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