Tidy shell shebangs with nix flakes

nix-runner is a tool for writing shell scripts that behave predictably on any machine that has the nix package manager installed. It allows you to precisely specify your dependencies with magic comments at the top of your script and leverages nix shell to run your script with them on your PATH. Facility is made for allowing you to specify script-local registry pins, which allows you to pull all of your dependencies from a specific version of nixpkgs (and update the pin in a single place). It was written due to impatience in waiting for NixOS/nix#5189 or a similar feature to land in nix proper.

It is recommended that you use nix run to invoke a pinned version of the nix-runner command, based on a commit hash (honestly, I would just take the HEAD of main at the time when you write your script). This should make your scripts behave predicatably even if the default behavior of nix-runner changes in a subsequent version, which is somewhat likely to happen. Internally, the nix-runner command will perform another invocation of nix shell with its own internally-pinned version of the nix command.

#!/usr/bin/env -S nix run 'github:clhodapp/nix-runner/7b56158f7ab9fd7806068c6571833210e063df19' --
#!pure
#!registry nixpkgs github:NixOS/nixpkgs/0080a93cdf255b27e466116250b14b2bcd7b843b
#!package nixpkgs#bash
#!package nixpkgs#coreutils
#!package nixpkgs#jq
#!package nixpkgs#nix
#!command bash

readonly temp_file=$(mktemp)

echo '{"city": "San Francisco", "state": "CA"}' > "${temp_file}"

jq '.state' "${temp_file}"

nix-runner magic comments are consecutive lines starting with #!, from the second line of the script. A current limitation is that magic comments must be grouped by type and specified in a particular order. For convenience, the order in this README will match the order that the magic comments need to be in.

Unset the existing $PATH, resulting in the specified dependencies being the only thing on the $PATH when the script runs. Highly recommended if you want to make your script more machine-independent.

Define an extra --option flag to be passed to the nix shell command that actually runs your script. For reproducibiilty, this will be the version of nix that has been "pinned" within nix-runner so the set of available options may differ slightly from what's available ambiently on your system.

Define a script-local nix registry. This is most useful to allow you to share a single pin across many packages (e.g. pin a specific nixpkgs hash that you can update in one place).

Specify a package that you want to put on your $PATH when your script runs, specified as flake installable. You can (are encouraged to!) leverage registries specified with #!registry to achieve predictable versioning, though you are completely free to e.g. track an unstable branch of nixpkgs dynamically. It is highly recommended that you include the script runner (e.g. bash) in the list of specified packages, as it allows you to take control of the version that will be used to run your script.

Specify the name of the shell you want to use to run your script. It is expected that <command> will be a simple command name (e.g. bash), which will be looked up on the $PATH that the nix-runner command sets up. Although nix-runner is intended to be used to run shell scripts, it should be technically possible to use it with any runner command that is packaged for nix and uses # as its comment character.

At present, the nix-runner command is itself a simple shell script, which uses sed to process magic comments. This approach is kind of hacky, doesn't allow for good error messages, and creates the limitation on the ordering described above. In the future, it's possible that it will be rewritten in a general-purpose language to resolve these limitations.