meta/recipes-devtools/update-alternatives

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[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >

<chapter id='overview-development-environment'>
<title>The Yocto Project Development Environment</title>

<para>
    This chapter takes a look at the Yocto Project development
    environment.
    The chapter provides Yocto Project Development environment concepts that
    help you understand how work is accomplished in an open source environment,
    which is very different as compared to work accomplished in a closed,
    proprietary environment.
</para>

<para>
    Specifically, this chapter addresses open source philosophy, source
    repositories, workflows, Git, and licensing.
</para>

<section id='open-source-philosophy'>
    <title>Open Source Philosophy</title>

    <para>
        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.
    </para>

    <para>
        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) who 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.
    </para>

    <para>
        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
        <trademark class='registered'>Windows</trademark> family of operating
        systems developed by
        <trademark class='registered'>Microsoft</trademark> Corporation.
    </para>

    <para>
        Wikipedia has a good historical description of the Open Source
        Philosophy
        <ulink url='http://en.wikipedia.org/wiki/Open_source'>here</ulink>.
        You can also find helpful information on how to participate in the
        Linux Community
        <ulink url='http://ldn.linuxfoundation.org/book/how-participate-linux-community'>here</ulink>.
    </para>
</section>

<section id='gs-the-development-host'>
    <title>The Development Host</title>

    <para>
        A development host or
        <ulink url='&YOCTO_DOCS_REF_URL;#hardware-build-system-term'>build host</ulink>
        is key to using the Yocto Project.
        Because the goal of the Yocto Project is to develop images or
        applications that run on embedded hardware, development of those
        images and applications generally takes place on a system not
        intended to run the software - the development host.
    </para>

    <para>
        You need to set up a development host in order to use it with the
        Yocto Project.
        Most find that it is best to have a native Linux machine function as
        the development host.
        However, it is possible to use a system that does not run Linux
        as its operating system as your development host.
        When you have a Mac or Windows-based system, you can set it up
        as the development host by using
        <ulink url='https://git.yoctoproject.org/cgit/cgit.cgi/crops/about/'>CROPS</ulink>,
        which leverages
        <ulink url='https://www.docker.com/'>Docker Containers</ulink>.
        Once you take the steps to set up a CROPS machine, you effectively
        have access to a shell environment that is similar to what you see
        when using a Linux-based development host.
        For the steps needed to set up a system using CROPS, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#setting-up-to-use-crops'>Setting Up to Use CROss PlatformS (CROPS)</ulink>"
        section in the Yocto Project Development Tasks Manual.
    </para>

    <para>
        If your development host is going to be a system that runs a Linux
        distribution, steps still exist that you must take to prepare the
        system for use with the Yocto Project.
        You need to be sure that the Linux distribution on the system is
        one that supports the Yocto Project.
        You also need to be sure that the correct set of host packages are
        installed that allow development using the Yocto Project.
        For the steps needed to set up a development host that runs Linux,
        see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#setting-up-a-native-linux-host'>Setting Up a Native Linux Host</ulink>"
        section in the Yocto Project Development Tasks Manual.
    </para>

    <para>
        Once your development host is set up to use the Yocto Project,
        several methods exist for you to do work in the Yocto Project
        environment:
        <itemizedlist>
            <listitem><para>
                <emphasis>Command Lines, BitBake, and Shells:</emphasis>
                Traditional development in the Yocto Project involves using the
                <ulink url='&YOCTO_DOCS_REF_URL;#build-system-term'>OpenEmbedded build system</ulink>,
                which uses BitBake, in a command-line environment from a shell
                on your development host.
                You can accomplish this from a host that is a native Linux
                machine or from a host that has been set up with CROPS.
                Either way, you create, modify, and build images and
                applications all within a shell-based environment using
                components and tools available through your Linux distribution
                and the Yocto Project.</para>

