Software Development (Git Users)

6.1. Software Development (Git Users)#

6.1.1. Browse the Git Repository Online#

You can browse all available repositories online by accessing https://gitlab.rtems.org/rtems/

6.1.2. Using the Git Repository#

The following examples demonstrate how to use the RTEMS’ Git repos. These examples are provided for the main rtems module, but they are also valid for the other modules.

First, we need to obtain our own local copy of the RTEMS Git repository:

git clone https://gitlab.rtems.org/rtems/rtos/rtems.git

This command will create a folder named rtems in the current directory. This folder will contain a full-featured RTEMS’ Git repository and the current HEAD revision checked out. Since all the history is available we can check out any release of RTEMS. Major RTEMS releases are available as separate branches in the repo.

To see all available remote branches issue the following command:

git branch -r

We can check out one of those remote branches (e.g. rtems-6.0 branch) using the command:

git checkout -b rtems60 origin/6.0

This will create a local branch named “rtems60”, containing the rtems-6.0 release, that will track the remote branch “rtems-6.0-branch” in origin (https://gitlab.rtems.org/rtems/rtos/rtems.git). The git branch command prints a list of the current local branches, indicating the one currently checked out.

If you want to switch between local branches:

git checkout <branch-name>

With time your local repository will diverge from the main RTEMS repository. To keep your local copy up to date you need to issue:

git pull origin

This command will update all your local branches with any new code revisions available on the central repository.

6.1.3. Making Changes#

Git allows you to make changes in the RTEMS source tree and track those changes locally. We recommend you make all your changes in local branches. If you are working on a few different changes or a progression of changes it is best to use a local branch for each change.

A branch for each change lets your repo’s master branch track the upstream RTEMS’ master branch without interacting with any of the changes you are working on. A completed change is emailed to the developer’s list for review and this can take time. While this is happening the upstream’s master branch may be updated and you may need to rebase your work and test again if you are required to change or update your patch. A local branch isolates a specific change from others and helps you manage the process.

First, you need to clone the repository:

git clone https://gitlab.rtems.org/rtems/rtos/rtems.git

Or if you already cloned it before, then you might want to update to the latest version before making your changes:

cd rtems
git pull

Create a local branch to make your changes in, in this example, the change is faster-context-switch:

git checkout -b faster-context-switch

Next, make your changes to files. If you add, delete ormove/rename files you need to inform Git

git add /some/new/file
git rm /some/old/file
git mv /some/old/file /some/new/file

When you’re satisfied with the changes you made, commit them (locally)

git commit -a

The -a flag commits all the changes that were made, but you can also control which changes to commit by individually adding files as you modify them by using. You can also specify other options to commit, such as a message with the -m flag.

git add /some/changed/files
git commit

Create a patch from your branch, in this case, we have two commits we want to send for review:

 git format-patch -2

There are new changes pushed to the RTEMS' master branch and our local branch
needs to be updated:

git checkout master
git pull
git checkout faster-context-switch
git rebase master

6.1.4. Working with Branches#

Branches facilitate trying out new code and creating patches.

The previous releases of RTEMS are available through remote branches. To check out a remote branch, first query the Git repository for the list of branches:

git branch -r

Then check out the desired remote branch, for example:

git checkout -b rtems60 origin/6.0

Or if you have previously checked out the remote branch then you should see it in your local branches:

git branch

You can change to an existing local branch easily:

git checkout rtems60

You can also create a new branch and switch to it:

git branch temporary
git checkout temporary

Or more concisely:

git checkout -b temporary

If you forget which branch you are on

git branch

shows you by placing a * next to the current one.

When a branch is no longer useful you can delete it.

git checkout master
git branch -d temporary

If you have unmerged changes in the old branch Git complains and you need to use -D instead of -d.

6.1.5. Viewing Changes#

To view all changes since the last commit:

git diff HEAD

To view all changes between the current branch and another branch, say master:

git diff master..HEAD

To view descriptions of committed changes:

git log

Or view the changeset for some file (or directory):

git log /some/file

To view the changesets made between two branches:

git log master..HEAD

Or for a more brief description use shortlog:

git shortlog master..HEAD

6.1.6. Reverting Changes#

To remove all (uncommitted) changes on a branch

git checkout -f

Or to selectively revert (uncommited) files, for example if you accidentally deleted ./some/file

git checkout -- ./some/file

or

git checkout HEAD ./some/file

To remove commits there are two useful options, reset and revert. git reset should only be used on local branches that no one else is accessing remotely. git revert is cleaner and is the right way to revert changes that have already been pushed/pulled remotely.

