Basic ideas
===========

Any time multiple versions of a document exist, whether due to a document
changing over time, or because multiple authors are working on it, some kind of
version control is needed.

Version control allows:

    * accessing any version from the original to the most recent;
    * seeing what has changed from one version to the next;
    * giving a label of some kind to distinguish a particular version.

Examples of version control systems
-----------------------------------

The simplest method of version control is probably the most widely used in
science: changing the file name.

.. figure:: http://www.phdcomics.com/comics/archive/phd052810s.gif
   :alt: Cartoon from "Piled Higher and Deeper" by Jorge Cham
   :target: http://www.phdcomics.com/comics.php?f=1323

.. figure:: http://www.phdcomics.com/comics/archive/phd101212s.gif
   :alt: Cartoon from "Piled Higher and Deeper" by Jorge Cham
   :target: http://www.phdcomics.com/comics.php?f=1531

   from *"Piled Higher and Deeper" by Jorge Cham*
   www.phdcomics.com


Other examples include:
    * "track changes" in Microsoft Word
    * Time Machine in Mac OS X
    * versioning in Dropbox, Google Drive
    * formal version control systems such as CVS, Subversion, Mercurial, Git

The importance of tracking projects, not individual files
---------------------------------------------------------

Early version control systems, such as CVS, track each file separately - each
file has its own version number. The same is true of Dropbox, Microsoft Word.

This is a problem when you make changes to several files at once, and the
changes in one file depend on changes in another.

In modern version control systems, and in backup-based systems such as
Time Machine, entire directory trees are tracked as a unit, which means that
each version corresponds to the state of an entire project at a point in time.

Advantages of formal version control systems
--------------------------------------------

* explicit version number for each version
* easy to switch between versions
* easy to see changes between versions
* tools to help merge incompatible changes

In the next sections, we will use Git_, probably the most widely used
modern version control system, to introduce the principles of version control.
We will demonstrate both Git's command-line interface and a graphical interface,
SourceTree_.

.. _Git: http://git-scm.com/
.. _SourceTree: http://www.sourcetreeapp.com