# 8. How to pull the updated version of your project from GitHub?

Occasionally you will want to fetch the project from the central repository and integrate it with your local repo. For example, when you want to sync the local repo with the latest updates that the other developers on your team have done.

To perform the synchronization, you need to use the Git pull command.

$ git pull

• Don't forget to cd into the local repo on your computer.

As a result, Git will integrate the changes from the remote repository to the local repo on your computer.

# 9. How to compare the local repo to the central repo?

Use Git diff command to compare a modified file in your repo with the version in the central repo:

    git diff [file_name]

For example, modify "index.php" file in the local repo and compare with the remote repo.

Use the Git status command to see the files that have been modified.

$ git status

Compare the local version of the file to the remote with the Git diff command and the name of the file.

$ git diff index.php

Let's finish this section of the tutorial by pushing the modified index.php file to Github:

$ git add index.php
$ git commit -m "Modified index.php"
$ git push

# 10. What are branches and how to use them?

The idea of branching is that you don't have to work on the same branch all the time. Instead, you can develop your project in a side-branch which is different from the master branch. In the side-branch, you can make all the changes without disturbing the master branch. And only if the changes are successful, you can merge the changes back to the master branch.

You always start with the master branch. To see the list of branches use Git branch command:

$ git branch

With the result:

* master

To create a new branch on your computer use Git branch command with the name of the new branch:

    git branch [new_branch_name]

Let's create a new branch with the name of "b2" inside the "exploring_git" repo:

$ git branch b2

Use Git branch command to see the list of branches:

$ git branch

With the result:

  b2
* master

The asterisk (*) before the branch name indicates the branch you are currently on.

Use Git checkout command to switch to the new branch (b2, in our example):

$ git checkout b2

Change the code in the new branch as desired. For example, add "script.php" file:

$ touch script.php

In the end, commit the new branch to github.com:

$ git add -A 
$ git commit -m "First commit to the b2 branch" 
$ git push --set-upstream origin b2

Pay attention that the commit is to the "b2" branch, not to the "master" branch.

Now return to the master branch, and verify that its content hasn't changed:

$ git checkout master
$ ls

You can see that the "master" branch is not affected by the side-branch. If you want the code to move from the side-branch to the master branch, you need to merge the branches, as you'll learn in the next section.

# 11. How to merge git branches?

Suppouse your side branch is complete, you can merge it into the master branch.

Switch into the branch you wish to merge into:

$ git checkout master

Inside the branch use Git merge command with the name of the side-branch:

    git merge [side-branch]

In our example:

$ git merge b2

Verify that the branches have merged successfully:

$ ls

With the result:

README.md  index.php  script.php

The "script.php" file originates from the "b2" branch.

Upload the merged repo to the central repository:

$ git add -A
$ git commit -m "Merged with branch b2"
$ git push

Now that the branches are merged, you may wish to delete the side-branch. Use Git branch -d to delete the branch.

$ git branch -d [side-branch]

In our example:

$ git branch -d b2

# 12. How to revert to a previous commit?

Each time that you upload your changes to GitHub, the service saves the previous versions, so you can revert to them whenever you like. How does it work? Each time that you make a new commit to the central repo, Github gives the version a unique id which you can later use to refer to the commit.

To get the list of all the commits, use Git log command:

$ git log

Now, your project's history needs to look something like this:

commit 06bc8f6a6507d766fbe2a808446f51e8fdef5797
Author: your_name 
Date:   Fri Oct 12 21:24:37 2018 +0300

    First commit to the b2 branch

commit c5ac7d5e3c7766a4f34f08d7ade05ee543193457
Author: your_name 
Date:   Tue Oct 12 21:10:04 2018 +0300

    Modified index.php

In each commit, the first line shows the id of the commit and the last line shows the comment.

Use the git checkout command with the unique id to load any old version to your computer. For example, you can revert to the "Modified index.php" commit with the checkout command and the unique id (your id is going to be different than mine):

$ git checkout c5ac7d5e3c7766a4f34f08d7ade05ee543193457

This makes a copy of the commit in your working directory.

To replace the content of the "master" branch with the copy of the old commit follow these 3 steps:

1. Make a branch out of the working directory. For example, create a new branch "b3" then checkout into it:
   

   $ git checkout -b b3

2. Change the name of the "master" branch to "old-master":
   

   $ git branch -m master old-master

3. Change the name of the "b3" branch to master:
   

   $ git branch -m b3 master

Switch to the "master" branch and verify that its content is compatible with the that of the old commit:

$ git checkout master
$ ls

Don't forget to push the local repo to the remote:

$ git add -A
$ git commit -m "Reverted to an old commit"
$ git push --force origin master

* Notice the --force flag which forces the commit to happen even if it removes previous commits.

# 13. How to solve merge conflicts?

Since working with Github allows multiple developers to work in parallel, each on their copy of the repository, merge conflicts may occur when two developers change the same file and try to upload the changes to the central repo. The way to solve the problem is by pulling the updated version from the central repository, updating the file with your changes, and pushing it back to github.com.

To simulate the problem download a second copy of the "exploring_git" repo to your computer, and place it in a different folder "exploring_git2".

If you can't remmember how to clone the repo read the previous section of this tutorial how to clone a remote repo.

cd into the local repo:

$ cd exploring_git2/exploring_git

Edit the "index.php" file by changing the content of the $x variable:

index.php

    <?php
    $x = 'Welcome to the Git and Github essentials for web developers and programmers repo';

Upload the changes that you have made in the second copy of the repo to github.com

$ git add -A
$ git commit -m "First commit from the second copy"
$ git push

***

Now get out of the second copy of the repo and cd into the first copy:

$ cd exploring_git

Try to change the same line then upload to the remote repo:

index.php

    <?php
    $x = 'Changes in the same line from the first repo';

$ git add -A
$ git commit -m "New upload from the original repo"
$ git push

The result might not be what you expect:

To https://github.com/[your_username]/exploring_git.git
 ! [rejected]        master -> master (fetch first)
error: failed to push some refs to 'https://github.com/[your_username]/exploring_git.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

What you see is a merge conflict in action.

To solve the problem do the following:

1. Pull the remote repo:
   

   $ git pull

   
   With the result:
   

   From github/[your_username]/exploring_git
    * branch            master     -> FETCH_HEAD
   Auto-merging index.php
   CONFLICT (content): Merge conflict in index.php
   Automatic merge failed; fix conflicts and then commit the result.

2. Open the "index.php" file and you will see something like this:
   

       <?php
       <<<<<<< HEAD
       $x = 'Changes in the same line from the first repo.';
       =======
       $x = 'Welcome to the Git and Github essentials for web developers and programmers repo';
       >>>>>>> 74934dd8ca1dc11dccccd98671d51e945c68f2f9

   
   
   The first line is from the current repo and the second from the other repo.
3. To resolve the conflict, decide which code to delete or merge the two versions.
   

       <?php
       $x = 'the merged line';

After you solved the problem, commit the merged file to Github as already explained.

# 14. How to delete your repo?

1. Click on the link to the repo that you want to delete at github.com
2. In the project page that opens click on the "settings" button.
3. In the settings page scroll to the danger zone, and click on the button "Delete this repository".
4. In the popup that opens enter the repo's name then click on the button "I understand the consequences, delete this repository".

That's it. You have successfully deleted your repo.