Software Engineering for Self-Directed Learners »

Git and GitHub

init: Getting started

Let's take your first few steps in your Git (with GitHub) journey.

0. Take a peek at the full picture(?). Optionally, if you are the sort who prefers to have some sense of the full picture before you get into the nitty-gritty details, watch the video in the panel below:

Git Overview


1. The first step is to install SourceTree, which is Git + a GUI for Git. If you prefer to use Git via the command line (i.e., without a GUI), you can install Git instead.

2. Next, initialize a repository. Let us assume you want to version control content in a specific directory. In that case, you need to initialize a Git repository in that directory. Here are the steps:

Create a directory for the repo (e.g., a directory named things).

commit: Saving changes to history

After initializing a repository, Git can help you with revision controlling files inside the working directory. However, it is not automatic. It is up to you to tell Git which of your changes (aka revisions) should be committed to its memory for later use. Saving changes into Git's memory in that way is often called committing and a change saved to the revision history is called a commit.

Working directory: the root directory revision-controlled by Git (e.g., the directory in which the repo was initialized).

Commit (noun): a change (aka a revision) saved in the Git revision history.
(verb): the act of creating a commit i.e., saving a change in the working directory into the Git revision history.

Here are the steps you can follow to learn how to work with Git commits:

1. Do some changes to the content inside the working directory e.g., create a file named fruits.txt in the things directory and add some dummy text to it.

2. Observe how the file is detected by Git.

3. Stage the changes to commit: Although Git has detected the file in the working directory, it will not do anything with the file unless you tell it to. Suppose you want to commit the current changes to the file. First, you should stage the file.

Stage (verb): Instructing Git to prepare a file for committing.

4. Commit the staged version of fruits.txt.

Note the existence of something called the master branch. Git allows you to have multiple branches (i.e. it is a way to evolve the content in parallel) and Git auto-creates a branch named master on which the commits go on by default.

5. Do a few more commits.

  1. Make some changes to fruits.txt (e.g. add some text and delete some text). Stage the changes, and commit the changes using the same steps you followed before. You should end up with something like this.

  2. Next, add two more files colors.txt and shapes.txt to the same working directory. Add a third commit to record the current state of the working directory.

6. See the revision graph: Note how commits form a path-like structure aka the revision tree/graph. In the revision graph, each commit is shown as linked to its 'parent' commit (i.e., the commit before it).

Omitting files from revision control

Often, there are files inside the Git working folder that you don't want to revision-control e.g., temporary log files. Follow the steps below to learn how to configure Git to ignore such files.

1. Add a file into your repo's working folder that you supposedly don't want to revision-control e.g., a file named temp.txt. Observe how Git has detected the new file.

2. Tell Git to ignore that file:

The .gitignore file

The .gitignore file tells Git which files to ignore when tracking revision history. That file itself can be either revision controlled or ignored.

  • To version control it (the more common choice – which allows you to track how the .gitignore file changes over time), simply commit it as you would commit any other file.

  • To ignore it, follow the same steps you followed above when you set Git to ignore the temp.txt file.

  • It supports file patterns e.g., adding temp/*.tmp to the .gitignore file prevents Git from tracking any .tmp files in the temp directory.

More information about the .gitignore file: git-scm.com/docs/gitignore

tag: Naming commits

Each Git commit is uniquely identified by a hash e.g., d670460b4b4aece5915caf5c68d12f560a9fe3e4. As you can imagine, using such an identifier is not very convenient for our day-to-day use. As a solution, Git allows adding a more human-readable tag to a commit e.g., v1.0-beta.

Here's how you can tag a commit in a local repo (e.g. in the samplerepo-things repo):

After adding a tag to a commit, you can use the tag to refer to that commit, as an alternative to using the hash.

diff: Comparing revisions

Git can show you what changed in each commit.

Git can also show you the difference between two points in the history of the repo.

checkout: Retrieving a specific revision

Git can load a specific version of the history to the working directory. Note that if you have uncommitted changes in the working directory, you need to stash them first to prevent them from being overwritten.

stash: Shelving changes temporarily

You can use Git's stash feature to temporarily shelve (or stash) changes you've made to your working copy so that you can work on something else, and then come back and re-apply the stashed changes later on. -- adapted from Atlassian

clone: Copying a repo

Given below is an example scenario you can try yourself to learn Git cloning.

