Nix-shell shebang: Difference between revisions

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imported>JakeHillion
→‎Python: Had no luck running this and [this](https://discourse.nixos.org/t/nix-shell-shebang-default-nix-no-such-file-or-directory/24246) suggested you need at least one package. Added `-p python3` and it worked.
imported>Milahu
add link: Nix Flakes, Part 2: Evaluation caching - Tweag
Line 117: Line 117:
* [https://discourse.nixos.org/t/speeding-up-nix-shell-shebang/4048 Speeding up nix-shell shebang]
* [https://discourse.nixos.org/t/speeding-up-nix-shell-shebang/4048 Speeding up nix-shell shebang]
* [https://github.com/xzfc/cached-nix-shell cached-nix-shell] - Instant startup time for nix-shell
* [https://github.com/xzfc/cached-nix-shell cached-nix-shell] - Instant startup time for nix-shell
* [https://www.tweag.io/blog/2020-06-25-eval-cache/ Nix Flakes, Part 2: Evaluation caching - Tweag]


== See also ==
== See also ==

Revision as of 20:10, 25 June 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-channels/archive/nixos-18.03.tar.gz

echo hello world

Performance

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

See also