--- /dev/null
+{% extends "_base.html" %}
+
+{% block file_metadata %}
+ <meta name="title" content="Version Control With Subversion" />
+{% endblock file_metadata %}
+
+{% block content %}
+ <ol class="toc">
+ <li><a href="#s:basics">Basic Use</a></li>
+ <li><a href="#s:merge">Merging Conflicts</a></li>
+ <li><a href="#s:rollback">Recovering Old Versions</a></li>
+ <li><a href="#s:setup">Setting up a Repository</a></li>
+ <li><a href="#s:provenance">Provenance</a></li>
+ <li><a href="#s:summary">Summing Up</a></li>
+ </ol>
+
+ <p>
+ Suppose that Wolfman and Dracula have been hired by Universal Monsters Inc.
+ to figure out where the company should put its next secret lair.
+ They want to be able to work on the plans at the same time,
+ but they have run into problems doing this in the past.
+ If they take turns,
+ each one will spend a lot of time waiting for the other to finish.
+ On the other hand,
+ if they work on their own copies and email changes back and forth
+ they know that things will be lost, overwritten, or duplicated.
+ </p>
+
+ <p>
+ The right solution is to use a <a href="glossary.html#version-control-system">version control system</a>
+ to manage their work.
+ Version control is better than mailing files back and forth because:
+ </p>
+
+ <ol>
+
+ <li>
+ It's hard (but not impossible) to accidentally overlook or overwrite someone's changes,
+ because the version control system highlights them automatically.
+ </li>
+
+ <li>
+ There are no arguments about whose copy is the most up to date.
+ </li>
+
+ <li>
+ Nothing that is committed to version control is ever lost.
+ This means it can be used like the "undo" feature in an editor,
+ and since all old versions of files are saved
+ it's always possible to go back in time to see exactly who wrote what on a particular day,
+ or what version of a program was used to generate a particular set of results.
+ </li>
+
+ </ol>
+
+ <p>
+ Version control systems do have one important shortcoming.
+ While it is easy for them to find, display, and merge differences in text files,
+ images, MP3s, PDFs, or Microsoft Word or Excel files aren't stored as text—they
+ use specialized binary data formats.
+ Most version control systems don't know how to deal with these formats,
+ so all they can say is, "These files differ."
+ The rest is up to you.
+ </p>
+
+ <p>
+ Even with this limitation,
+ version control is one of the most important concepts in this book.
+ The rest of this chapter will explore how to use Subversion,
+ a popular open source version control system.
+ </p>
+
+ <section id="s:basics">
+
+ <h2>Basic Use</h2>
+
+ <div class="understand" id="u:basics">
+ <h3>Understand:</h3>
+ <ul>
+ <li>Where version control stores information.</li>
+ <li>How to check out a working copy of a repository.</li>
+ <li>How to view the history of changes to a project.</li>
+ <li>Why working copies of different projects should not overlap.</li>
+ <li>How to add files to a project.</li>
+ <li>How to submit changes made locally to a project's master copy.</li>
+ <li>How to update a working copy to get changes made to the master.</li>
+ <li>How to check the status of a working copy.</li>
+ </ul>
+ </div>
+
+ <p>
+ A version control system keeps the master copy of a file
+ in a <a href="glossary.html#repository">repository</a>
+ located on a <a href="glossary.html#server">server</a>—a computer
+ that is never used directly by people,
+ but only by their programs
+ (<a href="#f:repository">Figure XXX</a>).
+ No-one ever edits the master copy directly.
+ Instead,
+ Wolfman and Dracula each have a <a href="glossary.html#working-copy">working copy</a>
+ on their own computer.
+ This lets them make whatever changes they want whenever they want.
+ </p>
+
+ <figure id="f:repository">
+ <img src="svn/repository.png" alt="A Version Control Repository" />
+ </figure>
+
+ <p id="a:commit">
+ As soon Wolfman is ready to share his changes,
+ he <a href="glossary.html#commit">commits</a> his work to the repository
+ (<a href="#f:workflow">Figure XXX</a>).
+ Dracula can then <a href="glossary.html#update">update</a> his working copy to get those changes.
+ And of course,
+ when Dracula finishes working on something,
+ he can commit and then Wolfman can update.
+ </p>
+
+ <figure id="f:workflow">
+ <img src="svn/workflow.png" alt="Version Control Workflow" />
+ </figure>
+
+ <p>
+ But what if Dracula and Wolfman make changes to the same part of their working copies?
+ Old-fashioned version control systems prevented this from happening
+ by <a href="glossary.html#lock">locking</a> the master copy
+ whenever someone was working on it.
+ This <a href="glossary.html#pessimistic-concurrency">pessimistic</a> strategy
+ guaranteed that a second person (or monster)
+ could never make changes to the same file at the same time,
+ but it also meant that people had to take turns.
