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+Basic invocation: `nix-shell -p [ pkgs ... ]`
+
+# Shebang
+```
+#! /usr/bin/env nix-shell
+#! nix-shell -i bash -p bash
+
+do stuff
+```
+
+Different interpreters can be set with the `-i` option such as python
+```
+#! /usr/bin/env nix-shell
+#! nix-shell -i python3 -p python3
+
+print("hello world")
+```
+
+The `-I` option can be used to pin nixpkgs
+```
+#! /usr/bin/env nix-shell
+#! nix-shell -i bash
+#! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/aed4b19d312525ae7ca9bceb4e1efe3357d0e2eb.tar.gz
+
+echo hello world
+```
+# Shell.nix
+Can be used to set up per dir environments, e.g using direnv to automatically activate them.
+## Python
+https://nixos.org/manual/nixpkgs/stable/#python
+
+```nix
+{ pkgs ? import <nixpkgs> {}}:
+
+pkgs.mkShell {
+    packages = [ pkgs.virtualenv ];
+}
+```
+
+```nix
+with import <nixpkgs> {};
+let
+	my_toolz = python311.pkgs.buildPythonPackage rec { 
+		pname = "toolz";
+		version = "0.10.0";
+		pyproject = true;
+		src = fetchPypi {
+			inherit pname version;
+			hash = "sha256-CP3V73yWSArRHBLUct4hrNMjWZlvaaUlkpm1QP66RWA=";
+		};
+		nativeBuildInputs = [
+			python311.pkgs.setuptools
+			python311.pkgs.wheel
+		]; # has no tests
+		doCheck = false;
+		meta = {
+			homepage = "https://github.com/pytoolz/toolz/";
+			description = "List processing tools and functional utilities";
+			# [...]
+		};
+	};
+in python311.withPackages (ps: with ps; [ numpy my_toolz ]) ).env
+```
+
+					The [[Import]] is required here because imports a nix expression from another source, in this case nixpkgs.
+It is not required at line two, because python310Packages itself is already in the local scope.
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