Say you want to test out a program, but don't want to go through all of the effort of adding it to your system configuration and rebuilding it. This is where the Nix shell comes in.

To temporarily bring a Nix package into the scope of your current terminal, you can use the nix-shell -p command. This lets you specify Nix packages which will be included in the "nix shell" environment, as well as your current packages.

$ nix-shell -p hello

[nix-shell:~]$ hello
Hello, world!

You are still able to modify the environment around you (e.g. create folders, edit files, open programs that are currently installed on your system). When you exit the nix shell (Using the exit command), that package is not present on your current system. For example, if I use nix-shell -p hello, the hello command is only present in the nix shell even if it wasn't installed with my main configuration.

To add multiple packages, include their names (as if you were adding it to your configuration.nix file) with a space between each one:

$ nix-shell -p hello gnome3.gnome-mahjongg vitetris

By adding the --pure tag to the nix-shell command, you are able to create an environment which contains none of the packages installed on your current system.

For example, if you have the following in your configuration.nix file:

environment.systemPackages = with pkgs; [
    hello
];

Running the hello command in a pure nix shell that doesn't have the hello package will produce the following result:

$ nix-shell --pure -p

[nix-shell:~]$ hello
The program 'hello' is currently not installed. It is provided by several packages. You can install it by typing one of the following:
    nix-env -iA nixos.hello
    nix-env -iA nixos.mbedtls
    nix-env -iA nixos.perkeep

When a package is downloaded temporarily for use in the Nix shell, it is stored in the Nix store (in /nix/store) until garbage collected. Until it is garbage collected, opening the same Nix shell for downloaded programs will be instant (as they're already downloaded).

As stated in the chapter on garbage collection, running the Nix garbage collector will remove any temporarily installed programs. Once garbage collected, in order to use them again, they will be redownloaded when a Nix shell is opened that requires packages which aren't in the Nix store.

When using Nix shells, it can sometimes be useful to keep track of what packages are currently installed in that instance of the Nix shell. A helpful tool is the any-nix-shell, which shows the information of temporarily installed packages when using the fish shell or the z shell.