6.1.7. git reset#

git reset is a powerful and tricky command that should only be used on local (un-pushed) branches): A good description of what it enables to do can be found here. The following are a few useful examples. Note that adding a ~ after HEAD refers to the most recent commit, and you can add a number after the ~ to refer to commits even further back; HEAD by itself refers to the current working directory (changes since the last commit).

git reset HEAD~

Will undo the last commit and unstage those changes. Your working directory will remain the same, therefore a git status will yield any changes you made plus the changes made in your last commit. This can be used to fix the last commit. You will need to add the files again.

git reset --soft HEAD~

Will just undo the last commit. The changes from the last commit will still be staged (just as if you finished git adding them). This can be used to amend the last commit (e.g. You forgot to add a file to the last commit).

git reset --hard HEAD~

Will revert everything, including the working directory, to the previous commit. This is dangerous and can lead to you losing all your changes; the --hard flag ignores errors.

git reset HEAD

Will unstage any change. This is used to revert a wrong git add. (e.g. You added a file that shouldn’t be there, but you haven’t ‘committed’)

Will revert your working directory to a HEAD state. You will lose any change you made to files after the last commit. This is used when you just want to destroy all changes you made since the last commit.

6.1.8. git revert#

git revert does the same as reset but creates a new commit with the reverted changes instead of modifying the local repository directly.

git revert HEAD

This will create a new commit which undoes the change in HEAD. You will be given a chance to edit the commit message for the new commit.

6.1.9. Merging Changes#

Suppose you commit changes in two different branches, branch1 and branch2, and want to create a new branch containing both sets of changes:

git checkout -b merged
git merge branch1
git merge branch2

Or you might want to bring the changes in one branch into the other:

git checkout branch1
git merge branch2

And now that branch2 is merged you might get rid of it:

git branch -d branch2

If you have done work on a branch, say branch1, and have gone out-of-sync with the remote repository, you can pull the changes from the remote repo and then merge them into your branch:

git checkout master
git pull
git checkout branch1
git merge master

If all goes well the new commits you pulled into your master branch will be merged into your branch1, which will now be up-to-date. However, if branch1 has not been pushed remotely then rebasing might be a good alternative to merging because the merge generates a commit.

6.1.10. Rebasing#

An alternative to the merge command is rebase, which replays the changes (commits) on one branch onto another. git rebase finds the common ancestor of the two branches, stores each commit of the branch you are on to temporary files and applies each commit in order.

For example

git checkout branch1
git rebase master

or more concisely

git rebase master branch1

will bring the changes of master into branch1, and then you can fast-forward master to include branch1 quite easily

git checkout master
git merge branch1

Rebasing makes a cleaner history than merging; the log of a rebased branch looks like a linear history as if the work was done serially rather than in parallel. A primary reason to rebase is to ensure commits apply cleanly on a remote branch, e.g. when submitting patches to RTEMS that you create by working on a branch in a personal repository. Using rebase to merge your work with the remote branch eliminates most integration work for the committer/maintainer.

There is one caveat to using rebase: Do not rebase commits that you have pushed to a public repository. Rebase abandons existing commits and creates new ones that are similar but different. If you push commits that others pull down, and then you rewrite those commits with git rebase and push them up again, the others will have to re-merge their work and trying to integrate their work into yours can become messy.

6.1.11. Accessing a Developer’s Repository#

RTEMS developers with Git commit access have personal repositories on https://gitlab.rtems.org/ that can be cloned to view cutting-edge development work shared there.

6.1.12. Commit Message Guidance#

The commit message associated with a change to any software project is of critical importance. It is the explanation of the change and the rationale for it. Future users looking back through the project history will rely on it. Even the author of the change will likely rely on it once they have forgotten the details of the change. It is important to make the message useful. Here are some guidelines followed by the RTEMS Project to help improve the quality of our commit messages.

  • When committing a change the first line is a summary. Please make it short while hinting at the nature of the change. You can discuss the change if you wish in a ticket that has a PR number which can be referenced in the commit message. After the first line, leave an empty line and add whatever required details you feel are needed.

  • Patches should be as single purpose as possible. This is reflected in the first line summary message. If you find yourself writing something like “Fixed X and Y”, “Updated A and B”, or similar, then evaluate whether the patch should really be a patch series rather than a single larger patch.

  • Format the commit message so it is readable and clear. If you have specific points related to the change make them with separate paragraphs and if you wish you can optionally uses a - marker with suitable indents and alignment to aid readability.

  • Limit the line length to less than 80 characters

  • Please use a real name with a valid email address. Please do not use pseudonyms or provide anonymous contributions.

  • Please do not use terms such as “Fix bug”, “With this change it works”, or “Bump hash”. If you fix a bug please state the nature of the bug and why this change fixes it. If a change makes something work then detail the reason. You do not need to explain the change line by line as the commits diff and associated ticket will.

  • If you change the formatting of source code in a repository please make that a separate patch and use “Formatting changes only” on the first line. Please indicate the reason or process. For example to “Conforming to code standing”, “Reverting to upstream format”, “Result of automatic formatting”.

  • Similarly, if addressing a spelling, grammar, or Doxygen issue, please put that in a commit by itself separate from technical changes.

An example commit message:

test/change: Test message on formatting of commits

- Shows a simple single first line

- Has an empty second line

- Shows the specifics of adding separate points in the commit message as
  separate paragraphs. It also shows a `-` separator and multilines
  that are less than the 80 character width

- Show a ticket update and close

Updates #9876
Closes #8765

The first line generally starts with a file or directory name which indicates the area in RTEMS to which the commit applies. For a patch series which impacts multiple BSPs, it is common to put each BSP into a separate patch. This improves the quality and specificity of the commit messages.

