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<appendix id='hello-world-example'>
    <title>Hello World Example</title>

    <section id='bitbake-hello-world'>
        <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 appendix demonstrates, in tutorial form, Hello
            World within the context of BitBake.
            The tutorial describes how to create a new project
            and the applicable metadata files necessary to allow
            BitBake to build it.
        </para>
    </section>

    <section id='example-obtaining-bitbake'>
        <title>Obtaining BitBake</title>

        <para>
            See the
            "<link linkend='obtaining-bitbake'>Obtaining BitBake</link>"
            section for information on how to obtain BitBake.
            Once you have the source code on your machine, the BitBake directory
            appears as follows:
            <literallayout class='monospaced'>
     $ 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
     -rw-rw-r--. 1 wmat wmat  2887 Nov 26 04:55 TODO
            </literallayout>
        </para>

        <para>
            At this point, you should have BitBake cloned to
            a directory that matches the previous listing except for
            dates and user names.
        </para>
    </section>

    <section id='setting-up-the-bitbake-environment'>
        <title>Setting Up the BitBake Environment</title>

        <para>
            First, you need to be sure that you can run BitBake.
            Set your working directory to where your local BitBake
            files are and run the following command:
            <literallayout class='monospaced'>
     $ ./bin/bitbake --version
     BitBake Build Tool Core version 1.23.0, bitbake version 1.23.0
            </literallayout>
            The console output tells you what version you are running.
        </para>

        <para>
            The recommended method to run BitBake is from a directory of your
            choice.
            To be able to run BitBake from any directory, you need to add the
            executable binary to your binary to your shell's environment
            <filename>PATH</filename> variable.
            First, look at your current <filename>PATH</filename> variable
            by entering the following:
            <literallayout class='monospaced'>
     $ echo $PATH
            </literallayout>
            Next, add the directory location for the BitBake binary to the
            <filename>PATH</filename>.
            Here is an example that adds the
            <filename>/home/scott-lenovo/bitbake/bin</filename> directory
            to the front of the <filename>PATH</filename> variable:
            <literallayout class='monospaced'>
     $ export PATH=/home/scott-lenovo/bitbake/bin:$PATH
            </literallayout>
            You should now be able to enter the <filename>bitbake</filename>
            command from the command line while working from any directory.
        </para>
    </section>

    <section id='the-hello-world-example'>
        <title>The Hello World Example</title>

        <para>
            The overall goal of this exercise is to build a
            complete "Hello World" example utilizing task and layer
            concepts.
            Because this is how modern projects such as OpenEmbedded and
            the Yocto Project utilize BitBake, the example
            provides an excellent starting point for understanding
            BitBake.
        </para>

        <para>
            To help you understand how to use BitBake to build targets,
            the example starts with nothing but the <filename>bitbake</filename>
            command, which causes BitBake to fail and report problems.
            The example progresses by adding pieces to the build to
            eventually conclude with a working, minimal "Hello World"
            example.
        </para>

        <para>
            While every attempt is made to explain what is happening during
            the example, the descriptions cannot cover everything.
            You can find further information throughout this manual.
            Also, you can actively participate in the
            <ulink url='http://lists.openembedded.org/mailman/listinfo/bitbake-devel'></ulink>
            discussion mailing list about the BitBake build tool.
        </para>

        <note>
            This example was inspired by and drew heavily from these sources:
            <itemizedlist>
                <listitem><para>
                    <ulink url="http://www.mail-archive.com/yocto@yoctoproject.org/msg09379.html">Mailing List post - The BitBake equivalent of "Hello, World!"</ulink>
                    </para></listitem>
                <listitem><para>
                    <ulink url="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>
                    </para></listitem>
            </itemizedlist>
        </note>

        <para>
            As stated earlier, the goal of this example
            is to eventually compile "Hello World".
            However, it is unknown what BitBake needs and what you have
            to provide in order to achieve that goal.
            Recall that BitBake utilizes three types of metadata files:
            <link linkend='configuration-files'>Configuration Files</link>,
            <link linkend='classes'>Classes</link>, and
            <link linkend='recipes'>Recipes</link>.
            But where do they go?
            How does BitBake find them?
            BitBake's error messaging helps you answer these types of questions
            and helps you better understand exactly what is going on.
        </para>

