# The Yocto Project Open Source Development Environment

This chapter helps you understand the Yocto Project as an open source development project. In general, working in an open source environment is very different from working in a closed, proprietary environment. Additionally, the Yocto Project uses specific tools and constructs as part of its development environment. This chapter specifically addresses open source philosophy, using the Yocto Project in a team environment, source repositories, Yocto Project terms, licensing, the open source distributed version control system Git, workflows, bug tracking, and how to submit changes.

## Open Source Philosophy

Open source philosophy is characterized by software development directed by peer production and collaboration through an active community of developers. Contrast this to the more standard centralized development models used by commercial software companies where a finite set of developers produces a product for sale using a defined set of procedures that ultimately result in an end product whose architecture and source material are closed to the public.

Open source projects conceptually have differing concurrent agendas, approaches, and production. These facets of the development process can come from anyone in the public (community) that has a stake in the software project. The open source environment contains new copyright, licensing, domain, and consumer issues that differ from the more traditional development environment. In an open source environment, the end product, source material, and documentation are all available to the public at no cost.

A benchmark example of an open source project is the Linux Kernel, which was initially conceived and created by Finnish computer science student Linus Torvalds in 1991. Conversely, a good example of a non-open source project is the Windows family of operating systems developed by Microsoft Corporation.

Wikipedia has a good historical description of the Open Source Philosophy here. You can also find helpful information on how to participate in the Linux Community here.

## Using the Yocto Project in a Team Environment

It might not be immediately clear how you can use the Yocto Project in a team environment, or scale it for a large team of developers. One of the strengths of the Yocto Project is that it is extremely flexible. Thus, you can adapt it to many different use cases and scenarios. However, these characteristics can cause a struggle if you are trying to create a working setup that scales across a large team.

To help with these types of situations, this section presents some of the project's most successful experiences, practices, solutions, and available technologies that work well. Keep in mind, the information here is a starting point. You can build off it and customize it to fit any particular working environment and set of practices.

### System Configurations

Systems across a large team should meet the needs of two types of developers: those working on the contents of the operating system image itself and those developing applications. Regardless of the type of developer, their workstations must be both reasonably powerful and run Linux.

#### Application Development

For developers who mainly do application level work on top of an existing software stack, here are some practices that work best:

- Use a pre-built toolchain that contains the software stack itself. Then, develop the application code on top of the stack. This method works well for small numbers of relatively isolated applications.

- When possible, use the Yocto Project plug-in for the Eclipse IDE and other pieces of Application Development Technology (ADT). For more information, see the "Application Development Workflow" section as well as the Yocto Project Application Developer's Guide.

- Keep your cross-development toolchains updated. You can do this through provisioning either as new toolchain downloads or as updates through a package update mechanism using `opkg` to provide updates to an existing toolchain. The exact mechanics of how and when to do this are a question for local policy.

- Use multiple toolchains installed locally into different locations to allow development across versions.

#### Core System Development

For core system development, it is often best to have the build system itself available on the developer workstations so developers can run their own builds and directly rebuild the software stack. You should keep the core system unchanged as much as possible and do your work in layers on top of the core system. Doing so gives you a greater level of portability when upgrading to new versions of the core system or Board Support Packages (BSPs). You can share layers amongst the developers of a particular project and contain the policy configuration that defines the project.

Aside from the previous best practices, there exists a number of tips and tricks that can help speed up core development projects:

- Use a Shared State Cache (sstate) among groups of developers who are on a fast network. The best way to share sstate is through a Network File System (NFS) share. The first user to build a given component for the first time contributes that object to the sstate, while subsequent builds from other developers then reuse the object rather than rebuild it themselves.

  Although it is possible to use other protocols for the sstate such as HTTP and FTP, you should avoid these. Using HTTP limits the sstate to read-only and FTP provides poor performance.

- Have autobuilders contribute to the sstate pool similarly to how the developer workstations contribute. For information, see the Autobuilders section.

- Build stand-alone tarballs that contain "missing" system requirements if for some reason developer workstations do not meet minimum system requirements such as latest Python versions, `chrpath`, or other tools. You can install and relocate the tarball exactly as you would the usual cross-development toolchain so that all developers can meet minimum version requirements on most distributions.

- Use a small number of shared, high performance systems for testing purposes (e.g. dual six core Xeons with 24GB RAM and plenty of disk space). Developers can use these systems for wider, more extensive testing while they continue to develop locally using their primary development system.

- Enable the PR Service when package feeds need to be incremental with continually increasing PR values. Typically, this situation occurs when you use or publish package feeds and use a shared state. You should enable the PR Service for all users who use the shared state pool. For more information on the PR Service, see the "Working With a PR Service".

### Source Control Management (SCM)

Keeping your Metadata and any software you are developing under the control of an SCM system that is compatible with the OpenEmbedded build system is advisable. Of the SCMs BitBake supports, the Yocto Project team strongly recommends using Git. Git is a distributed system that is easy to backup (each checkout is a backup in itself), allows you to work remotely, and then connects back to the infrastructure.

For information about BitBake and SCMs, see the BitBake manual located in the `bitbake/doc/manual` directory of the Source Directory.

It is relatively easy to set up Git services and create infrastructure like http://git.yoctoproject.org, which is based on server software called `gitolite` with `cgit` being used to generate the web interface that lets you view the repositories. The `gitolite` software identifies users using `ssh` keys and allows branch-based access controls to repositories that you can control as little or as much as necessary.

