This document describes how developers may contribute pull requests to an upstream repository and how upstream owners may merge pull requests from contributors according to the very popular fork and pull request workflow followed in many projects on GitHub.
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Every project has a main development branch where the developers push
commits on a day-to-day basis. Usually, the main development branch is
master but some projects choose to have develop or trunk or
another branch for day-to-day development activities. We refer to this
main development branch as main development branch throughout this
document to keep the text general. However in the command examples and
ASCII-diagrams, we use master as an example of the main development
branch.
We use the following placeholders in the command examples and ASCII-diagrams in this document:
GITHUB:github.comor domain name/hostname of your private GitHub Enterprise system.USERorCONTRIBUTOR: The user that forks an upstream repository, creates pull requests, and sends them to the upstream repository.UPSTREAM-OWNER: Owner of the upstream repository. This is the name of the user or organization that merges pull requests into the upstream repository.REPO: Repository name.FILES: One or more filenames to be staged for a commit.TOPIC-BRANCH: Feature-specific or bug-specific branch where a contributor develops her or his contribution. This is referred to as topic branch in the text.
These placeholders should be substituted with appropriate values while executing the commands.
Beginners to this workflow should always remember that a Git branch is not a container of commits, but rather a lightweight moving pointer that points to a commit in the commit history.
A---B---C
↑
(master)
When a new commit is made in a branch, its branch pointer simply moves to point to the last commit in the branch.
A---B---C---D
↑
(master)
A branch is merely a pointer to the tip of a series of commits. With this little thing in mind, seemingly complex operations like rebase and fast-forward merges become easy to understand and use.
The next section, Quick Reference, provides a brief summary of all the frequently used commands involved in creating and merging pull requests.
Here is a brief summary of all the commands used to create and merge pull requests in this document. This section serves as a quick reference.
# CREATE PULL REQUEST
# ===================
# Fork upstream and clone your fork.
git clone https://GITHUB/USER/REPO.git
cd REPO
git remote add upstream https://GITHUB/UPSTREAM-OWNER/REPO.git
git remote -v
# Work on pull request in a new topic branch.
git checkout -b TOPIC-BRANCH
git add FILES
git commit
git push origin TOPIC-BRANCH
# Go to your fork on GitHub, switch to the topic branch, and
# click *Compare & pull request*.
# Keep your fork's main development branch updated with upstream's.
git fetch upstream
git checkout master
git merge upstream/master
git push origin master
# Squash commits, e.g., last 3 commits in topic branch (optional).
git checkout TOPIC-BRANCH
git rebase -i HEAD~3
git push -f origin TOPIC-BRANCH
# Rebase topic branch on the main development branch (optional).
git checkout TOPIC-BRANCH
git rebase master
# Delete topic branch branch after pull request is merged.
git checkout master
git branch -D TOPIC-BRANCH
git push -d origin TOPIC-BRANCH
# MERGE PULL REQUEST (WITHOUT MERGE COMMIT)
# =========================================
# Clone upstream repo.
git clone https://GITHUB/UPSTREAM-OWNER/REPO.git
cd REPO
# Pull changes in pull request into a temporary branch.
git checkout -b pr
git pull https://GITHUB/CONTRIBUTOR/REPO.git TOPIC-BRANCH
# Rebase pull request on the main development branch.
git rebase master
# Merge pull request and delete temporary branch.
git checkout master
git merge pr
git branch -d pr
# Push the updated main development branch to upstream repo.
git push origin master
# MERGE PULL REQUEST (WITH MERGE COMMIT)
# ======================================
# Clone upstream repo.
git clone https://GITHUB/UPSTREAM-OWNER/REPO.git
cd REPO
# Pull changes in pull request into a temporary branch.
git checkout -b pr
git pull https://GITHUB/CONTRIBUTOR/REPO.git TOPIC-BRANCH
# Merge pull request and delete temporary branch
git checkout master
git merge --no-ff pr
git branch -d pr
# Push the updated main development branch to upstream repo.
git push origin master
The next two sections, Create Pull Request and Merge Pull Request, elaborate these commands in detail.
This section is meant for developers who contribute new commits to the upstream repository from their personal fork.
On GitHub, fork the upstream repository to your personal user account.
Then clone your fork from your personal GitHub user account to your
local system and set the upstream repository URL as a remote named
upstream.
git clone https://GITHUB/USER/REPO.git
cd REPO
git remote add upstream https://GITHUB/UPSTREAM-OWNER/REPO.git
git remote -v
Now the remote named upstream points to the upstream repository and
the remote named origin points to your fork.