Suppose you want to clone the sample repo samplerepo-things to your computer.

Note that the URL of the GitHub project is different from the URL you need to clone a repo in that GitHub project. e.g.

GitHub project URL: https://github.com/se-edu/samplerepo-things
Git repo URL: https://github.com/se-edu/samplerepo-things.git (note the .git at the end)

pull, fetch: Downloading data from other repos

Here's a scenario you can try in order to learn how to pull commits from another repo to yours.

1. Clone a repo (e.g., the repo used in [Git & GitHub → Clone]) to be used for this activity.

2. Delete the last few commits to simulate cloning the repo a few commits ago.

Now, your local repo state is exactly how it would be if you had cloned the repo 2 commits ago, as if somebody has added two more commits to the remote repo since you cloned it.

3. Pull from the other repo: To get those missing commits to your local repo (i.e. to sync your local repo with upstream repo) you can do a pull.

You can also do a fetch instead of a pull in which case the new commits will be downloaded to your repo but the working directory will remain at the current commit. To move the current state to the latest commit that was downloaded, you need to do a merge. A pull is a shortcut that does both those steps in one go.

Working with multiple remotes

When you clone a repo, Git automatically adds a remote repo named origin to your repo configuration. As you know, you can pull commits from that repo. As you know, a Git repo can work with remote repos other than the one it was cloned from.

To communicate with another remote repo, you can first add it as a remote of your repo. Here is an example scenario you can follow to learn how to pull from another repo:

push: Uploading data to other repos

Given below is a scenario you can try in order to learn how to push commits to a remote repo hosted on GitHub:

1. Fork an existing GitHub repo (e.g., samplerepo-things) to your GitHub account.

2. Clone the fork (not the original) to your computer.

3. Commit some changes in your local repo.

4. Push the new commits to your fork on GitHub

You can push to repos other than the one you cloned from, as long as the target repo and your repo have a shared history.

  1. Add the GitHub repo URL as a remote, if you haven't done so already.
  2. Push to the target repo.

You can even push an entire local repository to GitHub, to form an entirely new remote repository. For example, you created a local repo and worked with it for a while but now you want to upload it onto GitHub (as a backup or to share it with others). The steps are given below.

1. Create an empty remote repo on GitHub.

  1. Login to your GitHub account and choose to create a new Repo.

  2. In the next screen, provide a name for your repo but keep the Initialize this repo ... tick box unchecked.

  3. Note the URL of the repo. It will be of the form https://github.com/{your_user_name}/{repo_name}.git.
    e.g., https://github.com/johndoe/foobar.git (note the .git at the end)

2. Add the GitHub repo URL as a remote of the local repo. You can give it the name origin (or any other name).

3. Push the repo to the remote.

branch: Doing multiple parallel changes

Git supports branching, which allows you to do multiple parallel changes to the content of a repository.

A Git branch is simply a named label pointing to a commit. The HEAD label indicates which branch you are on. Git creates a branch named master by default. When you add a commit, it goes into the branch you are currently on, and the branch label (together with the HEAD label) moves to the new commit.

Given below is an illustration of how branch labels move as branches evolve.

  1. There is only one branch (i.e., master) and there is only one commit on it.
  2. A new commit has been added. The master and the HEAD labels have moved to the new commit.
  3. A new branch fix1 has been added. The repo has switched to the new branch too (hence, the HEAD label is attached to the fix1 branch).
  4. A new commit (c) has been added. The current branch label fix1 moves to the new commit, together with the HEAD label.
  5. The repo has switched back to the master branch.
  1. A new commit (d) has been added. The master label has moved to that commit.
  2. The repo has switched back to the fix1 branch and added a new commit (e) to it.
  3. The repo has switched to the master branch and the fix1 branch has been merged into the master branch, creating a merge commit f. The repo is currently on the master branch.

Follow the steps below to learn how to work with branches. You can use any repo you have on your computer (e.g. a clone of the samplerepo-things) for this.

0. Observe that you are normally in the branch called master.

1. Start a branch named feature1 and switch to the new branch.

2. Create some commits in the new branch. Just commit as per normal. Commits you add while on a certain branch will become part of that branch.
Note how the master label and the HEAD label moves to the new commit (The HEAD label of the local repo is represented as in SourceTree).