                <para>For a general flow of the build procedures, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#dev-building-a-simple-image'>Building a Simple Image</ulink>"
                section in the Yocto Project Development Tasks Manual.
                </para></listitem>
            <listitem><para>
                <emphasis>Board Support Package (BSP) Development:</emphasis>
                Development of BSPs involves using the Yocto Project to
                create and test layers that allow easy development of
                images and applications targeted for specific hardware.
                To development BSPs, you need to take some additional steps
                beyond what was described in setting up a development host.
                </para>

                <para>The
                <ulink url='&YOCTO_DOCS_BSP_URL;'>Yocto Project Board Support Package (BSP) Developer's Guide</ulink>
                provides BSP-related development information.
                For specifics on development host preparation, see the
                "<ulink url='&YOCTO_DOCS_BSP_URL;#preparing-your-build-host-to-work-with-bsp-layers'>Preparing Your Build Host to Work With BSP Layers</ulink>"
                section in the Yocto Project Board Support Package (BSP)
                Developer's Guide.
                </para></listitem>
            <listitem><para>
                <emphasis>Kernel Development:</emphasis>
                If you are going to be developing kernels using the Yocto
                Project you likely will be using <filename>devtool</filename>.
                A workflow using <filename>devtool</filename> makes kernel
                development quicker by reducing iteration cycle times.</para>

                <para>The
                <ulink url='&YOCTO_DOCS_KERNEL_DEV_URL;'>Yocto Project Linux Kernel Development Manual</ulink>
                provides kernel-related development information.
                For specifics on development host preparation, see the
                "<ulink url='&YOCTO_DOCS_KERNEL_DEV_URL;#preparing-the-build-host-to-work-on-the-kernel'>Preparing the Build Host to Work on the Kernel</ulink>"
                section in the Yocto Project Linux Kernel Development Manual.
                </para></listitem>
            <listitem><para>
                <emphasis>Using the <trademark class='trade'>Eclipse</trademark> IDE:</emphasis>
                One of two Yocto Project development methods that involves an
                interface that effectively puts the Yocto Project into the
                background is the popular Eclipse IDE.
                This method of development is advantageous if you are already
                familiar with working within Eclipse.
                Development is supported through a plugin that you install
                onto your development host.</para>

                <para>For steps that show you how to set up your development
                host to use the Eclipse Yocto Project plugin, see the
                "<ulink url='&YOCTO_DOCS_SDK_URL;#sdk-eclipse-project'>Developing Applications Using <trademark class='trade'>Eclipse</trademark></ulink>"
                Chapter in the Yocto Project Application Development and the
                Extensible Software Development Kit (eSDK) manual.
                </para></listitem>
            <listitem><para>
                <emphasis>Using Toaster:</emphasis>
                The other Yocto Project development method that involves an
                interface that effectively puts the Yocto Project into the
                background is Toaster.
                Toaster provides an interface to the OpenEmbedded build system.
                The interface enables you to configure and run your builds.
                Information about builds is collected and stored in a database.
                You can use Toaster to configure and start builds on multiple
                remote build servers.</para>

                <para>For steps that show you how to set up your development
                host to use Toaster and on how to use Toaster in general,
                see the
                <ulink url='&YOCTO_DOCS_TOAST_URL;'>Toaster User Manual</ulink>.
                </para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='yocto-project-repositories'>
    <title>Yocto Project Source Repositories</title>

    <para>
        The Yocto Project team maintains complete source repositories for all
        Yocto Project files at
        <ulink url='&YOCTO_GIT_URL;'></ulink>.
        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
        <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>,
        which is usually named "poky", allows
        you to make changes, contribute to the history, and ultimately enhance
        the Yocto Project's tools, Board Support Packages, and so forth.
    </para>

