Software Engineering for Self-Directed Learners
  • Git and Github

    Init

    Can create a local Git repo

    Soon you are going to take your first step in using Git. If you would like to see a quick overview of the full Git landscape before jumping in, watch the video below.

    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.

    Suppose you want to create a repository in an empty directory things. Here are the steps:

    Windows: Click FileClone/New…. Click on Create button.
    Mac: New...Create New Repository.

    Enter the location of the directory (Windows version shown below) and click Create.

    Go to the things folder and observe how a hidden folder .git has been created.

    Note: If you are on Windows, you might have to configure Windows Explorer to show hidden files.


    Open a Git Bash Terminal.

    If you installed SourceTree, you can click the Terminal button to open a GitBash terminal.

    Navigate to the things directory.

    Use the command git init which should initialize the repo.

    $ git init
    Initialized empty Git repository in c:/repos/things/.git/
    

    You can use the command ls -a to view all files, which should show the .git directory that was created by the previous command.

    $ ls -a
    .  ..  .git
    

    You can also use the git status command to check the status of the newly-created repo. It should respond with something like the bellow

    git status
    

    # On branch master
    #
    # Initial commit
    #
    nothing to commit (create/copy files and use "git add" to track)
    

    Commit

    Can commit using Git

    Create an empty repo.

    Create a file named fruits.txt in the working directory and add some dummy text to it.

    Working directory: The directory the repo is based in is called the working directory.

    Observe how the file is detected by Git.

    The file is shown as ‘unstaged’


    You can use the git status command to check the status of the working directory.

    git status
    

    # On branch master
    #
    # Initial commit
    #
    # Untracked files:
    #   (use "git add <file>..." to include in what will be committed)
    #
    #   a.txt
    nothing added to commit but untracked files present (use "git add" to track)
    

    Although git has detected the file in the working directory, it will not do anything with the file unless you tell it to. Suppose we want to commit the current state of the file. First, we should stage the file.

    Commit: Saving the current state of the working folder into the Git revision history.

    Stage: Instructing Git to prepare a file for committing.

    Select the fruits.txt and click on the Stage Selected button

    fruits.txt should appear in the Staged files panel now.


    You can use the stage or the add command (they are synonyms, add is the more popular choice) to stage files.

    git add fruits.txt
    git status
    

    # On branch master
    #
    # Initial commit
    #
    # Changes to be committed:
    #   (use "git rm --cached <file>..." to unstage)
    #
    #       new file:   fruits.txt
    #
    

    Now, you can commit the staged version of fruits.txt

    Click the Commit button, enter a commit message e.g. add fruits.txt in to the text box, and click Commit


    Use the commit command to commit. The -m switch is used to specify the commit message.

    git commit -m "add fruits.txt"
    

    You can use the log command to see the commit history

    git log
    

    commit 8fd30a6910efb28bb258cd01be93e481caeab846
    Author: … < … @... >
    Date:   Wed Jul 5 16:06:28 2017 +0800
    
      Add 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 creates a default branch named master on which the commits go on by default.

    Do 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.

    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.

    • Try Git is an online simulation/tutorial of Git basics. You can try its first few steps to solidify what you learned in this LO.

    Ignore

    Can set Git to ignore files

    Add a file names temp.txt to the things repo you created. Suppose we don’t want this file to be revision controlled by Git. Let’s instruct Git to ignore temp.txt

    The file should be currently listed under Unstaged files. Right-click it and choose Ignore…. Choose Ignore exact filename(s) and click OK.

    Observe that a file named .gitignore has been created in the working directory root and has the following line in it.

    temp.txt
    

    Create a file named .gitignore in the working directory root and add the following line in it.

    temp.txt
    

    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 changed over time), simply commit it as you would commit any other file.
    • To ignore it, follow the same steps we followed above when we set Git to ignore the temp.txt file.

    Tag

    Can tag commits using Git

    Let's tag a commit in a local repo you have (e.g. the sampelrepo-things repo)

    Right-click on the commit (in the graphical revision graph) you want to tag and choose Tag…

    Specify the tag name e.g. v1.0 and click Add Tag.

    The added tag will appear in the revision graph view.


    To add a tag to the current commit as v1.0,

    git tag –a v1.0
    

    To view tags

    git tag
    

    To learn how to add a tag to a past commit, go to the ‘Git Basics – Tagging’ page of the git-scm book and refer the ‘Tagging Later’ section.


    Checkout

    Can load a specific version of a Git repo

    Git can show you what changed in each commit.

    To see which files changed in a commit, click on the commit. To see what changed in a specific file in that commit, click on the file name.


    git show < part-of-commit-hash >
    

    Example:

    git show 251b4cf
    

    commit 5bc0e30635a754908dbdd3d2d833756cc4b52ef3
    Author: … < … >
    Date:   Sat Jul 8 16:50:27 2017 +0800
    
        fruits.txt: replace banana with berries
    
    diff --git a/fruits.txt b/fruits.txt
    index 15b57f7..17f4528 100644
    --- a/fruits.txt
    +++ b/fruits.txt
    @@ -1,3 +1,3 @@
     apples
    -bananas
    +berries
     cherries
    

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

    Select the two points you want to compare using Ctrl+Click.

    The same method can be used to compare the current state of the working directory (which might have uncommitted changes) to a point in the history.


    The diff command can be used to view the differences between two points of the history.

    • git diff : shows the changes (uncommitted) since the last commit
    • git diff 0023cdd..fcd6199: shows the changes between the points indicated by by commit hashes
    • git diff v1.0..HEAD: shows changes that happened from the commit tagged as v1.0 to the most recent commit.

    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.

     

    Tools → Git and GitHub →

    Stash

    You can use the 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 this

     

    Follow this article from SourceTree creators. Note the GUI shown in the article is slightly outdated but you should be able to map it to the current GUI.


     

    Follow this article from Atlassian.


    Double-click the commit you want to load to the working directory, or right-click on that commit and choose Checkout....

    Click OK to the warning about ‘detached HEAD’ (similar to below).

    The specified version is now loaded to the working folder, as indicated by the HEAD label. HEAD is a reference to the currently checked out commit.

    If you checkout a commit that come before the commit in which you added the .gitignore file, Git will now show ignored fiels as ‘unstaged modifications’ because at that stage Git hasn’t been told to ignore those files.

    To go back to the latest commit, double-click it.


    Use the checkout <commit-identifier> command to change the working directory to the state it was in at a specific past commit.

    • git checkout v1.0: loads the state as at commit tagged v1.0
    • git checkout 0023cdd: loads the state as at commit with the hash 0023cdd
    • git checkout HEAD~2: loads the state that is 2 commits behind the most recent commit

    For now, you can ignore the warning about ‘detached HEAD’.

    Use the checkout <branch-name> to go back to the most recent commit of the current branch (the default branch in git is named master)

    • git checkout master

    Stash

    Can use Git to stash files

    You can use the 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 this

     

    Follow this article from SourceTree creators. Note the GUI shown in the article is slightly outdated but you should be able to map it to the current GUI.


     

    Follow this article from Atlassian.


    Clone

    Can clone a remote repo

    Clone the sample repo samplerepo-things to your computer.

    Note that the URL of the Github project is different form 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)

    FileClone / New… and provide the URL of the repo and the destination directory.


    You can use the clone command to clone a repo.

    Follow instructions given here.


    Pull

    Can pull changes from a repo

    Clone the sample repo as explained in [Textbook Tools → Git & GitHub → Clone].

    Delete the last two commits to simulate cloning the repo 2 commits ago.

    Clone

    Can clone a remote repo

    Clone the sample repo samplerepo-things to your computer.

    Note that the URL of the Github project is different form 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)

    FileClone / New… and provide the URL of the repo and the destination directory.


    You can use the clone command to clone a repo.

    Follow instructions given here.


    Right-click the target commit (i.e. the commit that is 2 commits behind the tip) and choose Reset current branch to this commit.

    Choose the Hard - … option and click OK.

    This is what you will see.

    Note the following (cross refer the screenshot above):

    Arrow marked as a: The local repo is now at this commit, marked by the master label.
    Arrow marked as b: origin/master label shows what is the latest commit in the master branch in the remote repo.


    Use the reset command to delete commits at the tip of the revision history.

    git reset --hard HEAD~2
    

    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. To get those commits to your local repo (i.e. to sync your local repo with upstream repo) you can do a pull.

    Click the Pull button in the main menu, choose origin and master in the next dialog, and click OK.

    Now you should see something like this where master and origin/master are both pointing the same commit.


    git pull origin


    Push

    Can push to a remote repo

    1. Create a GitHub account if you don't have one yet.

    2. Fork the samplerepo-things to your GitHub account:

      Navigate to the on GitHub and click on the button on the top-right corner.

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

    4. Create some commits in your repo.

    5. Push the new commits to your fork on GitHub

    Click the Push button on the main menu, ensure the settings are as follows in the next dialog, and click the Push button on the dialog.


    Use the command git push origin master. Enter Github username and password when prompted.


    To push an existing local repo into a new remote repo on GitHub, first you need to 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

    Next, you can push the existing local repo to the new remote repo as follows:

    1. Open the local repo in SourceTree.
    2. Choose RepositoryRepository Settings menu option.
    3. Add a new remote to the repo with the following values.
      • Remote name: the name you want to assign to the remote repo. Recommended origin
      • URL/path: the URL of your repo (ending in .git) that you collected earlier.
      • Username: your GitHub username
    4. Now you can push your repo to the new remote the usual way.

