如何通过命令行将图像转换为灰度？

use one of: -monochrome or -colorspace gray options for imagemagick (convert).

If you have imagemagick installed,

convert source.jpg -colorspace Gray destination.jpg (true grayscale only)
convert source.jpg -monochrome destination.jpg (true black and white)
convert source.jpg -separate destination.jpg (separate into gray channels)

If you don't care about losing the original file: mogrify -colorspace Gray file.