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I'm taking a dd of a drive and trying to share it with a friend. It's rather big after dd (~8gb). When I tar -czvf my-image.img I can get it down to ~1.5gb. However, if I try to test unzip it, I see that the shasum hash of the unzipped file is different from the file I had originally before zip. Is there a way I can "lossless" zip a file?

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    You're already assuming that tar/gzip is normally lossy. No, that's not normal.
    – grawity_u1686
    Commented Aug 6, 2021 at 22:42
  • I don't understand. Are you saying there's no way to tar/gzip a file and expect it to be identical on unzip?
    – gh0st
    Commented Aug 6, 2021 at 22:47
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    We're saying what you described should compress losslessly in the first place. You're assuming it's lossy where by design it is not. In fact I haven't even heard of a way to make tar+gzip lossy on demand. Please edit and provide the exact commands you used to read, compress, copy, uncompress, compute hashes etc.; all relevant commands. The best we have for now is tar -czvf my-image.img and it's not even a complete command that would work.
    – Kamil Maciorowski
    Commented Aug 6, 2021 at 23:37
  • Compressing a data file is not at all the same thing as compressing a digital audio or video file. This is what the others are trying to say, because it seems you're making the error of conflating the two very different things.
    – music2myear
    Commented Aug 7, 2021 at 3:44

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As others have stated zip (and gzip & which is what you are actually using) are lossless. If the 2 files are not identical its because of corruption that should not be occurring or, more likely you gave implemented the command wrong.

The way you are attempting this is sub-optimal, the command you are looking for should be

  tar -czvf my-image.tar.gz my-image.img

And to decompress

  tar -zxf my-image.tar.gz

Of-course, the command you are using is designed to work to package and compress multiple files. As you only have a single file you dont need to use tar at all, you should just use gzip -

 gzip my-image.img

and to decompress

gunzip my-image.img.gz
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