Rotate An Image With ImageMagick

bash

# white (or possibly black) background
magick mogrify -rotate 45 a.png

# transparent background
magick mogrify -background 'rgba(0,0,0,0)' -rotate 45 a.png

# black background
magick mogrify -background 'rgb(0,0,0)' -rotate 45 a.png

Rotates the image 45 degrees clockwise
Rotates the image in place (i.e. it updates the file)
Expands the canvas to cover the points which extend out on the rotate
Puts a white background on area that is expanded