"failed with unknown exit code"

[William Carroll]

> nix-env -iA nixos.nix-index
> nix-index
+ querying available packages
error: querying available packages failed
caused by: nix-env failed with error: nix-env failed with unknown exit code
> echo $status
2

[Bob]

Bump, I got this too.

[Lin Hy]

out of mem maybe

[Alex Vorobiev]

I got this message as well. It was running out of memory.