6.1.13. Creating a Patch#

Before submitting a patch, please read Commit Message Guidance to become familiar with the commit message formatting we require.

The recommended way to create a patch is to branch the Git repository master and use one commit for each logical change. Then you can use git format-patch to turn your commits into patches and easily submit them.

git format-patch master

Creates a separate patch for each commit that has been made between the master branch and the current branch and writes them in the current directory. Use the -o flag to redirect the files to a different directory.

If you are re-submitting a patch that has previously been reviewed, you should specify a version number for your patch, for example, use

git format-patch -v2 ...

to indicate the second version of a patch, -v3 for a third, and so forth.

Also, in order to create a patch specifying the repo name in the patch message, you should use the``–subject-prefix`` flag. For example, if contributing to the rtems-docs repo, use

git format-patch --subject-prefix="PATCH rtems-docs" ...

You can set a default subject prefix for each repository locally, for example:

git config format.subjectprefix "PATCH rtems-docs"

Patches created using git format-patch are formatted so they can be emailed and rely on having Git configured with your name and email address, for example

git config --global user.name "Your Name"
git config --global user.email name@domain.com

Please use a real name, we do not allow pseudonyms or anonymous contributions.

6.1.14. Submitting a Patch#

Using git send-email you can easily contribute your patches. You will need to install git send-email first:

sudo yum install git-email

or

sudo dnf install git-email

or

sudo apt install git-email

Then you will need to configure an SMTP server. You could install one on your localhost, or you can connect to a mail server such as Gmail.

6.1.15. Configuring git send-email to use Gmail#

Configure Git to use Gmail:

git config --global sendemail.smtpserver smtp.gmail.com
git config --global sendemail.smtpserverport 587
git config --global sendemail.smtpencryption tls
git config --global sendemail.smtpuser your_email@gmail.com

It will ask for your password each time you use git send-email. Optionally you can also put it in your git config:

git config --global sendemail.smtppass your_password

6.1.16. Sending Email#

To send your patches just

git send-email /path/to/patch --to devel@rtems.org

To send multiple related patches (if you have more than one commit in your branch) specify a path to a directory containing all of the patches created by git format-patch. git send-email has some useful options such as:

  • --annotate to show/edit your patch

  • --cover-letter to prepend a summary

  • --cc=<address> to cc someone

You can configure the to address:

git config --global sendemail.to devel@rtems.org

So all you need is:

git send-email /path/to/patch

6.1.17. Manage Your Code#

You may prefer to keep your application and development work in a Git repository for all the good reasons that come with version control. For public repositories, you may like to try GitHub or BitBucket. RTEMS maintains mirrors on GitHub which can make synchronizing with upstream changes relatively simple. If you need to keep your work private, you can use one of those services with private repositories or manage your own server. The details of setting up a server are outside the scope of this document, but if you have a server with SSH access you should be able to find instructions on how to set up Git access. Once you have git configured on the server, adding repositories is a snap.

6.1.18. Private Servers#

In the following, replace @USER@ with your username on your server, @REPO@ with the name of your repository, and @SERVER@ with your server’s name or address.

To push a mirror to your private server, first create a bare repository on your server.

cd /home/@USER@
mkdir git
mkdir git/@REPO@.git
cd git/@REPO@.git
git --bare init

Now from your client machine (e.g. your work laptop/desktop), push a git, perhaps one you cloned from elsewhere, or one that you made locally with git init, by adding a remote and pushing:

git remote add @SERVER@ ssh://@SERVER@/home/@USER@/git/@REPO@.git
git push @SERVER@ master

You can replace the @SERVER@ with another name for your remote if you like. And now you can push other branches that you might have created. Now you can push and pull between your client and your server. Use SSH keys to authenticate with your server if you want to save on password typing; remember to put a passphrase on your SSH key if there is a risk the private key file might get compromised.

The following is an example scenario that might be useful for RTEMS users that uses a slightly different approach than the one just outlined:

ssh @SERVER@
mkdir git
git clone --mirror https://gitlab.rtems.org/rtems/rtos/rtems.git
## Add your ssh key to ~/.ssh/authorized_keys
exit
git clone ssh://@SERVER@/home/@USER@/git/rtems.git
cd rtems
git remote add upstream https://gitlab.rtems.org/rtems/rtos/rtems.git
git fetch upstream
git pull upstream master
git push
## If you want to track RTEMS on your personal master branch,
## you should only push changes to origin/master that you pull
## from upstream. The basic workflow should look something like:
git checkout master
git pull upstream master
git push
git checkout -b anewbranch
## Repeat: do work, git commit -a
git push origin anewbranch

## delete a remote branch
git push origin :anewbranch
## delete a local branch
git branch -d anewbranch

6.1.19. Learn more about Git#

Links to the sites with good Git information: