Git Branching (Media Engineering Architecture & Deployment)

# Git Branching

Learn how to work on isolated, parallel lines of development with [Git][git] branches.

This material is part of [architecture & deployment course](https://github.com/MediaComem/comem-archidep) for [Media Engineering](https://heig-vd.ch/formations/bachelor/filieres/ingenierie-des-medias).

**You will need**

* A Unix CLI
* [Git][git]

**Recommended reading**

* [Git introduction](../git/)

---
class: center, middle
## What is branching?

.breadcrumbs[<a href="#1">Git Branching</a>]

> Branching means you diverge from the main line of development and continue to do work without messing with that main line.

---
### Why use branches?

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#2">What is branching?</a>]

Git has a very powerful branching model that is very **lightweight and fast**: it encourages workflows that branch and merge often.

Many teams using Git create a **separate branch** to develop **each feature**.
This has many advantages:

* Each developer can work on his own feature, **isolated** from changes going on elsewhere
* They can pull in changes from the mainline **at their own pace**
* The team can choose **which features to release** and when

---
### What is a branch?

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#2">What is branching?</a>]

Remember that Git stores data as a series of snapshots.

<git-memoir name='internals' chapter='internals' controls='false' svg-height='137px'></git-memoir>

Each **commit** contains a pointer to the snapshot of the content you staged,
represented by the blue **T**ree objects above (as they refer to a *tree* of file snapshots).

Each commit also contains:

* The user name and e-mail or the author.
* The date at which the commit was created.
* A pointer to the previous commit (or commits).

---
#### Branches point to commits

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#2">What is branching?</a> > <a href="#4">What is a branch?</a>]

A branch is simply a lightweight, movable pointer to a commit.

<git-memoir name='branchingOneLine' chapter='commits' svg-height='137px'></git-memoir>

The default branch is `master`.
The special `HEAD` pointer indicates the current branch.

As you start making commits, the current branch pointer **automatically moves** forward to your latest commit.

---
### Example repository

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#2">What is branching?</a>]

We will use a prepared repository to illustrate branching.

```bash
$> cd /path/to/projects

$> git clone https://github.com/MediaComem/comem-webdev-git-branching-ex.git

$> cd comem-webdev-git-branching-ex
```

Remove the link to the remote repository (will we talk more about it in [Collaborating with Git](../git-collaborating/)):

```bash
$> git remote rm origin
```

As you can see if you type `git log`, there are some commits already.
Open the project with your favorite editor and open the `index.html` page in a browser.

---
class: center, middle
## Working with branches

.breadcrumbs[<a href="#1">Git Branching</a>]

---
### Showing branches on the command line

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

The `git log` command can show you a representation of the commit graph and its branches:

```bash
$> git log --oneline --decorate --graph --all
 * 4f94fa (HEAD -> master) Improve layout
 * 9ab3fd Fix addition
 * 387f12 First version
```

In fact, this command is so useful you should make an **alias**, as we will use it a lot in this tutorial:

```bash
$> git config --global alias.graph "log --oneline --graph --decorate --all"

$> git graph
 * 4f94fa (HEAD -> master) Improve layout
 * 9ab3fd Fix addition
 * 387f12 First version
```

---
### Create a new branch

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

> **Exercise:** our JavaScript calculator is missing some code.
> Let's create a branch to implement subtraction.

It's very fast and simple to create a new branch:

```bash
$> git branch feature-sub
```

<git-memoir name='branchingOneLine' chapter='branch' svg-height='137px'></git-memoir>

There is now a new pointer to the current commit.
Note that `HEAD` didn't move – we are still on the `master` branch.

---
#### Showing the current branch

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a> > <a href="#9">Create a new branch</a>]

You can use `git branch` without arguments to simply see the list of branches and which one you are currently on:

```bash
$> git branch
 * master
   feature-sub
```

---
### Switch branches

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

Now let's switch branches:

```bash
$> git checkout feature-sub
Switched to branch 'feature-sub'
```

<git-memoir name='branchingOneLine' chapter='checkout' svg-height='137px'></git-memoir>

This moves `HEAD` to point to the `feature-sub` branch.

