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## Chapter 20. Tutorial

## Hello, world

The simplest project that Boost.Build can construct is stored in `example/hello/` directory. The project is described by a file called `Jamfile` that contains:

    exe hello : hello.cpp ;

Even with this simple setup, you can do some interesting things. First of all, just invoking bjam will build the debug variant of the hello executable by compiling and linking `hello.cpp`. Now, to build the release variant of hello, invoke

    bjam release

Note that debug and release variants are created in different directories, so you can switch between variants or even build multiple variants at once, without any unneccessary recompilation. Let's extend the example by adding another line to our project's `Jamfile`:

    exe hello2 : hello.cpp ;

Now we can build both the debug and release variants of our project:

    bjam debug release

Note that two variants of hello2 are linked. Since we have already built both variants of hello, hello.cpp won't be recompiled; instead the existing object files will just be linked into the corresponding variants of hello2. Now let's remove all the built products:

    bjam --clean debug release

It's also possible to build or clean specific targets. The following two commands, respectively, build or clean only the debug version of hello2.

    bjam hello2
    bjam --clean hello2