BitBake is a program written in Python that interprets the metadata that makes up Poky. At some point, people wonder what actually happens when you type bitbake poky-image-sato. This section aims to give an overview of what happens behind the scenes from a BitBake perspective. It is worth noting that BitBake aims to be a generic "task" executor capable of handling complex dependency relationships. As such it has no real knowledge of what the tasks it is executing actually do. It just considers a list of tasks with dependencies and handles metadata consisting of variables in a certain format which get passed to the tasks.

The first thing BitBake does is work out its configuration by looking for a file called bitbake.conf. BitBake searches through the BBPATH environment variable looking for a conf/ directory containing a bitbake.conf file and adds the first bitbake.conf file found in BBPATH (similar to the PATH environment variable). For Poky, bitbake.conf is found in meta/conf/. In Poky, bitbake.conf lists other configuration files to include from a conf/ directory below the directories listed in BBPATH. In general the most important configuration file from a user's perspective is local.conf, which contains a users customized settings for Poky. Other notable configuration files are the distribution configuration file (set by the DISTRO variable) and the machine configuration file (set by the MACHINE variable). The DISTRO and MACHINE environment variables are both usually set in the local.conf file. Valid distribution configuration files are available in the meta/conf/distro/ directory and valid machine configuration files in the meta/conf/machine/ directory. Within the meta/conf/machine/include/ directory are various tune-*.inc configuration files which provide common "tuning" settings specific to and shared between particular architectures and machines. After the parsing of the configuration files some standard classes are included. In particular, base.bbclass is always included, as will any other classes specified in the configuration using the INHERIT variable. Class files are searched for in a classes subdirectory under the paths in BBPATH in the same way as configuration files. After the parsing of the configuration files is complete, the variable BBFILES is set, usually in local.conf, and defines the list of places to search for .bb files. By default this specifies the meta/packages/ directory within Poky, but other directories such as meta-extras/ can be included too. Adding extra content to BBFILES is best acheived through the use of BitBake "layers". BitBake parses each .bb file in BBFILES and stores the values of various variables. In summary, for each .bb file the configuration + base class of variables are set, followed by the data in the .bb file itself, followed by any inherit commands that .bb file might contain. Parsing .bb files is a time consuming process, so a cache is kept to speed up subsequent parsing. This cache is invalid if the timestamp of the .bb file itself has changed, or if the timestamps of any of the include, configuration or class files the .bb file depends on have changed.

Once all the .bb files have been parsed, BitBake will proceed to build "poky-image-sato" (or whatever was specified on the commandline) and looks for providers of that target. Once a provider is selected, BitBake resolves all the dependencies for the target. In the case of "poky-image-sato", it would lead to task-base.bb which in turn would lead to packages like Contacts, Dates, BusyBox and these in turn depend on glibc and the toolchain. Sometimes a target might have multiple providers and a common example is "virtual/kernel" that is provided by each kernel package. Each machine will often elect the best provider of its kernel with a line like the following in the machine configuration file: PREFERRED_PROVIDER_virtual/kernel = "linux-rp" The default PREFERRED_PROVIDER is the provider with the same name as the target. Understanding how providers are chosen is complicated by the fact multiple versions might be present. BitBake defaults to the highest version of a provider by default. Version comparisons are made using the same method as Debian. The PREFERRED_VERSION variable can be used to specify a particular version (usually in the distro configuration) but the order can also be influenced by the DEFAULT_PREFERENCE variable. By default files have a preference of "0". Setting the DEFAULT_PREFERENCE to "-1" will make a package unlikely to be used unless it was explicitly referenced and "1" makes it likely the package will be used. PREFERRED_VERSION overrides any DEFAULT_PREFERENCE. DEFAULT_PREFERENCE is often used to mark more experimental new versions of packages until they've undergone sufficient testing to be considered stable. The end result is that internally, BitBake has now built a list of providers for each target it needs in order of priority.

Each target BitBake builds consists of multiple tasks (e.g. fetch, unpack, patch, configure, compile etc.). For best performance on multi-core systems, BitBake considers each task as an independent entity with a set of dependencies. There are many variables that are used to signify these dependencies and more information can be found about these in the BitBake manual. At a basic level it is sufficient to know that BitBake uses the DEPENDS and RDEPENDS variables when calculating dependencies and descriptions of these variables are available through the links.