3. Switch to the master branch. Note how the changes you did in the feature1 branch are no longer in the working directory.

4. Add a commit to the master branch. Let’s imagine it’s a bug fix.
To keep things simple for the time being, this commit should not involve the same content that you changed in the feature1 branch. To be on the safe side, this commit can change an entirely different file.

5. Switch back to the feature1 branch (similar to step 3).

6. Merge the master branch to the feature1 branch, giving an end-result like the following. Also note how Git has created a merge commit.

The objective of that merge was to sync the feature1 branch with the master branch. Observe how the changes you did in the master branch (i.e. the imaginary bug fix) is now available even when you are in the feature1 branch.

Instead of merging master to feature1, an alternative is to rebase the feature1 branch. However, rebasing is an advanced feature that requires modifying past commits. If you modify past commits that have been pushed to a remote repository, you'll have to force-push the modified commit to the remote repo in order to update the commits in it.

7. Add another commit to the feature1 branch.

8. Switch to the master branch and add one more commit.

9. Merge feature1 to the master branch, giving and end-result like this:

10. Create a new branch called add-countries, switch to it, and add some commits to it (similar to steps 1-2 above). You should have something like this now:

Avoid this common rookie mistake!

Always remember to switch back to the master branch before creating a new branch. If not, your new branch will be created on top of the current branch.

11. Go back to the master branch and merge the add-countries branch onto the master branch (similar to steps 8-9 above). While you might expect to see something like the following,

... you are likely to see something like this instead:

That is because Git does a fast forward merge if possible. Seeing that the master branch has not changed since you started the add-countries branch, Git has decided it is simpler to just put the commits of the add-countries branch in front of the master branch, without going into the trouble of creating an extra merge commit.

It is possible to force Git to create a merge commit even if fast forwarding is possible.

Pushing a branch to a remote repo

Here's how to push a branch to a remote repo:

Dealing with merge conflicts

Merge conflicts happen when you try to combine two incompatible versions (e.g., merging a branch to another but each branch changed the same part of the code in a different way).

Here are the steps to simulate a merge conflict and use it to learn how to resolve merge conflicts.

0. Create an empty repo or clone an existing repo, to be used for this activity.

1. Start a branch named fix1 in the repo. Create a commit that adds a line with some text to one of the files.

2. Switch back to master branch. Create a commit with a conflicting change i.e. it adds a line with some different text in the exact location the previous line was added.

3. Try to merge the fix1 branch onto the master branch. Git will pause mid-way during the merge and report a merge conflict. If you open the conflicted file, you will see something like this:

COLORS
------
blue
<<<<<< HEAD
black
=======
green
>>>>>> fix1
red
white

4. Observe how the conflicted part is marked between a line starting with <<<<<< and a line starting with >>>>>>, separated by another line starting with =======.

Highlighted below is the conflicting part that is coming from the master branch:

blue
<<<<<< HEAD
black
=======
green
>>>>>> fix1
red

This is the conflicting part that is coming from the fix1 branch:

blue
<<<<<< HEAD
black
=======
green
>>>>>> fix1
red

5. Resolve the conflict by editing the file. Let us assume you want to keep both lines in the merged version. You can modify the file to be like this:

COLORS
------
blue
black
green
red
white

6. Stage the changes, and commit.

Creating PRs

Suppose you want to propose some changes to a GitHub repo (e.g., samplerepo-pr-practice) as a pull request (PR). Here is a scenario you can try in order to learn how to create PRs:

1. Fork the repo onto your GitHub account.

2. Clone it onto your computer.

3. Commit your changes e.g., add a new file with some contents and commit it.

  • Option A - Commit changes to the master branch
  • Option B - Commit to a new branch e.g., create a branch named add-intro (remember to switch to the master branch before creating a new branch) and add your commit to it.

4. Push the branch you updated (i.e., master branch or the new branch) to your fork, as explained here.

5. Initiate the PR creation:

  1. Go to your fork.

  2. Click on the Pull requests tab followed by the New pull request button. This will bring you to the 'Comparing changes' page.

