screengrab and video to GIF

# replace everything in UPPERCASE with appropriate values

# grab the screen in Linux:
ffmpeg -f x11grab -video_size WIDTHxHEIGHT -i :0.0+LEFT,TOP -t LENGTH -c:v libx264 -preset ultrafast -qp 0 -y OUT.mp4

# create an optimal GIF palette for all frames:
ffmpeg -i OUT.mp4 -vf palettegen -y PALETTE.png

# convert to GIF, using the palette:
ffmpeg -i OUT.mp4 -i PALETTE.png -filter_complex "[0:v][1:v]paletteuse=dither=none" -y OUT.gif