+ </p>
+
+ <p>
+ Most of today's version control systems use
+ an <a href="glossary.html#optimistic-concurrency">optimistic</a> strategy instead.
+ Nothing is ever locked—everyone is always allowed to edit their working copy.
+ This means that people can make changes to the same part of the paper,
+ but that's actually fairly uncommon in a well-run project,
+ and when it <em>does</em> happen,
+ the version control system helps people reconcile their changes.
+ </p>
+
+ <p>
+ For example,
+ if Wolfman and Dracula are making changes at the same time,
+ and Wolfman commits first,
+ his changes are simply copied to the repository
+ (<a href="#f:merge_first_commit">Figure XXX</a>):
+ </p>
+
+ <figure id="f:merge_first_commit">
+ <img src="svn/merge_first_commit.png" alt="Wolfman Commits First" />
+ </figure>
+
+ <p class="continue">
+ If Dracula now tries to commit something that would overwrite Wolfman's changes
+ the version control system stops him
+ and points out the <a href="glossary.html#conflict">conflict</a>
+ (<a href="#f:merge_second_commit">Figure XXX</a>):
+ </p>
+
+ <figure id="f:merge_second_commit">
+ <img src="svn/merge_second_commit.png" alt="Dracula Has a Conflict" />
+ </figure>
+
+ <p class="continue">
+ Dracula must <a href="glossary.html#resolve">resolve</a> that conflict
+ before the version control system will allow him to commit his work.
+ He can accept what Wolfman did,
+ replace it with what he has done,
+ or write something new that combines the two—that's up to him.
+ Once he has fixed things, he can go ahead and commit.
+ </p>
+
+ <p>
+ Let's start by looking at the basic workflow we use
+ when working with a version control system.
+ To keep things simple,
+ we'll assume that the Mummy has already put some notes in a version control repository
+ on the <code>universal.software-carpentry.org</code> server.
+ The full URL for this repository is <code>https://universal.software-carpentry.org/monsters</code>.
+ Every repository has an address like this that uniquely identifies the location of the master copy.
+ </p>
+
+ <p>
+ It's Monday night.
+ In order to get a working copy on his computer,
+ Dracula has to <a href="glossary.html#check-out">check out</a> a copy of the repository.
+ He only has to do this once per project:
+ once he has a working copy,
+ he can update it over and over again to get other people's work:
+ </p>
+
+ <div class="box">
+ <h3>There's More Than One Way To Do It</h3>
+
+ <p>
+ We will drive Subversion from the command line in our examples,
+ but if you prefer using a GUI,
+ there are many for you to choose from:
+ </p>
+
+ <ul>
+
+ <li>
+ <a href="http://tortoisesvn.net/">TortoiseSVN</a>
+ is integrated into the Windows desktop,
+ so there's no separate GUI as such.
+ </li>
+
+ <li>
+ <a href="http://rapidsvn.tigris.org/">RapidSVN</a> is free,
+ and runs on many platforms,
+ but some users report difficulties installing it.
+ </li>
+
+ <li>
+ Syntevo's <a href="http://www.syntevo.com/smartsvn/index.html">SmartSVN</a> isn't free,
+ but it costs less than most textbooks,
+ and is more stable (and has a friendlier interface) than RapidSVN.
+ </li>
+
+ </ul>
+
+ </div>
+
+ <p>
+ While in his home directory,
+ Dracula types the command:
+ </p>
+
+<pre>
+$ <span class="in">svn checkout https://universal.software-carpentry.org/monsters</span>
+</pre>
+
+ <p class="continue">
+ This creates a new directory called <code>monsters</code>
+ and fills it with a copy of the repository's contents
+ (<a href="#f:example_repo">Figure XXX</a>).
+ </p>
+
+<pre>
+<span class="out">A monsters/jupiter
+A monsters/mars
+A monsters/mars/mons-olympus.txt
+A monsters/mars/cydonia.txt
+A monsters/earth
+A monsters/earth/himalayas.txt
+A monsters/earth/antarctica.txt
+A monsters/earth/carlsbad.txt
+Checked out revision 6.</span>
+</pre>
+
+ <figure id="f:example_repo">
+ <img src="svn/example_repo.png" alt="Example Repository" />
+ </figure>
+
+ <p class="continue">
+ Dracula can then go into this directory
+ and use regular shell commands to view the files:
+ </p>
+
+<pre>
+$ <span class="in">cd monsters</span>
+$ <span class="in">ls</span>
+<span class="out">earth jupiter mars</span>
+$ <span class="in">ls *</span>
+<span class="out">earth:
+antarctica.txt carlsbad.txt himalayas.txt
+
+jupiter:
+
+mars:
+cydonia.txt mons-olympus.txt</span>
+</pre>
+
+ <div class="box">
+
+ <h3>Don't Let the Working Copies Overlap</h3>
+
+ <p>
+ It's very important that the working copies of different project do not overlap;
+ in particular,
+ we should never try to check out one project inside a working copy of another project.