        <para>
            Following is the complete "Hello World" example.
        </para>

        <orderedlist>
            <listitem><para><emphasis>Create a Project Directory:</emphasis>
                First, set up a directory for the "Hello World" project.
                Here is how you can do so in your home directory:
                <literallayout class='monospaced'>
     $ mkdir ~/hello
     $ cd ~/hello
                </literallayout>
                This is the directory that BitBake will use to do all of
                its work.
                You can use this directory to keep all the metafiles needed
                by BitBake.
                Having a project directory is a good way to isolate your
                project.
                </para></listitem>
            <listitem><para><emphasis>Run Bitbake:</emphasis>
                At this point, you have nothing but a project directory.
                Run the <filename>bitbake</filename> command and see what
                it does:
                <literallayout class='monospaced'>
     $ bitbake
     The BBPATH variable is not set and bitbake did not
     find a conf/bblayers.conf file in the expected location.
     Maybe you accidentally invoked bitbake from the wrong directory?
     DEBUG: Removed the following variables from the environment:
     GNOME_DESKTOP_SESSION_ID, XDG_CURRENT_DESKTOP,
     GNOME_KEYRING_CONTROL, DISPLAY, SSH_AGENT_PID, LANG, no_proxy,
     XDG_SESSION_PATH, XAUTHORITY, SESSION_MANAGER, SHLVL,
     MANDATORY_PATH, COMPIZ_CONFIG_PROFILE, WINDOWID, EDITOR,
     GPG_AGENT_INFO, SSH_AUTH_SOCK, GDMSESSION, GNOME_KEYRING_PID,
     XDG_SEAT_PATH, XDG_CONFIG_DIRS, LESSOPEN, DBUS_SESSION_BUS_ADDRESS,
     _, XDG_SESSION_COOKIE, DESKTOP_SESSION, LESSCLOSE, DEFAULTS_PATH,
     UBUNTU_MENUPROXY, OLDPWD, XDG_DATA_DIRS, COLORTERM, LS_COLORS
                </literallayout>
                The majority of this output is specific to environment variables
                that are not directly relevant to BitBake.
                However, the very first message regarding the
                <filename>BBPATH</filename> variable and the
                <filename>conf/bblayers.conf</filename> file
                is relevant.</para>
                <para>
                When you run BitBake, it begins looking for metadata files.
                The
                <link linkend='var-BBPATH'><filename>BBPATH</filename></link>
                variable is what tells BitBake where to look for those files.
                <filename>BBPATH</filename> is not set and you need to set it.
                Without <filename>BBPATH</filename>, Bitbake cannot
                find any configuration files (<filename>.conf</filename>)
                or recipe files (<filename>.bb</filename>) at all.
                BitBake also cannot find the <filename>bitbake.conf</filename>
                file.
                </para></listitem>
            <listitem><para><emphasis>Setting <filename>BBPATH</filename>:</emphasis>
                For this example, you can set <filename>BBPATH</filename>
                in the same manner that you set <filename>PATH</filename>
                earlier in the appendix.
                You should realize, though, that it is much more flexible to set the
                <filename>BBPATH</filename> variable up in a configuration
                file for each project.</para>
                <para>From your shell, enter the following commands to set and
                export the <filename>BBPATH</filename> variable:
                <literallayout class='monospaced'>
     $ BBPATH="<replaceable>projectdirectory</replaceable>"
     $ export BBPATH
                </literallayout>
                Use your actual project directory in the command.
                BitBake uses that directory to find the metadata it needs for
                your project.
                <note>
                    When specifying your project directory, do not use the
                    tilde ("~") character as BitBake does not expand that character
                    as the shell would.
                </note>
                </para></listitem>
            <listitem><para><emphasis>Run Bitbake:</emphasis>
                Now that you have <filename>BBPATH</filename> defined, run
                the <filename>bitbake</filename> command again:
                <literallayout class='monospaced'>
     $ bitbake
     ERROR: Traceback (most recent call last):
       File "/home/scott-lenovo/bitbake/lib/bb/cookerdata.py", line 163, in wrapped
         return func(fn, *args)
       File "/home/scott-lenovo/bitbake/lib/bb/cookerdata.py", line 173, in parse_config_file
         return bb.parse.handle(fn, data, include)
       File "/home/scott-lenovo/bitbake/lib/bb/parse/__init__.py", line 99, in handle
         return h['handle'](fn, data, include)
       File "/home/scott-lenovo/bitbake/lib/bb/parse/parse_py/ConfHandler.py", line 120, in handle
         abs_fn = resolve_file(fn, data)
       File "/home/scott-lenovo/bitbake/lib/bb/parse/__init__.py", line 117, in resolve_file
         raise IOError("file %s not found in %s" % (fn, bbpath))
     IOError: file conf/bitbake.conf not found in /home/scott-lenovo/hello