The setup of these services is beyond the scope of this manual. However, sites such as these exist that describe how to perform setup:

- Git documentation: Describes how to install `gitolite` on the server.

- The `gitolite` master index: All topics for `gitolite`.

- `cgit` index: A `README` file on how to create a fast web interface for Git.

### Autobuilders

Autobuilders are often the core of a development project. It is here that changes from individual developers are brought together and centrally tested and subsequent decisions about releases can be made. Autobuilders also allow for "continuous integration" style testing of software components and regression identification and tracking.

See "Welcome to the buildbot for the Yocto Project" for the Yocto Project's reference implementation that uses buildbot. The Yocto Project team has found this implementation works well in this role. A public example of this is the Yocto Project Autobuilders, which we use to test the overall health of the project.

The features of this system are:

- Highlights when commits break the build.

- Populates an sstate cache from which developers can pull rather than requiring local builds.

- Allows commit hook triggers, which trigger builds when commits are made.

- Allows triggering of automated image booting and testing under the QuickEMUlator (QEMU).

- Supports incremental build testing and from scratch builds.

- Shares output that allows developer testing and historical regression investigation.

- Creates output that can be used for releases.

- Allows scheduling of builds so that resources can be used efficiently.

### Policies and Change Flow

The Yocto Project itself uses a hierarchical structure and a pull model. Scripts exist to create and send pull requests (i.e. `create-pull-request` and `send-pull-request`). This model is in line with other open source projects where maintainers are responsible for specific areas of the project and a single maintainer handles the final "top-of-tree" merges.

You can also use a more collective push model. The `gitolite` software supports both the push and pull models quite easily.

As with any development environment, it is important to document the policy used as well as any main project guidelines so they are understood by everyone. It is also a good idea to have well structured commit messages, which are usually a part of a project's guidelines. Good commit messages are essential when looking back in time and trying to understand why changes were made.

If you discover that changes are needed to the core layer of the project, it is worth sharing those with the community as soon as possible. Chances are if you have discovered the need for changes, someone else in the community needs them also.

### Summary

This section summarizes the key recommendations described in the previous sections:

- Use Git as the source control system.

- Maintain your Metadata in layers that make sense for your situation. See the "Understanding and Creating Layers" section for more information on layers.

- Separate the project's Metadata and code by using separate Git repositories. See the "Yocto Project Source Repositories" section for information on these repositories. See the "Getting Set Up" section for information on how to set up various Yocto Project related Git repositories.

- Set up the directory for the shared state cache (`SSTATE_DIR`) where it makes sense. For example, set up the sstate cache on a system used by developers in the same organization and share the same source directories on their machines.

- Set up an Autobuilder and have it populate the sstate cache and source directories.

- The Yocto Project community encourages you to send patches to the project to fix bugs or add features. If you do submit patches, follow the project commit guidelines for writing good commit messages. See the "How to Submit a Change" section.

- Send changes to the core sooner than later as others likely run into the same issues. For some guidance on mailing lists to use, see the list in the "How to Submit a Change" section. For a description of the available mailing lists, see "Mailing Lists" section in the Yocto Project Reference Manual.

## Yocto Project Source Repositories

The Yocto Project team maintains complete source repositories for all Yocto Project files at . This web-based source code browser is organized into categories by function such as IDE Plugins, Matchbox, Poky, Yocto Linux Kernel, and so forth. From the interface, you can click on any particular item in the "Name" column and see the URL at the bottom of the page that you need to clone a Git repository for that particular item. Having a local Git repository of the Source Directory (poky) allows you to make changes, contribute to the history, and ultimately enhance the Yocto Project's tools, Board Support Packages, and so forth.

Conversely, if you are a developer that is not interested in contributing back to the Yocto Project, you have the ability to simply download and extract release tarballs and use them within the Yocto Project environment. All that is required is a particular release of the Yocto Project and your application source code.

For any supported release of Yocto Project, you can go to the Yocto Project Website and select the "Downloads" tab and get a tarball of the release. You can also go to this site to download any supported BSP tarballs. Unpacking the tarball gives you a hierarchical Source Directory that lets you develop using the Yocto Project.

Once you are set up through either tarball extraction or a checkout of Git repositories, you are ready to develop.

In summary, here is where you can get the project files needed for development:

- :SSource Repositories: This area contains IDE Plugins, Matchbox, Poky, Poky Support, Tools, Yocto Linux Kernel, and Yocto Metadata Layers. You can create local copies of Git repositories for each of these areas.

- :Index of /releases: This area contains index releases such as the Eclipse Yocto Plug-in, miscellaneous support, poky, pseudo, installers for cross-development toolchains, and all released versions of Yocto Project in the form of images or tarballs. Downloading and extracting these files does not produce a local copy of the Git repository but rather a snapshot of a particular release or image.

- "Downloads" page for the Yocto Project Website: Access this page by going to the website and then selecting the "Downloads" tab. This page allows you to download any Yocto Project release or Board Support Package (BSP) in tarball form. The tarballs are similar to those found in the Index of /releases: area.

## Yocto Project Terms

Following is a list of terms and definitions users new to the Yocto Project development environment might find helpful. While some of these terms are universal, the list includes them just in case:

- Append Files: Files that append build information to a recipe file. Append files are known as BitBake append files and `.bbappend` files. The OpenEmbedded build system expects every append file to have a corresponding recipe (`.bb`) file. Furthermore, the append file and corresponding recipe file must use the same root filename. The filenames can differ only in the file type suffix used (e.g. `formfactor_0.0.bb` and `formfactor_0.0.bbappend`).

