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What is Git ?

Git is a distributed version control system 1

1 http://git-scm.com/about

Setup

Some house-cleaning here. We assume of course you have Git installed, (hopefully >= 1.7.0).

First thing to do is to setup your identity. This identifies you to other people who download the project.

$ git config --global user.name "Your Name"
$ git config --global user.email your.email@example.com

As a helpful step, you may want to set Git to use your favourite editor

$ git config --global core.editor emacs

Starting your journey

First, clone this repository:

$ git clone https://github.com/kuahyeow/git-workshop.git

Once you have cloned your repository, you should now see a directory called git-workshop. This is your working directory

$ cd git-workshop
$ ls

Stuck? Ask for help from the workshop staff

For the curious, you should also see the .git subdirectory. This is where all your repository’s data and history is kept.

$ ls -a .git

You will see :

branches  config  description  HEAD  hooks  info  objects  refs

The staging area

Now, let’s try adding some files into the project. Create a couple of files.

Let’s create two files named bob.txt and alice.txt.

$ touch alice.txt bob.txt

Let’s use a mail analogy.

In Git, you first add content to the staging area by using git add. This is like putting the stuff you want to send into a cardboard box. You finalize the process and record it into the git index by using git commit. This is like sealing the box – it’s now ready to send.

Let’s add the files to the staging area

$ git add alice.txt bob.txt

Committing

You are now ready to commit. The -m flag allows you to enter a message to go with the commit at the same time.

$ git commit -m "I am adding two new files"

Stuck? Ask for help from the workshop staff

Let’s see what just happened

We should now have a new commit. To see all the commits so far, use git log

$ git log

The log should show all commits listed from most recent first to least recent. You would see various information like the name of the author, the date it was commited, a commit SHA number, and the message for the commit.

You should also see your most recent commit, where you added the two new files in the previous section. However git log does not show the files involved in each commit. To view more information about a commit, use git show.

$ git show

You should see something similar to:

commit 5a1fad96c8584b2c194c229de7e112e4c84e5089
Author: kuahyeow <kuahyeow@gmail.com>
Date:   Sun Jul 17 19:13:42 2011 +1200

    I am adding two new files

diff --git a/alice.txt b/alice.txt
new file mode 100644
index 0000000..e69de29
diff --git a/bob.txt b/bob.txt
new file mode 100644
index 0000000..e69de29

Stuck? Ask for help from the workshop staff

A necessary digression

In this section, we are going to add more changes, and try to recover from mistakes.

Be forewarned, this next step is going to be hard. We will need to add some content to alice.txt.

Open alice.txt and type in your favourite line from a song, or:

e.g. Lorem ipsum Sed ut perspiciatis, unde omnis iste natus error sit voluptatem accusantium doloremque laudantium

Then save the file

What did we change? A very useful command is git diff. This is very useful to see exactly what changes you have done.

$ git diff

You should see something like the following:


diff --git a/alice.txt b/alice.txt
index e69de29..2aedcab 100644
--- a/alice.txt
+++ b/alice.txt
@@ -0,0 +1 @@
+Lorem ipsum Sed ut perspiciatis, unde omnis iste natus error sit voluptatem accusantium doloremque laudantium

Stuck? Ask for help from the workshop staff

Staging area again

Now let’s add our modified file, alice.txt to the staging area. Do you remember how ?

Next, check the status of alice.txt. Is it in the staging area now?

Stuck? Ask for help from the workshop staff

Undoing

Let’s say we did not like putting Lorem ipsum into alice.txt. One advantage of a staging area is to enable us to back out before we commit – which is a bit harder to back out of. Remembering the mail analogy – it’s easier to take mail out of the cardboard box before you seal it than after.

Here’s how to back out of the staging area :

$ git reset HEAD alice.txt
Unstaged changes after reset:
M	alice.txt

Compare the git status now to the git status from the previous section. How does it differ?

Stuck? Ask for help from the workshop staff

Your staging area should now be empty. What’s happened to the Lorem Ipsum changes? It’s still there. We are now back to the state just before we added this file to staging area. Going back to the mail analogy, we just took our letter out of the box.

Undoing II

Sometimes we did not like what we have done and we wish to go back to the last recorded state. In this case, we wish to go back to the state just before we added the Lorrem ipsum text to alice.txt.

To accomplish this, we use git checkout, like so:

$ git checkout alice.txt

You have now un-done your changes. Your file is now empty.

Stuck? Ask for help from the workshop staff

Branching

Most large code bases have at least two branches – a ‘live’ branch and a ‘development’ branch. The live branch is code which is OK to be deployed on to a website, or downloaded by customers. The development branch allows developers to work on features which might not be bug free. Only once everyone is happy with the development branch would it be merged with the live branch.

Creating a branch in Git is easy. The git branch command, when used by itself, will list the branches you currently have

$ git branch

The * should indicate the current branch you are on, which is master.

If you wish to start another branch, use git checkout -b (new-branch-name) :

$ git checkout -b exp1

Try git branch again to check which branch you are currently on:

$ git branch
  exp1
* master

The new branch is now created. Now let’s work in that branch. To switch to the new branch:

$ git checkout exp1

git checkout (branch-name) is used to switch branches.