+ The reason is that Subversion stories information about
+ the current state of a working copy
+ in special sub-directories called <code>.svn</code>:
+ </p>
+
+<pre>
+$ <span class="in">pwd</span>
+<span class="out">/home/vlad/monsters</span>
+$ <span class="in">ls -a</span>
+<span class="out">. .. .svn earth jupiter mars</span>
+$ <span class="in">ls -F .svn</span>
+<span class="out">entries prop-base/ props/ text-base/ tmp/</span>
+</pre>
+
+ <p class="continue">
+ If two working copies overlap,
+ the files in the <code>.svn</code> directories for one repository
+ will be clobbered by the other repository's <code>.svn</code> files,
+ and Subversion will become hopelessly confused.
+ </p>
+
+ </div>
+
+ <p>
+ Dracula can find out more about the history of the project
+ using Subversion's <code>log</code> command:
+ </p>
+
+<pre>
+$ <span class="in">svn log</span>
+<span class="out">------------------------------------------------------------------------
+r6 | mummy | 2010-07-26 09:21:10 -0400 (Mon, 26 Jul 2010) | 1 line
+
+Damn the budget---the Jovian moons would be a _perfect_ place for a lair.
+------------------------------------------------------------------------
+r5 | mummy | 2010-07-26 09:19:39 -0400 (Mon, 26 Jul 2010) | 1 line
+
+The budget might not even stretch to a deep-sea lair... :-(
+------------------------------------------------------------------------
+r4 | mummy | 2010-07-26 09:17:46 -0400 (Mon, 26 Jul 2010) | 1 line
+
+Budget cuts may force us to reconsider Earth as a base.
+------------------------------------------------------------------------
+r3 | mummy | 2010-07-26 09:14:14 -0400 (Mon, 26 Jul 2010) | 1 line
+
+Converting to wiki-formatted text.
+------------------------------------------------------------------------
+r2 | mummy | 2010-07-26 09:11:55 -0400 (Mon, 26 Jul 2010) | 1 line
+
+Hide near the face in Cydonia, perhaps?
+------------------------------------------------------------------------
+r1 | mummy | 2010-07-26 09:08:23 -0400 (Mon, 26 Jul 2010) | 1 line
+
+Thoughts on Mons Olympus (probably too obvious)
+------------------------------------------------------------------------</span>
+</pre>
+
+ <p class="continue">
+ Subversion displays a summary of all the changes made to the project so far.
+ This list includes the
+ <a href="glossary.html#revision-number">revision number</a>,
+ the name of the person who made the change,
+ the date the change was made,
+ and whatever comment the user provided when the change was submitted.
+ As we can see,
+ the <code>monsters</code> project is currently at revision 6,
+ and all changes so far have been made by the Mummy.
+ </p>
+
+ <p>
+ Notice how detailed the comments on the updates are.
+ Good comments are as important in version control as they are in coding.
+ Without them, it can be very difficult to figure out who did what, when, and why.
+ We can use comments like "Changed things" and "Fixed it" if we want,
+ or even no comments at all,
+ but we'll only be making more work for our future selves.
+ </p>
+
+ <p>
+ Another thing to notice is that the revision number applies to the whole repository,
+ not to a particular file.
+ When we talk about "version 61" we mean
+ "the state of all files and directories at that point."
+ Older version control systems like CVS gave each file a new version number when it was updated,
+ which meant that version 38 of one file could correspond in time to version 17 of another
+ (<a href="#f:version_numbering">Figure XXX</a>).
+ Experience shows that
+ global version numbers that apply to everything in the repository
+ are easier to manage than
+ per-file version numbers,
+ so that's what Subversion uses.
+ </p>
+
+ <figure id="f:version_numbering">
+ <img src="svn/version_numbering.png" alt="Version Numbering in CVS and Subversion" />
+ </figure>
+
+ <p>
+ A couple of cubicles away,
+ Wolfman also runs <code>svn checkout</code>
+ to get a working copy of the repository.
+ He also gets version 6,
+ so the files on his machine are the same as the files on Dracula's.
+ While he is looking through the files,
+ Dracula decides to add some information to the repository about Jupiter's moons.
+ Using his favorite editor,
+ he creates a file in the <code>jupiter</code> directory called <code>moons.txt</code>,
+ and fills it with information about Io, Europa, Ganymede, and Callisto:
+ </p>
+
+<pre src="svn/moons_initial.txt">
+Name Orbital Radius Orbital Period Mass Radius
+Io 421.6 1.769138 893.2 1821.6
+Europa 670.9 3.551181 480.0 1560.8
+Ganymede 1070.4 7.154553 1481.9 2631.2
+Calisto 1882.7 16.689018 1075.9 2410.3
+</pre>
+
+ <p>
+ After double-checking his data,
+ he wants to commit the file to the repository so that everyone else on the project can see it.
