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.

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 -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 cloned to a directory that matches the previous listing except for dates and user names.

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. First, run BitBake to make sure it's working. From the BitBake source code directory, issue the following command: $ ./bin/bitbake --version BitBake Build Tool Core version 1.23.0, bitbake version 1.23.0 You are now ready to use BitBake. A final step to make development easier is to add the executable binary to your environment PATH. 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 using this form: $ export PATH=<path-to-bitbake-executable>:$PATH This will add the directory to the beginning of your PATH environment variable. You should now be able to enter the bitbake command at the command line to run BitBake. For a more permanent solution assuming you are running the BASH shell, edit ~/.bashrc and add the following to the end of that file: PATH=<path-to-bitbake-executable>:$PATH 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 that directory does not exist, create it, and then restart Vim.

The following example leaps directly into how BitBake works. While every attempt is made to explain what is happening, not everything can be covered. You can find further information in the "Syntax and Operators" chapter. 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. 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 good way to understand anything is to walk through the steps that take you to where you want to be and observe first principles. BitBake allows us to do this through the -D or Debug command-line parameter. The goal is to eventually compile a "Hello World" example. However, it is unknown what is needed 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. First, set up a directory for the "Hello World" project. Here is how you can do so in your home directory: $ mkdir ~/dev $ mkdir ~/dev/hello $ cd ~/dev/hello Within this new, empty directory, run BitBake with debugging output and see what happens: $ bitbake -DDD 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, DEFAULTS_PATH, XDG_SEAT_PATH, XDG_CONFIG_DIRS, LESSOPEN, DBUS_SESSION_BUS_ADDRESS, _, XDG_SESSION_COOKIE, DESKTOP_SESSION, LESSCLOSE, GNOME_KEYRING_PID, 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 is relevant and you need to rectify it by setting BBPATH. When you run BitBake, it begins looking for metadata files. The BBPATH variable is what tells BitBake where to look. You could set BBPATH in the same manner that you set PATH as shown earlier. However, it is much more flexible to set the BBPATH variable for each project. Without BBPATH, Bitbake cannot find any configuration files (.conf) or recipe files (.bb) at all. BitBake also cannot find the bitbake.conf file. It is standard practice to organize the project's directory tree to include both a conf/ and classes/ directory. You need to add those directories to your project: $ mkdir conf classes Once those directories are in place, you can copy the sample configuration files provided in the BitBake source tree to their appropriate directories. First, change to the BitBake source tree directory and then copy the directories: 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 Once you have copied these files into your project, you can now get back to resolving the BBPATH issue. The first configuration file that BitBake looks for is always bblayers.conf. With this knowledge, you know that to resolve your BBPATH error you can add a conf/bblayers.conf file to the project source tree and populate it with the BBPATH variable declaration. From your project source tree: $ vim conf/bblayers.conf Now add the following to the empty bblayers.conf file: BBPATH := "${TOPDIR}" Now, from the root of your project directory, 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, 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, DEFAULTS_PATH, XDG_SEAT_PATH, XDG_CONFIG_DIRS, LESSOPEN, DBUS_SESSION_BUS_ADDRESS, _, XDG_SESSION_COOKIE, DESKTOP_SESSION, LESSCLOSE, GNOME_KEYRING_PID, UBUNTU_MENUPROXY, OLDPWD, XDG_DATA_DIRS, COLORTERM, LS_COLORS DEBUG: Found bblayers.conf (/home/scott-lenovo/dev/hello/conf/bblayers.conf) DEBUG: LOAD /home/scott-lenovo/dev/hello/conf/bblayers.conf DEBUG: LOAD /home/scott-lenovo/dev/hello/conf/bitbake.conf DEBUG: BB configuration INHERITs:0: inheriting /home/scott-lenovo/dev/hello/classes/base.bbclass DEBUG: BB /home/scott-lenovo/dev/hello/classes/base.bbclass: handle(data, include) DEBUG: LOAD /home/scott-lenovo/dev/hello/classes/base.bbclass DEBUG: Clearing SRCREV cache due to cache policy of: clear DEBUG: Using cache in '/home/scott-lenovo/dev/hello/tmp/cache/local_file_checksum_cache.dat' DEBUG: Using cache in '/home/scott-lenovo/dev/hello/tmp/cache/bb_codeparser.dat' DEBUG: Features set [3] (was [3]) From this point forward in the example, the environment variable removal messages are ignored and omitted. Examine the relevant DEBUG messages: