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<title>Git Magic - Chapter 1. Introduction</title>
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<div class="toc">
<ul class="toc">
<li><b>Git Magic</b></li>
<li>
<span class="preface"><a href=
"index.html">Preface</a></span>
<ul>
<li><span class="section"><a href=
"index.html#thanks">Thanks!</a></span></li>
<li><span class="section"><a href=
"index.html#license">License</a></span></li>
</ul>
</li>
<li>
<span class="chapter"><a href="ch01.html">1.
Introduction</a></span>
<ul>
<li><span class="section"><a href=
"ch01.html#work_is_play">Work is Play</a></span></li>
<li><span class="section"><a href=
"ch01.html#version_control">Version
Control</a></span></li>
<li><span class="section"><a href=
"ch01.html#distributed_control">Distributed
Control</a></span></li>
<li><span class="section"><a href=
"ch01.html#a_silly_superstition">A Silly
Superstition</a></span></li>
<li><span class="section"><a href=
"ch01.html#merge_conflicts">Merge
Conflicts</a></span></li>
</ul>
</li>
<li>
<span class="chapter"><a href="ch02.html">2. Basic
Tricks</a></span>
<ul>
<li><span class="section"><a href=
"ch02.html#saving_state">Saving State</a></span></li>
<li><span class="section"><a href=
"ch02.html#add_delete_rename">Add, Delete,
Rename</a></span></li>
<li><span class="section"><a href=
"ch02.html#advanced_undo_redo">Advanced
Undo/Redo</a></span></li>
<li><span class="section"><a href=
"ch02.html#reverting">Reverting</a></span></li>
<li><span class="section"><a href=
"ch02.html#changelog_generation">Changelog
Generation</a></span></li>
<li><span class="section"><a href=
"ch02.html#downloading_files">Downloading
Files</a></span></li>
<li><span class="section"><a href=
"ch02.html#the_bleeding_edge">The Bleeding
Edge</a></span></li>
<li><span class="section"><a href=
"ch02.html#instant_publishing">Instant
Publishing</a></span></li>
<li><span class="section"><a href=
"ch02.html#what_have_i_done">What Have I
Done?</a></span></li>
<li><span class="section"><a href=
"ch02.html#exercise">Exercise</a></span></li>
</ul>
</li>
<li>
<span class="chapter"><a href="ch03.html">3. Cloning
Around</a></span>
<ul>
<li><span class="section"><a href=
"ch03.html#sync_computers">Sync
Computers</a></span></li>
<li><span class="section"><a href=
"ch03.html#classic_source_control">Classic Source
Control</a></span></li>
<li><span class="section"><a href=
"ch03.html#secret_source">Secret Source</a></span></li>
<li><span class="section"><a href=
"ch03.html#bare_repositories">Bare
repositories</a></span></li>
<li><span class="section"><a href=
"ch03.html#push_versus_pull">Push versus
pull</a></span></li>
<li><span class="section"><a href=
"ch03.html#forking_a_project">Forking a
Project</a></span></li>
<li><span class="section"><a href=
"ch03.html#ultimate_backups">Ultimate
Backups</a></span></li>
<li><span class="section"><a href=
"ch03.html#light_speed_multitask">Light-Speed
Multitask</a></span></li>
<li><span class="section"><a href=
"ch03.html#guerilla_version_control">Guerilla Version
Control</a></span></li>
<li><span class="section"><a href=
"ch03.html#mercurial">Mercurial</a></span></li>
<li><span class="section"><a href=
"ch03.html#bazaar">Bazaar</a></span></li>
<li><span class="section"><a href=
"ch03.html#why_i_use_git">Why I use Git</a></span></li>
</ul>
</li>
<li>
<span class="chapter"><a href="ch04.html">4. Branch
Wizardry</a></span>
<ul>
<li><span class="section"><a href=
"ch04.html#the_boss_key">The Boss Key</a></span></li>
<li><span class="section"><a href=
"ch04.html#dirty_work">Dirty Work</a></span></li>
<li><span class="section"><a href=
"ch04.html#quick_fixes">Quick Fixes</a></span></li>
<li><span class="section"><a href=
"ch04.html#merging">Merging</a></span></li>
<li><span class="section"><a href=
"ch04.html#uninterrupted_workflow">Uninterrupted
Workflow</a></span></li>
<li><span class="section"><a href=
"ch04.html#reorganizing_a_medley">Reorganizing a
Medley</a></span></li>
<li><span class="section"><a href=
"ch04.html#managing_branches">Managing