    <para>
        For any supported release of Yocto Project, you can also go to the
        <ulink url='&YOCTO_HOME_URL;'>Yocto Project Website</ulink> and
        select the "DOWNLOADS" item from the "SOFTWARE" menu and get a
        released tarball of the <filename>poky</filename> repository, any
        supported BSP tarball, or Yocto Project tools.
        Unpacking these tarballs gives you a snapshot of the released
        files.
        <note><title>Notes</title>
            <itemizedlist>
                <listitem><para>
                    The recommended method for setting up the Yocto Project
                    <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>
                    and the files for supported BSPs
                    (e.g., <filename>meta-intel</filename>) is to use
                    <link linkend='git'>Git</link> to create a local copy of
                    the upstream repositories.
                    </para></listitem>
                <listitem><para>
                    Be sure to always work in matching branches for both
                    the selected BSP repository and the Source Directory
                    (i.e. <filename>poky</filename>) repository.
                    For example, if you have checked out the "master" branch
                    of <filename>poky</filename> and you are going to use
                    <filename>meta-intel</filename>, be sure to checkout the
                    "master" branch of <filename>meta-intel</filename>.
                    </para></listitem>
            </itemizedlist>
        </note>
    </para>

    <para>
        In summary, here is where you can get the project files needed for
        development:
        <itemizedlist>
            <listitem><para id='source-repositories'>
                <emphasis>
                <ulink url='&YOCTO_GIT_URL;'>Source Repositories:</ulink>
                </emphasis>
                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.</para>

                <para>
                <imagedata fileref="figures/source-repos.png" align="center" width="6in" depth="4in" />
                For steps on how to view and access these upstream Git
                repositories, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#accessing-source-repositories'>Accessing Source Repositories</ulink>"
                Section in the Yocto Project Development Tasks Manual.
                </para></listitem>
            <listitem><para><anchor id='index-downloads' />
                <emphasis>
                <ulink url='&YOCTO_DL_URL;/releases/'>Index of /releases:</ulink>
                </emphasis>
                This is an index of releases such as
                the <trademark class='trade'>Eclipse</trademark>
                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.</para>

                <para>
                <imagedata fileref="figures/index-downloads.png" align="center" width="6in" depth="3.5in" />
                For steps on how to view and access these files, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#accessing-index-of-releases'>Accessing Index of Releases</ulink>"
                section in the Yocto Project Development Tasks Manual.
                </para></listitem>
            <listitem><para id='downloads-page'>
                <emphasis>"DOWNLOADS" page for the
                <ulink url='&YOCTO_HOME_URL;'>Yocto Project Website</ulink>:
                </emphasis></para>

                <para>The Yocto Project website includes a "DOWNLOADS" page
                accessible through the "SOFTWARE" menu that allows you to
                download any Yocto Project release, tool, and Board Support
                Package (BSP) in tarball form.
                The tarballs are similar to those found in the
                <ulink url='&YOCTO_DL_URL;/releases/'>Index of /releases:</ulink>
                area.</para>

                <para>
                <imagedata fileref="figures/yp-download.png" align="center" width="6in" depth="4in" />
                For steps on how to use the "DOWNLOADS" page, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#using-the-downloads-page'>Using the Downloads Page</ulink>"
                section in the Yocto Project Development Tasks Manual.
                </para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='gs-git-workflows-and-the-yocto-project'>
    <title>Git Workflows and the Yocto Project</title>

    <para>
        Developing using the Yocto Project likely requires the use of
        <link linkend='git'>Git</link>.
        Git is a free, open source distributed version control system
        used as part of many collaborative design environments.
        This section provides workflow concepts using the Yocto Project and
        Git.
        In particular, the information covers basic practices that describe
        roles and actions in a collaborative development environment.
        <note>
            If you are familiar with this type of development environment, you
            might not want to read this section.
        </note>
    </para>

    <para>
        The Yocto Project files are maintained using Git in "branches"
        whose Git histories track every change and whose structures
        provide branches for all diverging functionality.
        Although there is no need to use Git, many open source projects do so.
    <para>