  Information in append files overrides the information in the similarly-named recipe file. For an example of an append file in use, see the "Using .bbappend Files" section.

- BitBake: The task executor and scheduler used by the OpenEmbedded build system to build images. For more information on BitBake, see the BitBake documentation in the `bitbake/doc/manual` directory of the Source Directory.

- Build Directory: This term refers to the area used by the OpenEmbedded build system for builds. The area is created when you `source` the setup environment script that is found in the Source Directory (i.e. `&OE_INIT_FILE;`). The `TOPDIR` variable points to the Build Directory.

  You have a lot of flexibility when creating the Build Directory. Following are some examples that show how to create the directory:

  - Create the Build Directory in your current working directory and name it `build`. This is the default behavior.

             $ source &OE_INIT_PATH;

  - Provide a directory path and specifically name the build directory. This next example creates a Build Directory named `YP-&POKYVERSION;` in your home directory within the directory `mybuilds`. If `mybuilds` does not exist, the directory is created for you:

             $ source &OE_INIT_PATH; $HOME/mybuilds/YP-&POKYVERSION;

  - Provide an existing directory to use as the Build Directory and use the default `build` name.

             $ source &OE_INIT_PATH; $HOME/mybuilds/

- Build System: In the context of the Yocto Project, this term refers to the OpenEmbedded build system used by the project. This build system is based on the project known as "Poky." For some historical information about Poky, see the Poky term.

- Classes: Files that provide for logic encapsulation and inheritance so that commonly used patterns can be defined once and then easily used in multiple recipes. Class files end with the `.bbclass` filename extension.

- Configuration File: Configuration information in various `.conf` files provides global definitions of variables. The `conf/local.conf` configuration file in the Build Directory contains user-defined variables that affect each build. The `meta-yocto/conf/distro/poky.conf` configuration file defines Yocto "distro" configuration variables used only when building with this policy. Machine configuration files, which are located throughout the Source Directory, define variables for specific hardware and are only used when building for that target (e.g. the `machine/beagleboard.conf` configuration file defines variables for the Texas Instruments ARM Cortex-A8 development board). Configuration files end with a `.conf` filename extension.

- Cross-Development Toolchain: A collection of software development tools and utilities that allow you to develop software for targeted architectures. This toolchain contains cross-compilers, linkers, and debuggers that are specific to an architecture. You can use the OpenEmbedded build system to build a cross-development toolchain installer that, when run, installs the toolchain that contains the development tools you need to cross-compile and test your software. The Yocto Project ships with images that contain installers for toolchains for supported architectures as well. Sometimes this toolchain is referred to as the meta-toolchain.

  Following is a list of toolchain recipes with brief descriptions of each:

  - `gcc-cross-initial`: The initial compiler needed to bootstrap the toolchain that runs on the host and is used to build software for the target. This tool is a "native" package.

  - `gcc-cross-intermediate`: The second stage of the bootstrap process that runs on the host and builds software for the target. This tool is a "native" package.

  - `gcc-cross`: The the final stage of the bootstrap process that results in the cross compiler that runs on the host and builds software for the target. If you are replacing the cross compiler toolchain with a custom version, this is what you must replace. This tool is a "native" package.

  - `gcc-runtime`: Runtime libraries from the toolchain bootstrapping process. This tool produces a binary for the target.

  - `gcc-crosssdk-initial/intermediate`: Stage 1 and 2 of the a cross compiler that runs on the host and builds for the SDK. Often the SDK is not the same target as the host. This tool is a "native" binary.

  - `gcc-crosssdk`: The final stage of the SDK compiler. This tool is a "native" binary. The tool runs on the host and builds for the SDK.

  - `gcc-cross-canadian`: The compiler that runs on the SDK machine and is included with the SDK that builds software for the target. This tool is a "nativesdk" package.

- Image: An image is the result produced when BitBake processes a given collection of recipes and related Metadata. Images are the binary output that run on specific hardware or QEMU and for specific use cases. For a list of the supported image types that the Yocto Project provides, see the "Images" chapter in the Yocto Project Reference Manual.

- Layer: A collection of recipes representing the core, a BSP, or an application stack. For a discussion on BSP Layers, see the "BSP Layers" section in the Yocto Project Board Support Packages (BSP) Developer's Guide.

- Metadata: The files that BitBake parses when building an image. In general, Metadata includes recipes, classes, and configuration files. In the context of the kernel ("kernel Metadata"), it refers to Metadata in the `meta` branches of the kernel source Git repositories.

- OE-Core: A core set of Metadata originating with OpenEmbedded (OE) that is shared between OE and the Yocto Project. This Metadata is found in the `meta` directory of the Source Directory.

- Package: In the context of the Yocto Project, this term refers to the packaged output from a baked recipe. A package is generally the compiled binaries produced from the recipe's sources. You "bake" something by running it through BitBake.

  It is worth noting that the term "package" can, in general, have subtle meanings. For example, the packages referred to in the "The Packages" section are compiled binaries that when installed add functionality to your Linux distribution.

  Another point worth noting is that historically within the Yocto Project, recipes were referred to as packages - thus, the existence of several BitBake variables that are seemingly mis-named, (e.g. `PR`, `PRINC`, `PV`, and `PE`).

