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Is it possible to download a file nested in a zip file, without downloading the entire zip file?

Is it possible to download a file nested in a zip file, without downloading the entire zip archive?

For example from a url that could look like:

https://www.any.com/zipfile.zip?dir1\dir2\ZippedFileName.txt
Solution

  • Depending on if you are asking whether there is a simple way of implementing this on the server-side or a way of using standard protocols so you can do it from the client-side, there are different answers:

    Doing it with the server's intentional support

    Optimally, you implement a handler on the server that accepts a query string to any file download similar to your suggestion (I would however include a variable name, example: ?download_partial=dir1/dir2/file ). Then the server can just extract the file from the ZIP archive and serve just that (maybe via a compressed stream if the file is large).

    If this is the path you are going and you update the question with the technology used on the server, someone may be able to answer with suggested code.

    But on with the slightly more fun way...

    Doing it opportunistically if the server cooperates a little

    There are two things that conspire to make this a bit feasible, but only worth it if the ZIP file is massive in comparison to the file you want from it.

    1. ZIP files have a directory that says where in the archive each file is. This directory is present at the end of the archive.

    2. HTTP servers optionally allow download of only a range of a response.

    So, if we issue a HEAD request for the URL of the ZIP file: HEAD /path/file.zip we may get back a header Accept-Ranges: bytes and a header Content-Length that tells us the length of the ZIP file. If we have those then we can issue a GET request with the header (for example) Range: bytes=1000000-1024000 which would give us part of the file.

    The directory of files is towards the end of the archive, so if we request a reasonable block from the end of the file then we will likely get the central directory included. We then look up the file we want, and know where it is located in the large ZIP file.

    We can then request just that range from the server, and decompress the result...