## 6.5. Running a Task

Tasks can either be a shell task or a Python task. For shell tasks, BitBake writes a shell script to `${WORKDIR}/temp/run.do_taskname.pid` and then executes the script. The generated shell script contains all the exported variables, and the shell functions with all variables expanded. Output from the shell script goes to the file `${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.

For Python tasks, BitBake executes the task internally and logs information to the controlling terminal. Future versions of BitBake will write the functions to files similar to the way shell tasks are handled. Logging will be handled in way similar to shell tasks as well.

Once all the tasks have been completed BitBake exits.

When running a task, BitBake tightly controls the execution environment of the build tasks to make sure unwanted contamination from the build machine cannot influence the build. Consequently, if you do want something to get passed into the build task's environment, you must take a few steps:

1. Tell BitBake to load what you want from the environment into the data store. You can do so through the `BB_ENV_EXTRAWHITE` variable. For example, assume you want to prevent the build system from accessing your `$HOME/.ccache` directory. The following command tells BitBake to load `CCACHE_DIR` from the environment into the data store:

            export BB_ENV_EXTRAWHITE="$BB_ENV_EXTRAWHITE CCACHE_DIR"

2. Tell BitBake to export what you have loaded into the environment store to the task environment of every running task. Loading something from the environment into the data store (previous step) only makes it available in the datastore. To export it to the task environment of every running task, use a command similar to the following in your `local.conf` or distro configuration file:

            export CCACHE_DIR

### Note

A side effect of the previous steps is that BitBake records the variable as a dependency of the build process in things like the shared state checksums. If doing so results in unnecessary rebuilds of tasks, you can whitelist the variable so that the shared state code ignores the dependency when it creates checksums. For information on this process, see the `BB_HASHBASE_WHITELIST` example in the "Checksums (Signatures)" section.