- Poky: The term "poky" can mean several things. In its most general sense, it is an open-source project that was initially developed by OpenedHand. With OpenedHand, poky was developed off of the existing OpenEmbedded build system becoming a build system for embedded images. After Intel Corporation acquired OpenedHand, the project poky became the basis for the Yocto Project's build system. Within the Yocto Project source repositories, `poky` exists as a separate Git repository that can be cloned to yield a local copy on the host system. Thus, "poky" can refer to the local copy of the Source Directory used to develop within the Yocto Project.

- Recipe: A set of instructions for building packages. A recipe describes where you get source code and which patches to apply. Recipes describe dependencies for libraries or for other recipes, and they also contain configuration and compilation options. Recipes contain the logical unit of execution, the software/images to build, and use the `.bb` file extension.

- Source Directory: This term refers to the directory structure created as a result of either downloading and unpacking a Yocto Project release tarball or creating a local copy of the `poky` Git repository `git://git.yoctoproject.org/poky`. Sometimes you might hear the term "poky directory" used to refer to this directory structure.

  The OpenEmbedded build system does not support file or directory names that contain spaces. Be sure that the Source Directory you use does not contain these types of names.

  The Source Directory contains BitBake, Documentation, Metadata and other files that all support the Yocto Project. Consequently, you must have the Source Directory in place on your development system in order to do any development using the Yocto Project.

  For tarball expansion, the name of the top-level directory of the Source Directory is derived from the Yocto Project release tarball. For example, downloading and unpacking `&YOCTO_POKY_TARBALL;` results in a Source Directory whose top-level folder is named `&YOCTO_POKY;`. If you create a local copy of the Git repository, then you can name the repository anything you like. Throughout much of the documentation, `poky` is used as the name of the top-level folder of the local copy of the poky Git repository. So, for example, cloning the `poky` Git repository results in a local Git repository whose top-level folder is also named `poky`.

  It is important to understand the differences between the Source Directory created by unpacking a released tarball as compared to cloning `git://git.yoctoproject.org/poky`. When you unpack a tarball, you have an exact copy of the files based on the time of release - a fixed release point. Any changes you make to your local files in the Source Directory are on top of the release. On the other hand, when you clone the `poky` Git repository, you have an active development repository. In this case, any local changes you make to the Source Directory can be later applied to active development branches of the upstream `poky` Git repository.

  Finally, if you want to track a set of local changes while starting from the same point as a release tarball, you can create a local Git branch that reflects the exact copy of the files at the time of their release. You do this by using Git tags that are part of the repository.

  For more information on concepts related to Git repositories, branches, and tags, see the "Repositories, Tags, and Branches" section.

- Tasks: Arbitrary groups of software Recipes. You use tasks to hold recipes that, when built, usually accomplish a single task. For example, a task could contain the recipes for a company’s proprietary or value-add software. Or, the task could contain the recipes that enable graphics. A task is really just another recipe. Because task files are recipes, they end with the `.bb` filename extension.

- Upstream: A reference to source code or repositories that are not local to the development system but located in a master area that is controlled by the maintainer of the source code. For example, in order for a developer to work on a particular piece of code, they need to first get a copy of it from an "upstream" source.

## Licensing

Because open source projects are open to the public, they have different licensing structures in place. License evolution for both Open Source and Free Software has an interesting history. If you are interested in this history, you can find basic information here:

- Open source license history

- Free software license history

In general, the Yocto Project is broadly licensed under the Massachusetts Institute of Technology (MIT) License. MIT licensing permits the reuse of software within proprietary software as long as the license is distributed with that software. MIT is also compatible with the GNU General Public License (GPL). Patches to the Yocto Project follow the upstream licensing scheme. You can find information on the MIT license at here. You can find information on the GNU GPL here.

When you build an image using the Yocto Project, the build process uses a known list of licenses to ensure compliance. You can find this list in the Source Directory at `meta/files/common-licenses`. Once the build completes, the list of all licenses found and used during that build are kept in the Build Directory at `tmp/deploy/images/licenses`.

If a module requires a license that is not in the base list, the build process generates a warning during the build. These tools make it easier for a developer to be certain of the licenses with which their shipped products must comply. However, even with these tools it is still up to the developer to resolve potential licensing issues.

The base list of licenses used by the build process is a combination of the Software Package Data Exchange (SPDX) list and the Open Source Initiative (OSI) projects. SPDX Group is a working group of the Linux Foundation that maintains a specification for a standard format for communicating the components, licenses, and copyrights associated with a software package. OSI is a corporation dedicated to the Open Source Definition and the effort for reviewing and approving licenses that are OSD-conformant.

You can find a list of the combined SPDX and OSI licenses that the Yocto Project uses here.

For information that can help you to maintain compliance with various open source licensing during the lifecycle of a product created using the Yocto Project, see the "Maintaining Open Source License Compliance During Your Product's Lifecycle" section.

## Git

The Yocto Project uses Git, which is a free, open source distributed version control system. Git supports distributed development, non-linear development, and can handle large projects. It is best that you have some fundamental understanding of how Git tracks projects and how to work with Git if you are going to use the Yocto Project for development. This section provides a quick overview of how Git works and provides you with a summary of some essential Git commands.

For more information on Git, see . If you need to download Git, go to .

### Repositories, Tags, and Branches

As mentioned earlier in the section "Yocto Project Source Repositories", the Yocto Project maintains source repositories at . If you look at this web-interface of the repositories, each item is a separate Git repository.