+ The first step is to add the file to his working copy using <code>svn add</code>:
+ </p>
+
+<pre>
+$ <span class="in">svn add jupiter/moons.txt</span>
+<span class="out">A jupiter/moons.txt</span>
+</pre>
+
+ <p>
+ Adding a file is not the same as creating it—he has already done that.
+ Instead,
+ the <code>svn add</code> command tells Subversion to add the file to
+ the list of things it's supposed to manage.
+ It's quite common,
+ particularly in programming projects,
+ to have backup files or intermediate files in a directory
+ that aren't worth storing in the repository.
+ This is why version control requires us to explicitly tell it which files are to be managed.
+ </p>
+
+ <p>
+ Once he has told Subversion to add the file,
+ Dracula can go ahead and commit his changes to the repository.
+ He uses the <code>-m</code> flag to provide a one-line message explaining what he's doing;
+ if he didn't,
+ Subversion would open his default editor
+ so that he could type in something longer.
+ </p>
+
+<pre>
+$ <span class="in">svn commit -m "Some basic facts about the Galilean moons of Jupiter." jupiter/moons.txt</span>
+<span class="out">Adding jupiter/moons.txt
+Transmitting file data .
+Committed revision 7.</span>
+</pre>
+
+ <p class="continue">
+ When Dracula runs this command,
+ Subversion establishes a connection to the server,
+ copies over his changes,
+ and updates the revision number from 6 to 7
+ (<a href="#f:updated_repo">Figure XXX</a>).
+ Again,
+ this version number applies to the <em>whole</em> repository,
+ not just to files that have changed.
+ </p>
+
+ <figure id="f:updated_repo">
+ <img src="svn/updated_repo.png" alt="Updated Repository" />
+ </figure>
+
+ <p id="a:define-head">
+ Back in his cubicle,
+ Wolfman uses <code>svn update</code> to update his working copy.
+ It tells him that a new file has been added
+ and brings his working copy up to date with version 7 of the repository,
+ because this is now the most recent revision
+ (also called the <a href="glossary.html#head">head</a>).
+ <code>svn update</code> updates an existing working copy,
+ rather than checking out a new one.
+ While <code>svn checkout</code> is usually only run once per project per machine,
+ <code>svn update</code> may be run many times a day.
+ </p>
+
+ <p>
+ Looking in the new file <code>jupiter/moons.txt</code>,
+ Wolfman notices that Dracula has misspelled "Callisto"
+ (it is supposed to have two L's.)
+ Wolfman edits that line of the file:
+ </p>
+
+<pre src="svn/moons_spelling.txt">
+Name Orbital Radius Orbital Period Mass Radius
+Io 421.6 1.769138 893.2 1821.6
+Europa 670.9 3.551181 480.0 1560.8
+Ganymede 1070.4 7.154553 1481.9 2631.2
+<span class="highlight">Callisto 1882.7 16.689018 1075.9 2410.3</span>
+</pre>
+
+ <p class="continue">
+ He also adds a line about Amalthea,
+ which he thinks might be a good site for a secret lair despite its small size:
+ </p>
+
+<pre src="svn/moons_amalthea.txt">
+Name Orbital Radius Orbital Period Mass Radius
+<span class="highlight">Amalthea 181.4 0.498179 0.075 125.0</span>
+Io 421.6 1.769138 893.2 1821.6
+Europa 670.9 3.551181 480.0 1560.8
+Ganymede 1070.4 7.154553 1481.9 2631.2
+Callisto 1882.7 16.689018 1075.9 2410.3
+</pre>
+
+ <p class="continue">
+ uses the <code>svn status</code> command to check that he hasn't accidentally changed anything else:
+ </p>
+
+<pre>
+$ <span class="in">svn status</span>
+<span class="out">M jupiter/moons.txt</span>
+</pre>
+
+ <p class="continue">
+ and then runs <code>svn commit</code>.
+ Since has hasn't used the <code>-m</code> flag to provide a message on the command line,
+ Subversion launches his default editor and shows him:
+ </p>
+
+<pre>
+
+--This line, and those below, will be ignored--
+
+M jupiter/moons.txt
+</pre>
+
+ <p>
+ He changes this to be
+ </p>
+
+<pre>
+1. Fixed typo in moon's name: 'Calisto' -> 'Callisto'.
+2. Added information about Amalthea.
+--This line, and those below, will be ignored--
+
+M jupiter/moons.txt
+</pre>
+
+ <p class="continue">
+ When he saves this temporary file and exits the editor,
+ Subversion commits his changes:
+ </p>
+
+<pre>
+<span class="out">Sending jupiter/moons.txt
+Transmitting file data .
+Committed revision 8.</span>
+</pre>
+
+ <p class="continue">
+ Note that since Wolfman didn't specify a particular file to commit,
+ Subversion commits <em>all</em> of his changes.
+ This is why he ran the <code>svn status</code> command first.
