niff

niff is a script that compares two Nix expressions and determines which attributes changed.

• niff diff compares expr to --against
• niff path returns a local path to expr
• niff pr determines a diff between the head of the PR and the base of the PR.

Furthermore, diff and pr have an --output argument for controlling the format of the output:

• --output=list is the default and produces a list of attribute names
• --output=attrs produces a single string that prepends -A for each attribute name, thereby making it easy to use for nix-build

Examples

diff

Check one remote archive against another:

niff diff https://github.com/NixOS/nixpkgs/archive/master.zip https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz

Check a PR against a remote archive (note that the hashtag needs to be escaped):

niff diff pr://28167 https://github.com/NixOS/nixpkgs/archive/master.zip

Check a PR against a local expression:

niff diff pr://28167 ../nixpkgs

Instead of generating a list of attributes its possible to output them together with -A

niff diff pr://28167 ../nixpkgs` --output attrs

pr

To check a PR (note that with niff pr the scheme is not necessary):

niff pr 28167

path

To get a local path to the HEAD of a PR:

niff path pr://28167

To get a local path to a NIX_PATH prefix:

niff path nixpath://nixpkgs

How does it work?

1. Fetch archives with nix-prefetch-url --unpack and keep the store path, or use a given path to local Nix expression
2. Run nix-env -qaP to determine a list of attributes that are provided
3. Compare lists of each expression and output

Arguments

Valid arguments are

• an url, e.g. https://https://github.com/NixOS/nixpkgs/archive/master.zip.
• a local or absolute path, e.g. ../path/to/nixpkgs. Maps to file:///abs/path/to/nixpkgs.

Available schemes

General schemes.

Scheme	Example	Explanation
file://	file:///abs/path/to/expression	Local expression or archive
https://	https://github.com/NixOS/nixpkgs/archive/master.zip	Remote archive
nixpath://	nixpath://nixpkgs	NIX_PATH prefix
github://	github://org/repo/ref	GitHub reference

Nixpkgs-specific.

Scheme	Example	Explanation
channel://	channel:///nixos-unstable	Nixpkgs channel
ref://	ref://staging	Reference in nixpkgs repo
pr://	pr://28032	Nixpkgs pull request