     ERROR: Unable to parse conf/bitbake.conf: file conf/bitbake.conf not found in /home/scott-lenovo/hello
                </literallayout>
                This sample output shows that BitBake could not find the
                <filename>conf/bitbake.conf</filename> file in the project
                directory.
                This file is the first thing BitBake must find in order
                to build a target.
                And, since the project directory for this example is
                empty, you need to provide a <filename>conf/bitbake.conf</filename>
                file.
                </para></listitem>
            <listitem><para><emphasis>Creating <filename>conf/bitbake.conf</filename>:</emphasis>
                The <filename>conf/bitbake.conf</filename> includes a number of
                configuration variables BitBake uses for metadata and recipe
                files.
                For this example, you need to create the file in your project directory
                and define some key BitBake variables.
                For more information on the <filename>bitbake.conf</filename>,
                see
                <ulink url='http://hambedded.org/blog/2012/11/24/from-bitbake-hello-world-to-an-image/#an-overview-of-bitbakeconf'></ulink>
                </para>
                <para>Use the following commands to create the <filename>conf</filename>
                directory in the project directory:
                <literallayout class='monospaced'>
     $ mkdir conf
                </literallayout>
                From within the <filename>conf</filename> directory, use
                some editor to create the <filename>bitbake.conf</filename>
                so that it contains the following:
                <literallayout class='monospaced'>
     TMPDIR  = "${<link linkend='var-TOPDIR'>TOPDIR</link>}/tmp"
     <link linkend='var-CACHE'>CACHE</link>   = "${TMPDIR}/cache"
     <link linkend='var-STAMP'>STAMP</link>   = "${TMPDIR}/stamps"
     <link linkend='var-T'>T</link>       = "${TMPDIR}/work"
     <link linkend='var-B'>B</link>       = "${TMPDIR}"
                </literallayout>
                The <filename>TMPDIR</filename> variable establishes a directory
                that BitBake uses for build output and intermediate files (other
                than the cached information used by the
                <link linkend='setscene'>Setscene</link> process.
                Here, the <filename>TMPDIR</filename> directory is set to
                <filename>hello/tmp</filename>.
                <note><title>Tip</title>
                    You can always safely delete the <filename>tmp</filename>
                    directory in order to rebuild a BitBake target.
                    The build process creates the directory for you
                    when you run BitBake.
                </note></para>
                <para>For information about each of the other variables defined in this
                example, click on the links to take you to the definitions in
                the glossary.
                </para></listitem>
            <listitem><para><emphasis>Run Bitbake:</emphasis>
                After making sure that the <filename>conf/bitbake.conf</filename>
                file exists, you can run the <filename>bitbake</filename>
                command again:
                <literallayout class='monospaced'>
$ bitbake
ERROR: Traceback (most recent call last):
  File "/home/scott-lenovo/bitbake/lib/bb/cookerdata.py", line 163, in wrapped
    return func(fn, *args)
  File "/home/scott-lenovo/bitbake/lib/bb/cookerdata.py", line 177, in _inherit
    bb.parse.BBHandler.inherit(bbclass, "configuration INHERITs", 0, data)
  File "/home/scott-lenovo/bitbake/lib/bb/parse/parse_py/BBHandler.py", line 92, in inherit
    include(fn, file, lineno, d, "inherit")
  File "/home/scott-lenovo/bitbake/lib/bb/parse/parse_py/ConfHandler.py", line 100, in include
    raise ParseError("Could not %(error_out)s file %(fn)s" % vars(), oldfn, lineno)
ParseError: ParseError in configuration INHERITs: Could not inherit file classes/base.bbclass

