Your set of files that you work on locally is sometimes called a working directory, working copy, working tree or local checkout.
A tree is the directory structure and contents of your files, or a snapshot of them from a particular point in time.
We've already talked about how a repository is the version control data for your codebase. Now we'll go into a little more detail about what kind of information git stores and what that means for how to use it.
Every repository contains:
-
object database -- the history of all commits and the relationships between them.
-
one or more branches -- a branch is a container that has a name and stores set of commits. For now all you need to know is that every repository is created with a default branch named master that all of your commits are being stored on.
-
HEAD -- a reference to the current commit on the current branch. A reference is like a bookmark to a commit. Since we're not doing anything fancy,
HEADwill point to the most recent commit on themasterbranch for the forseeable future. -
the tree -- a snapshot of your files at the time of the
HEADcommit. -
the index -- a snapshot of your files at the time of the
HEADcommit plus the changes you've staged for commit.
Conceptually, your repository looks a little something like this:
+------------------------+ +----------------------+
| object-db | | index |
| +---------------------+ +----------------------+
HEAD ---> master ----+->| c331b4d gitign...------------+----------------+ |
(ref) (branch) | | 6a338d8 Format... | | | | c331b4d tree | |
| | 02c7a7a Minor ... | | | +----------------+ |
| | 382421e Finish... | | | | pypet.py | |
| | fa69673 Clean ... | | | | README.md | |
| | 92241db Reorga... | | | | .replit | |
| | fb090a5 Add Py... | | | | ... | |
| | ... | | | +----------------+ |
| +-------------------+ | | |
+------------------------+ | +----------------+ |
| | staging | |
| +----------------+ |
| | +LICENCE.md | |
| | +bin/ | |
| | ... | |
| +----------------+ |
+----------------------+
But wait, there's more!
There are actually at least two repositories of your files.
- The one on Repl.it. Even though it is not actually on your physical computer, it is where you're editing the files so we'll refer to it as a local repository.
- The remote repository on Github.
- Eventually you may also have an actually-local local repository on your computer.
Git is a special kind of version control system called Distributed version control. As opposed to Centralized Version Control Systems that have one central repository that all other repositories were required to communicate with in order to commit changes. In git on the other hand, every repo is self- contained--that means anyone who has a local checkout of a repo can make commits and look up changes to their hearts content without having to consult any other repo.
Additionally, every repo can send changes to or receive changes from any other compatable repo.
Imagine we all worked on the same codebase. The difference between the two systems would be somthing like this:
|
Centeralized Version Control | Decentralized Version Control
|
| Jayson
Central Repo | / | \
------------ | / | \
/ | \ | Alissa---+---Github
/ | \ | \ | /
Alissa Jayson Sean | \ | /
| \Sean_/
Even though something like that is technically possible, in practice we'll use Github as a hub to sync between different repos.
The way I use it looks something like this:
`Laptop <----------> Github <-----------> Repl.it`
When you have a remote repository that you want to send changes to, it is sometimes refereed to as an upstream repo or just a remote repository. For us, that will be the Github repo.
Git gives our first remote repository the default name of origin. A special kind of branch is also created called a tracking branch. Its job is to store a copy of the object database for the cooresponding branch from the remote repo.
So it turns out, our repo actually looks more like this.
+------------------------+ +----------------------+
| object-db | | index |
| +-------------------+ | +----------------------+
HEAD ---> master ------+->| c331b4d gitign...------------+----------------+ |
(ref) (branch) | | 6a338d8 Format... | | | | c331b4d tree | |
| | 02c7a7a Minor ... | | | +----------------+ |
| | 382421e Finish... | | | | pypet.py | |
| | fa69673 Clean ... | | | | README.md | |
| | 92241db Reorga... | | | | .replit | |
| | fb090a5 Add Py... | | | | ... | |
| | ... | | | +----------------+ |
| +-------------------+ | | |
| | | +----------------+ |
| | | | staging | |
| | | +----------------+ |
| | | | +LICENCE.md | |
| | | | +bin/ | |
| | | | ... | |
| | | +----------------+ |
| | +----------------------+
| +-------------------+ |
remotes/origin/head ---+->| 112e375 Add ca..------------+-----------------+
(tracking branch) | | 6a338d8 Format... | | | 112e375 tree |
| | 02c7a7a Minor ... | | +----------------+
| | 382421e Finish... | | | pypet.py |
| | fa69673 Clean ... | | | docs/ |
| | 92241db Reorga... | | | .replit |
| | fb090a5 Add Py... | | | ... |
| | ... | | +----------------+
| +-------------------+ |
+------------------------+
It's time to zoom in on commits. A commit is a set of changes recorded in git. Sometimes also called a revision or a change log. Every commit has:
- a SHA or hash -- a 40-character unique identifier that points to a
the rest of the data related to the commit. Basically, a commit ID. Here's an
example:
d16085b3b913e5bc5e351c0a7461051e9973629a
-
a patch -- the file detailing the exact changes that were made in a format that git and other programs understand. It's very similar to the output of
git diff. -
the tree -- a snapshot of your files at the time of the commit.
-
Historical data detailing the ancestery of the commit.
-
Metadata like the date, author, and commit message.
Now that you are a little more familiar with the pieces and how they fit togther, let's talk about what that means.
There are now potentially many different versions of your files that may exist at any given time:
- your working directory
- the index
- head
HEAD, the ref- its tree
- its object-database
- for the master branch
- for the tracking branch to remotes/origin/master
- origin
HEAD- its tree
- its
object-database- for the master branch
Here are the possible states of local file changes.
+------------------------------------------+----------------+-----------------------------------------+
| status | where | description |
+------------------------------------------+----------------+-----------------------------------------+
| untracked | | working-tree | a new file |
| tracked | modified | working-tree | changes to a file |
| tracked | staged | index | changes queued up for the next commit |
| tracked | committed/unmodified | head | changes git has recorded |
+--------------+---------------------------+----------------+-----------------------------------------+
Note: It is possible for a file to be both staged AND modified at the same
time. This can happen if you stage a commit, then make additional changes to
the same file. It is actually the chunks of changes that are tracked, rather
than the file as a whole.
Here are the possible states of the local repository relative to the remote repository.
+---------------+---------------------------------------------------------------+
| status | description |
+---------------+---------------------------------------------------------------+
| up to date | local and remote are the same |
| behind | remote changes not in local |
| ahead | local changes not in remote |
| diverged | local changes not in remote AND remote changes not in local |
+---------------+---------------------------------------------------------------+
And finally, here is the workflow and how states change for each action.
+--------------+--------------+--------------------------------------+-----------------------------+
| working tree | index | head | origin |
+--------------+--------------+------------+----------+--------------|--------+----------+---------+
| local files | snapshot | ref | snapshot | db | db | snapshot | ref |
+--------------+--------------+------------+----------+--------------|--------+----------+---------+
| | | | | | | | |
| [git add ----------->] | | | | | | |
| | | | | | | | |
| [git commit -a ----->|----------->|----------->|----------->] | | | |
| | | | | | | | |
| | | | | | | | |
| | [git commit ---------->|--+---->|----+------>] | | | |
| | | | | | | | |
| | | | | [git fetch <--------] | | |
| | | | | | | | |
| | | [git pull <-----------+-----|<-----------------------------------] |
| | | | | | | | |
| | | | | | | | |
| | | [git push <------------------------------>|---+---->|----+-------] |
| | | | | | | | |
| | | | | | | | |
+--------------+--------------+------------+----------+--------------+--------+----------+---------+