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CSC301 - Tutorial 1, using Git & GitHub

Let's go through a short session of using Git. We will talk about the internals of Git, and its terminology as we go along.

Basics

Let's clone a remote repo

git clone REPO-URL

Check the status of your repo

git status

At this point, we have a clone (i.e. files and full history) of the remote repository on our local machine.
Let's create a new file (in the working directory)

echo "Hi" > a.txt

Add the file (to the index/staging-area)

git add a.txt

Check the status of your repo, and see what's different

git status

Now that Git knows that we are interested in including a.txt in our next commit, let's commit the file (to the history)

git commit -m "Adding a.txt"

Check the status of your repo again, and see what's different this time

git status

Let's stop for a second, and see what happened here:

  • A repo is a graph of commit objects.
  • A commit object is a snapshot of the codebase, with some extra metadata.
  • When we run git commit, a new commit object gets created and added to the graph.
    An edge is created from this new commit to its parent (i.e. The most recent commit before we ran git commit).

At this point, a.txt is committed to your local repo (i.e. "the clone"), let's push our changes to the remote repository (i.e. "the origin"):

git push origin master

You're probably wondering what the origin master arguments are ...

  • origin - The name of the repo we're pushing our changes to.
    • Git allows you to define remotes, a remote is a name that is associated with a URL of a remote repository.
    • When you clone a repo, Git automatically creates the origin remote, and associates it with the URL of the repository that you cloned.
  • master - Which branch (of the remote repository) we're pushing the changes to.
    By default, a Git repo starts with a single branch, called master.
    We'll talk more about branches soon.

Notice that you can push changes to other repos, not just the one you cloned. Let's try that as follows:

  • Fork (i.e. clone on GitHub) the remote repo, using a different GitHub account.
  • Add a remote called other and set its URL to the URL of our forked repo.
  • Try to push some changes to other.
  • Notice: We will need to grant our user push permissions.

Summary

  • We can clone a repo.
  • We can work with a local clone (i.e. commit changes).
  • When we are ready, we can push the changes to a remote repo.
    • The remote repo doesn't have to be the one we originally cloned - As long as you have push permissions, and the changes that you are pushing "make sense to Git", you're good.
    • If Git cannot safely push the changes, it will ask you to pull the changes from the remote repo, resolve any conflicts, commit your conflict resolution(s), and only then it will allow you to push the changes back.

Branching

So far, we've pushed changes between different repositories. That is, each repo was a single sequence of commit objects. Branching is used to work on different sequences in parallel (in a single same repo).

Let's create (and switch to) a new branch

git checkout -b feature1

We use the checkout command to switch between branches

git checkout master
git checkout feature1

Let's make some changes in our branch

echo "Bye" > b.txt
git add b.txt
git commit -m "Adding b.txt to the feature1 branch"

Notice that the changes exist only in the feature1 branch

git checkout master
ls
git checkout feature1
ls

Modifying files

Let's modify a.txt in the feature1 branch

echo "Modification 1" >> a.txt
git commit -m "Modifying a.txt in feature1 branch"

Oops, Git complains that there is nothing to commit (and give us a hint).
Git doesn't assume you want to commit modified files by default. For those of you who are used to SVN, this behaviour may seem a little odd.
We have two options now, we can either run git add a.txt, or we can run git commit with the -a option, which tells Git to add all (tracked) modified files to this commit.

git commit -a -m "Modifying a.txt in feature1 branch"

At this point feature1 branch doesn't exist on the remote repo (origin), let's push it there

git push origin feature1

Merging & Resolving Conflicts

Next, let's switch back to the master branch, and make other modifications to a.txt

git checkout master
echo "Modification 2" >> a.txt
git commit -m "Modifying a.txt in master"

Now, let's try to merge feature1 into master (we need to be on the master branch in order to do that, which we are).

git merge feature1

Since there are conflicts, Git "plays it safe" - You will see a message asking you to resolve conflicts, and commit the changes.

The working directory, index and history

This is a good time to dive a little deeper into how a Git repository works (and its terminology).

Each Git repo keeps tracks of three things:

  • Working directory (aka working tree) - The files on your local system, at their current state.
  • Index (aka Staging Area) - All the changes that will be committed with the next commit.
  • History - The graph of commit objects that were committed.

Just to be clear - These 3 components are a part of a single repo, they have nothing to do with a remote repo.
A remote repo has its own working directory, index and history. When you push changes to a remote repo, you're updating that repo's history.

Let's revisit the Git commands we've already used:

  • We add changes from the working directory to the index.
  • We commit changes from the index to the history.
  • A branch is simply a pointer to a commit object in the history (i.e. a pointer to a node in the graph).
  • HEAD is a pointer to (the most recent commit of) the current branch.

For more details, here is a nice visual tutorial/guide.

Undoing Things

There are multiple ways to undo things in Git - revert and reset are two different commands, the main difference between them is:

  • revert creates a new commit object in the graph.
  • reset doesn't create new commit objects, it changes the HEAD pointer (and optionally changes the working directory and/or index).

For this tutorial, we will only use reset.

  • Use git reset --soft COMMIT-ID to move HEAD to the specified commit, without changing the index or the working directory.
  • Use git reset COMMIT-ID to move HEAD to the specified commit, update the index, but not the working directory.
  • Use git reset --hard COMMIT-ID to move HEAD to the specified commit, and update both the index and working directory.

Let's see an example ...

git log

With git log we can see the id's of all commit objects in the history. We will use the id of the most recent commit (i.e. HEAD) when we run git reset.

Let's commit some changes

touch c.txt
git add c.txt
git commit c.txt
git status

Let's start with

git reset --soft COMMIT-ID
git status

At this point, HEAD points to the most recent commit before c.txt was added.
Notice that the index/staging-area did not change. That is, c.txt is staged, and ready to be committed.

git commit -m "Committing changes after a soft reset"
git status

Now, let's try the default reset behaviour.

git reset COMMIT-ID
git status

This time, the index changed as well, and c.txt is no longer staged.
If we run git commit at this point, Git will complain that there is nothing to be committed.
Let's add c.txt to the index again, and commit it.

git add c.txt
git commit -m "Adding c.txt again"

Now, let's try a hard reset.

git reset --hard COMMIT-ID
git status
ls

This time, HEAD, the index and the working directory were all updated. When we run ls, we see that c.txt is gone.

Summary

  • Git's model is a little more complex than SVN. A single repository keeps track of 3 things
    • Working directory
    • Index
    • History
  • git add submits changes from the working directory to the index.
  • git commit submits changes from the index to the history.
  • git commit -a add alls modified files from the working directory to the index, and commits them to the history.
  • git reset --soft is used to undo changes in the history (but not the index or the working directory).
  • git reset is used to undo changes in the history and the index (but not the working directory).
  • git reset --hard is used to undo changes in the history, index and working directory.

OK, that's enough Git for now, let's quickly mention two useful GitHub features.

Markdown

Markdown is a convenient Wiki-like language. That is, you write text, and GitHub formats it nicely.
The page you're reading right now was written using Markdown.

Quick example:

 * A List item
 * Another list item

The text above will be displayed as

  • A List item
  • Another list item

GitHub uses its own modified Markdown syntax. You can/should learn more about GitHub's Markdown.

Side note: If your repository contains a file called README.md, GitHub will display the content of this file (formatted nicely) in the repo's homepage on GitHub.

Issue Management

GitHub has a convenient, and fairly simple issue/ticket management system. Some of its key features are:

  • Issues are either opened or closed.
  • You can define custom labels, and attach any number of labels to a ticket.
  • You can use Markdown in the ticket body/description.
  • You can assign tickets to users.