+ </p>
+
+ <div class="box" id="b:basics:transaction">
+
+ <h3>Working With Multiple Files</h3>
+
+ <p>
+ Our example only includes one file,
+ but version control can work on any number of files at once.
+ For example,
+ if Wolfman noticed that a dozen data files had the same incorrect header,
+ he could change it in all 12 files,
+ then commit all those changes at once.
+ This is actually the best way to work:
+ every logical change to the project should be a single commit,
+ and every commit should include everything involved in one logical change.
+ </p>
+
+ </div>
+
+ <p>
+ That night,
+ when Dracula rises from his coffin to start work,
+ the first thing he wants to do is get Wolfman's changes.
+ Before updating his working copy with <code>svn update</code>,
+ though,
+ he wants to see the differences between what he has
+ and what he <em>will</em> have if he updates.
+ To do this,
+ Dracula uses <code>svn diff</code>.
+ When run without arguments,
+ it compares what's in his working copy to what he started with,
+ and shows no differences:
+ </p>
+
+<pre>
+$ <span class="in">svn diff</span>
+$
+</pre>
+
+ <p class="continue">
+ To compare his working copy to the master,
+ Dracula uses <code>svn diff -r HEAD</code>.
+ The <code>-r</code> flag is used to specify a revision,
+ while <code>HEAD</code> means
+ "<a href="#a:define-head">the latest version of the master</a>".
+ </p>
+
+<pre>
+$ <span class="in">svn diff -r HEAD</span>
+<span class="out">--- moons.txt(revision 8)
++++ moons.txt(working copy)
+@@ -1,5 +1,6 @@
+ Name Orbital Radius Orbital Period Mass Radius
++Amalthea 181.4 0.498179 0.075 125.0
+ Io 421.6 1.769138 893.2 1821.6
+ Europa 670.9 3.551181 480.0 1560.8
+ Ganymede 1070.4 7.154553 1481.9 2631.2
+-Calisto 1882.7 16.689018 1075.9 2410.3
++Callisto 1882.7 16.689018 1075.9 2410.3
+</span>
+</pre>
+
+ <p class="continue">
+ After looking over the changes,
+ Dracula goes ahead and does the update.
+ </p>
+
+ <div class="box">
+ <h3>Reading a Diff</h3>
+
+ <p>
+ The output of <code>diff</code> isn't particularly user-friendly,
+ but actually isn't that hard to figure out.
+ The first two lines:
+ </p>
+
+<pre>
+--- moons.txt(revision 9)
++++ moons.txt(working copy)
+</pre>
+
+ <p class="continue">
+ signal that '-' will be used to show content from revision 9
+ and '+' to show content from the user's working copy.
+ The next line, with the '@' markers,
+ indicates where lines were inserted or removed.
+ This isn't really intended for human consumption:
+ a variety of other software tools will use this information.
+ </p>
+
+ <p>
+ The most important parts of what follows are the lines marked with '+' and '-',
+ which show insertions and deletions respectively.
+ Here,
+ we can see that the line for Amalthea was inserted,
+ and that the line for Callisto was changed
+ (which is indicated by an add and a delete right next to one another).
+ Many editors and other tools can display diffs like this in a two-column display,
+ highlighting changes.
+ </p>
+
+ </div>
+
+ <p>
+ This is a very common workflow,
+ and is the basic heartbeat of most developers' days.
+ To recap,
+ the steps are:
+ </p>
+
+ <ol>
+
+ <li>
+ Check to see if there are changes in the repository to download.
+ </li>
+
+ <li>
+ Update our working copy with those changes.
+ </li>
+
+ <li>
+ Do our own work.
+ </li>
+
+ <li>
+ Commit our changes to the repository so that other people can get them.
+ </li>
+
+ </ol>
+
+ <p>
+ It's worth noticing here how important Wolfman's comments about his changes were.
+ It's hard to see the difference between "Calisto" with one 'L' and "Callisto" with two,
+ even if the line containing the difference has been highlighted.
+ Without Wolfman's comments,
+ Dracula might have wasted time wondering what the difference was.
+ </p>
+
+ <p>
+ In fact,
+ Wolfman should probably have committed his two changes separately,
+ since there's no logical connection between
+ fixing a typo in Callisto's name
+ and adding information about Amalthea to the same file.
+ Just as a function or program should do one job and one job only,
+ a single commit to version control should have a single logical purpose so that it's easier to find,
+ understand,
+ and if necessary undo later on.