    </para>
        For the Yocto Project, a key individual called the "maintainer" is
        responsible for the integrity of the "master" branch of a given Git
        repository.
        The "master" branch is the “upstream” repository from which final or
        most recent builds of a project occur.
        The maintainer is responsible for accepting changes from other
        developers and for organizing the underlying branch structure to
        reflect release strategies and so forth.
        <note>
            For information on finding out who is responsible for (maintains)
            a particular area of code in the Yocto Project, see the
            "<ulink url='&YOCTO_DOCS_DEV_URL;#how-to-submit-a-change'>Submitting a Change to the Yocto Project</ulink>"
            section of the Yocto Project Development Tasks Manual.
        </note>
    </para>

    <para>
        The Yocto Project <filename>poky</filename> Git repository also has an
        upstream contribution Git repository named
        <filename>poky-contrib</filename>.
        You can see all the branches in this repository using the web interface
        of the
        <ulink url='&YOCTO_GIT_URL;'>Source Repositories</ulink> organized
        within the "Poky Support" area.
        These branches hold changes (commits) to the project that have been
        submitted or committed by the Yocto Project development team and by
        community members who contribute to the project.
        The maintainer determines if the changes are qualified to be moved
        from the "contrib" branches into the "master" branch of the Git
        repository.
    </para>

    <para>
        Developers (including contributing community members) create and
        maintain cloned repositories of upstream branches.
        The cloned 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 change to the appropriate "contrib" repository.
    </para>

    <para>
        Developers are responsible for keeping their local repository
        up-to-date with whatever upstream branch they are working against.
        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 development host before
        anything is pushed to a "contrib" area and examined at the maintainer’s
        level.
    </para>

    <para>
        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 an upstream branch.
        This process is called “submitting a patch” or "submitting a change."
        For information on submitting patches and changes, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#how-to-submit-a-change'>Submitting a Change to the Yocto Project</ulink>"
        section in the Yocto Project Development Tasks Manual.
    </para>

    <para>
        In summary, a single point of entry
        exists for changes into a "master" or development branch of the
        Git repository, which is controlled by the project’s maintainer.
        And, a set of developers exist 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.
    </para>

    <para>
        <imagedata fileref="figures/git-workflow.png" width="6in" depth="3in" align="left" scalefit="1" />
    </para>

    <para>
        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
        <ulink url='http://book.git-scm.com'>Git Community Book</ulink>.
        <itemizedlist>
            <listitem><para>
                <emphasis>Make Small Changes:</emphasis>
                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.
                </para></listitem>
            <listitem><para>
                <emphasis>Make Complete Changes:</emphasis>
                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.
                </para></listitem>
            <listitem><para>
                <emphasis>Use Branches Liberally:</emphasis>
                It is very easy to create, use, and delete local branches in
                your working Git repository on the development host.
                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.
                </para></listitem>
            <listitem><para>
                <emphasis>Merge Changes:</emphasis>
                The <filename>git merge</filename> 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.
                </para></listitem>
            <listitem><para>
                <emphasis>Manage Branches:</emphasis>
                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.
                </para></listitem>
            <listitem><para>
                <emphasis>Use Push and Pull:</emphasis>
                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
                <filename>create-pull-request</filename> and
                <filename>send-pull-request</filename> that ship with the
                release to facilitate this workflow.
                You can find these scripts in the <filename>scripts</filename>
                folder of the
                <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>.
                For information on how to use these scripts, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#pushing-a-change-upstream'>Using Scripts to Push a Change Upstream and Request a Pull</ulink>"
                section in the Yocto Project Development Tasks Manual.
                </para></listitem>
            <listitem><para>
                <emphasis>Patch Workflow:</emphasis>
                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
                <filename>git format-patch</filename> and
                <filename>git send-email</filename>.
                For information on how to use these scripts, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#how-to-submit-a-change'>Submitting a Change to the Yocto Project</ulink>"
                section in the Yocto Project Development Tasks Manual.
                </para></listitem>
        </itemizedlist>
    </para>
</section>