Based on the generated list of providers and the dependency information, BitBake can now calculate exactly which tasks it needs to run and in what order. The build now starts with BitBake forking off threads up to the limit set in the BB_NUMBER_THREADS variable as long as there are tasks ready to run, i.e. tasks with all their dependencies met. As each task completes, a timestamp is written to the directory specified by the STAMPS variable (usually build/tmp/stamps/*/). On subsequent runs, BitBake looks at the STAMPS directory and will not rerun tasks its already completed unless a timestamp is found to be invalid. Currently, invalid timestamps are only considered on a per .bb file basis so if for example the configure stamp has a timestamp greater than the compile timestamp for a given target the compile task would rerun but this has no effect on other providers depending on that target. This could change or become configurable in future versions of BitBake. Some tasks are marked as "nostamp" tasks which means no timestamp file will be written and the task will always rerun. Once all the tasks have been completed BitBake exits.

It's worth noting what BitBake does to run a task. A task can either be a shell task or a python task. For shell tasks, BitBake writes a shell script to ${WORKDIR}/temp/run.do_taskname.pid and then executes the script. The generated shell script contains all the exported variables, and the shell functions with all variables expanded. Output from the shell script is sent to the file ${WORKDIR}/temp/log.do_taskname.pid. Looking at the expanded shell functions in the run file and the output in the log files is a useful debugging technique. Python functions are executed internally to BitBake itself and logging goes to the controlling terminal. Future versions of BitBake will write the functions to files in a similar way to shell functions and logging will also go to the log files in a similar way.

To quote from "bitbake --help": Usage: bitbake [options] [package ...] Executes the specified task (default is 'build') for a given set of BitBake files. It expects that BBFILES is defined, which is a space separated list of files to be executed. BBFILES does support wildcards. Default BBFILES are the .bb files in the current directory. Options: --version show program's version number and exit -h, --help show this help message and exit -b BUILDFILE, --buildfile=BUILDFILE execute the task against this .bb file, rather than a package from BBFILES. -k, --continue continue as much as possible after an error. While the target that failed, and those that depend on it, cannot be remade, the other dependencies of these targets can be processed all the same. -a, --tryaltconfigs continue with builds by trying to use alternative providers where possible. -f, --force force run of specified cmd, regardless of stamp status -c CMD, --cmd=CMD Specify task to execute. Note that this only executes the specified task for the providee and the packages it depends on, i.e. 'compile' does not implicitly call stage for the dependencies (IOW: use only if you know what you are doing). Depending on the base.bbclass a listtasks tasks is defined and will show available tasks -r FILE, --read=FILE read the specified file before bitbake.conf -v, --verbose output more chit-chat to the terminal -D, --debug Increase the debug level. You can specify this more than once. -n, --dry-run don't execute, just go through the motions -S, --dump-signatures don't execute, just dump out the signature construction information -p, --parse-only quit after parsing the BB files (developers only) -d, --disable-psyco disable using the psyco just-in-time compiler (not recommended) -s, --show-versions show current and preferred versions of all packages -e, --environment show the global or per-package environment (this is what used to be bbread) -g, --graphviz emit the dependency trees of the specified packages in the dot syntax -I EXTRA_ASSUME_PROVIDED, --ignore-deps=EXTRA_ASSUME_PROVIDED Assume these dependencies don't exist and are already provided (equivalent to ASSUME_PROVIDED). Useful to make dependency graphs more appealing -l DEBUG_DOMAINS, --log-domains=DEBUG_DOMAINS Show debug logging for the specified logging domains -P, --profile profile the command and print a report -u UI, --ui=UI userinterface to use --revisions-changed Set the exit code depending on whether upstream floating revisions have changed or not

As well as the containing the parsing and task/dependency handling code, BitBake also contains a set of "fetcher" modules which allow fetching of source code from various types of sources. Example sources might be from disk with the metadata, from websites, from remote shell accounts or from SCM systems like cvs/subversion/git. The fetchers are usually triggered by entries in SRC_URI. Information about the options and formats of entries for specific fetchers can be found in the BitBake manual. One useful feature for certain SCM fetchers is the ability to "auto-update" when the upstream SCM changes version. Since this requires certain functionality from the SCM only certain systems support it, currently Subversion, Bazaar and to a limited extent, Git. It works using the SRCREV variable. See the developing with an external SCM based project section for more information.