ERROR: Unable to parse base: ParseError in configuration INHERITs: Could not inherit file classes/base.bbclass
                </literallayout>
                In the sample output, BitBake could not find the
                <filename>classes/base.bbclass</filename> file.
                You need to create that file next.
                </para></listitem>
            <listitem><para><emphasis>Creating <filename>classes/base.bbclass</filename>:</emphasis>
                BitBake uses class files to provide common code and functionality.
                The minimally required class for BitBake is the
                <filename>classes/base.bbclass</filename> file.
                The <filename>base</filename> class is implicitly inherited by
                every recipe.
                BitBake looks for the class in the <filename>classes</filename>
                directory of the project (i.e <filename>hello/classes</filename>
                in this example).
                </para>
                <para>Create the <filename>classes</filename> directory as follows:
                <literallayout class='monospaced'>
     $ cd $HOME/hello
     $ mkdir classes
                </literallayout>
                Move to the <filename>classes</filename> directory and then
                create the <filename>base.bbclass</filename> file by inserting
                this single line:
                <literallayout class='monospaced'>
     addtask build
                </literallayout>
                The minimal task that BitBake runs is the
                <filename>do_build</filename> task.
                This is all the example needs in order to build the project.
                Of course, the <filename>base.bbclass</filename> can have much
                more depending on which build environments BitBake is
                supporting.
                For more information on the <filename>base.bbclass</filename> file,
                you can look at
                <ulink url='http://hambedded.org/blog/2012/11/24/from-bitbake-hello-world-to-an-image/#tasks'></ulink>.
                </para></listitem>
            <listitem><para><emphasis>Run Bitbake:</emphasis>
                After making sure that the <filename>classes/base.bbclass</filename>
                file exists, you can run the <filename>bitbake</filename>
                command again:
                <literallayout class='monospaced'>
     $ bitbake
     Nothing to do.  Use 'bitbake world' to build everything, or run 'bitbake --help' for usage information.
                </literallayout>
                BitBake is finally reporting no errors.
                However, you can see that it really does not have anything
                to do.
                You need to create a recipe that gives BitBake something to do.
                </para></listitem>
            <listitem><para><emphasis>Creating a Layer:</emphasis>
                While it is not really necessary for such a small example,
                it is good practice to create a layer in which to keep your
                code separate from the general metadata used by BitBake.
                Thus, this example creates and uses a layer called "mylayer".
                <note>
                    You can find additional information on adding a layer at
                    <ulink url='http://hambedded.org/blog/2012/11/24/from-bitbake-hello-world-to-an-image/#adding-an-example-layer'></ulink>.
                </note>
                </para>
                <para>Minimally, you need a recipe file and a layer configuration
                file in your layer.
                The configuration file needs to be in the <filename>conf</filename>
                directory inside the layer.
                Use these commands to set up the layer and the <filename>conf</filename>
                directory:
                <literallayout class='monospaced'>
     $ cd $HOME
     $ mkdir mylayer
     $ cd mylayer
     $ mkdir conf
                </literallayout>
                Move to the <filename>conf</filename> directory and create a
                <filename>layer.conf</filename> file that has the following:
                <literallayout class='monospaced'>
     BBPATH .= ":${<link linkend='var-LAYERDIR'>LAYERDIR</link>}"

     <link linkend='var-BBFILES'>BBFILES</link> += "${LAYERDIR}/*.bb"