In the fork and pull request workflow, a contributor must never push
commits to upstream. A contributor must only push commits to origin.
Work on a new pull request in a new topic branch and commit to your
fork. Remember to use a meaningful name instead of TOPIC-BRANCH in the
commands below.
git checkout -b TOPIC-BRANCH
git add FILES
git commit
git push origin TOPIC-BRANCH
Create pull request via GitHub web interface as per the following steps:
- Go to your fork on GitHub.
- Switch to the topic branch.
- Click Compare & pull request.
- Click Create pull request.
Wait for an upstream developer to review and merge your pull request.
If there are review comments to be addressed, continue working on your
branch, commiting, optionally squashing and rebasing them, and pushing
them to the topic branch of origin (your fork). Any changes to the
topic branch automatically become available in the pull request.
In the fork and pull request workflow, a contributor should never commit anything to the main development branch of personal fork. This makes it very easy to keep the main development branch of your fork in sync with that of the upstream repository. This is explained in the next subsection.
As new pull requests get merged into the upstream's main development branch, the main development branch of your fork begins falling behind it.
The commands below show how to update your fork's main development branch with the new commits in the upstream's main development branch.
git fetch upstream
git checkout master
git merge upstream/master
git push origin master
The git merge command above simply fast-forwards the main development
branch from an earlier commit to the last commit in the upstream's main
development branch.
When your main development branch falls behind the upstream's main development branch, the upstream's main development branch extends linearly from the last commit in your main development branch, provided that there are no additional commits to your main development branch that has caused it to diverge.
E---F---G (TOPIC-BRANCH)
/
A---B---C---D (master)
\
H---I---J (upstream/master)
With such a commit history, when the upstream's main development branch is merged into your main development branch, the merge is done by simply fast-forwarding the pointer of your main development branch to the last commit in the upstream's main development branch.
E---F---G (TOPIC-BRANCH)
/
A---B---C---D
\
H---I---J (upstream/master, master)
After the merge is complete, the upstream's main development branch and your main development branch point to the same commit.
This is an optional step to keep the commit history concise.
After developing the required feature or bug-fix in a topic branch, the developer or a reviewer may notice issues in the work that need to be addressed before the pull request can be merged into the upstream repository. This may lead to multiple new commits in the topic branch that should ideally have been part of the first commit that implemented that feature or bug-fix.
E---F---G (TOPIC-BRANCH)
/
A---B---C---D (master)
It may be a good idea to squash such multiple commits in the topic branch into a single coherent commit with all changes for the feature or bug-fix being developed.
E' (TOPIC-BRANCH)
/
A---B---C---D (master)
The following example shows how to squash the last 3 commits into a single commit and publish the squashed commit to the pull request.
git checkout TOPIC-BRANCH
git rebase -i HEAD~3
git push -f origin TOPIC-BRANCH
This brings up an editor with the last 3 commits ordered from earliest
to last. Leave the first commit untouched. Replace pick with squash
in the next two lines, save, and quit the editor.
This brings up an editor again. Clean up the commit message, save, and quit the editor.
The -f (force) option in the git push command is necessary only if
you are pushing to an already existing pull request branch because doing
so overwrites the history of the branch. Normally, overwriting history
is strictly discouraged but this is one of the rare scenarios where it
is safe to overwrite the commit history because the commits are being
pushed to a personal branch in a personal fork without affecting the
upstream repository.
Note: Even if the pull request developer does not squash commits, the upstream developers always have the option to squash the commits themselves before merging the pull request into the upstream repository.
This is an optional step to keep the commit history as linear as possible.
The main development branch may have diverged since the topic branch was created.
E---F---G (TOPIC-BRANCH)
/
A---B---C---D (master)
It may be a good idea to move the commits in the topic branch and place them on top of the main development branch, so that the topic branch extends linearly from the last commit in the main development branch.
E'---F'---G' (TOPIC-BRANCH)
/
A---B---C---D (master)
The following command shows how to rebase the topic branch on the main development branch.
git checkout TOPIC-BRANCH
git rebase master
Note: Even if the the pull request developer does not rebase commits, the upstream developers always have the option to rebase the commits themselves before merging the pull request into upstream.
Once the upstream developer merges your pull request, you may delete the topic branch from your local system as well as from your fork.
git checkout master
git branch -D TOPIC-BRANCH
git push -d origin TOPIC-BRANCH
The next section, [Merge Pull Request], explains how an upstream owner can merge pull request from contributors into the upstream repository.
