.. meta::
    :description: Version control system
    :author: Serhii Horodilov
    :keywords: version, control, system, vcs, git

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                          About version control (Git)
*******************************************************************************

Version control is a system that records changes to a file or set of files over
time so that you can recall specific versions later. It allows you to revert
selected files back to a previous state, revert the entire project back to
a previous state, compare changes over time, see who last modified something
that might be causing a problem, who introduced an issue and when, and more.

In a :abbr:`DVCS (Distributed Version Control Systems)` clients don't just
check out the latest snapshot of the files; rather, they fully mirror
the repository, including its full history. Thus, if any server dies, and
these systems were collaborating via that server, any of the client
repositories can be copied back up to the server to restore it. Every clone
is really a full backup of all the data.

.. figure:: /../assets/img/vcs-dvcs.svg
    :align: center

    Distributed version control diagram

.. rubric:: What is Git?

The major difference between Git and any other VCS is the way Git thinks about
its data. Conceptually, most other systems store information as a list of
file-based changes. Git doesn't think of or store its data this way.
Instead, Git thinks of its data more like a series of snapshots of a miniature
filesystem.

This is an important distinction between Git and nearly all other VCSs.
It makes Git reconsider almost every aspect of version control that most
other systems copied from the previous generation. This makes Git more like
a mini filesystem with some incredibly powerful tools built on top of it,
rather than simply a VCS.

.. figure:: /../assets/img/git-snapshots.svg
    :align: center

    Storing data as snapshots of the project over time

.. rubric:: Local operations

Most operations in Git need only local files and resources to operate --
generally no information is needed from another computer on your network.
Because you have the entire history of the project right there on your local
disk, most operations seem almost instantaneous.

.. rubric:: Integrity

Everything in Git is checksummed before it is stored and is then referred
to by that checksum. This means it's impossible to change the contents of
any file or directory without Git knowing about it.

.. rubric:: Repository files states

Git has three main states that your files can reside in:
**modified**, **staged**, and **committed**:

-   Modified means that you have changed the file but have not committed it
    to your database yet.
-   Staged means that you have marked a modified file in its current version
    to go into your next commit snapshot.
-   Committed means that the data is safely stored in your local database.

.. only:: html

    .. mermaid:: /../assets/mermaid/git/states.mmd
        :align: center

.. only:: latex

    .. figure:: /../assets/mermaid/git/states.mmd.png
        :align: center

The basic Git workflow goes something like this:

#.  You modify files in your working tree.
#.  You selectively stage just those changes you want to be part of your next
    commit, which adds only those changes to the staging area.
#.  You do a commit, which takes the files as they are in the staging area and
    stores that snapshot permanently to your Git directory.

Command line
============

There are a lot of different ways to use Git. There are the original
command-line tools, and there are many graphical user interfaces of varying
capabilities. For one, the command line is the only place you can run all Git
commands -- most of the GUIs implement only a partial subset of Git
functionality for simplicity. If you know how to run the command-line version,
you can probably also figure out how to run the GUI version, while the opposite
is not necessarily true. Also, while your choice of graphical client is
a matter of personal taste, all users will have the command-line tools
installed and available.