# A BitBake Hello World

## BitBake Hello World

The simplest example commonly used to demonstrate any new programming language or tool is the Hello World example. This chapter demonstrates, in tutorial form, Hello World within the context of BitBake. This tutorial describes how to create a new Project and the applicable metadata files necessary to allow BitBake to build it.

## Obtaining BitBake

Please refer to Chapter 1 Section 1.7 for the various methods to obtain BitBake. Once the source code is on your machine the BitBake directory will appear as follows:

         $ ls -al
         total 100
         drwxrwxr-x. 9 wmat wmat  4096 Jan 31 13:44 .
         drwxrwxr-x. 3 wmat wmat  4096 Feb  4 10:45 ..
         -rw-rw-r--. 1 wmat wmat   365 Nov 26 04:55 AUTHORS
         drwxrwxr-x. 2 wmat wmat  4096 Nov 26 04:55 bin
         drwxrwxr-x. 4 wmat wmat  4096 Jan 31 13:44 build
         -rw-rw-r--. 1 wmat wmat 16501 Nov 26 04:55 ChangeLog
         drwxrwxr-x. 2 wmat wmat  4096 Nov 26 04:55 classes
         drwxrwxr-x. 2 wmat wmat  4096 Nov 26 04:55 conf
         drwxrwxr-x. 3 wmat wmat  4096 Nov 26 04:55 contrib
         -rw-rw-r--. 1 wmat wmat 17987 Nov 26 04:55 COPYING
         drwxrwxr-x. 3 wmat wmat  4096 Nov 26 04:55 doc
         -rw-rw-r--. 1 wmat wmat    69 Nov 26 04:55 .gitignore
         -rw-rw-r--. 1 wmat wmat   849 Nov 26 04:55 HEADER
         drwxrwxr-x. 5 wmat wmat  4096 Jan 31 13:44 lib
         -rw-rw-r--. 1 wmat wmat   195 Nov 26 04:55 MANIFEST.in
         -rwxrwxr-x. 1 wmat wmat  3195 Jan 31 11:57 setup.py
         -rw-rw-r--. 1 wmat wmat  2887 Nov 26 04:55 TODO

At this point you should have BitBake extracted or cloned to a directory and it should match the directory tree above. Please note that you'll see your username wherever "wmat" appears above.

## Setting Up the BitBake Environment

The recommended method to run BitBake is from a directory of your choice. The directory can be within your home directory or in `/usr/local`, depending on your preference. Let's run BitBake now to make sure it's working.

From the BitBake source code directory, issue the following command:

         $ ./bin/bitbake --version
         BitBake Build Tool Core version 1.19.0, bitbake version
         1.19.0

You're now ready to use BitBake.

A final step to make development easier is to add the executable binary to your environment `PATH`. First, have a look at your current `PATH` variable. If I check mine, I get:

         $ echo $PATH
         /home/wmat/bin:/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:
         /usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

Now add the directory location for the BitBake binary to the `PATH` with:

         $ export PATH={path to the bitbake executable}:$PATH

This will add the directory to the beginning of your PATH environment variable. For example, on my machine:

         $ export PATH=/media/wmat/Backups/dev/bitbake/bin:$PATH
         /media/wmat/Backups/dev/bitbake/bin:/home/wmat/bin:
         /usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:
         /usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games

Now, you should be able to simply enter the `bitbake` command at the command line to run bitbake. For a more permanent solution and assuming you are running the BASH shell, edit `~/.bashrc` and add the following to the end of that file:

         PATH={path to the bitbake executable}:$PATH

Note that if you're a Vim user, you will find useful Vim configuration contributions in the `contrib/vim` directory. Copy the files from that directory to your `/home/yourusername/.vim` directory. If it doesn't exist, create it, and restart Vim.

## The Hello World Example

The following example leaps directly into how BitBake works. Every attempt is made to explain what is happening, however, further information can be found in the Metadata chapter.

The overall goal of this exercise is to create a Hello World example utilizing concepts used to build and construct a complete example application including Tasks and Layers. This is how modern projects such as OpenEmbedded and the Yocto Project utilize BitBake, therefore it provides an excellent starting point for understanding BitBake.

It should be noted that this chapter was inspired by and draws heavily from several sources:

- Mailing List post - The BitBake equivalent of "Hello, World!"

- Hambedded Linux blog post - From Bitbake Hello World to an Image

### A Reverse Walkthrough

One of the best means to understand anything is to walk through the steps to where we want to be by observing first principles. BitBake allows us to do this through the -D or Debug command line parameter. We know we want to eventually compile a HelloWorld example, but we don't know what we need to do that. Remember that BitBake utilizes three types of metadata files: Configuration Files, Classes, and Recipes. But where do they go, how does BitBake find them, etc. etc.? Hopefully we can use BitBake's error messaging to figure this out and better understand exactly what's going on.