<section id='git'>
    <title>Git</title>

    <para>
        The Yocto Project makes extensive use of 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.
        <note><title>Notes</title>
            <itemizedlist>
                <listitem><para>
                    For more information on Git, see
                    <ulink url='http://git-scm.com/documentation'></ulink>.
                    </para></listitem>
                <listitem><para>
                    If you need to download Git, it is recommended that you add
                    Git to your system through your distribution's "software
                    store" (e.g. for Ubuntu, use the Ubuntu Software feature).
                    For the Git download page, see
                    <ulink url='http://git-scm.com/download'></ulink>.
                    </para></listitem>
                <listitem><para>
                    For information beyond the introductory nature in this
                    section, see the
                    "<ulink url='&YOCTO_DOCS_DEV_URL;#locating-yocto-project-source-files'>Locating Yocto Project Source Files</ulink>"
                    section in the Yocto Project Development Tasks Manual.
                    </para></listitem>
            </itemizedlist>
        </note>
    </para>

    <section id='repositories-tags-and-branches'>
        <title>Repositories, Tags, and Branches</title>

        <para>
            As mentioned briefly in the previous section and also in the
            "<link linkend='gs-git-workflows-and-the-yocto-project'>Git Workflows and the Yocto Project</link>"
            section, the Yocto Project maintains source repositories at
            <ulink url='&YOCTO_GIT_URL;'></ulink>.
            If you look at this web-interface of the repositories, each item
            is a separate Git repository.
        </para>

        <para>
            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.
        </para>

        <para>
            A Git repository represents all development efforts for a given
            project.
            For example, the Git repository <filename>poky</filename> contains
            all changes and developments for that repository 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.
        </para>

        <para>
            You can create a local copy of any repository by "cloning" it
            with the <filename>git clone</filename> 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
            "<ulink url='&YOCTO_DOCS_DEV_URL;#locating-yocto-project-source-files'>Locating Yocto Project Source Files</ulink>"
            section in the Yocto Project Development Tasks Manual.
        </para>

        <para>
            It is important to understand that Git tracks content change and
            not files.
            Git uses "branches" to organize different development efforts.
            For example, the <filename>poky</filename> repository has
            several branches that include the current "&DISTRO_NAME_NO_CAP;"
            branch, the "master" branch, and many branches for past
            Yocto Project releases.
            You can see all the branches by going to
            <ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/'></ulink> and
            clicking on the
            <filename><ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/refs/heads'>[...]</ulink></filename>
            link beneath the "Branch" heading.
        </para>

        <para>
            Each of these branches represents a specific area of development.
            The "master" branch represents the current or most recent
            development.
            All other branches represent offshoots of the "master" branch.
        </para>

        <para>
            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 upstream 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, consider the following example Git commands:
            <literallayout class='monospaced'>
     $ cd ~
     $ git clone git://git.yoctoproject.org/poky
     $ cd poky
     $ git checkout -b &DISTRO_NAME_NO_CAP; origin/&DISTRO_NAME_NO_CAP;
            </literallayout>
            In the previous example after moving to the home directory, the
            <filename>git clone</filename> command creates a
            local copy of the upstream <filename>poky</filename> Git repository.
            By default, Git checks out the "master" branch for your work.
            After changing the working directory to the new local repository
            (i.e. <filename>poky</filename>), the
            <filename>git checkout</filename> command creates
            and checks out a local branch named "&DISTRO_NAME_NO_CAP;", which
            tracks the upstream "origin/&DISTRO_NAME_NO_CAP;" branch.
            Changes you make while in this branch would ultimately affect
            the upstream "&DISTRO_NAME_NO_CAP;" branch of the
            <filename>poky</filename> repository.
        </para>

        <para>
            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 particular
            development branch at the time you created your local branch,
            which could be different from the files in the "master" branch
            of the upstream repository.
            In other words, creating and checking out a local branch based on
            the "&DISTRO_NAME_NO_CAP;" branch name is not the same as
            checking out the "master" branch in the repository.
            Keep reading to see how you create a local snapshot of a Yocto
            Project Release.
        </para>

