# Hello, World!

We start by getting acquainted with the birch program that you have just installed, using it to create, build and run a simple Birch program. We will work from the terminal. If you are unfamiliar with Unix commands, that’s okay, everything you need to type will be provided as we go.

We start by creating a new package. From the terminal, create a new directory and change into it:

mkdir HelloWorld
cd HelloWorld


Initialize a new Birch package with the following command:

birch init --package HelloWorld


This creates the standard files and subdirectories for a Birch package, including (list them with ls):

• src/ for source code,
• config/ for configuration files (setting various options for a model and/or inference method),
• input/ for input files (your data sets),
• output/ for output files (the answers!),
• and a number of other meta files in the base directory such as README.md and LICENSE.

A new package contains just one source file, src/hello.birch, containing a program called hello that prints exactly what you think it does (view it with cat src/hello.birch).

Build the package with:

birch build


then run the hello program with:

birch hello


If you see Hello, World! on the terminal then everything is in order!

This is the typical workflow: whenever you make a change, run birch build to rebuild the package before running it again. The first time always takes a little longer than subsequent calls. Under the hood, the command is running a Birch-to-C++ source-to-source compiler, setting up a GNU Autotools build system (using autoconf, automake and libtool), and compiling the C++ sources to binaries. You will see the output of these steps on the terminal.

Tip

When building, Birch will create a number of additional files in the current working directory. If you ever want to delete these files, use:

birch clean


Tip

Now is the best time to set up version control. For Git:

git init

Typically, files in src/, config/ and input/ are tracked, while those in output/ are not, as they are derived from the others when running programs.