+ </p>
+
+ <div class="keypoints" id="k:basics">
+ <h3>Summary</h3>
+ <ul>
+ <li>Version control is a better way to manage shared files than email or shared folders.</li>
+ <li>The master copy is stored in a repository.</li>
+ <li>Nobody ever edits the master directory: instead, each person edits a local working copy.</li>
+ <li>People share changes by committing them to the master or updating their local copy from the master.</li>
+ <li idea="paranoia">The version control system prevents people from overwriting each other's work by forcing them to merge concurrent changes before committing.</li>
+ <li idea="perf">It also keeps a complete history of changes made to the master so that old versions can be recovered reliably.</li>
+ <li>Version control systems work best with text files, but can also handle binary files such as images and Word documents.</li>
+ <li>Every repository is identified by a URL.</li>
+ <li>Working copies of different repositories may not overlap.</li>
+ <li>Each changed to the master copy is identified by a unique revision number.</li>
+ <li>Revisions identify snapshots of the entire repository, not changes to individual files.</li>
+ <li idea="perf">Each change should be commented to make the history more readable.</li>
+ <li>Commits are transactions: either all changes are successfully committed, or none are.</li>
+ <li>The basic workflow for version control is update-change-commit.</li>
+ <li><code>svn add <em>things</em></code> tells Subversion to start managing particular files or directories.</li>
+ <li><code>svn checkout <em>url</em></code> checks out a working copy of a repository.</li>
+ <li><code>svn commit -m "<em>message</em>" <em>things</em></code> sends changes to the repository.</li>
+ <li><code>svn diff</code> compares the current state of a working copy to the state after the most recent update.</li>
+ <li><code>svn diff -r HEAD</code> compares the current state of a working copy to the state of the master copy.</li>
+ <li><code>svn history</code> shows the history of a working copy.</li>
+ <li><code>svn status</code> shows the status of a working copy.</li>
+ <li><code>svn update</code> updates a working copy from the repository.</li>
+ </ul>
+ </div>
+
+ </section>
+
+ <section id="s:merge">
+
+ <h2>Merging Conflicts</h2>
+
+ <div class="understand" id="u:merge">
+ <h3>Understand:</h3>
+ <ul>
+ <li>What a conflict in an update is.</li>
+ <li>How to resolve conflicts when updating.</li>
+ </ul>
+ </div>
+
+ <p>
+ Dracula and Wolfman have both synchronized their working copies of <code>monsters</code>
+ with version 8 of the repository.
+ Dracula now edits his copy to change Amalthea's radius
+ from a single number to a triple to reflect its irregular shape:
+ </p>
+
+<pre src="svn/moons_dracula_triple.txt">
+Name Orbital Radius Orbital Period Mass Radius
+<span class="highlight">Amalthea 181.4 0.498179 0.075 131 x 73 x 67</span>
+Io 421.6 1.769138 893.2 1821.6
+Europa 670.9 3.551181 480.0 1560.8
+Ganymede 1070.4 7.154553 1481.9 2631.2
+Callisto 1882.7 16.689018 1075.9 2410.3
+</pre>
+
+ <p class="continue">
+ He then commits his work,
+ creating revision 9 of the repository
+ (<a href="#f:after_dracula_commits">Figure XXX</a>).
+ </p>
+
+ <figure id="f:after_dracula_commits">
+ <img src="svn/after_dracula_commits.png" alt="After Dracula Commits" />
+ </figure>
+
+ <p>
+ But while he is doing this,
+ Wolfman is editing <em>his</em> copy
+ to add information about two other minor moons,
+ Himalia and Elara:
+ </p>
+
+<pre src="svn/moons_wolfman_extras.txt">
+Name Orbital Radius Orbital Period Mass Radius
+Amalthea 181.4 0.498179 0.075 131
+Io 421.6 1.769138 893.2 1821.6
+Europa 670.9 3.551181 480.0 1560.8
+Ganymede 1070.4 7.154553 1481.9 2631.2
+Callisto 1882.7 16.689018 1075.9 2410.3
+<span class="highlight">Himalia 11460 250.5662 0.095 85.0
+Elara 11740 259.6528 0.008 40.0</span>
+</pre>
+
+ <p>
+ When Wolfman tries to commit his changes to the repository,
+ Subversion won't let him:
+ </p>
+
+<pre>
+$ <span class="in">svn commit -m "Added data for Himalia, Elara"</span>
+<span class="out">Sending jupiter/moons.txt
+svn: Commit failed (details follow):
+svn: File or directory 'moons.txt' is out of date; try updating
+svn: resource out of date; try updating</span>
+</pre>
+
+ <p class="continue">
+ The reason is that
+ Wolfman's changes were based on revision 8,
+ but the repository is now at revision 9,
+ and the file that Wolfman is trying to overwrite
+ is different in the later revision.
+ (Remember,
+ one of version control's main jobs is to make sure that
+ people don't trample on each other's work.)
+ Wolfman has to update his working copy to get Dracula's changes before he can commit.
+ Luckily,
+ Dracula edited a line that Wolfman didn't change,
+ so Subversion can merge the differences automatically.
+ </p>
+
+ <p>
+ This does <em>not</em> mean that Wolfman's changes have been committed to the repository:
+ Subversion only does that when it's ordered to.
