Nix Tutorial 0 : Testing Nix Source Code from Tutorials - wimsio/universities GitHub Wiki

Learn how to try out Nix code—interactively, with files, online, and more! This guide is hands-on, practical, and works for any code sample you find.

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1. Using the Nix REPL
2. Testing Code in Files (nix eval & nix-instantiate)
3. Playing Online: Nix Playground
4. Building Derivations and Packages
5. Testing Nix Shells and Environments
6. Pro Tips
7. Summary Table
8. Glossary
9. Need More Help?

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The Nix REPL is the fastest way to test Nix expressions, functions, and data.

• Start the REPL:

  nix repl

• Try expressions:

  nix-repl> let x = 2; in x + 3
  5

• Check types or builtins:

  nix-repl> builtins.isList [1 2 3]
  true

Use the REPL for:

• Exploring syntax
• Quick checks
• Understanding built-in functions

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For anything more than a quick one-liner (functions, sets, derivations), write it in a .nix file and evaluate it.

1. Create test.nix:

   let x = 10; in x * 2

2. Run:

   nix-instantiate --eval test.nix
   # Output: 20

   Add --json for JSON output, or --strict for strict evaluation.

• Evaluate a file:

  nix eval --file test.nix

• Run a one-liner:

  nix eval --expr 'let a = 5; in a * 3'
  # Output: 15

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• Visit https://play.nix.dev/
• Paste code from any tutorial
• Click "Evaluate" and view results instantly

No install needed—perfect for beginners or sharing code!

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If your tutorial uses derivation or stdenv.mkDerivation:

1. Create hello.nix:

   derivation {
     name = "hello";
     system = builtins.currentSystem;
     builder = "/bin/sh";
     args = [ "-c" "echo Hello > $out" ];
   }

2. Build:

   nix-build hello.nix
   # Symlink `result` points to the build output

3. Check output:

   cat result
   # Should print "Hello"

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For shell.nix or flake.nix environments:

1. Create shell.nix:

   { pkgs ? import <nixpkgs> {} }:
   pkgs.mkShell {
     buildInputs = [ pkgs.curl pkgs.git ];
   }

2. Enter the shell:

   nix-shell
   # curl and git are now available in your shell!

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• Run nix repl '<nixpkgs>' for full access to the Nix Packages library.
• Use import ./file.nix to load other files in REPL or files.
• Use Nix Pills for step-by-step guided practice.
• Try multi-line and let ... in ... everywhere, including in the REPL.

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Task	Tool/Command	Best For
Quick checks / eval	nix repl	Explore expressions
Evaluate file	nix eval --file test.nix	File-based code
Run one-liner	nix eval --expr 'expr'	Fast checks
Build derivation	nix-build file.nix	Build outputs
Online playground	play.nix.dev	No local install needed
Test dev env / shell	nix-shell (with shell.nix)	Custom dev environments

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Term	Meaning
REPL	Interactive Read-Eval-Print Loop—try out code live
Derivation	A build recipe for the Nix store (result of derivation or mkDerivation)
Nix Shell	Temporary environment with specific packages (using shell.nix)
nix-instantiate	Evaluates Nix expressions, mainly for legacy/testing
nix eval	Modern tool for evaluating Nix expressions or files
Nixpkgs	The official package set for Nix
Flake	Standardized project definition file (experimental feature)

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• Try code at play.nix.dev.
• Have a tutorial snippet you’re stuck on? 👉 Paste it here and I’ll walk you through testing it—step by step!
• See also: Nix Manual, NixOS Wiki

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