`writeShellApplication`

The recommended way to create Derivation using shell scripts.

The function generates a Derivation for a shell script specified as the value for text attribute.
runtimeInputs: packages to be made available to the shell application’s PATH.
Uses shellcheck to statically analyze your bash script for issues.

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nixpkgs

nixpkgs (https://github.com/NixOS/nixpkgs) is a monorepo containing a collection of Nix packages. It also includes various utility Nix functions (like writeShellApplication), as well as the NixOS Linux distribution (including the Module System).
Nixifying a Haskell project using nixpkgs

This flake defines a flake app that can be run using nix run. This app is simply a shell script that starts a Postgres server. nixpkgs provides the convenient writeShellApplication function to generate such a script. Note that "${script}" provides the path in the nix/store where the application is located.
Introduction to module system

This is a simple flake that exposes a package (a writeShellApplication Derivation wrapping lsd), that can be `nix run`ed to list the contents of the root directory.