  3. Set the appropriate target repo and the branch that should receive your PR, using the base repository and base dropdowns. e.g.,
    base repository: se-edu/samplerepo-pr-practice base: master

    Normally, the default value shown in the dropdown is what you want but in case your fork has e.g., the repo you forked from is also a fork of a another repo, which means both of those are considered upstream repos of your forkmultiple upstream repos, the default may not be what you want.

  4. Indicate which repo:branch contains your proposed code, using the head repository and compare dropdowns. e.g.,
    head repository: myrepo/samplerepo-pr-practice compare: master

6. Verify the proposed code: Verify that the diff view in the page shows the exact change you intend to propose. If it doesn't, commit the new code and push to the branchupdate the branch as necessary.

7. Submit the PR:

  1. Click the Create pull request button.

  2. Fill in the PR name and description e.g.,
    Name: Add an introduction to the README.md
    Description:

    Add some paragraph to the README.md to explain ...
    Also add a heading ...
    
  3. If you want to indicate that the PR you are about to create is 'still work in progress, not yet ready', click on the dropdown arrow in the Create pull request button and choose Create draft pull request option.

  4. Click the Create pull request button to create the PR.

  5. Go to the receiving repo to verify that your PR appears there in the Pull requests tab.

The next step of the PR life cycle is the PR review. The members of the repo that received your PR can now review your proposed changes.

  • If they like the changes, they can merge the changes to their repo, which also closes the PR automatically.
  • If they don't like it at all, they can simply close the PR too i.e., they reject your proposed change.
  • In most cases, they will add comments to the PR to suggest further changes. When that happens, GitHub will notify you.

You can update the PR along the way too. Suppose PR reviewers suggested a certain improvement to your proposed code. To update your PR as per the suggestion, you can simply modify the code in your local repo, commit the updated code to the same master branch, and push to your fork as you did earlier. The PR will auto-update accordingly.

Sending PRs using the master branch is less common than sending PRs using separate branches. For example, suppose you wanted to propose two bug fixes that are not related to each other. In that case, it is more appropriate to send two separate PRs so that each fix can be reviewed, refined, and merged independently. But if you send PRs using the master branch only, both fixes (and any other change you do in the master branch) will appear in the PRs you create from it.

To create another PR while the current PR is still under review, create a new branch (remember to switch back to the master branch first), add your new proposed change in that branch, and create a new PR following the steps given above.

It is possible to create PRs within the same repo e.g., you can create a PR from branch feature-x to the master branch, within the same repo. Doing so will allow the code to be reviewed by other developers (using PR review mechanism) before it is merged.

Reviewing PRs

The PR review stage is a dialog between the PR author and members of the repo that received the PR, in order to refine and eventually merge the PR.

Given below are some steps you can follow when reviewing a PR.

1. Locate the PR:

  1. Go to the GitHub page of the repo.
  2. Click on the Pull requests tab.
  3. Click on the PR you want to review.

2. Read the PR description. It might contain information relevant to reviewing the PR.

3. Click on the Files changed tab to see the diff view.

4. Add review comments:

  1. Hover over the line you want to comment on and click on the icon that appears on the left margin. That should create a text box for you to enter your comment.
    To mark multiple lines, click-and-drag the icon.
  2. Enter your comment.
    This page @SE-EDU/guides has some best practices PR reviewers can follow.
  3. After typing in the comment, click on the Start a review button (not the Add single comment button. This way, your comment is saved but not visible to others yet. It will be visible to others only when you have finished the entire review.

  4. Repeat the above steps to add more comments.

5. Submit the review:

  1. When there are no more comments to add, click on the Review changes button (on the top right of the diff page).
  2. Type in an overall comment about the PR, if any. e.g.,
    Overall, I found your code easy to read for the most part except a few places
    where the nesting was too deep. I noted a few minor coding standard violations
    too. Some of the classes are getting quite long. Consider splitting into
    smaller classes if that makes sense.
    
    LGTM is often used in such overall comments, to indicate Looks good to merge.
    nit is another such term, used to indicate minor flaws e.g., LGTM, almost. Just a few nits to fix..
  3. Choose Approve, Comment, or Request changes option as appropriate and click on the Submit review button.

Merging PRs

Let's look at the steps involved in merging a PR, assuming the PR has been reviewed, refined, and approved for merging already.

Preparation: If you would like to try merging a PR yourself, you can create a dummy PR in the following manner.