Branches</a></span></li>
<li><span class="section"><a href=
"ch04.html#temporary_branches">Temporary
Branches</a></span></li>
<li><span class="section"><a href=
"ch04.html#work_how_you_want">Work How You
Want</a></span></li>
</ul>
</li>
<li>
<span class="chapter"><a href="ch05.html">5. Lessons of
History</a></span>
<ul>
<li><span class="section"><a href=
"ch05.html#i_stand_corrected">I Stand
Corrected</a></span></li>
<li><span class="section"><a href=
"ch05.html#and_then_some">… And Then
Some</a></span></li>
<li><span class="section"><a href=
"ch05.html#local_changes_last">Local Changes
Last</a></span></li>
<li><span class="section"><a href=
"ch05.html#rewriting_history">Rewriting
History</a></span></li>
<li><span class="section"><a href=
"ch05.html#making_history">Making
History</a></span></li>
<li><span class="section"><a href=
"ch05.html#where_did_it_all_go_wrong">Where Did It All
Go Wrong?</a></span></li>
<li><span class="section"><a href=
"ch05.html#who_made_it_all_go_wrong">Who Made It All Go
Wrong?</a></span></li>
<li><span class="section"><a href=
"ch05.html#personal_experience">Personal
Experience</a></span></li>
</ul>
</li>
<li>
<span class="chapter"><a href="ch06.html">6. Multiplayer
Git</a></span>
<ul>
<li><span class="section"><a href=
"ch06.html#who_am_i">Who Am I?</a></span></li>
<li><span class="section"><a href=
"ch06.html#git_over_ssh_http">Git Over SSH,
HTTP</a></span></li>
<li><span class="section"><a href=
"ch06.html#git_over_anything">Git Over
Anything</a></span></li>
<li><span class="section"><a href=
"ch06.html#patches_the_global_currency">Patches: The
Global Currency</a></span></li>
<li><span class="section"><a href=
"ch06.html#sorry_we_8217_ve_moved">Sorry, We’ve
Moved</a></span></li>
<li><span class="section"><a href=
"ch06.html#remote_branches">Remote
Branches</a></span></li>
<li><span class="section"><a href=
"ch06.html#multiple_remotes">Multiple
Remotes</a></span></li>
<li><span class="section"><a href=
"ch06.html#my_preferences">My
Preferences</a></span></li>
</ul>
</li>
<li>
<span class="chapter"><a href="ch07.html">7. Git
Grandmastery</a></span>
<ul>
<li><span class="section"><a href=
"ch07.html#source_releases">Source
Releases</a></span></li>
<li><span class="section"><a href=
"ch07.html#commit_what_changed">Commit What
Changed</a></span></li>
<li><span class="section"><a href=
"ch07.html#my_commit_is_too_big">My Commit Is Too
Big!</a></span></li>
<li><span class="section"><a href=
"ch07.html#the_index_git_8217_s_staging_area">The
Index: Git’s Staging Area</a></span></li>
<li><span class="section"><a href=
"ch07.html#don_8217_t_lose_your_head">Don’t Lose Your
HEAD</a></span></li>
<li><span class="section"><a href=
"ch07.html#head_hunting">HEAD-hunting</a></span></li>
<li><span class="section"><a href=
"ch07.html#building_on_git">Building On
Git</a></span></li>
<li><span class="section"><a href=
"ch07.html#daring_stunts">Daring Stunts</a></span></li>
<li><span class="section"><a href=
"ch07.html#preventing_bad_commits">Preventing Bad
Commits</a></span></li>
</ul>
</li>
<li>
<span class="chapter"><a href="ch08.html">8. Secrets
Revealed</a></span>
<ul>
<li><span class="section"><a href=
"ch08.html#invisibility">Invisibility</a></span></li>
<li><span class="section"><a href=
"ch08.html#integrity">Integrity</a></span></li>
<li><span class="section"><a href=
"ch08.html#intelligence">Intelligence</a></span></li>
<li><span class="section"><a href=
"ch08.html#indexing">Indexing</a></span></li>
<li><span class="section"><a href=
"ch08.html#git_8217_s_origins">Git’s
Origins</a></span></li>
<li><span class="section"><a href=
"ch08.html#the_object_database">The Object
Database</a></span></li>
<li><span class="section"><a href=
"ch08.html#blobs">Blobs</a></span></li>
<li><span class="section"><a href=
"ch08.html#trees">Trees</a></span></li>
<li><span class="section"><a href=
"ch08.html#commits">Commits</a></span></li>
<li><span class="section"><a href=
"ch08.html#indistinguishable_from_magic">Indistinguishable
From Magic</a></span></li>
</ul>
</li>
<li>