First, let's begin by setting up a directory for our HelloWorld project. I'll do this in my home directory and change into that directory:

         $ mkdir ~/dev/hello && cd ~/dev/hello

Within this new, empty directory, let's run BitBake with Debugging output and see what happens:

         $ bitbake -DDD
         The BBPATH variable is not set
         DEBUG: Removed the following variables from the environment:
         GNOME_DESKTOP_SESSION_ID, LESSOPEN, WINDOWID,
         GNOME_KEYRING_CONTROL, DISPLAY, SSH_AGENT_PID, LANG,
         XDG_SESSION_PATH, XAUTHORITY, LANGUAGE, SESSION_MANAGER,
         SHLVL, MANDATORY_PATH, COMPIZ_CONFIG_PROFILE, TEXTDOMAIN,
         GPG_AGENT_INFO, SSH_AUTH_SOCK, XDG_RUNTIME_DIR,
         COMPIZ_BIN_PATH, GDMSESSION, DEFAULTS_PATH, TEXTDOMAINDIR,
         XDG_SEAT_PATH, XDG_CONFIG_DIRS, XDG_CURRENT_DESKTOP,
         DBUS_SESSION_BUS_ADDRESS, _, XDG_SESSION_COOKIE,
         DESKTOP_SESSION, LESSCLOSE, GNOME_KEYRING_PID,
         UBUNTU_MENUPROXY, OLDPWD, GTK_MODULES, XDG_DATA_DIRS,
         COLORTERM, LS_COLORS

The majority of this output is specific to environment variables that are not directly relevant to BitBake. However, the very first message `The BBPATH variable is not set` is and needs to be rectified. So how do we set the BBPATH variable?

When BitBake is run it begins looking for metadata files. The BBPATH variable is what tells BitBake where to look. It is possible to set BBPATH as an environment variable as you did above for the BitBake exexcutable's PATH. However, it's much more flexible to set the BBPATH variable for each project, as this allows for greater flexibility.

Without BBPATH Bitbake will not find any `.conf` files or recipe files at all. It will also not find `bitbake.conf`. Note the reference to `conf/`. It is standard practice to organize the project's directory tree to include a `conf/` and a `classes/` directory. Add those now to your project directory:

         $ mkdir conf classes

Now let's copy the sample configuration files provided in the BitBake source tree to their appropriate conf and classes directory. Change to the BitBake source tree directory and:

         cp conf/bitbake.conf ~/dev/hello/conf/
         cp classes/base.bbclass ~/dev/hello/classes/

At this point your project directory structure should look like the following:

         ~/dev/hello$ tree
         .
         ├── classes
         │   └── base.bbclass
         └── conf
         └── bitbake.conf

But what about BBPATH, we still haven't set it?

The first configuration file that BitBake looks for is always `bblayers.conf`. With this knowledge we know that to resolve our BBPATH error we can add a `conf/bblayers.conf` file to our project source tree and populate it with the BBPATH variable declaration. From your project source tree:

         $ vim conf/bblayers.conf

Add the following to the empty bblayers.conf file:

         BBPATH := "${TOPDIR}"

Now from the root of our project directory, let's run BitBake again and see what happens:

         $ bitbake -DDD
         Nothing to do.  Use 'bitbake world' to build everything, or run
         'bitbake --help' for usage information.
         DEBUG: Removed the following variables from the environment:
         GNOME_DESKTOP_SESSION_ID, LESSOPEN, WINDOWID,
         GNOME_KEYRING_CONTROL, DISPLAY, SSH_AGENT_PID, LANG,
         XDG_SESSION_PATH, XAUTHORITY, LANGUAGE, SESSION_MANAGER,
         SHLVL, MANDATORY_PATH, COMPIZ_CONFIG_PROFILE, TEXTDOMAIN,
         GPG_AGENT_INFO, SSH_AUTH_SOCK, XDG_RUNTIME_DIR,
         COMPIZ_BIN_PATH, GDMSESSION, DEFAULTS_PATH, TEXTDOMAINDIR,
         XDG_SEAT_PATH, XDG_CONFIG_DIRS, XDG_CURRENT_DESKTOP,
         DBUS_SESSION_BUS_ADDRESS, _, XDG_SESSION_COOKIE,
         DESKTOP_SESSION, LESSCLOSE, GNOME_KEYRING_PID, UBUNTU_MENUPROXY,
         OLDPWD, GTK_MODULES, XDG_DATA_DIRS, COLORTERM, LS_COLORS
         DEBUG: Found bblayers.conf (/home/wmat/dev/hello/conf/
         bblayers.conf)
         DEBUG: LOAD /home/wmat/dev/hello/conf/bblayers.conf
         DEBUG: LOAD /home/wmat/dev/hello/conf/bitbake.conf
         DEBUG: BB configuration INHERITs:0: inheriting /home/wmat/dev/
         hello/classes/base.bbclass
         DEBUG: BB /home/wmat/dev/hello/classes/base.bbclass: handle
         (data, include)
         DEBUG: LOAD /home/wmat/dev/hello/classes/base.bbclass
         DEBUG: Clearing SRCREV cache due to cache policy of: clear
         DEBUG: Using cache in '/home/wmat/dev/hello/tmp/cache/
         local_file_checksum_cache.dat'
         DEBUG: Using cache in '/home/wmat/dev/hello/tmp/cache/
         bb_codeparser.dat'

From this point forward, the environment variable removal messages will be ignored and omitted. Let's examine the relevant DEBUG messages: