Nix-shell shebang: Difference between revisions

Revision as of 23:48, 4 July 2023

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 -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());
}

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

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