Add a file to the nix store (permanently)

Add a file to the store

The fancy new nix 3 command line has the store subcommand which lets you manipulate the store. nix store --help reveals that there are the following subcommands:

· nix store add-file - add a regular file to the Nix store
· nix store add-path - add a path to the Nix store

Surely one of these is what we want, right? Maybe! For reasons I don’t really understand these two commands are missing some of the features the older command had… For that reason the best tool to use (in my experience) is nix-prefetch-url. The man pages say about everything you want, here’s what I find useful.

• It is meant to be run with urls, local files will have a url structure like file:/absolute/path/to/file.zip
• The name of the prefetched file has to match the name of the file in your derivation, not just the hash. This took me a hot minute to figure out. You can force the name of the imported file to be different than the name of the original file with the --name option.
• If the file is an archive you probably want the --unpack flag, this will make it compatible with fetchzip and the like.
• To get the path of the file in the store (which you will need to register a GC root) you need the --print-path flag.

Keep that file safe

Great! We got a file into the store, problem: this file has nothing pointing to it, the nix garbage collector is liable to delete it at a moments notice. To stop this we need to register a GC root. As far as I can tell there are no commands in place to do this for you. Let alone do it atomically¹. Turns out a gc root is just a symlink in /nix/var/nix/gcroots pointing into the nix store. So this is as easy as:

ln -st /nix/var/nix/gcroots /nix/store/<path>

A worked example

In this example I am pushing a zip file of the lovely butterick fonts into the nix store so I can reference that zip file to install the fonts. First we push the file into the nix store.

$ nix-prefetch-url \
	--unpack \
	--print-path \
	file:/home/ethan/downloads/butterick

1j62qgsvyyza2yzam5rzalky7lgdyxj9gngrzyz0zpqkrnwpz5cx
/nix/store/sb9856q6l3yhzykh83m39rrlsmrhxklz-butterick.zip

Then we register the GC root

$ sudo ln -st \
	/nix/var/nix/gcroots \
	/nix/store/sb9856q6l3yhzykh83m39rrlsmrhxklz-butterick.zip

I like the SRI hash format used with most nix derivations for some reason… so we convert the hash from the prefetch step to SRI using nix-hash.

$ nix-hash \
	--to-sri \
	--type sha256 \
	1j62qgsvyyza2yzam5rzalky7lgdyxj9gngrzyz0zpqkrnwpz5cx

sha256-nZV/uc0T3w++//nZl2T37dHjJ1U/l6q+F+p7v/XDwsg=

Finally we can write a nix expression to reference this file.

fetchzip {
    url = " ";
    # name has to match the file name...
    name = "butterick.zip"; 
    hash = "sha256-nZV/uc0T3w++//nZl2T37dHjJ1U/l6q+F+p7v/XDwsg=";
}