Nothing else happened because `HEAD` is still pointing to the same commit as `master`.

> **Exercise:** you can now implement the subtraction in `subtraction.js`.

---
### Commit on a branch

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

Once you're done, it's time to add and commit your changes.

```bash
$> git add subtraction.js
$> git commit -m "Implement subtraction"
```

As you commit, the current branch (the one pointed to by `HEAD`), moves forward to the new commit:

<git-memoir name='branchingOneLine' chapter='commit-on-a-branch' svg-height='137px'></git-memoir>

---
### Switch back to `master`

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

> **Exercise:** oops, you just noticed that addition is not working correctly.
> You need to make a bugfix, but you don't want to mix that code with the new subtraction feature.
> Let's **go back to `master`**.

```bash
$> git checkout master
Switched to branch 'master'
```

---
#### Checkout behavior

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a> > <a href="#13">Switch back to `master`</a>]

Two things happened when you ran `git checkout master`:

* The `HEAD` pointer was **moved** back to the `master` branch.
* The files in your working directory were **reverted** back to the snapshot that `master` points to.

<git-memoir name='branchingOneLine' chapter='back-to-master' svg-height='137px'></git-memoir>

You have essentially **rewinded** the work you've done in `feature-sub`, and are working on an **older version** of the project.

---
### Create another branch

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

> **Exercise:** let's a create a new branch to fix the bug.

You can create a new branch *and* switch to it in one command:

```bash
$> git checkout -b fix-add
Switched to a new branch 'fix-add'
```

<git-memoir name='branchingOneLine' chapter='another-branch' svg-height='137px'></git-memoir>

Nothing has changed yet because `fix-add` still points to the same commit as `master`.

---
### Work on a separate branch

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

> **Exercise:** fix addition in `addition.js` and commit your changes.

```bash
$> git add addition.js
$> git commit -m "Fix addition"
[fix-add 2817bc] Fix addition
 1 file changed, 1 insertion(+), 1 deletion(-)
```

<git-memoir name='branching' chapter='divergent-history' svg-height='275px'></git-memoir>

---
#### Divergent history

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a> > <a href="#16">Work on a separate branch</a>]

Now your project history has **diverged**.

The changes in `feature-sub` and `fix-add` are **isolated**.
You can **switch back and forth** between the branches with `git checkout`.

<git-memoir name='branching' chapter='switch-branches' svg-height='250px'></git-memoir>

Every time you checkout one of these branches,
the files in your **working directory** are updated to reflect the state of the corresponding commit, or snapshot.

---
### Merging

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

Now that you've tested your fix and made sure it works,
you want to **bring those changes** back **into the `master` branch**.

Git's `merge` command can do that for you,
but it can only **bring changes** from another branch **into the current branch**,
not the other way around.

So you must first switch to the `master` branch:

```bash
$> git checkout master
```

<git-memoir name='branching' chapter='fast-forward-merge-checkout' svg-height='200px'></git-memoir>

---
### Merge a branch

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

Now that you are on the correct branch,
you can **merge** the changes from `fix-add`:

```bash
$> git merge fix-add
Updating 4f94fa..2817bc
Fast-forward
 addition.js | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)
```

Notice the term **fast-forward**.

---
### Fast-forward

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

The `fix-add` branch pointed to a commit **directly ahead** of the commit `master` pointed to.
There is no divergent history, so Git simply has to **moves the pointer forward**.
This is what is called a **fast-forward**.