This section is meant for lead developers who own the upstream repository and merge pull requests from contributors to it.
There are two popular methods to merge commits: one that does not introduce an additional merge commit, and another that does.
Both method are perfectly acceptable. Both methods are discussed below.
Which method you choose depends on whether you want to maintain a concise commit history consisting only of development commits or if you want to introduce additional merge commits for every merge into your commit history.
Clone the upstream repository to your local system.
git clone https://GITHUB/UPSTREAM-OWNER/REPO.git
cd REPO
Create a temporary branch (pr for example) to pull the contribution
(pull request) from the CONTRIBUTOR's branch in it:
git checkout -b pr
git pull https://GITHUB/CONTRIBUTOR/REPO.git TOPIC-BRANCH
If the main development branch has diverged from the branch in the pull request, move the commits in the pull request and place them on top of the main development branch, so that the branch in the pull request extends linearly from the main development branch.
git rebase master
See Rebase Commits section above for more details.
Squash multiple commits in the pull request into fewer commits (if desired). See Squash Commits section above for more details.
After sufficient testing, merge the commits in the pull request into the main development branch and remove the pull request branch.
git checkout master
git merge pr
git branch -d pr
Finally, push the current state of the main development branch to the upstream repository.
git push origin master
The git merge command above simply fast-forwards the main development
branch from an earlier commit to the last commit in the pull request
thereby making both the main development branch and the pull request
branch point to the same commit. This achieves the merge from the pull
request branch into the main development branch without creating a new
merge commit.
After a pull request branch is rebased on the main development branch, the pull request branch becomes a linear extension of the main development branch.
E'---F'---G' (pr)
/
A---B---C---D (master)
With such a commit history, when the pull request branch is now merged into the main development branch, the merge is done by simply fast-forwarding the pointer of the main development branch to the last commit in the pull request branch.
E'---F'---G' (pr, master)
/
A---B---C---D
Clone the upstream repository to your local system.
git clone https://GITHUB/UPSTREAM-OWNER/REPO.git
cd REPO
Create a temporary branch (pr for example) to pull the contribution
(pull request) from the CONTRIBUTOR's branch in it:
git checkout -b pr
git pull https://GITHUB/CONTRIBUTOR/REPO.git TOPIC-BRANCH
Squash multiple commits in the pull request into one commit (if desired). See Squash Commits section above for more details.
After sufficient testing, merge the commits in pull request into the main development branch.
git checkout master
git merge --no-ff pr
git branch -d pr
Finally, push the current state of the main development branch (with the commits from the pull request in it) to the upstream repository.
git push origin master
The --no-ff (no fast-forward) option in the git merge command
ensures that a merge commit is always created even when a fast-forward
merge is possible.
E'---F'---G' (pr)
/ \
A---B---C---D-------------H (master)
This is a bonus section that describes a few aliases and commands that may be useful during day-to-day development activities. When git merges crash and burn, these aliases and commands may be useful to check the current state of the repository.
The following commands create aliases to run git log with various subsets of
{--pretty, --graph, --all} options to display commit logs in a compact
form, i.e., one line per log.
# Define
FORMAT="%C(auto)%h %C(magenta)%ad %C(cyan)%an%C(auto)%d %s"
PRETTY="--pretty=format:'$FORMAT' --date=short"
git config --global alias.lga "log --graph --all $PRETTY"
git config --global alias.lg "log --graph $PRETTY"
git config --global alias.la "log --all $PRETTY"
git config --global alias.ll "log $PRETTY"
git config --global alias.lf "log --pretty=fuller --stat"
# Use aliases
git lga
git lg
git la
git ll
git lf
While git diff shows the unstaged changes in the working directory, it
is necessary to use the --cached option with git diff to see the
changes staged for the next commit. The following command creates a
convenient alias for this option.
# Define aliases
git config --global alias.diffc "diff --cached"
git config --global alias.dc "diff --cached"
# Use aliases
git diffc
git dc
The following commands provides aliases to list branches with verbose information. The second alias includes remote branches too in the output.
# Define aliases
git config --global alias.br "branch -vv"
git config --global alias.brr "branch -vva"
# Use aliases
git br
git brr
Find a common ancestor of two branches with this command. It helps to find the commit after which two branches began diverging.
git merge-base upstream/master TOPIC-BRANCH
Enter one of the commands below in your command shell depending on your operating system, or just open the URL with your web browser.
open https://xkcd.com/1597/
xdg-open https://xkcd.com/1597/
start https://xkcd.com/1597/
Copyright © 2018 Susam Pal
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