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.

Examples

Bash

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

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

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}"))

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-channels/archive/nixos-18.03.tar.gz

echo hello world

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

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