Git repositories use branching techniques that track content change (not files) within a project (e.g. a new feature or updated documentation). Creating a tree-like structure based on project divergence allows for excellent historical information over the life of a project. This methodology also allows for an environment from which you can do lots of local experimentation on projects as you develop changes or new features.

A Git repository represents all development efforts for a given project. For example, the Git repository `poky` contains all changes and developments for Poky over the course of its entire life. That means that all changes that make up all releases are captured. The repository maintains a complete history of changes.

You can create a local copy of any repository by "cloning" it with the Git `clone` command. When you clone a Git repository, you end up with an identical copy of the repository on your development system. Once you have a local copy of a repository, you can take steps to develop locally. For examples on how to clone Git repositories, see the "Getting Set Up" section.

It is important to understand that Git tracks content change and not files. Git uses "branches" to organize different development efforts. For example, the `poky` repository has `bernard`, `edison`, `denzil`, `danny` and `master` branches among others. You can see all the branches by going to and clicking on the `[...]` link beneath the "Branch" heading.

Each of these branches represents a specific area of development. The `master` branch represents the current or most recent development. All other branches represent off-shoots of the `master` branch.

When you create a local copy of a Git repository, the copy has the same set of branches as the original. This means you can use Git to create a local working area (also called a branch) that tracks a specific development branch from the source Git repository. in other words, you can define your local Git environment to work on any development branch in the repository. To help illustrate, here is a set of commands that creates a local copy of the `poky` Git repository and then creates and checks out a local Git branch that tracks the Yocto Project &DISTRO; Release (&DISTRO_NAME;) development:

         $ cd ~
         $ git clone git://git.yoctoproject.org/poky
         $ cd poky
         $ git checkout -b &DISTRO_NAME; origin/&DISTRO_NAME;

In this example, the name of the top-level directory of your local Source Directory is `poky`, and the name of that local working area (local branch) you just created and checked out is `&DISTRO_NAME;`. The files in your local repository now reflect the same files that are in the `&DISTRO_NAME;` development branch of the Yocto Project's `poky` upstream repository. It is important to understand that when you create and checkout a local working branch based on a branch name, your local environment matches the "tip" of that development branch at the time you created your local branch, which could be different than the files at the time of a similarly named release. In other words, creating and checking out a local branch based on the `&DISTRO_NAME;` branch name is not the same as cloning and checking out the `master` branch. Keep reading to see how you create a local snapshot of a Yocto Project Release.

Git uses "tags" to mark specific changes in a repository. Typically, a tag is used to mark a special point such as the final change before a project is released. You can see the tags used with the `poky` Git repository by going to and clicking on the `[...]` link beneath the "Tag" heading.

Some key tags are `bernard-5.0`, `denzil-7.0`, and `&DISTRO_NAME;-&POKYVERSION;`. These tags represent Yocto Project releases.

When you create a local copy of the Git repository, you also have access to all the tags. Similar to branches, you can create and checkout a local working Git branch based on a tag name. When you do this, you get a snapshot of the Git repository that reflects the state of the files when the change was made associated with that tag. The most common use is to checkout a working branch that matches a specific Yocto Project release. Here is an example:

         $ cd ~
         $ git clone git://git.yoctoproject.org/poky
         $ cd poky
         $ git checkout -b my-&DISTRO_NAME;-&POKYVERSION; &DISTRO_NAME;-&POKYVERSION;

In this example, the name of the top-level directory of your local Yocto Project Files Git repository is `poky`. And, the name of the local branch you have created and checked out is `my-&DISTRO_NAME;-&POKYVERSION;`. The files in your repository now exactly match the Yocto Project &DISTRO; Release tag (`&DISTRO_NAME;-&POKYVERSION;`). It is important to understand that when you create and checkout a local working branch based on a tag, your environment matches a specific point in time and not the entire development branch.

### Basic Commands

Git has an extensive set of commands that lets you manage changes and perform collaboration over the life of a project. Conveniently though, you can manage with a small set of basic operations and workflows once you understand the basic philosophy behind Git. You do not have to be an expert in Git to be functional. A good place to look for instruction on a minimal set of Git commands is here. If you need to download Git, you can do so here.

If you don’t know much about Git, you should educate yourself by visiting the links previously mentioned.

The following list briefly describes some basic Git operations as a way to get started. As with any set of commands, this list (in most cases) simply shows the base command and omits the many arguments they support. See the Git documentation for complete descriptions and strategies on how to use these commands:

- `git init`: Initializes an empty Git repository. You cannot use Git commands unless you have a `.git` repository.

- `git clone`: Creates a clone of a repository. During collaboration, this command allows you to create a local repository that is on equal footing with a fellow developer’s repository.

- `git add`: Stages updated file contents to the index that Git uses to track changes. You must stage all files that have changed before you can commit them.

- `git commit`: Creates a "commit" that documents the changes you made. Commits are used for historical purposes, for determining if a maintainer of a project will allow the change, and for ultimately pushing the change from your local Git repository into the project’s upstream (or master) repository.

- `git status`: Reports any modified files that possibly need staged and committed.

- `git checkout <branch-name>`: Changes your working branch. This command is analogous to "cd".

- `git checkout –b <working-branch>`: Creates a working branch on your local machine where you can isolate work. It is a good idea to use local branches when adding specific features or changes. This way if you do not like what you have done you can easily get rid of the work.

