# Hello World Example

## 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.

## 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.

## 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.

## 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 discussion mailing list about the BitBake build tool.

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.

   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 .

   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}"

   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, click on the links 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".

   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 the links 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 these variables, 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.

    BitBake finds the `printhello` recipe and successfully runs the task.

    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.