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source/new-flake/bash-flake/main.adoc0

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+= Bash
+
+In this section we will create a very simple flake, and then make several improvements.
+To follow along, open a new terminal shell.
+Start with an empty directory and create a git repository.
+
+....
+$ mkdir my-project
+$ cd my-project
+$ git init
+....
+
+This will be a very simple development project,
+so that we can focus on how to use Nix.
+We want to package the following script.
+
+////
+$ cp ../cow-hello.sh .
+////
+
+[source,bash,linenums]
+.cow-hello.sh
+....
+$# cat cow-hello.sh
+....
+
+Add the script to your directory and we will test it.
+
+....
+$ chmod +x cow-hello.sh
+$ ./cow-hello.sh     # Fails
+....
+
+This probably failed on your machine -- why?
+The `cow-hello.sh` script depends on `cowsay`,
+which isn't part of your default environment (unless you specifically configure Nix or NixOS to include it).
+We can create a suitable temporary environment for a quick test:
+
+....
+$ nix shell nixpkgs#cowsay
+$ ./cow-hello.sh     # Succeeds
+....
+
+However, that approach becomes inconvenient once you have more dependencies.
+Let's exit to the original (default) environment,
+and see that `cowsay` is no longer available.
+
+....
+$ exit
+$ ./cow-hello.sh     # Fails again
+....
+
+For a single script like this, we could modify it by replacing the first line with a "Nix shebang",
+as shown later in <<#shebang>>;
+then the script would run in any Nix environment.
+But for this exercise, we will package it as a "proper" development project.
+
+We want to define the development environment we need for this project,
+so that we can recreate it at any time.
+Furthermore, anyone else who wants to work on our project will be able to recreate
+the same development environment we used.
+This avoids complaints of "but it works on my machine!"
+
+////
+$ cp ../flake.nix .
+////
+
+Create the file `flake.nix` as shown below.
+If you're not on an `x86_64-linux` system, modify the highlighted lines accordingly.
+
+[NOTE]
+====
+If you're not sure of the correct string so use for your system,
+run the following command.
+
+    nix eval --impure --raw --expr 'builtins.currentSystem'
+====
+
+[source,nix,linenums,highlight="10,12,14"]
+.flake.nix (version 1)
+....
+$# cat flake.nix
+....
+
+The `description` part is just a short description of the package.
+The `inputs` section should be familiar from <<#flake-inputs>>.
+If we ignore parts of the long package names, the `outputs` section looks like this:
+
+[source,subs=quotes]
+....
+    devShells = {
+      x86_64-linux.default =
+        _blahblah_.mkShell {
+          packages = [
+            _blahblah_.cowsay
+          ];
+        }; # mkShell
+    }; # devShells
+....
+
+This says, in effect, that to create a `default` shell for the `x86_64-linux` architecture,
+call the `mkShell` command and tell it you need the `cowsay` package.
+We can refactor the code to eliminate some duplication.
+
+[source,nix]
+....
+    devShells = {
+      x86_64-linux.default =
+        let
+          pkgs = nixpkgs.legacyPackages.x86_64-linux;
+        in
+          pkgs.mkShell {
+            packages = [
+              pkgs.cowsay
+            ];
+          }; # mkShell
+    }; # devShells
+....
+
+After a bit more refactoring, here's the entire flake.
+You may want to compare this carefully to the original version,
+in order to reassure yourself that the definitions are equivalent.
+
+////
+$ cp ../flake-3.nix flake.nix
+////
+
+[source,nix,linenums]
+.flake.nix (version 2)
+....
+$# cat flake.nix
+....
+
+Let's enter the shell.
+
+....
+$# echo '$ nix develop     # Fails'
+$# nix develop --command sh
+....
+
+Because we haven't added `flake.nix` to the git repository, it's essentially invisible to Nix.
+Let's correct that and try again.
+
+....
+$ git add flake.nix
+$# echo '$ nix develop'
+$# nix develop --command sh
+....
+
+The warning is because the changes haven't been committed to the git repository yet.
+We changed `flake.nix` of course, but when we ran `nix develop` it automatically created a `flake.lock` file.
+Don't worry about the warnings for now.
+We can see that `cowsay` is now available, and our script runs.
+
+....
+$ ./cow-hello.sh     # Succeeds
+....
+
+We created an appropriate development environment, and tested our script.
+Now we are ready to package it.
+Add the new lines below to `flake.nix`.
+
+////
+$ cp ../flake-4.nix flake.nix
+////
+
+[source,nix,linenums,highlight="23..48"]
+.flake.nix (version 3)
+....
+$# cat flake.nix
+....
+
+
+MAYBE INTRODUCE GETATTRS EARLIER
+
+CONTINUE HERE
+
+CONTINUE HERE
+
+CONTINUE HERE
+
+CONTINUE HERE
+
+....
+$ nix run
+....
+
+== Supporting multiple architectures
+
+////
+Good adoc0 scripts clean up after themselves.
+$ cd .. ;  rm -rf my-project # clean up
+////