It requires the programs `curl`, `xml2json`, and `jq`.
It also requires the `bash` interpreter.
If any of these dependencies are not present on the system running the script, it will fail partially or altogether.
With Nix, we can declare all dependencies explicitly, and produce a script that will always run on any machine that supports Nix and the required packages taken from Nixpkgs.
## The script
A [shebang] is the first line of a script starting with `#!`.
It determines which program to use for running the script.
It takes a command name as argument and will run the first executable by that name it finds in the directories listed in the environment variable `$PATH`.
The command `xml2json` is provided by the package `python3Packages.xmljson`, while the commands `jq` and `curl` are provided by packages of the same name.
The parameter of `-I` refers to a specific Git commit of the Nixpkgs repository.
This ensures that the script will always run with the exact same packages versions, everywhere.
- {ref}`reading-nix-language` to learn about the Nix language, which is used to declare packages and configurations.
- {ref}`declarative-reproducible-envs` to create reproducible shell environments with a declarative configuration file.
- [Garbage Collection](https://nixos.org/manual/nix/stable/package-management/garbage-collection.html) – free up storage used by the programs made available through Nix