    1. Navigate to the folder containing the local repo.
    2. Set the new remote repo as a remote of the local repo.
      command: git remote add {remote_name} {remote_repo_url}
      e.g., git remote add origin https://github.com/johndoe/foobar.git
    3. Push to the new remote the usual way. You can use the -u flag to inform Git that you wish to track the branch.
      e.g., git push -u origin master

    Branch

    Can use Git branching

    0. Observe that you are normally in the branch called master. For this, you can take any repo you have on your computer (e.g. a clone of the samplerepo-things).


    git status
    

    on branch master
    

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

    Click on the Branch button on the main menu. In the next dialog, enter the branch name and click Create Branch

    Note how the feature1 is indicated as the current branch.


    You can use the branch command to create a new branch and the checkout command to switch to a specific branch.

    git branch feature1
    git checkout feature1
    

    One-step shortcut to create a branch and switch to it at the same time:

    git checkout –b feature1
    

    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.

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

    Double-click the master branch


    git checkout master
    

    4. Add a commit to the master branch. Let’s imagine it’s a bug fix.

    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 below. Also note how Git has created a merge commit.

    Right-click on the master branch and choose merge master into the current branch. Click OK in the next dialog.


    git merge master
    

    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.

    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:

    Right-click on the feature1 branch and choose Merge....


    git merge feature1
    

    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:

    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 below,

    ... 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.

    Tick the box shown below when you merge a branch:


    Use the --no-ff switch (short for no fast forward):

    git merge --no-ff add-countries
    

    Merge Conflicts

    Can use Git to resolve merge conflicts

    1. Start a branch named fix1 in a local 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 =======.

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

    
    <<<<<<< HEAD
    black
    =======
    
    

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

    
    =======
    green
    >>>>>>> fix1
    
    

    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.

    Create PRs

    Can create PRs on GitHub

    1. Fork the samplerepo-pr-practice onto your GitHub account. Clone it onto your computer.

    2. Create a branch named add-intro in your clone. Add a couple of commits which adds/modifies an Introduction section to the README.md. Example:

    
    # Introduction
    Creating Pull Requsts (PRs) is needed when using RCS in a multi-person projects.
    This repo can be used to practice creating PRs.
    
    

    3. Push the add-intro branch to your fork.


    git push origin add-intro
    

    4. Create a Pull Request from the add-intro branch in your fork to the master branch of the same fork (i.e. your-user-name/samplerepo-pr-practice, not se-edu/samplerepo-pr-practice), as described below.

    4a. Go to the GitHub page of your fork (i.e. https://github.com/{your_username}/samplerepo-pr-practice), click on the Pull Requests tab, and then click on New Pull Request button.

    4b. Select base fork and head fork as follows:

    • base fork: your own fork (i.e. {your user name}/samplerepo-pr-practice, NOT se-edu/samplerepo-pr-practice)
    • head fork: your own fork.

    The base fork is where changes should be applied. The head fork contains the changes you would like to be applied.

    4c. (1) Set the base branch to master and head branch to add-intro, (2) confirm the diff contains the changes you propose to merge in this PR (i.e. confirm that you did not accidentally include extra commits in the branch), and (3) click the Create pull request button.

    4d. (1) Set PR name, (2) set PR description, and (3) Click the Create pull request button.

    A common newbie mistake when creating branch-based PRs is to mix commits of one PR with another. To learn how to avoid that mistake, you are encouraged to continue and create another PR as explained below.  

    5. In your local repo, create a new branch add-summary off the master branch.

    When creating the new branch, it is very important that you switch back to the master branch first. If not, the new branch will be created off the current branch add-intro. And that is how you end up having commits of the first PR in the second PR as well.

    6. Add a commit in the add-summary branch that adds a Summary section to the README.md, in exactly the same place you added the Introduction section earlier.

    7. Push the add-summary to your fork and create a new PR similar to before.

    Manage PRs

    Can review and merge PRs on GitHub

    1. Go to GitHub page of your fork and review the add-intro PR you created previously in [Tools → Git & GitHub → Create PRs] to simulate the PR being reviewed by another developer, as explained below. Note that some features available to PR reviewers will be unavailable to you because you are also the author of the PR.

     

    1. Fork the samplerepo-pr-practice onto your GitHub account. Clone it onto your computer.

    2. Create a branch named add-intro in your clone. Add a couple of commits which adds/modifies an Introduction section to the README.md. Example:

    
    # Introduction
    Creating Pull Requsts (PRs) is needed when using RCS in a multi-person projects.
    This repo can be used to practice creating PRs.
    