+ Wolfman's changes are still in his working copy,
+ and <em>only</em> in his working copy.
+ But since Wolfman's version of the file now includes
+ the lines that Dracula added,
+ Wolfman can go ahead and commit them as usual to create revision 10.
+ </p>
+
+ <p>
+ Wolfman's working copy is now in sync with the master,
+ but Dracula's is one behind at revision 9.
+ At this point,
+ they independently decide to add measurement units
+ to the columns in <code>moons.txt</code>.
+ Wolfman is quicker off the mark this time;
+ he adds a line to the file:
+ </p>
+
+<pre src="svn/moons_wolfman_units.txt">
+Name Orbital Radius Orbital Period Mass Radius
+<span class="highlight"> (10**3 km) (days) (10**20 kg) (km)</span>
+Amalthea 181.4 0.498179 0.075 131 x 73 x 67
+Io 421.6 1.769138 893.2 1821.6
+Europa 670.9 3.551181 480.0 1560.8
+Ganymede 1070.4 7.154553 1481.9 2631.2
+Callisto 1882.7 16.689018 1075.9 2410.3
+Himalia 11460 250.5662 0.095 85.0
+Elara 11740 259.6528 0.008 40.0
+</pre>
+
+ <p class="continue">
+ and commits it to create revision 11.
+ While he is doing this,
+ though,
+ Dracula inserts a different line at the top of the file:
+ </p>
+
+<pre src="svn/moons_dracula_units.txt">
+Name Orbital Radius Orbital Period Mass Radius
+<span class="highlight"> * 10^3 km * days * 10^20 kg * km</span>
+Amalthea 181.4 0.498179 0.075 131 x 73 x 67
+Io 421.6 1.769138 893.2 1821.6
+Europa 670.9 3.551181 480.0 1560.8
+Ganymede 1070.4 7.154553 1481.9 2631.2
+Callisto 1882.7 16.689018 1075.9 2410.3
+Himalia 11460 250.5662 0.095 85.0
+Elara 11740 259.6528 0.008 40.0
+</pre>
+
+ <p>
+ Once again,
+ when Dracula tries to commit,
+ Subversion tells him he can't.
+ But this time,
+ when Dracula does updates his working copy,
+ he doesn't just get the line Wolfman added to create revision 11.
+ There is an actual conflict in the file,
+ so Subversion asks Dracula what he wants to do:
+ </p>
+
+<pre src="svn/moons_dracula_conflict.txt">
+$ <span class="in">svn update</span>
+<span class="out">Conflict discovered in 'jupiter/moons.txt'.
+Select: (p) postpone, (df) diff-full, (e) edit,
+ (mc) mine-conflict, (tc) theirs-conflict,
+ (s) show all options:</span>
+</pre>
+
+ <p>
+ Dracula choose <code>p</code> for "postpone",
+ which tells Subversion that he'll deal with the problem later.
+ Once the update is finished,
+ he opens <code>moons.txt</code> in his editor and sees:
+ </p>
+
+<pre>
+ Name Orbital Radius Orbital Period Mass
++<<<<<<< .mine
+ + * 10^3 km * days * 10^20 kg
++=======
++ (10**3 km) (days) (10**20 kg)
++>>>>>>> .r11
+ Amalthea 181.4 0.498179 0.074
+ Io 421.6 1.769138 893.2
+ Europa 670.9 3.551181 480.0
+ Ganymede 1070.4 7.154553 1481.9
+ Callisto 1882.7 16.689018 1075.9
+</pre>
+
+ <p class="continue">
+ As we can see,
+ Subversion has inserted
+ <a href="glossary.html#conflict-marker">conflict markers</a>
+ in <code>moons.txt</code>
+ wherever there is a conflict.
+ The line <code><<<<<<< .mine</code> shows the start of the conflict,
+ and is followed by the lines from the local copy of the file.
+ The separator <code>=======</code> is then
+ followed by the lines from the repository's file that are in conflict with that section,
+ while <code>>>>>>>> .r11</code> marks the end of the conflict.
+ </p>
+
+ <p>
+ Before he can commit,
+ Dracula has to edit his copy of the file to get rid of those markers.
+ He changes it to:
+ </p>
+
+<pre src="svn/moons_dracula_resolved.txt">
+Name Orbital Radius Orbital Period Mass Radius
+<span class="highlight"> (10^3 km) (days) (10^20 kg) (km)</span>
+Amalthea 181.4 0.498179 0.075 131 x 73 x 67
+Io 421.6 1.769138 893.2 1821.6
+Europa 670.9 3.551181 480.0 1560.8
+Ganymede 1070.4 7.154553 1481.9 2631.2
+Callisto 1882.7 16.689018 1075.9 2410.3
+Himalia 11460 250.5662 0.095 85.0
+Elara 11740 259.6528 0.008 40.0
+</pre>
+
+ <p class="continue">
+ then uses the <code>svn resolved</code> command to tell Subversion that
+ he has fixed the problem.
