blog.lazkani.io-20200902-historical/posts/revision-control/git-branching-and-merging.org

#+BEGIN_COMMENT
.. title: Git! Branching and Merging
.. date: 2019-08-01
.. slug: git-branching-and-merging
.. updated: 2019-08-01
.. status: published
.. tags: git, revision-control
.. category: revision-control
.. authors: Elia el Lazkani
.. description: Explaining branches, branching and merging strategies.
.. type: text
#+END_COMMENT

In the previous post about /git/, we had a look at what /git/ is and got our feet wet with a bit of it.
In this post, I will be moving forward with the topic, I will be talking about branches, how to work with them and finally what merging is and how it works.

{{{TEASER_END}}}

* Requirements

The same requirement we had from the last post, obviously /git/.

* Branching and Merging

** What is a branch?

/git/ [[https://git-scm.com/book/en/v1/Git-Branching-What-a-Branch-Is][documentation]] describes it as:

#+BEGIN_QUOTE
"A branch in Git is simply a lightweight movable pointer to one of the[se] commits."
#+END_QUOTE

Usually, people coming from /svn/ think of *branches* differently. In /git/, a branch is simply a pointer to a commit.

So let's verify that claim to see if it's true.

Remember our example repository from the last post ? We'll be using it here.

First let's create a new branch.

#+BEGIN_EXAMPLE
 $ git checkout -b mybranch
 Switched to a new branch 'mybranch'
#+END_EXAMPLE

That was simple, wasn't it ?
Alright, let's test our hypothesis.

#+BEGIN_EXAMPLE
 $ git log
 commit 643a353370d74c26d7cbf5c80a0d73988a75e09e (HEAD -> mybranch, master)
 Author: John Doe <johndoe@example.com>
 Date:   Thu Aug 1 19:50:45 2019 +0200

     Second commit
#+END_EXAMPLE

The commit is, of course, different because this is a different computer with a different repository from scratch. Anyway, it seems from the log message that both /mybranch/ and /master/ are pointing to same commit /SHA/. Technically they are pointing to *HEAD*.

Now let's continue and add a new commit.

#+BEGIN_EXAMPLE
 $ echo "" >> README.md
 $ git add README.md
 $ git commit -m "Adding an empty line"
 [mybranch b30f4e0] Adding an empty line
  1 file changed, 1 insertion(+)
#+END_EXAMPLE

After this last commit, let's check the log

#+BEGIN_EXAMPLE
 $ git log
 commit b30f4e0fa8f3b5c9f041c9ad1be982b2fed80851 (HEAD -> mybranch)
 Author: John Doe <johndoe@example.com>
 Date:   Thu Aug 1 20:28:05 2019 +0200

      Adding an empty line

 commit 643a353370d74c26d7cbf5c80a0d73988a75e09e (master)
 Author: John Doe <johndoe@example.com>
 Date:   Thu Aug 1 19:50:45 2019 +0200

      Second commit
#+END_EXAMPLE

From reading the output of log, we can see that the /master/ branch points to a different commit than /mybranch/.

To visualize this, let's look at it in a different way.

#+BEGIN_EXAMPLE
 $ git log --graph --oneline --all
 * b30f4e0 (HEAD -> mybranch) Adding an empty line
 * 643a353 (master) Second commit
#+END_EXAMPLE

What the above suggests is that our two branches have different contents at this stage. In other words, if I switch back to the /master/ branch what do you think we will find in =README.md= ?

#+BEGIN_EXAMPLE
 $ git checkout master
 Switched to branch 'master'
 $ cat README.md
 # Example

 This is an example repository.
 This repository is trying to give you a hands on experience with git to complement the post.
 $
#+END_EXAMPLE

And if we switch back to /mybranch/.

#+BEGIN_EXAMPLE
 $ git checkout mybranch
 Switched to branch 'mybranch'
 $ cat README.md
 # Example

 This is an example repository.
 This repository is trying to give you a hands on experience with git to complement the post.

 $
#+END_EXAMPLE


Let's add another commit to make easier to see the changes than an empty line.