- `git branch`: Reports existing local branches and tells you the branch in which you are currently working.

- `git branch -D <branch-name>`: Deletes an existing local branch. You need to be in a local branch other than the one you are deleting in order to delete `<branch-name>`.

- `git pull`: Retrieves information from an upstream Git repository and places it in your local Git repository. You use this command to make sure you are synchronized with the repository from which you are basing changes (.e.g. the master branch).

- `git push`: Sends all your committed local changes to an upstream Git repository (e.g. a contribution repository). The maintainer of the project draws from these repositories when adding changes to the project’s master repository or other development branch.

- `git merge`: Combines or adds changes from one local branch of your repository with another branch. When you create a local Git repository, the default branch is named "master". A typical workflow is to create a temporary branch for isolated work, make and commit your changes, switch to your local master branch, merge the changes from the temporary branch into the local master branch, and then delete the temporary branch.

- `git cherry-pick`: Choose and apply specific commits from one branch into another branch. There are times when you might not be able to merge all the changes in one branch with another but need to pick out certain ones.

- `gitk`: Provides a GUI view of the branches and changes in your local Git repository. This command is a good way to graphically see where things have diverged in your local repository.

- `git log`: Reports a history of your changes to the repository.

- `git diff`: Displays line-by-line differences between your local working files and the same files in the upstream Git repository that your branch currently tracks.

## Workflows

This section provides some overview on workflows using Git. In particular, the information covers basic practices that describe roles and actions in a collaborative development environment. Again, if you are familiar with this type of development environment, you might want to just skip this section.

The Yocto Project files are maintained using Git in a "master" branch whose Git history tracks every change and whose structure provides branches for all diverging functionality. Although there is no need to use Git, many open source projects do so. For the Yocto Project, a key individual called the "maintainer" is responsible for the "master" branch of a given Git repository. The "master" branch is the “upstream” repository where the final builds of the project occur. The maintainer is responsible for allowing changes in from other developers and for organizing the underlying branch structure to reflect release strategies and so forth.

For information on finding out who is responsible (maintains) for a particular area of code, see the "How to Submit a Change" section.

The project also has contribution repositories known as "contrib" areas. These areas temporarily hold changes to the project that have been submitted or committed by the Yocto Project development team and by community members that contribute to the project. The maintainer determines if the changes are qualified to be moved from the "contrib" areas into the "master" branch of the Git repository.

Developers (including contributing community members) create and maintain cloned repositories of the upstream "master" branch. These repositories are local to their development platforms and are used to develop changes. When a developer is satisfied with a particular feature or change, they "push" the changes to the appropriate "contrib" repository.

Developers are responsible for keeping their local repository up-to-date with "master". They are also responsible for straightening out any conflicts that might arise within files that are being worked on simultaneously by more than one person. All this work is done locally on the developer’s machines before anything is pushed to a "contrib" area and examined at the maintainer’s level.

A somewhat formal method exists by which developers commit changes and push them into the "contrib" area and subsequently request that the maintainer include them into "master" This process is called “submitting a patch” or "submitting a change." For information on submitting patches and changes, see the "How to Submit a Change" section.

To summarize the environment: we have a single point of entry for changes into the project’s "master" branch of the Git repository, which is controlled by the project’s maintainer. And, we have a set of developers who independently develop, test, and submit changes to "contrib" areas for the maintainer to examine. The maintainer then chooses which changes are going to become a permanent part of the project.

While each development environment is unique, there are some best practices or methods that help development run smoothly. The following list describes some of these practices. For more information about Git workflows, see the workflow topics in the Git Community Book.

- Make Small Changes: It is best to keep the changes you commit small as compared to bundling many disparate changes into a single commit. This practice not only keeps things manageable but also allows the maintainer to more easily include or refuse changes.

  It is also good practice to leave the repository in a state that allows you to still successfully build your project. In other words, do not commit half of a feature, then add the other half as a separate, later commit. Each commit should take you from one buildable project state to another buildable state.

- Use Branches Liberally: It is very easy to create, use, and delete local branches in your working Git repository. You can name these branches anything you like. It is helpful to give them names associated with the particular feature or change on which you are working. Once you are done with a feature or change and have merged it into your local master branch, simply discard the temporary branch.

- Merge Changes: The `git merge` command allows you to take the changes from one branch and fold them into another branch. This process is especially helpful when more than a single developer might be working on different parts of the same feature. Merging changes also automatically identifies any collisions or "conflicts" that might happen as a result of the same lines of code being altered by two different developers.

- Manage Branches: Because branches are easy to use, you should use a system where branches indicate varying levels of code readiness. For example, you can have a "work" branch to develop in, a "test" branch where the code or change is tested, a "stage" branch where changes are ready to be committed, and so forth. As your project develops, you can merge code across the branches to reflect ever-increasing stable states of the development.

- Use Push and Pull: The push-pull workflow is based on the concept of developers "pushing" local commits to a remote repository, which is usually a contribution repository. This workflow is also based on developers "pulling" known states of the project down into their local development repositories. The workflow easily allows you to pull changes submitted by other developers from the upstream repository into your work area ensuring that you have the most recent software on which to develop. The Yocto Project has two scripts named `create-pull-request` and `send-pull-request` that ship with the release to facilitate this workflow. You can find these scripts in the `scripts` folder of the Source Directory. For information on how to use these scripts, see the "Using Scripts to Push a Change Upstream and Request a Pull" section.

