6. Hello World Example
**********************


6.1. BitBake Hello World
========================

The simplest example commonly used to demonstrate any new programming
language or tool is the “Hello World” 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.


6.2. Obtaining BitBake
======================

See the Obtaining BitBake section for information on how to obtain
BitBake. Once you have the source code on your machine, the BitBake
directory appears 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
   -rw-rw-r--. 1 wmat wmat  2887 Nov 26 04:55 TODO

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


6.3. Setting Up the BitBake Environment
=======================================

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:

   $ ./bin/bitbake --version
   BitBake Build Tool Core version 1.23.0, bitbake version 1.23.0

The console output tells you what version you are running.

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
"PATH" variable. First, look at your current "PATH" variable by
entering the following:

   $ echo $PATH

Next, add the directory location for the BitBake binary to the "PATH".
Here is an example that adds the "/home/scott-lenovo/bitbake/bin"
directory to the front of the "PATH" variable:

   $ export PATH=/home/scott-lenovo/bitbake/bin:$PATH

You should now be able to enter the "bitbake" command from the command
line while working from any directory.


6.4. The Hello World Example
============================

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.

To help you understand how to use BitBake to build targets, the
example starts with nothing but the "bitbake" 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.

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 https://lists.openembedded.org/g/bitbake-devel
discussion mailing list about the BitBake build tool.

Note:

  This example was inspired by and drew heavily from Mailing List post
  - The BitBake equivalent of “Hello, World!”.

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: Configuration Files, Classes,
and Recipes. 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.

Following is the complete “Hello World” example.

1. **Create a Project Directory:** First, set up a directory for the
   “Hello World” project. Here is how you can do so in your home
   directory:

      $ mkdir ~/hello
      $ cd ~/hello

   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.

2. **Run BitBake:** At this point, you have nothing but a project
   directory. Run the "bitbake" command and see what it does:

      $ 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

   The majority of this output is specific to environment variables
   that are not directly relevant to BitBake. However, the very first
   message regarding the "BBPATH" variable and the
   "conf/bblayers.conf" file is relevant.

   When you run BitBake, it begins looking for metadata files. The
   *BBPATH* variable is what tells BitBake where to look for those
   files. "BBPATH" is not set and you need to set it. Without
   "BBPATH", BitBake cannot find any configuration files (".conf") or
   recipe files (".bb") at all. BitBake also cannot find the
   "bitbake.conf" file.

3. **Setting BBPATH:** For this example, you can set "BBPATH" in the
   same manner that you set "PATH" earlier in the appendix. You should
   realize, though, that it is much more flexible to set the "BBPATH"
   variable up in a configuration file for each project.

   From your shell, enter the following commands to set and export the
   "BBPATH" variable:

      $ BBPATH="projectdirectory"
      $ export BBPATH

   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.

4. **Run BitBake:** Now that you have "BBPATH" defined, run the
   "bitbake" command again:

      $ 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

   This sample output shows that BitBake could not find the
   "conf/bitbake.conf" 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 "conf/bitbake.conf" file.

5. **Creating conf/bitbake.conf:** The "conf/bitbake.conf" 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 "bitbake.conf" file, see
   http://git.openembedded.org/bitbake/tree/conf/bitbake.conf.

   Use the following commands to create the "conf" directory in the
   project directory:

      $ mkdir conf

   From within the "conf" directory, use some editor to create the
   "bitbake.conf" so that it contains the following:

      PN  = "${@bb.parse.BBHandler.vars_from_file(d.getVar('FILE', False),d)[0] or 'defaultpkgname'}"

      TMPDIR  = "${TOPDIR}/tmp"
      CACHE   = "${TMPDIR}/cache"
      STAMP   = "${TMPDIR}/${PN}/stamps"
      T       = "${TMPDIR}/${PN}/work"
      B       = "${TMPDIR}/${PN}"

   Note:

     Without a value for PN , the variables STAMP , T , and B ,
     prevent more than one recipe from working. You can fix this by
     either setting PN to have a value similar to what OpenEmbedded
     and BitBake use in the default bitbake.conf file (see previous
     example). Or, by manually updating each recipe to set PN . You
     will also need to include PN as part of the STAMP , T , and B
     variable definitions in the local.conf file.

   The "TMPDIR" variable establishes a directory that BitBake uses for
   build output and intermediate files other than the cached
   information used by the Setscene process. Here, the "TMPDIR"
   directory is set to "hello/tmp".

