In ImageMagick's compare command, the NULL: output is required to suppress the default output of the difference image. However, if you only want the metric (like AE, Absolute Error) and don't need the visual difference image, you can redirect the output to /dev/null (on Unix-like systems) or NUL (on Windows).
bashcompare -metric AE 0.png 1.png /dev/null
bashcompare -metric AE 0.png 1.png NUL
This way, the difference image is discarded, and only the metric is printed to the console. The NULL: is specific to ImageMagick's internal syntax, but using the system's null device achieves the same result.
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