- Patch Workflow: This workflow allows you to notify the maintainer through an email that you have a change (or patch) you would like considered for the "master" branch of the Git repository. To send this type of change, you format the patch and then send the email using the Git commands `git format-patch` and `git send-email`. For information on how to use these scripts, see the "How to Submit a Change" section.

## Tracking Bugs

The Yocto Project uses its own implementation of Bugzilla to track bugs. Implementations of Bugzilla work well for group development because they track bugs and code changes, can be used to communicate changes and problems with developers, can be used to submit and review patches, and can be used to manage quality assurance. The home page for the Yocto Project implementation of Bugzilla is &YOCTO_BUGZILLA_URL;.

Sometimes it is helpful to submit, investigate, or track a bug against the Yocto Project itself such as when discovering an issue with some component of the build system that acts contrary to the documentation or your expectations. Following is the general procedure for submitting a new bug using the Yocto Project Bugzilla. You can find more information on defect management, bug tracking, and feature request processes all accomplished through the Yocto Project Bugzilla on the wiki page here.

1. Always use the Yocto Project implementation of Bugzilla to submit a bug.

2. When submitting a new bug, be sure to choose the appropriate Classification, Product, and Component for which the issue was found. Defects for the Yocto Project fall into one of six classifications: Yocto Project Components, Infrastructure, Build System & Metadata, Documentation, QA/Testing, and Runtime. Each of these Classifications break down into multiple Products and, in some cases, multiple Components.

3. Use the bug form to choose the correct Hardware and Architecture for which the bug applies.

4. Indicate the Yocto Project version you were using when the issue occurred.

5. Be sure to indicate the Severity of the bug. Severity communicates how the bug impacted your work.

6. Provide a brief summary of the issue. Try to limit your summary to just a line or two and be sure to capture the essence of the issue.

7. Provide a detailed description of the issue. You should provide as much detail as you can about the context, behavior, output, and so forth that surrounds the issue. You can even attach supporting files for output from logs by using the "Add an attachment" button.

8. Be sure to copy the appropriate people in the "CC List" for the bug. See the "How to Submit a Change" section for information about finding out who is responsible for code.

9. Submit the bug by clicking the "Submit Bug" button.

## How to Submit a Change

Contributions to the Yocto Project and OpenEmbedded are very welcome. Because the system is extremely configurable and flexible, we recognize that developers will want to extend, configure or optimize it for their specific uses. You should send patches to the appropriate mailing list so that they can be reviewed and merged by the appropriate maintainer.

Before submitting any change, be sure to find out who you should be notifying. Several methods exist through which you find out who you should be copying or notifying:

- Maintenance File: Examine the `maintainers.inc` file, which is located in the Source Directory at `meta-yocto/conf/distro/include`, to see who is responsible for code.

- Board Support Package (BSP) README Files: For BSP maintainers of supported BSPs, you can examine individual BSP `README` files. Alternatively, you can examine the `MAINTAINERS` file, which is found in the `meta-intel`, for a list of all supported BSP maintainers.

- Search by File: Using Git, you can enter the following command to bring up a short list of all commits against a specific file:

           git shortlog -- <filename>

  Just provide the name of the file for which you are interested. The information returned is not ordered by history but does include a list of all committers grouped by name. From the list, you can see who is responsible for the bulk of the changes against the file.

For a list of the Yocto Project and related mailing lists, see the "Mailing lists" section in the Yocto Project Reference Manual.

Here is some guidance on which mailing list to use for what type of change:

- For changes to the core Metadata, send your patch to the openembedded-core mailing list. For example, a change to anything under the `meta` or `scripts` directories should be sent to this mailing list.

- For changes to BitBake (anything under the `bitbake` directory), send your patch to the bitbake-devel mailing list.

- For changes to `meta-yocto`, send your patch to the poky mailing list.

- For changes to other layers hosted on `yoctoproject.org` (unless the layer's documentation specifies otherwise), tools, and Yocto Project documentation, use the yocto mailing list.

- For additional recipes that do not fit into the core Metadata, you should determine which layer the recipe should go into and submit the change in the manner recommended by the documentation (e.g. README) supplied with the layer. If in doubt, please ask on the yocto or openembedded-devel mailing lists.

When you send a patch, be sure to include a "Signed-off-by:" line in the same style as required by the Linux kernel. Adding this line signifies that you, the submitter, have agreed to the Developer's Certificate of Origin 1.1 as follows:

         Developer's Certificate of Origin 1.1

         By making a contribution to this project, I certify that:

         (a) The contribution was created in whole or in part by me and I
             have the right to submit it under the open source license
             indicated in the file; or

         (b) The contribution is based upon previous work that, to the best
             of my knowledge, is covered under an appropriate open source
             license and I have the right under that license to submit that
             work with modifications, whether created in whole or in part
             by me, under the same open source license (unless I am
             permitted to submit under a different license), as indicated
             in the file; or

         (c) The contribution was provided directly to me by some other
             person who certified (a), (b) or (c) and I have not modified
             it.

         (d) I understand and agree that this project and the contribution
             are public and that a record of the contribution (including all
             personal information I submit with it, including my sign-off) is
             maintained indefinitely and may be redistributed consistent with
             this project or the open source license(s) involved.

In a collaborative environment, it is necessary to have some sort of standard or method through which you submit changes. Otherwise, things could get quite chaotic. One general practice to follow is to make small, controlled changes. Keeping changes small and isolated aids review, makes merging/rebasing easier and keeps the change history clean when anyone needs to refer to it in future.

