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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<chapter id='hello'>
	<title>A BitBake Hello World</title>
	<section>
		<title>BitBake Hello World</title>
		<para>The simplest example commonly used to demonstrate any new
			programming language or tool is the
			<ulink url="http://en.wikipedia.org/wiki/Hello_world_program">Hello World</ulink> 
			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.
		</para>
	</section>

<section>
	<title>Obtaining BitBake</title>
	<para>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:
		<screen>
			$ 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
		</screen>
		</para>
		
		<para>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.
		</para>
</section>

<section>
	<title>Setting Up the BitBake Environment</title>
	<para>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:
		<screen>$ ./bin/bitbake --version
			BitBake Build Tool Core version 1.19.0, bitbake version
			 1.19.0
		</screen>
		You're now ready to use BitBake.
	</para>
	<para>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:
	<screen>$ echo $PATH	
	/home/wmat/bin:/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:
	/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
	</screen>
	Now add the directory location for the BitBake binary to the PATH
	with:
	<screen>$ export PATH={path to the bitbake executable}:$PATH
	</screen>
	This will add the directory to the beginning of your PATH environment
	variable.
	For example, on my machine:
	<screen>$ export PATH=/media/wmat/Backups/dev/bitbake/bin:$PATH</screen>
	<screen>$ echo $PATH
	/media/wmat/Backups/dev/bitbake/bin:/home/wmat/bin:
	/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:
	/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
	</screen>
	Now, you should be able to simply enter the <screen>bitbake</screen>
	command at the command line to run bitbake.
	For a more permanent solution and assuming you are running the BASH
	shell, edit <screen>~/.bashrc</screen> and add the following to the end
	of that file:
	<screen>PATH={path to the bitbake executable}:$PATH</screen>
	</para>
	<para>Note that if you're a Vim user, you will find useful
			Vim configuration contributions in the 
			<emphasis>contrib/vim</emphasis> 
			directory.
			Copy the files from that directory to your 
			<emphasis>/home/yourusername/.vim</emphasis> 
			directory.
			If it doesn't exist, create it, and restart Vim.
		</para>
	</section>
	
	<section>
		<title>The Hello World Example</title>
		<para>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.
		</para>
		<para>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.
		</para>
		<para>It should be noted that this chapter was inspired by
			and draws heavily from several sources:
			<itemizedlist>
				<listitem>
					<ulink href="http://www.mail-archive.com/yocto@yoctoproject.org/msg09379.html">Mailing List post - The BitBake equivalent of "Hello, World!"</ulink>
				</listitem>
				<listitem>
					<ulink href="http://hambedded.org/blog/2012/11/24/from-bitbake-hello-world-to-an-image/">Hambedded Linux blog post - From Bitbake Hello World to an Image</ulink>
				</listitem>
			</itemizedlist>
		</para>
		<section>
		<title>A Reverse Walkthrough</title>
		<para>
		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.
		</para>
		
		<para>
		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:
		<screen>$mkdir ~/dev/hello && cd ~/dev/hello</screen>
		Within this new, empty directory, let's run BitBake with
		Debugging output and see what happens:
		<screen>$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
                </screen>
                The majority of this output is specific to environment variables
                that are not directly relevant to BitBake.  However, the very
                first message <screen>The BBPATH variable is not set</screen>
                is and needs to be rectified.  So how do we set the BBPATH
                variable?
                </para>
                <para>
                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.
                </para>
                <para>
                Without BBPATH Bitbake will not find any conf/<filename>.conf
                files or recipe files at all.
                It will also not find bitbake.conf.
                Note the reference to conf/<filename>.
                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.
                <screen>$ mkdir conf classes</screen>
                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:
                <screen>cp conf/bitbake.conf ~/dev/hello/conf/
                cp classes/base.bbclass ~/dev/hello/classes/
                </screen>
                At this point your project directory structure should look like
                the following:
                <screen>
                ~/dev/hello$ tree
                .
                ├── classes
                │   └── base.bbclass
                └── conf
                    └── bitbake.conf
                    </screen>
                </para>
                <para>
                But what about BBPATH, we still haven't set it?               
                </para>
                <para>
                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 <screen>conf/bblayers.conf</screen> file to our
                project source tree and populate it with the BBPATH variable
                declaration.
                From your project source tree:
                <screen>$ vim conf/bblayers.conf</screen>
                Add the following to the empty bblayers.conf file:
                <screen>BBPATH := "${TOPDIR}"</screen>                
                </para>
                <para>
                Now from the root of our project directory, let's run BitBake
                again and see what happens:
                <screen>:~/dev/hello$ 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'
                </screen>
                NOTE: From this point forward, the environment variable
                removal messages will be ignored and omitted.
                Let's examine the relevant DEBUG messages:
                
                
                </para>
                </section>

	</section>

</chapter>