     <link linkend='var-BBFILE_COLLECTIONS'>BBFILE_COLLECTIONS</link> += "mylayer"
     <link linkend='var-BBFILE_PATTERN'>BBFILE_PATTERN_mylayer</link> := "^${LAYERDIR_RE}/"
                </literallayout>
                For information on these variables, click the links
                to go to the definitions in the glossary.</para>
                <para>You need to create the recipe file next.
                Inside your layer at the top-level, use an editor and create
                a recipe file named <filename>printhello.bb</filename> that
                has the following:
                <literallayout class='monospaced'>
     <link linkend='var-DESCRIPTION'>DESCRIPTION</link> = "Prints Hello World"
     <link linkend='var-PN'>PN</link> = 'printhello'
     <link linkend='var-PV'>PV</link> = '1'

     python do_build() {
        bb.plain("********************");
        bb.plain("*                  *");
        bb.plain("*  Hello, World!   *");
        bb.plain("*                  *");
        bb.plain("********************");
     }
                </literallayout>
                The recipe file simply provides a description of the
                recipe, the name, version, and the <filename>do_build</filename>
                task, which prints out "Hello World" to the console.
                For more information on these variables, follow the links
                to the glossary.
                </para></listitem>
            <listitem><para><emphasis>Run Bitbake With a Target:</emphasis>
                Now that a BitBake target exists, run the command and provide
                that target:
                <literallayout class='monospaced'>
     $ cd $HOME/hello
     $ bitbake printhello
     ERROR: no recipe files to build, check your BBPATH and BBFILES?

     Summary: There was 1 ERROR message shown, returning a non-zero exit code.
                </literallayout>
                We have created the layer with the recipe and the layer
                configuration file but it still seems that BitBake cannot
                find the recipe.
                BitBake needs a <filename>conf/bblayers.conf</filename> that
                lists the layers for the project.
                Without this file, BitBake cannot find the recipe.
                </para></listitem>
            <listitem><para><emphasis>Creating <filename>conf/bblayers.conf</filename>:</emphasis>
                BitBake uses the <filename>conf/bblayers.conf</filename> file
                to locate layers needed for the project.
                This file must reside in the <filename>conf</filename> directory
                of the project (i.e. <filename>hello/conf</filename> for this
                example).</para>
                <para>Set your working directory to the <filename>hello/conf</filename>
                directory and then create the <filename>bblayers.conf</filename>
                file so that it contains the following:
                <literallayout class='monospaced'>
     BBLAYERS ?= " \
       /home/&lt;you&gt;/mylayer \
       "
                </literallayout>
                You need to provide your own information for
                <filename>you</filename> in the file.
                </para></listitem>
            <listitem><para><emphasis>Run Bitbake With a Target:</emphasis>
                Now that you have supplied the <filename>bblayers.conf</filename>
                file, run the <filename>bitbake</filename> command and provide
                the target:
                <literallayout class='monospaced'>
     $ bitbake printhello
     Parsing recipes: 100% |##################################################################################|
     Time: 00:00:00
     Parsing of 1 .bb files complete (0 cached, 1 parsed). 1 targets, 0 skipped, 0 masked, 0 errors.
     NOTE: Resolving any missing task queue dependencies
     NOTE: Preparing RunQueue
     NOTE: Executing RunQueue Tasks
     ********************
     *                  *
     *  Hello, World!   *
     *                  *
     ********************
     NOTE: Tasks Summary: Attempted 1 tasks of which 0 didn't need to be rerun and all succeeded.
                </literallayout>
                BitBake finds the <filename>printhello</filename> recipe and
                successfully runs the task.
                <note>
                    After the first execution, re-running
                    <filename>bitbake printhello</filename> again will not
                    result in a BitBake run that prints the same console
                    output.
                    The reason for this is that the first time the
                    <filename>printhello.bb</filename> recipe's
                    <filename>do_build</filename> task executes
                    successfully, BitBake writes a stamp file for the task.
                    Thus, the next time you attempt to run the task
                    using that same <filename>bitbake</filename> command,
                    BitBake notices the stamp and therefore determines
                    that the task does not need to be re-run.
                    If you delete the <filename>tmp</filename> directory
                    or run <filename>bitbake -c clean printhello</filename>
                    and then re-run the build, the "Hello, World!" message will
                    be printed again.
                </note>
                </para></listitem>
        </orderedlist>
    </section>
</appendix>
