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If you want to create a flattened version from multi-layer Photoshop files with the help of ImageMagick use the following command:
convert image.psd image.png

Or for batch converting multiple images:
for i (*.psd) convert $i'' ${i%.*}.png

To refer to the other layers use the numbers greater than 0.

Note that the for loop above was tested with zsh, if ...

One fast way to generate grayscale images named *.grayscale.png from multiple *.png images with ImageMagick is to use the zsh command for i (*.png) convert -colorspace Gray $i ${i%.*}.grayscale.png. If you want to override the existing images use for i (*.png) convert -colorspace Gray $i $i instead.

The zsh option short_loops has to be set ...