About ScreenSELect
This tool allows you to select a rectangle on your screen which then can be used to record that portion of the screen via ffmpeg (x11grab) using the command
ffmpeg -f x11grab -framerate 25 $(./ssel) $(date+%Y-%m-%d_%H-%M_%S).mp4
You can compile the program using
gcc -Wall -lX11 ssel.c -o ssel
Credits
The source code is from this forum post. I just commented the code that grabs the keyboard.