Advanced Git: A talk on the finer parts of Git. Covering basic to somewhat advanced Git usage for development tasks. Goes into some detail on some parts of Git that may confuse many

2. VCS / SCM
Source control /Version control is the management
of changes to documents, computer programs and
other collections of information
git is a source code management system.

3. WHAT GIT IS NOT
* git is neither Github or Bitbucket

4. SOME BENEFITS OF GIT

5. DATA REDUNDANCY AND REPLICATION
Since there are multiple full repositories on multiple machines
(instead of one central repository), the risk of project loss due to
server failure or some other catastrophe is much lower.
HIGH AVAILABILITY
High availability -- Server down? Network down? No problem.
Work in your repo (commit, view history, branch, etc) to your
heart's content.Try that with Subversion!

6. SUPERIOR DISK UTILIZATION AND NETWORK
PERFORMANCE
Git efficiently compresses the objects in the .git directory, saving
you space.
COLLABORATION FRIENDLY
Git makes it easy to collaborate on projects with groups of
contributors and with great services like Bitbucket, Gitlab and (of
course) Github, git's collaboration friendliness is even further
magnified.

7. SETUP
git config —global user.name “John Doe”
git config —global user.email “johndoe@example.co”

8. INITIALIZING A GIT REPO
$ git init
A git repo is usually a directory that is tracked by git. Git tracks the states
of the files in the directory and with various commands, informs you of
the status of you repository.
To start tracking a directory (to initialize a git repo):

9. The stages of any file in a git repository

10. How git thinks about its data.
* One key difference that we need to mention a bit is the way git thinks about
data in comparison to how other VCS. Most other systems (CVS, Subversion,
Perforce etc.) store information as a list of file-based changes
fig: file-based changes

11. * Git thinks of its data more like a set of snapshots of a miniature filesystem.
Every time you commit, or save the state of your project in Git, it basically takes a
picture of what all your files look like at that moment and stores a reference to
that snapshot.
fig: changes as snapshots

12. Simply put, a branch in Git is a lightweight movable pointer to a
commit.
Commits are snapshots of your repository.These snapshots usually
contain/reference other git object types.
COMMITS
BRANCHES

13. The image shows how
a commit is
represented in the
repository
Pointer to that root tree and all the commit metadata

14. (some) Git commands

15. Git status
Arguably the most typed command on the git cli, it displays paths that have
differences between the index file and the current HEAD commit, paths that have
differences between the working tree and the index file, and paths in the working
tree that are not (yet) tracked by Git (and are not ignored by ‘gitignore’)
$ git status

16. Git add
In order to begin tracking a new file, you use the command git add.
What this command does is to update the ‘index’ using the current content found in
the working tree to prepare the content stages for the next commit.
$ git add <file_name>
Suppose you have a lot of changes in a file and you don’t want to commit all the
changes at once:
$ git add <file_name> -p
The ‘-p’ flag interactively chooses chunks in the file to add to the ‘index’ or staging area.

17. Git commit
Stores the current contents of the index in a new commit along with a log message
from the user describing the changes.
$ git commit
If you have changes in the working directory and you really want to add all files with
a simple commit message
$ git commit —amend
To change/update the last commit - in the event that you don’t like the commit
message or to add another file to the same commit.
$ git commit -am “my super awesome commit message”
* Note that any file you have in the index gets added when you use this commit option and yeah, it rewrites history so
use carefully.

18. $ git cat-file -p <commit_sha>
Since we should now have a commit, lets take a minute to examine it - or
any random commit on your system

19. After a few hours of
moving bits around
and a bunch of
commits, our repo
‘kinda’ looks like this
*conceptually*
Remember: Git thinks of its data more like a set of snapshots
of a miniature filesystem

20. *The “master” branch in Git is not a special branch. It is exactly like any other branch.The only reason
nearly every repository has one is that the git init command creates it by default and most people
don’t bother to change it.
Which when looked at from a birds eye view, looks ‘something’ like this:

21. $ git branch testing

22. * By default, the `git branch` command makes the tip of the
branch the HEAD
Now we have 2 branches pointing to the same commit object
f30ab…

23. $ git checkout master
$ git branch testing
$ git show-ref master
$ git show-ref testing
Let’s confirm that both branches actually point to the
same commit object
* We’ll talk a bit about ‘checkout’ soon.

24. Since we’ve seen that a branch is simply a pointer to a commit
object - identified by a ‘sha’, it’s fairly easy to select commits as
bases for our new branches.
$ git branch <branch_name> [<start-point>] (source: man git-branch)
Creating a branch with a start point.
$ git branch feature/authentication-patch 34ac55…

25. HEAD
How does Git know what
branch you’re currently
on?
Easy, It keeps a special
pointer called HEAD
The HEAD is just a pointer to
the currently checked out branch.
$ cat .git/HEAD

26. $ git checkout testing
* ’checking out’ the testing branch switches HEAD to the testing branch

27. CHECKOUT
Git checkout is a very useful command that alters the working tree.
Updates files in the working tree to match the version in the index or the specified
tree. If no paths are given, git checkout will also update HEAD to set the specified
branch as the current branch.
It moves HEAD to a different branch (or commit) and updates the working
directory to match it.
$ git checkout <branch_name>

28. Note that any uncommitted change will be lost during a checkout so Git
forces you to stash or commit before checking out to another branch.
This is also the reason why when you need to remove the changes made
on a file you use:
$ git checkout <file_name>

29. Detached HEAD
The HEAD becomes ‘detached’ when you checkout to a valid commit
that isn’t a branch.
$ git checkout 34ac55…
This can be used to quickly check inspect an old version of your
project.
* Being in the ‘detached head’ state can however be dangerous
because if you checkout to a grandparent of HEAD for instance and
then make commits, you loose being able to get back the commits
even after switching to other branches.