    

    3. Push the add-intro branch to your fork.


    git push origin add-intro
    

    4. Create a Pull Request from the add-intro branch in your fork to the master branch of the same fork (i.e. your-user-name/samplerepo-pr-practice, not se-edu/samplerepo-pr-practice), as described below.

    4a. Go to the GitHub page of your fork (i.e. https://github.com/{your_username}/samplerepo-pr-practice), click on the Pull Requests tab, and then click on New Pull Request button.

    4b. Select base fork and head fork as follows:

    • base fork: your own fork (i.e. {your user name}/samplerepo-pr-practice, NOT se-edu/samplerepo-pr-practice)
    • head fork: your own fork.

    The base fork is where changes should be applied. The head fork contains the changes you would like to be applied.

    4c. (1) Set the base branch to master and head branch to add-intro, (2) confirm the diff contains the changes you propose to merge in this PR (i.e. confirm that you did not accidentally include extra commits in the branch), and (3) click the Create pull request button.

    4d. (1) Set PR name, (2) set PR description, and (3) Click the Create pull request button.

    A common newbie mistake when creating branch-based PRs is to mix commits of one PR with another. To learn how to avoid that mistake, you are encouraged to continue and create another PR as explained below.  

    5. In your local repo, create a new branch add-summary off the master branch.

    When creating the new branch, it is very important that you switch back to the master branch first. If not, the new branch will be created off the current branch add-intro. And that is how you end up having commits of the first PR in the second PR as well.

    6. Add a commit in the add-summary branch that adds a Summary section to the README.md, in exactly the same place you added the Introduction section earlier.

    7. Push the add-summary to your fork and create a new PR similar to before.

    1a. Go to the respective PR page and click on the Files changed tab. 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.

    1b. Enter some dummy comment and click on Start a review button.

    1c. Add a few more comments in other places of the code.

    1d. Click on the Review Changes button, enter an overall comment, and click on the Submit review button.

    2. Update the PR to simulate revising the code based on reviewer comments. Add some more commits to the add-intro branch and push the new commits to the fork. Observe how the PR is updated automatically to reflect the new code.

    3. Merge the PR. Go to the GitHub page of the respective PR, scroll to the bottom of the Conversation tab, and click 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.

    4. Sync the local repo with the remote repo. Because of the merge you did on the GitHub, the master branch of your fork is now ahead of your local repo by one commit. To sync the local repo with the remote repo, pull the master branch to the local repo.

    git checkout master
    git pull origin master
    

    Observe how the add-intro branch is now merged to the master branch in your local repo as well.

    5. De-conflict the add-summary PR that you created earlier. Note that GitHub page for the add-summary PR is now showing a conflict (when you scroll to the bottom of that page, you should see a message This branch has conflicts that must be resolved). You can resolve it locally and update the PR accordingly, as explained below.

    5a. Switch to the add-summary branch. To make that branch up-to-date with the master branch, merge the master branch to it, which will surface the merge conflict. Resolve it and complete the merge.

    5b. Push the updated add-summary branch to the fork. That will remove the 'merge conflicts' warning in the GitHub page of the PR.

    6. Merge the add-summary PR using the GitHub interface, similar to how you merged the previous PR.

    Note that you could have merged the add-summary branch to the master branch locally before pushing it to GitHub. In that case, the PR will be merged on GitHub automatically to reflect that the branch has been merged already.

    Forking Workflow

    Can follow Forking Workflow

    This activity is best done as a team. If you are learning this alone, you can simulate a team by using two different browsers to log into GitHub using two different accounts.

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

    2. Each team member: create PRs via own fork

      • Fork that repo from your team org to your own GitHub account.
      • Create a PR to add a file yourName.md  (e.g. jonhDoe.md containing a brief resume of yourself  (branch → commit → push → create PR)
    3. For each PR: review, update, and merge.

      • A team member (not the PR author): Review the PR by adding comments (can be just dummy comments).
      • PR author: Update the PR by pushing more commits to it, to simulate updating the PR based on review comments.
      • Another team member: Merge the PR using the GitHub interface.
      • 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.
    4. Create conflicting PRs.

      • Each team member: Create a PR to add yourself under the Team Members section in the README.md.
      • One member: in the master branch, remove John Doe and Jane Doe from the README.md, commit, and push to the main repo.
    5. Merge conflicting PRs one at a time. Before merging a PR, you’ll have to resolve conflicts. Steps:

      • [Optional] A member can inform the PR author (by posting a comment) that there is a conflict in the PR.
      • PR author: Pull the master branch from the repo in your team org. Merge the pulled master branch to your PR branch. Resolve the merge conflict that crops up during the merge. Push the updated PR branch to your fork.
      • Another member or the PR author: When GitHub does not indicate a conflict anymore, you can go ahead and merge the PR.