How do I see the download size of an update before downloading?

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1

I have a metered data connection so I must conserve bandwidth and consider what to download carefully.

After updating my flake.lock, sometimes I must download a lot of stuff to complete the update. Rather than finding out after-the-fact, I want to see the required download size before spending much bandwidth. Is there a nixos-rebuild option for this or any other way to find out?

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2

There’s dry-run but it doesn’t have size info.

3

Yeah, good point. Do you know if there’s any other data I could cross-reference those paths with to find out the size of each item?

4

--log-format bar or --log-format bar-with-logs display expected download size in the bar while building. this flag is supported by nixos-rebuild too

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5

Fantastic! For reference, it looks like this:

$ nixos-rebuild dry-build --log-format bar-with-logs
building the system configuration...
these 36 derivations will be built:
  /nix/store/dm96z1v95a96vkbvrlg7g32bf853n3cr-system-path.drv
#
# -- snip --
#
these 4 paths will be fetched (34.04 MiB download, 135.16 MiB unpacked):
  /nix/store/477fy383kb2w6dck1zv1g7j18d1hblvi-fractal-10.1
  /nix/store/746xbcfs67iw6qpm0k226k28nb9iflf7-imagemagick-7.1.1-47
  /nix/store/sia7mkbc7hqwl271fcagpa7w91pk6102-kitty-0.41.1
  /nix/store/pbqh8sgahb71w0vs9jlks8gs3bjy22mg-kitty-0.41.1-terminfo
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