30. GIT OBJECTS

31. • Blobs
The git “blob” type is just a bunch of bytes that could be anything, like a text file, source
code, or a picture, etc.
• Trees
A git tree is like a filesystem directory.A git tree can point to, or include Git “blob”
objects or other git trees.
• Commits
A git commit object includes:
Information about who committed the change/check-in/commit, a pointer to the git tree
object that represents the git repository when the commit was done and the parent
commit to this commit (so we can easily find out the situation at the previous commit).
• Tags
A git tag object points to any git commit object. A git tag can be used to refer to a
specific tree, rather than having to remember or use the hash of the tree.

32. Inside .git
HEAD
config
description
hooks
pre-push.sample
post-update.sample
pre-commit.sample
info
objects
refs
[ a text file that is shown as project description in web frontend ]
[ repository wide configuration ]
[ text file showing currently checked out branch ]
[ where git hooks are stored ]
* Git hooks are scripts that run
automatically every time a particular
event occurs in a Git repository
[ use this file to ignore files (like .gitignore but isn’t versioned) ]exclude
[ Git’s internal warehouse for blobs, indexed by SHAs - where the ’objects’ are stored ]
[ Where refs (including) branches live ]
heads
remotes
tags
stash
[ where your git branches are managed ]
[ remote branches ]
[ … here be the tags ]
[ an ascii text file that has your stash SHAs ]

33. (other) Git commands

34. Git clone
The git clone command copies an existing Git repository to a new directory.
The “working copy” is a full-fledged Git repository—it has its own history, manages its
own files, and is a completely isolated environment from the original repository.
$ git clone <repo_url>
<repo_url> must be a valid ssh, git, ftp or http(s) url

35. …or even on a flash drive
$ git clone /<path_to_source> /<new_path>
$ git clone /path/to/source/repo /path/on/flash/drive
You can clone from and to your local filesystem.
* of course, the flash drive should be mounted

36. Git merge
The git merge command incorporates changes from the named commits
(since the time their histories diverged from the current branch)
into the current branch
Say we have a repository with 2 branches as shown below:
* see the git documentation for more options ( http://git-scm.com/book)

37. $ git checkout master
$ git merge iss53
Instead of just moving the branch pointer forward, Git creates a new snapshot that results from this
merge and automatically creates a new commit that points to it.
This type of commit referred to as a merge commit, and is special in that it has more than one
parent.
The above commands leave our repository in a new state.
fig: merge commit

38. Merge conflicts
Sometimes, the merge between 2 branches may not go smoothly and you’ll have something
called a merge conflict.
$ git merge iss53
Auto-merging index.html
CONFLICT (content): Merge conflict in index.html
Automatic merge failed; fix conflicts and then commit the result.
If your changes on the iss53 branch modified the same part of a file as a new
change on the branch being merged unto, you’ll get a merge conflict.

39. $ git status
On branch master
You have unmerged paths.
(fix conflicts and run "git commit")
Unmerged paths:
(use "git add <file>..." to mark resolution)
both modified: index.html
no changes added to commit (use "git add" and/or "git commit -a”)
Running ‘git status’ will give some insight into what is happening.
Despite trying to merge branches, we still have ‘unmerged paths’.This is a merge conflict and git
status shows the files that have conflicts under ‘Unmerged Paths’.

40. Git helps in resolving conflicts by adding conflict-resolution markers to the files
that have conflicts, so you can open them manually and resolve those conflicts.
Resolving Conflicts
Your file will contain sections that have markers and the conflicted file will look
something like:
<<<<<<< HEAD:index.html
<div id="footer">contact : email.support@example.com</div>
=======
<div id="footer">
please contact us at support@example.com
</div>
>>>>>>> iss53:index.html
* you can then stage your updated files and commit to complete the merge.

41. Git show
This command shows you information about a git object - commits, blobs, stashes,
branches etc.
$ git show [<branch_name>] | [<sha>]
Git blame
Show what revision and author last modified each line of a file.The blame command is
a Git feature, designed to help you determine who made changes to a file.
$ git blame <file_name>
* see the git documentation for more options ( http://git-scm.com/book)

42. Git cherry-pick
Apply the changes introduced by some existing commits.
You’ll often find, when using branches, how easy it is to commit to a different branch (say,
testing branch) than the one intended (master).
$ git cherry-pick testing
[ apply to the current branch the change introduced at the tip of the testing branch ]
$ git cherry-pick testing..hotfix
[ apply to the current branch the changes introduced in ‘hotfix’ that are not in ‘testing’ ]
* see the git documentation for more options ( http://git-scm.com/book)

43. Git reset
git reset is used to move the tip of the branch to a different commit.This can be used to remove
commits from the current branch.
Recall: when you checkout a branch, it changes HEAD to point to the new branch ref,
populates your Index with the snapshot of that commit, then copies the contents of
the Index into your Working Directory.
file.txt has been modified and the repo is in a clean state.
For instance, if we had our repository as such,

44. $ git reset —-soft HEAD^
** Reset always moves
what HEAD points to.
That means, from a clean repo, if
we’re on the master branch and
we run git reset 93536a4,
master will point to 9e5e6a4.

45. $ git reset —-mixed HEAD^
** Reset always moves
what HEAD points to.
That means, from a clean repo, if
we’re on the master branch and
we run git reset —mixed
93536a4, master will point to.