<git-memoir name='branching' chapter='fast-forward-merge' svg-height='275px'></git-memoir>

---
### Delete a branch

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

> **Exercise:** now that we've brought our fix back into `master`, we don't need the `fix-add` branch anymore.
  Let's delete it.

```bash
$> git branch -d fix-add
Deleted branch fix-add (was 2817bc).
```

<git-memoir name='branching' chapter='delete-branch' svg-height='275px'></git-memoir>

---
### Continue working on a feature branch

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

> **Exercise:** let's switch back to our `feature-sub` branch and finish our work.
  Write a comment for the subtract function and commit your changes.

```bash
$> git checkout feature-sub
(Write your comment...)
$> git add subtraction.js
$> git commit -m "Comment subtract function"
```

<git-memoir name='branching' chapter='work-on-feature-branch' svg-height='225px'></git-memoir>

---
### Merge a divergent history

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a>]

<git-memoir name='branching' chapter='work-on-feature-branch' controls='false' svg-height='200px'></git-memoir>

Now that we're happy with our new subtraction feature, we want to **merge** it into `master` as well.
But the `feature-sub` branch has **diverged from some older point compared to `master`**, so Git cannot do a fast-forward:

* `feature-sub` points to commit `f92ab0` which contains our feature
* `master` points to commit `2817bc` which contains the addition fix
* Commit `4f94fa` is the common ancestor

Git will do a **three-way merge** instead, combining together the changes of `master` and `feature-sub` (compared to the common ancestor).
A **new commit** will be created representing that state.

---
#### Merge commit message

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a> > <a href="#23">Merge a divergent history</a>]

> **Exercise:** switch back to the `master` branch and merge `feature-sub` into it.

```bash
$> git checkout master
$> git merge feature-sub
Merge made by the 'recursive' strategy.
 subtraction.js | 5 ++++-
  1 file changed, 4 insertions(+), 1 deletion(-)
```

Git will need to create a new commit when you run the merge command,
so it will **open the configured editor** (vim by default) with a generated commit message:

```txt
 Merge branch 'feature-sub'

# Please enter a commit message to explain why this merge is necessary,
 # especially if it merges an updated upstream into a topic branch.
 #
 # Lines starting with '#' will be ignored, and an empty message aborts
 # the commit.
```

Type `:wq` (**w**rite and **q**uit) to save and exit.

---
#### Merge commit

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a> > <a href="#23">Merge a divergent history</a>]

You can see the new **merge commit** that Git has created.
It is a special commit in that it has more than one parent:

<git-memoir name='branching' chapter='merge' svg-height='275px'></git-memoir>

---
#### Delete `feature-sub`

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#7">Working with branches</a> > <a href="#23">Merge a divergent history</a>]

Now that you're done, you can delete `feature-sub`:

```bash
$> git branch -d feature-sub
```

<git-memoir name='branching' chapter='delete-feature-sub' svg-height='275px'></git-memoir>

---
## Merge conflicts

.breadcrumbs[<a href="#1">Git Branching</a>]

Occasionally, the merge process doesn't go smoothly:
if the **same line(s) in the same file(s)** was modified in two diverging branches and you merge them together,
Git can't know which is the correct version.

Let's pretend that a colleague of yours also implemented the subtraction function but in a different way than you did.

---
### Find the common ancestor

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#27">Merge conflicts</a>]

We want to make it look as if your colleague did his work **at the same time** as you.
Let's find the original starting point (the common ancestor where `feature-sub` and `fix-add` diverged) and start a new branch from there:

```bash
$> git graph
 *   04fb82 (HEAD -> master) Merge branch 'feature-sub'
 |\
 | * f92ab0 Comment subtract function
 * | 2817bc Fix addition
 | * 712ff2 Implement subtraction
 |/
 * `4f94fa` (origin/master, origin/HEAD) Comment add function
 * 9ab3fd Simplify addition and subtraction implementation
 * 387f12 First version
```

Make a copy of that commit hash.

---
### Create a branch "in the past"

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#27">Merge conflicts</a>]

You can create a branch at any point in the project's history by passing an additional commit reference to `git checkout`:

```bash
$> git checkout -b better-sub 4f94fa
```

<git-memoir name='branching' chapter='checkout-past' svg-height='250px'></git-memoir>

`HEAD` also moved since we used the `-b` option.