        <para>
            Git uses "tags" to mark specific changes in a repository branch
            structure.
            Typically, a tag is used to mark a special point such as the final
            change (or commit) before a project is released.
            You can see the tags used with the <filename>poky</filename> Git
            repository by going to
            <ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/'></ulink> and
            clicking on the
            <filename><ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/refs/tags'>[...]</ulink></filename>
            link beneath the "Tag" heading.
        </para>

        <para>
            Some key tags for the <filename>poky</filename> repository are
            <filename>jethro-14.0.3</filename>,
            <filename>morty-16.0.1</filename>,
            <filename>pyro-17.0.0</filename>, and
            <filename>&DISTRO_NAME_NO_CAP;-&POKYVERSION;</filename>.
            These tags represent Yocto Project releases.
        </para>

        <para>
            When you create a local copy of the Git repository, you also
            have access to all the tags in the upstream repository.
            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:
            <literallayout class='monospaced'>
     $ cd ~
     $ git clone git://git.yoctoproject.org/poky
     $ cd poky
     $ git fetch --tags
     $ git checkout tags/rocko-18.0.0 -b my_rocko-18.0.0
            </literallayout>
            In this example, the name of the top-level directory of your
            local Yocto Project repository is <filename>poky</filename>.
            After moving to the <filename>poky</filename> directory, the
            <filename>git fetch</filename> command makes all the upstream
            tags available locally in your repository.
            Finally, the <filename>git checkout</filename> command
            creates and checks out a branch named "my-rocko-18.0.0" that is
            based on the upstream branch whose "HEAD" matches the
            commit in the repository associated with the "rocko-18.0.0" tag.
            The files in your repository now exactly match that particular
            Yocto Project release as it is tagged in the upstream Git
            repository.
            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 (i.e. from the "tip" of the branch backwards).
        </para>
    </section>

    <section id='basic-commands'>
        <title>Basic Commands</title>

        <para>
            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
            <ulink url='http://git-scm.com/documentation'>here</ulink>.
        </para>

