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 | xml2json | jq .

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 determines which program to use for running an interpreted script.

We will use the shebang line #!/usr/bin/env nix-shell.

env is a program available on most modern Unix-like operating systems at the file system path /usr/bin/env. 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.

We use nix-shell as a shebang interpreter. It takes the following parameters relevant for our use case:

  • -i tells which program to use for interpreting the rest of the file

  • --pure excludes most environment variables when the script is run

  • -p lists packages that should be present in the interpreter’s environment

  • -I explicitly sets the search path for packages

More details on the options can be found in the nix-shell reference documentation.

Create a file named 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=

curl | xml2json | jq .

The first line is a standard shebang. The additional shebang lines are a Nix-specific construct.

We specify bash as the interpreter for the rest of the file with the -i option.

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.

With the -p option we specify the packages required for the script to run. The command xml2json is provided by the package python3Packages.xmljson, while bash, jq, and curl are provided by packages of the same name. cacert must be present for SSL authentication to work.


Use 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

Run the script:


Next steps#