---
### Make a conflicting change

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#27">Merge conflicts</a>]

Now edit `subtraction.js` and implement subtraction again, but in a different way.
For example:

```js
function subtract(a, b) {
  return -b + a;
}
```

Note that if you try to check out the `master` branch at this point,
Git won't let you do it because the state of `subtraction.js` is different in that branch:

```bash
$> git checkout master
error: Your local changes to the following files would be overwritten by checkout:
  subtraction.js
Please commit your changes or stash them before you switch branches.
Aborting
```

Commit your changes:

```bash
$> git add subtraction.js
$> git commit -m "Implemented a better subtract"
```

---
#### The state before merging

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#27">Merge conflicts</a> > <a href="#30">Make a conflicting change</a>]

Viewing the graph of commits, it's clear that the change has been made **in parallel** with our earlier changes:

<git-memoir name='branching' chapter='conflicting-change' svg-height='300px'></git-memoir>

---
### Merge the conflicting branch

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#27">Merge conflicts</a>]

Go back to `master` and merge `better-sub`:

```bash
$> git checkout master
$> git merge better-sub
Auto-merging subtraction.js
CONFLICT (content): Merge conflict in subtraction.js
Recorded preimage for 'subtraction.js'
Automatic merge failed; fix conflicts and then commit the result.
```

Git tells you that a **content conflict** has occurred in `subtraction.js`.

The merge has failed and no new commit has been created.

---
### Check the status of the conflict

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#27">Merge conflicts</a>]

Let's see what `git status` tells us:

```bash
$> git status
On branch master
You have unmerged paths.
  (fix conflicts and run "git commit")
  (use "git merge --abort" to abort the merge)

Unmerged paths:
  (use "git add <file>..." to mark resolution)

both modified:   subtraction.js

no changes added to commit (use "git add" and/or "git commit -a")
```

* Git tells you that the merge is **not complete**:
  * You can either fix the conflicts and run `git commit` to end the merge, or cancel the whole thing with `git merge --abort`
* `subtraction.js` was modified in **both** the **current branch** and the **branch we are trying to merge in**
* You can use `git add <file>` to **mark the conflicts in a file as resolved**

---
### Inspect the conflicted file

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#27">Merge conflicts</a>]

Let's see what's in `subtraction.js`:

```js
/**
 * Takes two numbers a and b, and returns the result of subtracting b from a.
 */
function subtract(a, b) {
<<<<<<< HEAD
  return a - b;
=======
  return -b + a;
>>>>>>> better-sub
}

calculate('subtraction', subtract);
```

Notice two things here:

* Git has **successfully merged the comment** on the subtract function, since only one person changed these lines.
* Git could not merge the line with the computation, because the changes in the two branches conflict.
  It has added **conflict markers** to help you solve the issue.

---
### Conflict markers

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#27">Merge conflicts</a>]

Take a closer look at the conflict markers:

```txt
<<<<<<< HEAD
  return a - b;
=======
  return -b + a;
>>>>>>> better-sub
```

* The section between `<<<<<<< HEAD` and `=======` is the content that was present in the current branch (`HEAD`) before you merged.
* The section between `=======` and `>>>>>>> better-sub` is the content that is being merged in from the `better-sub` branch.

Since Git cannot know which is better, it's **your responsibility** to:

* Remove the version you don't want
* Remove the marker conflicts

```js
return -b + a;
```

---
### Mark the conflict as resolved

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#27">Merge conflicts</a>]

Now that you have fixed the conflict, do as instructed by Git and add the file to the staging area:

```bash
$> git add subtraction.js
$> git status
On branch master
All conflicts fixed but you are still merging.
  (use "git commit" to conclude merge)

Changes to be committed:

modified:   subtraction.js
```

Git tells you that all conflicts have been resolved but that you still need to **commit** to end the merge:

```bash
$> git commit -m "Merge better-sub into master"
```

If you do not specify a commit message with `-m`, Git will generate one for you and open the configured editor (vim by default) for you to check and/or change the message.
Type `:wq` to exit from vim and make the commit.