        <para>
            The following list of Git commands 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 it supports.
            See the Git documentation for complete descriptions and strategies
            on how to use these commands:
            <itemizedlist>
                <listitem><para>
                    <emphasis><filename>git init</filename>:</emphasis>
                    Initializes an empty Git repository.
                    You cannot use Git commands unless you have a
                    <filename>.git</filename> repository.
                    </para></listitem>
                <listitem><para id='git-commands-clone'>
                    <emphasis><filename>git clone</filename>:</emphasis>
                    Creates a local clone of a Git repository that is on
                    equal footing with a fellow developer’s Git repository
                    or an upstream repository.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git add</filename>:</emphasis>
                    Locally 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.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git commit</filename>:</emphasis>
                    Creates a local "commit" that documents the changes you
                    made.
                    Only changes that have been staged can be committed.
                    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 repository.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git status</filename>:</emphasis>
                    Reports any modified files that possibly need to be
                    staged and gives you a status of where you stand regarding
                    local commits as compared to the upstream repository.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git checkout</filename> <replaceable>branch-name</replaceable>:</emphasis>
                    Changes your local working branch and in this form
                    assumes the local branch already exists.
                    This command is analogous to "cd".
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git checkout –b</filename> <replaceable>working-branch</replaceable> <replaceable>upstream-branch</replaceable>:</emphasis>
                    Creates and checks out a working branch on your local
                    machine.
                    The local branch tracks the upstream branch.
                    You can use your local branch to isolate your work.
                    It is a good idea to use local branches when adding
                    specific features or changes.
                    Using isolated branches facilitates easy removal of
                    changes if they do not work out.
                    </para></listitem>
                <listitem><para><emphasis><filename>git branch</filename>:</emphasis>
                    Displays the existing local branches associated with your
                    local repository.
                    The branch that you have currently checked out is noted
                    with an asterisk character.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git branch -D</filename> <replaceable>branch-name</replaceable>:</emphasis>
                    Deletes an existing local branch.
                    You need to be in a local branch other than the one you
                    are deleting in order to delete
                    <replaceable>branch-name</replaceable>.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git pull --rebase</filename>:</emphasis>
                    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).
                    The "--rebase" option ensures that any local commits you
                    have in your branch are preserved at the top of your
                    local branch.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git push</filename> <replaceable>repo-name</replaceable> <replaceable>local-branch</replaceable><filename>:</filename><replaceable>upstream-branch</replaceable>:</emphasis>
                    Sends all your committed local changes to the upstream Git
                    repository that your local repository is tracking
                    (e.g. a contribution repository).
                    The maintainer of the project draws from these repositories
                    to merge changes (commits) into the appropriate branch
                    of project's upstream repository.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git merge</filename>:</emphasis>
                    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 that is
                    based off "master" that you would use for isolated work.
                    You would make your changes in that isolated branch,
                    stage and commit them locally, switch to the "master"
                    branch, and then use the <filename>git merge</filename>
                    command to apply the changes from your isolated branch
                    into the currently checked out branch (e.g. "master").
                    After the merge is complete and if you are done with
                    working in that isolated branch, you can safely delete
                    the isolated branch.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git cherry-pick</filename> <replaceable>commits</replaceable>:</emphasis>
                    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.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>gitk</filename>:</emphasis>
                    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.
                    <note>
                        You need to install the <filename>gitk</filename>
                        package on your development system to use this
                        command.
                    </note>
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git log</filename>:</emphasis>
                    Reports a history of your commits to the repository.
                    This report lists all commits regardless of whether you
                    have pushed them upstream or not.
                    </para></listitem>
                <listitem><para>
                    <emphasis><filename>git diff</filename>:</emphasis>
                    Displays line-by-line differences between a local
                    working file and the same file as understood by Git.
                    This command is useful to see what you have changed
                    in any given file.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>
</section>

<section id='licensing'>
    <title>Licensing</title>

    <para>
        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:
        <itemizedlist>
            <listitem><para>
                <ulink url='http://en.wikipedia.org/wiki/Open-source_license'>Open source license history</ulink>
                </para></listitem>
            <listitem><para>
                <ulink url='http://en.wikipedia.org/wiki/Free_software_license'>Free software license history</ulink>
                </para></listitem>
        </itemizedlist>
    </para>

    <para>
        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
        <ulink url='http://www.opensource.org/licenses/mit-license.php'>here</ulink>.
        You can find information on the GNU GPL
        <ulink url='http://www.opensource.org/licenses/LGPL-3.0'>here</ulink>.
    </para>

    <para>
        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
        <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>
        at <filename>meta/files/common-licenses</filename>.
        Once the build completes, the list of all licenses found and used
        during that build are kept in the
        <ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>
        at <filename>tmp/deploy/licenses</filename>.
    </para>

    <para>
        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.
    </para>

    <para>
        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.
        <ulink url='http://spdx.org'>SPDX Group</ulink> 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.
        <ulink url='http://opensource.org'>OSI</ulink> is a corporation
        dedicated to the Open Source Definition and the effort for reviewing
        and approving licenses that conform to the Open Source Definition
        (OSD).
    </para>

    <para>
        You can find a list of the combined SPDX and OSI licenses that the
        Yocto Project uses in the
        <filename>meta/files/common-licenses</filename> directory in your
        <ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>.
    </para>

    <para>
        For information that can help you maintain compliance with various
        open source licensing during the lifecycle of a product created using
        the Yocto Project, see the
        "<ulink url='&YOCTO_DOCS_DEV_URL;#maintaining-open-source-license-compliance-during-your-products-lifecycle'>Maintaining Open Source License Compliance During Your Product's Lifecycle</ulink>"
        section in the Yocto Project Development Tasks Manual.
    </para>
</section>
</chapter>
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