According to the manual:

The command nix-shell will build the dependencies of the specified derivation, but not the derivation itself. It will then start an interactive shell in which all environment variables defined by the derivation path have been set to their corresponding values, and the script $stdenv/setup has been sourced. This is useful for reproducing the environment of a derivation for development.

How does that help me?

Ephemeral shells⌗

As an example - I need to search local directories quickly for specific string:

ag 'nix-shell' .

but I don’t have the silver-searcher installed. Running the above will result in:

The program 'ag' is not in your PATH. You can make it available in an
ephemeral shell by typing:
  nix-shell -p silver-searcher

Using that suggestion, once it completes, I will be dropped into a temporary shell where i can repeat the search and it will work as expected. Exit the shell and the silver-searcher will be gone.

Development environments⌗

As an alternative to setting up and running a containerised development environment using docker or podman etc.

E.g. setting up and running a Symfony project⌗

Given i have already installed the Symfony cli tool

symfony check:requirements

will result in

no PHP binaries detected

create an ephemeral shell that has php binary:

nix-shell -p php

check it is available:

php -v

PHP 8.1.24 (cli) (built: Sep 26 2023 23:43:49) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.1.24, Copyright (c) Zend Technologies
    with Zend OPcache v8.1.24, Copyright (c), by Zend Technologies

create new symfony project:

symfony new symfony-sandbox

[OK] Your project is now ready in /home/john/development/web/symfony-sandbox

In order to have the changes persist after exiting this nix-shell you need to write requirements to file

touch shell.nix

add the following content:

{ pkgs ? import <nixpkgs> {} }:
  pkgs.mkShell {
    # nativeBuildInputs is usually what you want -- tools you need to run
    nativeBuildInputs = with pkgs.buildPackages; [
      php81
    ];
}

and exit the temporary shell.

Should be back in a normal terminal shell (check prompt differences between the two). Try and run any of the symfony-cli commands then you’ll get the ’no PHP binaries detected’ error again. Not to worry:

nix-shell

then

symfony server:start

 [OK] Web server listening
      The Web server is using PHP FPM 8.1.24
      http://127.0.0.1:8000

Bonus - using direnv⌗

Add a .envrc file to project root directory with following content:

use nix

Install direnv by adding it to package list in main configuration file; E.g.:

with pkgs; [
    direnv
]

add the following to programs.zsh section

initExtra = ''
    eval "$(direnv hook zsh)"
''

rebuild config

sudo nixos-rebuild switch

Then from the same directory that contains the .envrc file:

direnv allow .

Now anytime you cd in/out of project dir the environment shell will be loaded and unloaded automatically.

See here for more info.