---
#### The state after merging

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#27">Merge conflicts</a> > <a href="#36">Mark the conflict as resolved</a>]

Finally, delete the branch:

```bash
$> git branch -d better-sub
```

The latest commit on `master` now includes the changes from all lines of development:

<git-memoir name='branching' chapter='merge-conflicting-change' svg-height='300px'></git-memoir>

---
## Merge file conflicts

.breadcrumbs[<a href="#1">Git Branching</a>]

Sometimes it's not just the contents of the file that are in conflict:
you could have **modified the file** in your branch, and a colleague could have **deleted it** in another branch.
Let's again pretend to be another colleague starting from the same point:

```bash
$> git checkout -b cleanup 4f94fa
```

<git-memoir name='branching' chapter='conflicting-file-change-checkout' svg-height='250px'></git-memoir>

---
### Make a conflicting file change

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#38">Merge file conflicts</a>]

This time, this colleague decided to delete `subtraction.js` in his branch because he doesn't like to see files with incomplete code:

```bash
$> rm subtraction.js
$> git add --all
$> git commit -m "Remove incomplete implementations"
```

<git-memoir name='branching' chapter='conflicting-file-change' svg-height='300px'></git-memoir>

---
### Merge the conflicting branch

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#38">Merge file conflicts</a>]

Let's try to merge that branch into `master`:

```bash
$> git checkout master
$> git merge cleanup
CONFLICT (modify/delete): subtraction.js deleted in cleanup
  and modified in HEAD. Version HEAD of subtraction.js left in tree.
Automatic merge failed; fix conflicts and then commit the result.
```

Git tells you immediately that there is a conflict and that:

* `subtraction.js` was **deleted** in the `cleanup` branch
* `subtraction.js` was **modified** in the current branch (`HEAD`)
* Git **doesn't know** whether it should apply the deletion or the modification, so it left the modified file for you to check

---
### Check the status of the file conflict

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#38">Merge file conflicts</a>]

Let's see what `git status` tells us:

```bash
$> git status
On branch master
You have unmerged paths.
  (fix conflicts and run "git commit")
  (use "git merge --abort" to abort the merge)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)

deleted by them: subtraction.js

no changes added to commit (use "git add" and/or "git commit -a")
```

Again, Git gives us some information:

* `subtraction.js` was **deleted by "them"**, meaning that it was deleted by someone else in the branch you're trying to merge in
  (if *you* had deleted it and they had modified it, it would be *deleted by "us"*)
* Use either `git add` or `git rm` to mark the conflict as resolved

---
### Resolving the file conflict

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#38">Merge file conflicts</a>]

You have to choose whether you want to:

* Keep the modified file (use `git add`)
* Delete it (use `git rm`)

Let's keep it:

```bash
$> git add subtraction.js
$> git status
On branch master
All conflicts fixed but you are still merging.
  (use "git commit" to conclude merge)
```

As instructed, use `git commit` to complete the merge:

```bash
$> git commit -m "Merge cleanup (kept implemented subtraction.js)"
```

Finally, delete the `cleanup` branch:

```bash
$> git branch -d cleanup
```

---
### Final state

.breadcrumbs[<a href="#1">Git Branching</a> > <a href="#38">Merge file conflicts</a>]

And you're done!

<git-memoir name='branching' chapter='merge-conflicting-file-change' svg-height='325px'></git-memoir>

---
## Resources

.breadcrumbs[<a href="#1">Git Branching</a>]

* [Git branching][branching]
* [Advanced merging][advanced-merging]
* [Understanding branches in Git][understanding-branches]

[advanced-merging]: https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging
[branching]: https://git-scm.com/book/en/v2/Git-Branching-Branches-in-a-Nutshell
[git]: https://git-scm.com
[understanding-branches]: https://blog.thoughtram.io/git/rebase-book/2015/02/10/understanding-branches-in-git.html