When you make a commit, you must follow certain standards established by the OpenEmbedded and Yocto Project development teams. For each commit, you must provide a single-line summary of the change and you should almost always provide a more detailed description of what you did (i.e. the body of the commit message). The only exceptions for not providing a detailed description would be if your change is a simple, self-explanatory change that needs no further description beyond the summary. Here are the guidelines for composing a commit message:

- Provide a single-line, short summary of the change. This summary is typically viewable in the "shortlist" of changes. Thus, providing something short and descriptive that gives the reader a summary of the change is useful when viewing a list of many commits. This short description should be prefixed by the recipe name (if changing a recipe), or else the short form path to the file being changed.

- For the body of the commit message, provide detailed information that describes what you changed, why you made the change, and the approach you used. It may also be helpful if you mention how you tested the change. Provide as much detail as you can in the body of the commit message.

- If the change addresses a specific bug or issue that is associated with a bug-tracking ID, include a reference to that ID in your detailed description. For example, the Yocto Project uses a specific convention for bug references - any commit that addresses a specific bug should include the bug ID in the description (typically at the beginning) as follows:

           Fixes YOCTO #<bug-id>

           <detailed description of change>

Where <bug-id> is replaced with the specific bug ID from the Yocto Project Bugzilla instance.

You can find more guidance on creating well-formed commit messages at this OpenEmbedded wiki page: .

The next two sections describe general instructions for both pushing changes upstream and for submitting changes as patches.

### Using Scripts to Push a Change Upstream and Request a Pull

The basic flow for pushing a change to an upstream "contrib" Git repository is as follows:

- Make your changes in your local Git repository.

- Stage your changes by using the `git add` command on each file you changed.

- Commit the change by using the `git commit` command and push it to the "contrib" repository. Be sure to provide a commit message that follows the project’s commit message standards as described earlier.

- Notify the maintainer that you have pushed a change by making a pull request. The Yocto Project provides two scripts that conveniently let you generate and send pull requests to the Yocto Project. These scripts are `create-pull-request` and `send-pull-request`. You can find these scripts in the `scripts` directory within the Source Directory.

  Using these scripts correctly formats the requests without introducing any whitespace or HTML formatting. The maintainer that receives your patches needs to be able to save and apply them directly from your emails. Using these scripts is the preferred method for sending patches.

  For help on using these scripts, simply provide the `-h` argument as follows:

           $ ~/poky/scripts/create-pull-request -h
           $ ~/poky/scripts/send-pull-request -h

You can find general Git information on how to push a change upstream in the Git Community Book.

### Using Email to Submit a Patch

You can submit patches without using the `create-pull-request` and `send-pull-request` scripts described in the previous section. However, keep in mind, the preferred method is to use the scripts.

Depending on the components changed, you need to submit the email to a specific mailing list. For some guidance on which mailing list to use, see the list in the "How to Submit a Change" section. For a description of the available mailing lists, see the "Mailing Lists" section in the Yocto Project Reference Manual.

Here is the general procedure on how to submit a patch through email without using the scripts:

- Make your changes in your local Git repository.

- Stage your changes by using the `git add` command on each file you changed.

- Commit the change by using the `git commit --signoff` command. Using the `--signoff` option identifies you as the person making the change and also satisfies the Developer's Certificate of Origin (DCO) shown earlier.

  When you form a commit, you must follow certain standards established by the Yocto Project development team. See the earlier section "How to Submit a Change" for Yocto Project commit message standards.

- Format the commit into an email message. To format commits, use the `git format-patch` command. When you provide the command, you must include a revision list or a number of patches as part of the command. For example, either of these two commands takes your most recent single commit and formats it as an email message in the current directory:

           $ git format-patch -1

  or

           $ git format-patch HEAD~

  After the command is run, the current directory contains a numbered `.patch` file for the commit.

  If you provide several commits as part of the command, the `git format-patch` command produces a series of numbered files in the current directory – one for each commit. If you have more than one patch, you should also use the `--cover` option with the command, which generates a cover letter as the first "patch" in the series. You can then edit the cover letter to provide a description for the series of patches. For information on the `git format-patch` command, see `GIT_FORMAT_PATCH(1)` displayed using the `man git-format-patch` command.

  If you are or will be a frequent contributor to the Yocto Project or to OpenEmbedded, you might consider requesting a contrib area and the necessary associated rights.

- Import the files into your mail client by using the `git send-email` command.

  In order to use `git send-email`, you must have the the proper Git packages installed. For Ubuntu and Fedora the package is `git-email`.

  The `git send-email` command sends email by using a local or remote Mail Transport Agent (MTA) such as `msmtp`, `sendmail`, or through a direct `smtp` configuration in your Git `config` file. If you are submitting patches through email only, it is very important that you submit them without any whitespace or HTML formatting that either you or your mailer introduces. The maintainer that receives your patches needs to be able to save and apply them directly from your emails. A good way to verify that what you are sending will be applicable by the maintainer is to do a dry run and send them to yourself and then save and apply them as the maintainer would.

  The `git send-email` command is the preferred method for sending your patches since there is no risk of compromising whitespace in the body of the message, which can occur when you use your own mail client. The command also has several options that let you specify recipients and perform further editing of the email message. For information on how to use the `git send-email` command, see `GIT-SEND-EMAIL(1)` displayed using the `man git-send-email` command.