together but the pictures from the time are quite good. Most importantly, you feel the rhythm a lot more with the Fish chiming in.
This next video is of Country Joe singing solo at Woodstock.
The first show day, Friday, found most performers either trapped at their hotel in a surrounding village, or trying to get to the stage area. Joe, who had come down to see what was going on, happened to be onstage at the exact moment Richie Havens was finishing his set. A guitar was found, a set was organized and after four or five songs, he decided to "do the Rag," which he had intended to perform with the band later in the festival.
The cheer, "Gimme an F," "Gimme an I," had by that time evolved into, "Gimme an F," "Gimme an U," etc.