  1. Fork any repo (e.g., samplerepo-pr-practice).
  2. Clone in to your computer.
  3. Create a new branch e.g., (feature1) and add some commits to it.
  4. Push the new branch to the fork.
  5. Create a PR from that branch to the master branch in your fork. Yes, it is possible to create a PR within the same repo.

1. Locate the PR to be merged in your repo's GitHub page.

2. Click on the Conversation tab and scroll to the bottom. You'll see a panel containing the PR status summary.

3. If the PR is not merge-able in the current state, the Merge pull request will not be green. Here are the possible reasons and remedies:

  • Problem: The PR code is out-of-date, indicated by the message This branch is out-of-date with the base branch. That means the repo's master branch has been updated since the PR code was last updated.
    • If the PR author has allowed you to update the PR and you have sufficient permissions, GitHub will allow you to update the PR simply by clicking the Update branch on the right side of the 'out-of-date' error message. If that option is not available, post a message in the PR requesting the PR author to update the PR.
  • Problem: There are merge conflicts, indicated by the message This branch has conflicts that must be resolved. That means the repo's master branch has been updated since the PR code was last updated, in a way that the PR code conflicts with the current master branch. Those conflicts must be resolved before the PR can be merged.
    • If the conflicts are simple, GitHub might allow you to resolve them using the Web interface.
    • If that option is not available, post a message in the PR requesting the PR author to update the PR.

3. Merge the PR by clicking on the Merge pull request button, followed by the Confirm merge button. You should see a Pull request successfully merged and closed message after the PR is merged.

  • You can choose between three merging options by clicking on the down-arrow in the Merge pull request button. If you are new to Git and GitHub, the Create merge commit options are recommended.

Next, sync your local repos (and forks). Merging a PR simply merges the code in the upstream remote repository in which it was merged. The PR author (and other members of the repo) needs to pull the merged code from the upstream repo to their local repos and push the new code to their respective forks to sync the fork with the upstream repo.

Forking workflow

You can follow the steps in the simulation of a forking workflow given below to learn how to follow such a workflow.

This activity is best done as a team.

Step 1. One member: set up the team org and the team repo.

  1. Create a GitHub organization for your team. The org name is up to you. We'll refer to this organization as team org from now on.

  2. Add a team called developers to your team org.

  3. Add team members to the developers team.

  4. Fork se-edu/samplerepo-workflow-practice to your team org. We'll refer to this as the team repo.

  5. Add the forked repo to the developers team. Give write access.

Step 2. Each team member: create PRs via own fork.

  1. Fork that repo from your team org to your own GitHub account.

  2. Create a branch named add-{your name}-info (e.g. add-johnTan-info) in the local repo.

  3. Add a file yourName.md into the members directory (e.g., members/jonhTan.md) containing some info about you into that branch.

  4. Push that branch to your fork.

  5. Create a PR from that branch to the master branch of the team repo.

Step 3. For each PR: review, update, and merge.

  1. [A team member (not the PR author)] Review the PR by adding comments (can be just dummy comments).

  2. [PR author] Update the PR by pushing more commits to it, to simulate updating the PR based on review comments.

  3. [Another team member] Approve and merge the PR using the GitHub interface.

  4. [All members] Sync your local repo (and your fork) with upstream repo. In this case, your upstream repo is the repo in your team org.

Step 4. Create conflicting PRs.

  1. [One member]: Update README: In the master branch, remove John Doe and Jane Doe from the README.md, commit, and push to the main repo.

  2. [Each team member] Create a PR to add yourself under the Team Members section in the README.md. Use a new branch for the PR e.g., add-johnTan-name.

Step 5. Merge conflicting PRs one at a time. Before merging a PR, you’ll have to resolve conflicts.

  1. [Optional] A member can inform the PR author (by posting a comment) that there is a conflict in the PR.

  2. [PR author] Resolve the conflict locally:

    1. Pull the master branch from the repo in your team org.
    2. Merge the pulled master branch to your PR branch.
    3. Resolve the merge conflict that crops up during the merge.
    4. Push the updated PR branch to your fork.
  3. [Another member or the PR author]: Merge the de-conflicted PR: When GitHub does not indicate a conflict anymore, you can go ahead and merge the PR.