Nix{,OS,pkgs} bag of tricks

Last updated at 2023-06-23.

This is my personal collection of somewhat unknown, but occasionally useful, features of the Nix ecosystem, in no particular order.

Nix language

• __curPos evaluates to an attrset containing the current file and line/column. It is handled directly in the parser.
• builtins.unsafeGetAttrPos name attrset returns position information for a specified attribute in an attrset. Both of these should be used with care, as the same expression at a different position will return different values.

Nixpkgs

• Setting the NIX_DEBUG environment variable in a derivation enables extra logging output during the build. Setting it to 6 or higher enables set -x: nix-build -E '(import <nixpkgs> {}).hello.overrideAttrs (_: { NIX_DEBUG = 6; })'
• The module system that backs NixOS can be used standalone, without any predefined modules. This is also how the Nixpkgs's config is implemented: nix-repl> x = lib.evalModules { modules = [{ options.foo = lib.mkOption { type = lib.types.str; }; } { foo = "bar"; } ]; } nix-repl> x.config.foo "bar"
• If you have a callPackage-style out-of-tree package, you can build it with the script maintainers/scripts/nix-call-package from the nixpkgs source tree. Note: This script will build your package by importing <nixpkgs>, and not by importing the source tree in which the script is stored! $ cat foo.nix { runCommand }: runCommand "foo" {} "echo hello world > $out" $ ~/src/nixpkgs/maintainers/scripts/nix-call-package ./foo.nix warning: you did not specify '--add-root'; the result might be removed by the garbage collector warning: you did not specify '--add-root'; the result might be removed by the garbage collector /nix/store/gnr862zavclmh273w4h1h6v78b6jaix3-foo
• nix-shell -p, without any further arguments, is a quick and dirty way to get into a development shell with a properly configured C compiler etc. available.

NixOS

• /etc/hosts management:
  • networking.hosts lets you add to your hosts file
  • environment.etc.hosts.mode = "0644"; makes your hosts file writable
• systemd.tmpfiles.rules lets you declaratively create file system structures (e.g. directories, symlinks, etc.). Adding a rule without an expiration date removes the "temporary" aspect of tmpfiles.