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doc/manual/src/expressions/simple-expression.md

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+# A Simple Nix Expression
+
+This section shows how to add and test the [GNU Hello
+package](http://www.gnu.org/software/hello/hello.html) to the Nix
+Packages collection. Hello is a program that prints out the text “Hello,
+world\!”.
+
+To add a package to the Nix Packages collection, you generally need to
+do three things:
+
+1.  Write a Nix expression for the package. This is a file that
+    describes all the inputs involved in building the package, such as
+    dependencies, sources, and so on.
+
+2.  Write a *builder*. This is a shell script\[1\] that actually builds
+    the package from the inputs.
+
+3.  Add the package to the file `pkgs/top-level/all-packages.nix`. The
+    Nix expression written in the first step is a *function*; it
+    requires other packages in order to build it. In this step you put
+    it all together, i.e., you call the function with the right
+    arguments to build the actual package.
+
+<!-- end list -->
+
+1.  In fact, it can be written in any language, but typically it's a
+    `bash` shell script.