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A tar file doesn't have a central directory like a zip, so you do have to search all the file headers. However, each file header contains the length of the file it describes, which lets you seek past the content of any files you don't care about if the tar isn't stored in a way that would prevent seeking.

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That would work if your tar was uncompressed. The lengths in the tar are meaningless in a compressed file.

That's not true, xz allows efficient decompression of arbitrary byte ranges inside the file.

Does it let you seek by a decompressed byte count, or or does it let you decode arbitrary compressed byte ranges?

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