<span class="appendix"><a href="apa.html">A. Git
Shortcomings</a></span>
<ul>
<li><span class="section"><a href=
"apa.html#sha1_weaknesses">SHA1
Weaknesses</a></span></li>
<li><span class="section"><a href=
"apa.html#microsoft_windows">Microsoft
Windows</a></span></li>
<li><span class="section"><a href=
"apa.html#unrelated_files">Unrelated
Files</a></span></li>
<li><span class="section"><a href=
"apa.html#who_8217_s_editing_what">Who’s Editing
What?</a></span></li>
<li><span class="section"><a href=
"apa.html#file_history">File History</a></span></li>
<li><span class="section"><a href=
"apa.html#initial_clone">Initial Clone</a></span></li>
<li><span class="section"><a href=
"apa.html#volatile_projects">Volatile
Projects</a></span></li>
<li><span class="section"><a href=
"apa.html#global_counter">Global
Counter</a></span></li>
<li><span class="section"><a href=
"apa.html#empty_subdirectories">Empty
Subdirectories</a></span></li>
<li><span class="section"><a href=
"apa.html#initial_commit">Initial
Commit</a></span></li>
<li><span class="section"><a href=
"apa.html#interface_quirks">Interface
Quirks</a></span></li>
</ul>
</li>
<li><span class="appendix"><a href="apb.html">B.
Translating This Guide</a></span></li>
</ul>
</div>
<div class="content">
<div class="chapter">
<div class="titlepage">
<div>
<div>
<h1 class="title"><a id="introduction" name=
"introduction"></a>Chapter 1. Introduction</h1>
</div>
</div>
</div>
<p>I’ll use an analogy to introduce version control. See
<a class="ulink" href=
"http://en.wikipedia.org/wiki/Revision_control" target=
"_top">the Wikipedia entry on revision control</a> for a saner
explanation.</p>
<div class="section">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="work_is_play" name=
"work_is_play"></a>Work is Play</h2>
</div>
</div>
</div>
<p>I’ve played computer games almost all my life. In
contrast, I only started using version control systems as an
adult. I suspect I’m not alone, and comparing the two may
make these concepts easier to explain and understand.</p>
<p>Think of editing your code, or document, as playing a
game. Once you’ve made a lot of progress, you’d like to save.
To do so, you click on the <span class=
"emphasis"><em>Save</em></span> button in your trusty
editor.</p>
<p>But this will overwrite the old version. It’s like those
old school games which only had one save slot: sure you could
save, but you could never go back to an older state. Which
was a shame, because your previous save might have been right
at an exceptionally fun part of the game that you’d like to
revisit one day. Or worse still, your current save is in an
unwinnable state, and you have to start again.</p>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="version_control" name=
"version_control"></a>Version Control</h2>
</div>
</div>
</div>
<p>When editing, you can <span class="emphasis"><em>Save
As…</em></span> a different file, or copy the file somewhere
first before saving if you want to savour old versions. You
can compress them too to save space. This is a primitive and
labour-intensive form of version control. Computer games
improved on this long ago, many of them providing multiple
automatically timestamped save slots.</p>
<p>Let’s make the problem slightly tougher. Say you have a
bunch of files that go together, such as source code for a
project, or files for a website. Now if you want to keep an
old version you have to archive a whole directory. Keeping
many versions around by hand is inconvenient, and quickly
becomes expensive.</p>
<p>With some computer games, a saved game really does consist
of a directory full of files. These games hide this detail
from the player and present a convenient interface to manage
different versions of this directory.</p>
<p>Version control systems are no different. They all have
nice interfaces to manage a directory of stuff. You can save
the state of the directory every so often, and you can load
any one of the saved states later on. Unlike most computer
games, they’re usually smart about conserving space.
