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I also had to specify a command line option -mobi-keep-original-images, since I wanted to convert epub to mobi format using: ebook-convert ~/test.epub ~/test.mobi -mobi-keep-original-images
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Installing some missing python modules (which you figure out by running the convert command ebook-convert inputfile outputfile), in my case: python3-msgpack
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The steps involved extracting these directories from the calibre package (deb, rpm, whatever): /usr/bin/ebook-convert I've managed to cut out ebook-convert from Calibre (which, for who knows what reasons, requires Qt for image manipulation) and got a command-line only interface for it, to be able to use the tool on a headless/server machine.
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