Let’s perform some commits now,

$ echo 'some content' > test.txt
$ git add test.txt
$ git commit -m "Added experimental txt"

Now, let’s compare them to the master branch. Use git diff

$ git diff master

Basically what the above output says is that test.txt is present on the exp1 branch, but is absent on the master branch.

Stuck? Ask for help from the workshop staff

Now you see me, now you don’t

Git is good enough to handle your files when you switch between branches. Switch back to the master branch

Try switching back to the master branch (Hint: It’s the same command we used to switch to the exp1 branch above)

Now, where’s our test.txt file ?

$ ls
README.textile	alice.txt	bob.txt		gamow.txt

As you can see the new file you created in the other branch has disappeared. Not to worry, it is safely tucked away, and will re-appear when you switch back to that branch.

Now, switch back to the exp1 branch, and check that the test.txt is now present.

Stuck? Ask for help from the workshop staff

Merging

We now try out merging. Eventually you will want to merge two branches together after the conclusion of work.
git merge allows you to do that.

Git merging works by first switching the branch you want to into, and then running the command to merge the other branch in.

We now want to merge our exp1 branch into master. First, switch to the master branch.

git checkout master

Next, we merge the exp1 branch into master :

$ git merge exp1

Do you see the following output ?

Merge made by recursive.
 test.txt |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 test.txt

You have to be in the branch you want merge into and then you always specify the branch you want to merge.

At this point, you can also try out gitk to visualize the changes and how the two branches have merged

Merge Conflicts

Git is pretty good at merging automagically, even when the same file is edited. There are however, some situations where the same line of code is edited there is no way a computer can figure out how to merge.
This will trigger a conflict which you will have to fix.

We now practise fixing merge conflicts. Recall that conflicts are caused by merges which affect the same block of code.

Here’s a branch I prepared earlier. The branch is called alpher. Run the code below to set it up (don’t worry if you can’t understand it)

$ git checkout alpher

You should now have a new branch called alpher. Try merging that branch into master now and fix the ensuing conflict.

Stuck? Ask for help from the workshop staff

Fixing a conflict

You should see a conflict with the gamow.txt file. This means that the same line of text was edited and committed on both the master branch and the alpher branch. The output below basically tells you the current situation :

Auto-merging gamow.txt
CONFLICT (content): Merge conflict in gamow.txt
Automatic merge failed; fix conflicts and then commit the result.

If you open the gamow.txt file, you will see something similar as below:

$ cat gamow.txt
<<<<<<< HEAD
It was eventually recognized that most of the heavy elements observed in the present universe are the result of stellar nucleosynthesis (http://en.wikipedia.org/wiki/Stellar_nucleosynthesis) in stars, a theory largely developed by Bethe.
=======

http://en.wikipedia.org/wiki/Stellar_nucleosynthesis
Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances. For the creation of elements during the explosion of a star, the term supernova nucleosynthesis is used.
>>>>>>> alpher

Git uses pretty much standard conflict resolution markers. The top part of the block, which is everything between <<<<<< HEAD and ====== is what was in your current branch.
The bottom half is the version that is present from the alpher branch. To resolve the conflict, you either choose one side or merge them as you see fit.

For example, I might decide to choose the version from the alpher branch.

Now, try to fix the merge conflict. Pick the text that you think is better (Ask for help if stumped)

Once I have done that, I can then mark the conflict as fixed by using git add and git commit.

Stuck? Ask for help from the workshop staff

$ git add gamow.txt
$ git commit -m "Fixed conflict"

Congratulations. You have fixed the conflict. All is good in the world.

Fin

You have learnt :

  1. Clone a repository
  2. Commit files
  3. Check status
  4. Check diff
  5. Undoing changes
  6. Branching and merging
  7. Fixing conflicts

Part II

GitHub

But, wait. There’s more. What about this distributed sharing thing with Git ?

To be able to share, we’ll need a server to host our git repositiories. GitHub (github.com) is probably the easiest place to begin with.

Login or sign up with GitHub

Go sign up for an account at GitHub; Or login into your GitHub account if you had previously signed up.

Hint: You may need to setup git cache your GitHub password – see https://help.github.com/articles/set-up-git

Then come back here, we’ll wait.

Create your first GitHub repository

A repository (repo) is a place where you would store your code. You were practising on your very own repo just now in Part 1!

The following tutorial will show you how to create a GitHub repo – which you can then share with others

Then come back here, we’ll wait.

Fork a repo

Go to this tutorial

Then come back here, we’ll wait.

Let’s collaborate !

In which you will be collaborating with your team-mates using GitHub…

Fin

You have learnt:

  1. Forking a repo at GitHub
  2. Git push
  3. Git pull

References and Further reading

I throughly recommend these resources to continue your Git practice:

Author

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License
http://creativecommons.org/licenses/by-nc-sa/3.0/
Author: Thong Kuah
Contributors: Nick Malcolm

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