Typically, only a few files change from version to version,
and not by much. Storing the differences instead of entire
new copies saves room.</p>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="distributed_control" name=
"distributed_control"></a>Distributed Control</h2>
</div>
</div>
</div>
<p>Now imagine a very difficult computer game. So difficult
to finish that many experienced gamers all over the world
decide to team up and share their saved games to try to beat
it. Speedruns are real-life examples: players specializing in
different levels of the same game collaborate to produce
amazing results.</p>
<p>How would you set up a system so they can get at each
other’s saves easily? And upload new ones?</p>
<p>In the old days, every project used centralized version
control. A server somewhere held all the saved games. Nobody
else did. Every player kept at most a few saved games on
their machine. When a player wanted to make progress, they’d
download the latest save from the main server, play a while,
save and upload back to the server for everyone else to
use.</p>
<p>What if a player wanted to get an older saved game for
some reason? Maybe the current saved game is in an unwinnable
state because somebody forgot to pick up an object back in
level three, and they want to find the latest saved game
where the game can still be completed. Or maybe they want to
compare two older saved games to see how much work a
particular player did.</p>
<p>There could be many reasons to want to see an older
revision, but the outcome is the same. They have to ask the
central server for that old saved game. The more saved games
they want, the more they need to communicate.</p>
<p>The new generation of version control systems, of which
Git is a member, are known as distributed systems, and can be
thought of as a generalization of centralized systems. When
players download from the main server they get every saved
game, not just the latest one. It’s as if they’re mirroring
the central server.</p>
<p>This initial cloning operation can be expensive,
especially if there’s a long history, but it pays off in the
long run. One immediate benefit is that when an old save is
desired for any reason, communication with the central server
is unnecessary.</p>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="a_silly_superstition" name=
"a_silly_superstition"></a>A Silly Superstition</h2>
</div>
</div>
</div>
<p>A popular misconception is that distributed systems are
ill-suited for projects requiring an official central
repository. Nothing could be further from the truth.
Photographing someone does not cause their soul to be stolen.
Similarly, cloning the master repository does not diminish
its importance.</p>
<p>A good first approximation is that anything a centralized
version control system can do, a well-designed distributed
system can do better. Network resources are simply costlier
than local resources. While we shall later see there are
drawbacks to a distributed approach, one is less likely to
make erroneous comparisons with this rule of thumb.</p>
<p>A small project may only need a fraction of the features
offered by such a system, but using systems that scale poorly
for tiny projects is like using Roman numerals for
calculations involving small numbers.</p>
<p>Moreover, your project may grow beyond your original
expectations. Using Git from the outset is like carrying a
Swiss army knife even though you mostly use it to open
bottles. On the day you desperately need a screwdriver you’ll
be glad you have more than a plain bottle-opener.</p>
</div>
<div class="section">
<div class="titlepage">
<div>
<div>
<h2 class="title"><a id="merge_conflicts" name=
"merge_conflicts"></a>Merge Conflicts</h2>
</div>
</div>
</div>
<p>For this topic, our computer game analogy becomes too
thinly stretched. Instead, let us again consider editing a
document.</p>
<p>Suppose Alice inserts a line at the beginning of a file,
and Bob appends one at the end of his copy. They both upload
their changes. Most systems will automatically deduce a
reasonable course of action: accept and merge their changes,
so both Alice’s and Bob’s edits are applied.</p>
<p>Now suppose both Alice and Bob have made distinct edits to
the same line. Then it is impossible to proceed without human
intervention. The second person to upload is informed of a
<span class="emphasis"><em>merge conflict</em></span>, and
must choose one edit over another, or revise the line
entirely.</p>
<p>More complex situations can arise. Version control systems
handle the simpler cases themselves, and leave the difficult
cases for humans. Usually their behaviour is
configurable.</p>
</div>
</div>
</div>
</div><div class="footer"><a href="/~blynn/">Ben Lynn</a></div>
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