#+BEGIN_EXAMPLE
 $ echo "Let's add a line to mybranch." >> README.md
 $ git add README.md
 $ git commit -m "Adding more commits to mybranch"
 [mybranch f25dd5d] Adding more commits to mybranch
  1 file changed, 1 insertion(+)
#+END_EXAMPLE

Now let's check the tree again.

#+BEGIN_EXAMPLE
 $ git log --graph --oneline --all
 * f25dd5d (HEAD -> mybranch) Adding more commits to mybranch
 * b30f4e0 Adding an empty line
 * 643a353 (master) Second commit
#+END_EXAMPLE

Let's also check the difference between our /master/ branch and /mybranch/.

#+BEGIN_EXAMPLE
 $ git diff master mybranch
 diff --git a/README.md b/README.md
 index b4734ad..f07e71e 100644
 --- a/README.md
 +++ b/README.md
 @@ -2,3 +2,5 @@

  This is an example repository.
  This repository is trying to give you a hands on experience with git to complement the post.
 +
 +Let's add a line to mybranch.
#+END_EXAMPLE


The =+= suggests an addition and =-= suggests a deletion of a line. As we can see from the =+= shown before the two lines added to the =README.md= file, /mybranch/ has these additions.

You can read more about /git/ branches in the /git/ [[https://git-scm.com/book/en/v1/Git-Branching-What-a-Branch-Is][documentation]] page.

** What is merging ?
That's all fine so far, but how do I get these changes from /mybranch/ to the /master/ branch ?

The answer to that is also as easy as all the steps taken so far. /git/ merges *from* a branch you specify *to* the branch you are currently on.

#+BEGIN_EXAMPLE
 $ # Checking which branch we are on
 $ git branch
   master
 * mybranch
 $ # We are on mybranch and we need to put these changes into master
 $ # First we need to move to our master branch
 $ git checkout master
 Switched to branch 'master'
 $ # Now we can merge from mybranch
 $ git merge mybranch
 Updating 643a353..f25dd5d
 Fast-forward
  README.md | 2 ++
  1 file changed, 2 insertions(+)
#+END_EXAMPLE

As we can see. The changes in /mybranch/ have been merged into the /master/ branch.

#+BEGIN_EXAMPLE
 $ git log
 commit f25dd5da3e6f91d117177782a5811d5086f66799 (HEAD -> master, mybranch)
 Author: John Doe <johndoe@example.com>
 Date:   Thu Aug 1 20:43:57 2019 +0200

     Adding more commits to mybranch

 commit b30f4e0fa8f3b5c9f041c9ad1be982b2fed80851
 Author: John Doe <johndoe@example.com>
 Date:   Thu Aug 1 20:28:05 2019 +0200

     Adding an empty line

 commit 643a353370d74c26d7cbf5c80a0d73988a75e09e
 Author: John Doe <johndoe@example.com>
 Date:   Thu Aug 1 19:50:45 2019 +0200

     Second commit
#+END_EXAMPLE


* Merging Strategies
I'll explain to you how I like to work and my personal merging strategy. I will keep out some details as they use concepts that are more advanced than what has been discussed so far.

** /master/ branch
To me, the /master/ branch stays always up to date with the *remote* /master/ branch. In other words, I do not make commits against the /master/ branch in the project I'm working on.

** branch
If I want to work on the project, I start by updating the /master/ branch and then branching it as we've seen before. The name of the branch is always indicative on what it holds, or what kind of work I am doing on it.

As long as I am working on my dev branch, I keep updating the /master/ branch and then porting the changes into my dev branch. This way, at the end the code is compatible and I am testing with the latest version of the code. This is very helpful and makes merging later a breeze.

** merging
After my work is done, I push my branch to the remote server and ask for the maintainer of the project to merge my changes into the /master/ branch after reviewing it, of course. To explain this in a very simple manner, all that mumbo jumpo talk previously simply means someone else did the merge into master.


* Conclusion
In this post, I talked about what are branches. We went ahead and worked a little bit with branches and then mentioned merging. At the end of the post I talked a bit about my merging strategy.

In the next post, I will be talking about remotes.