   Tip:

     You can always safely delete the tmp directory in order to
     rebuild a BitBake target. The build process creates the directory
     for you when you run BitBake.

   For information about each of the other variables defined in this
   example, check *PN*, *TOPDIR*, *CACHE*, *STAMP*, *T* or *B* to take
   you to the definitions in the glossary.

6. **Run BitBake:** After making sure that the "conf/bitbake.conf"
   file exists, you can run the "bitbake" command again:

      $ 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

   In the sample output, BitBake could not find the
   "classes/base.bbclass" file. You need to create that file next.

7. **Creating classes/base.bbclass:** BitBake uses class files to
   provide common code and functionality. The minimally required class
   for BitBake is the "classes/base.bbclass" file. The "base" class is
   implicitly inherited by every recipe. BitBake looks for the class
   in the "classes" directory of the project (i.e "hello/classes" in
   this example).

   Create the "classes" directory as follows:

      $ cd $HOME/hello
      $ mkdir classes

   Move to the "classes" directory and then create the "base.bbclass"
   file by inserting this single line: addtask build The minimal task
   that BitBake runs is the "do_build" task. This is all the example
   needs in order to build the project. Of course, the "base.bbclass"
   can have much more depending on which build environments BitBake is
   supporting.

8. **Run BitBake:** After making sure that the "classes/base.bbclass"
   file exists, you can run the "bitbake" command again:

      $ bitbake
      Nothing to do. Use 'bitbake world' to build everything, or run 'bitbake --help' for usage information.

   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.

9. **Creating a Layer:** 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 layers in the “Layers”
     section.

   Minimally, you need a recipe file and a layer configuration file in
   your layer. The configuration file needs to be in the "conf"
   directory inside the layer. Use these commands to set up the layer
   and the "conf" directory:

      $ cd $HOME
      $ mkdir mylayer
      $ cd mylayer
      $ mkdir conf

   Move to the "conf" directory and create a "layer.conf" file that
   has the following:

       BBPATH .= ":${LAYERDIR}"
       BBFILES += "${LAYERDIR}/\*.bb"
       BBFILE_COLLECTIONS += "mylayer"
      `BBFILE_PATTERN_mylayer := "^${LAYERDIR_RE}/"

   For information on these variables, click on *BBFILES*, *LAYERDIR*,
   *BBFILE_COLLECTIONS* or *BBFILE_PATTERN_mylayer* to go to the
   definitions in the glossary.

   You need to create the recipe file next. Inside your layer at the
   top-level, use an editor and create a recipe file named
   "printhello.bb" that has the following:

      DESCRIPTION = "Prints Hello World"
      PN = 'printhello'
      PV = '1'

      python do_build() {
         bb.plain("********************");
         bb.plain("*                  *");
         bb.plain("*  Hello, World!   *");
         bb.plain("*                  *");
         bb.plain("********************");
      }

   The recipe file simply provides a description of the recipe, the
   name, version, and the "do_build" task, which prints out “Hello
   World” to the console. For more information on *DESCRIPTION*, *PN*
   or *PV* follow the links to the glossary.

10. **Run BitBake With a Target:** Now that a BitBake target exists,
    run
       the command and provide that target:

          $ 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.

       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 "conf/bblayers.conf" that lists the
       layers for the project. Without this file, BitBake cannot find
       the recipe.

11. **Creating conf/bblayers.conf:** BitBake uses the
       "conf/bblayers.conf" file to locate layers needed for the
       project. This file must reside in the "conf" directory of the
       project (i.e. "hello/conf" for this example).

       Set your working directory to the "hello/conf" directory and
       then create the "bblayers.conf" file so that it contains the
       following:

          BBLAYERS ?= " \
              /home/<you>/mylayer \
          "

       You need to provide your own information for "you" in the file.

12. **Run BitBake With a Target:** Now that you have supplied the
       "bblayers.conf" file, run the "bitbake" command and provide the
       target:

          $ 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.

       Note:

         After the first execution, re-running bitbake printhello
         again will not result in a BitBake run that prints the same
         console output. The reason for this is that the first time
         the printhello.bb recipe’s do_build task executes
         successfully, BitBake writes a stamp file for the task. Thus,
         the next time you attempt to run the task using that same
         bitbake command, BitBake notices the stamp and therefore
         determines that the task does not need to be re-run. If you
         delete the tmp directory or run bitbake -c clean printhello
         and then re-run the build, the “Hello, World!” message will
         be printed again.
