Reproducible interpreted scripts#
In this tutorial, you will learn how to use Nix to create and run reproducible interpreted scripts, also known as shebang scripts.
A trivial script with non-trivial dependencies#
Take the following script, which fetches the content XML of a URL, converts it to JSON, and formats it for better readability:
#! /bin/bash curl https://github.com/NixOS/nixpkgs/releases.atom | xml2json | jq .
It requires the programs
It also requires the
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.
A shebang determines which program to use for running an interpreted script.
We will use the shebang line
env is a program available on most modern Unix-like operating systems at the file system path
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
nix-shell as a shebang interpreter.
It takes the following parameters relevant for our use case:
-itells which program to use for interpreting the rest of the file
--pureexcludes most environment variables when the script is run
-plists packages that should be present in the interpreter’s environment
-Iexplicitly sets the search path for packages
More details on the options can be found in the
nix-shell reference documentation.
Create a file named
nixpkgs-releases.sh with the following content:
#!/usr/bin/env nix-shell #! nix-shell -i bash --pure #! nix-shell -p bash cacert curl jq python3Packages.xmljson #! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/2a601aafdc5605a5133a2ca506a34a3a73377247.tar.gz curl https://github.com/NixOS/nixpkgs/releases.atom | xml2json | jq .
The first line is a standard shebang. The additional shebang lines are a Nix-specific construct.
bash as the interpreter for the rest of the file with the
We enable the
--pure option to prevent the script from implicitly using programs that may already exist on the system that will run the script.
-p option we specify the packages required for the script to run.
xml2json is provided by the package
curl are provided by packages of the same name.
cacert must be present for SSL authentication to work. Use search.nixos.org to find packages providing the program you need.
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.
Make the script executable:
chmod +x nixpkgs-releases.sh
Run the script:
Nix language basics to learn about the Nix language, which is used to declare packages and configurations.
Declarative shell environments with shell.nix to create reproducible shell environments with a declarative configuration file.
Garbage Collection – free up storage used by the programs made available through Nix