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## Chapter 23. 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 `Jamroot` 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 `hello` executable by compiling and linking `hello.cpp`. By default, debug variant is built. 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 unnecessary recompilation. Let's extend the example by adding another line to our project's `Jamroot`:

    exe hello2 : hello.cpp ;

Now let us build both the debug and release variants of our project again:

    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