Nix-shell shebang

You can use nix-shell as a script interpreter to

• run scripts in arbitrary languages
• provide dependencies with Nix

To do this, start the script with multiple shebang (#!) lines.
The first shebang line is always #! /usr/bin/env nix-shell.
The second shebang line declares the script language and the script dependencies.

As of Nix 2.19.0 you can also use the new CLI nix shell and flakes to define shebangs. See docs.

Examples

Bash

To run bash scripts, set the interpreter with -i bash

#! /usr/bin/env nix-shell
#! nix-shell -i bash -p bash

echo hello world

You can use nix-shell -p ... to add dependencies:

#! /usr/bin/env nix-shell
#! nix-shell -i bash -p imagemagick cowsay

# scale image by 50%
convert "$1" -scale 50% "$1.s50.jpg" &&
cowsay "done $1.q50.jpg"

Python

#! /usr/bin/env nix-shell
#! nix-shell -i python3 -p python3

print("hello world")

#! /usr/bin/env nix-shell
#! nix-shell -i python3 -p python3Packages.pillow python3Packages.ansicolor

# scale image by 50%
import sys, PIL.Image, ansicolor
path = sys.argv[1]
image = PIL.Image.open(path)
factor = 0.5
image = image.resize((round(image.width * factor), round(image.height * factor)))
path = path + ".s50.jpg"
image.save(path)
print(ansicolor.green(f"done {path}"))

Rust

No dependencies

#!/usr/bin/env nix-shell
#![allow()] /*
#!nix-shell -i bash -p rustc
rsfile="$(readlink -f $0)"
binfile="/tmp/$(basename "$rsfile").bin"
rustc "$rsfile" -o "$binfile" --edition=2021 && exec "$binfile" $@ || exit $?
*/
fn main() {
    for argument in std::env::args().skip(1) {
        println!("{}", argument);
    };
    println!("{}", std::env::var("HOME").expect(""));
}

With dependencies

uses rust-script

#!/usr/bin/env nix-shell
//! ```cargo
//! [dependencies]
//! time = "0.1.25"
//! ```
/*
#!nix-shell -i rust-script -p rustc -p rust-script -p cargo
*/
fn main() {
    for argument in std::env::args().skip(1) {
        println!("{}", argument);
    };
    println!("{}", std::env::var("HOME").expect(""));
    println!("{}", time::now().rfc822z());
}

Haskell

#! /usr/bin/env nix-shell
#! nix-shell -p "haskellPackages.ghcWithPackages (p: with p; [turtle])" -i runghc

{-# LANGUAGE OverloadedStrings #-}

import Turtle

main = echo "Hello world!"

Pinning nixpkgs

To pin nixpkgs to a specific version, add a third shebang line:

#! /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

Flake

It is also possible to make it work for flake like in:

#!/usr/bin/env -S nix shell nixpkgs#bash nixpkgs#hello nixpkgs#cowsay --command bash

hello | cowsay

The doc mentions that it should be possible to run more complex commands using multiple lines, but it does not work for me as reported here.

Performance

TODO ... why the startup delay? how to make it faster?

• Speeding up nix-shell shebang
• cached-nix-shell - Instant startup time for nix-shell
• Nix Flakes, Part 2: Evaluation caching - Tweag

See also

• nix-shell: Use As a #!-Interpreter in nix manual
• nix-shell: Use As a #!-Interpreter man page
• nix-shell and Shebang Lines
• Spice up with Nix: Scripts with magical dependencies