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# pkgs.mkBinaryCache {#sec-pkgs-binary-cache}
`pkgs.mkBinaryCache` is a function for creating Nix flat-file binary caches.
Such a cache exists as a directory on disk, and can be used as a Nix substituter by passing `--substituter file:///path/to/cache` to Nix commands.
Nix packages are most commonly shared between machines using [HTTP, SSH, or S3](https://nixos.org/manual/nix/stable/package-management/sharing-packages.html), but a flat-file binary cache can still be useful in some situations.
For example, you can copy it directly to another machine, or make it available on a network file system.
It can also be a convenient way to make some Nix packages available inside a container via bind-mounting.
`mkBinaryCache` expects an argument with the `rootPaths` attribute.
`rootPaths` must be a list of derivations.
The transitive closure of these derivations' outputs will be copied into the cache.
::: {.note}
This function is meant for advanced use cases.
The more idiomatic way to work with flat-file binary caches is via the [nix-copy-closure](https://nixos.org/manual/nix/stable/command-ref/nix-copy-closure.html) command.
You may also want to consider [dockerTools](#sec-pkgs-dockerTools) for your containerization needs.
:::
[]{#sec-pkgs-binary-cache-example}
:::{.example #ex-mkbinarycache-copying-package-closure}
# Copying a package and its closure to another machine with `mkBinaryCache`
The following derivation will construct a flat-file binary cache containing the closure of `hello`.
```nix
{ mkBinaryCache, hello }:
mkBinaryCache {
rootPaths = [hello];
}
```
Build the cache on a machine.
Note that the command still builds the exact nix package above, but adds some boilerplate to build it directly from an expression.
```shellSession
$ nix-build -E 'let pkgs = import <nixpkgs> {}; in pkgs.callPackage ({ mkBinaryCache, hello }: mkBinaryCache { rootPaths = [hello]; }) {}'
/nix/store/azf7xay5xxdnia4h9fyjiv59wsjdxl0g-binary-cache
```
Copy the resulting directory to another machine, which we'll call `host2`:
```shellSession
$ scp result host2:/tmp/hello-cache
```
At this point, the cache can be used as a substituter when building derivations on `host2`:
```shellSession
$ nix-build -A hello '<nixpkgs>' \
--option require-sigs false \
--option trusted-substituters file:///tmp/hello-cache \
--option substituters file:///tmp/hello-cache
/nix/store/zhl06z4lrfrkw5rp0hnjjfrgsclzvxpm-hello-2.12.1
```
:::
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