imagemagick Utilities' Syntax and Examples in bash for commandline Image Conversion, Creation, and Parsing (https://imagemagick.org/Usage/)

bash-imagemagick.sh

# Create Multiscale ICO Image (filename.ico) from a given JPEG Image (filename.jpg)
convert -background transparent filename.jpg -define icon:auto-resize=16,24,32,48,64,72,96,128,256 filename.ico

# Convert MP4 Animated Video (animated.mp4) to Animated GIF (filename.gif)
convert filename.mp4 filename.gif

# Convert MP4 Animated Video (animated.mp4) to Animated GIF (filename.gif)
#   with size restriction to width of 300 [px] while preserving aspect-ratio
#   (Note that re-scaling may degrade quality)
convert -resize 300x filename.mp4 filename.gif

# Convert MP4 Animated Video (animated.mp4) to Animated GIF (filename.gif)
#   with size restriction to height of 240 [px] while preserving aspect-ratio
#   (Note that re-scaling may degrade quality)
convert -resize x240 filename.mp4 filename.gif

See more discussion and examples related to this gist on my personal website: https://www.tommypkeane.com/articles/bash/imagemagick.html