+ Subversion will now let him commit to create revision 12.
+ </p>
+
+ <div class="box">
+
+ <h3>Auxiliary Files</h3>
+
+ <p>
+ When Dracula did his update and Subversion detected the conflict in <code>moons.txt</code>,
+ it created three temporary files to help Dracula resolve it.
+ The first is called <code>moons.txt.r9</code>;
+ it is the file as it was in Dracula's local copy
+ before he started making changes,
+ i.e., the common ancestor for his work
+ and whatever he is in conflict with.
+ </p>
+
+ <p>
+ The second file is <code>moons.txt.r11</code>.
+ This is the most up-to-date revision from the repository—the
+ file as it is including Wolfman's changes.
+ The third temporary file, <code>moons.txt.mine</code>,
+ is the file as it was in Dracula's working copy before he did the Subversion update.
+ </p>
+
+ <p>
+ Subversion creates these auxiliary files primarily
+ to help people merge conflicts in binary files.
+ It wouldn't make sense to insert <code><<<<<<<</code>
+ and <code>>>>>>>></code> characters into an image file
+ (it would almost certainly result in a corrupted image).
+ The <code>svn resolved</code> command deletes these three extra files
+ as well as telling Subversion that the conflict has been taken care of.
+ </p>
+
+ </div>
+
+ <p>
+ Some power users prefer to work with interpolated conflict markers directly,
+ but for the rest of us,
+ there are several tools for displaying differences and helping to merge them,
+ including <a href="http://diffuse.sourceforge.net/">Diffuse</a> and <a href="http://winmerge.org/">WinMerge</a>.
+ If Dracula launches Diffuse,
+ it displays his file,
+ the common base that he and Wolfman were working from,
+ and Wolfman's file in a three-pane view
+ (<a href="#f:diff_viewer">Figure XXX</a>):
+ </p>
+
+ <figure id="f:diff_viewer">
+ <img src="svn/diff_viewer.png" alt="A Difference Viewer" />
+ </figure>
+
+ <p class="continue">
+ Dracula can use the buttons to merge changes from either of the edited versions
+ into the common ancestor,
+ or edit the central pane directly.
+ Again,
+ once he is done,
+ he uses <code>svn resolved</code> and <code>svn commit</code>
+ to create revision 12 of the repository.
+ </p>
+
+ <p>
+ In this case, the conflict was small and easy to fix.
+ However, if two or more people on a team are repeatedly creating conflicts for one another,
+ it's usually a signal of deeper communication problems:
+ either they aren't talking as often as they should, or their responsibilities overlap.
+ If used properly,
+ the version control system can help the team find and fix these issues
+ so that it will be more productive in future.
+ </p>
+
+ <div class="box">
+
+ <h3>Working With Multiple Files</h3>
+
+ <p>
+ As mentioned <a href="#a:transaction">earlier</a>,
+ every logical change to a project should result in a single commit,
+ and every commit should represent one logical change.
+ This is especially true when resolving conflicts:
+ the work done to reconcile one person's changes with another are often complicated,
+ so it should be a single entry in the project's history,
+ with other, later, changes coming after it.
+ </p>
+
+ </div>
+
+ <div class="keypoints" id="k:merge">
+ <h3>Summary</h3>
+ <ul>
+ <li>Conflicts must be resolved before a commit can be completed.</li>
+ <li>Subversion puts markers in text files to show regions of conflict.</li>
+ <li>For each conflicted file, Subversion creates auxiliary files containing the common parent, the master version, and the local version.</li>
+ <li><code>svn resolve <em>files</em></code> tells Subversion that conflicts have been resolved.</li>
+ </ul>
+ </div>
+
+ </section>
+
+ <section id="s:rollback">
+
+ <h2>Recovering Old Versions</h2>
+
+ <div class="understand" id="u:rollback">
+ <h3>Understand:</h3>
+ <ul>
+ <li>How to undo changes to a working copy.</li>
+ <li>How to recover old versions of files.</li>
+ <li>What a branch is.</li>
+ </ul>
+ </div>
+
+ <p>
+ Now that we have seen how to merge files and resolve conflicts,
+ we can look at how to use version control as an "infinite undo".
+ Suppose that when Wolfman starts work late one night,
+ his copy of <code>monsters</code> is in sync with the head at revision 12.
+ He decides to edit the file <code>moons.txt</code>;
+ unfortunately, he forgot that there was a full moon,
+ so his changes don't make a lot of sense:
+ </p>
+
+<pre src="svn/poetry.txt">
+Just one moon can make me growl
+Four would make me want to howl
+...
+</pre>
+
+ <p>
+ When he's back in human form the next day,
+ he wants to undo his changes.
+ Without version control, his choices would be grim:
+ he could try to edit them back into their original state by hand
+ (which for some reason hardly ever seems to work),
+ or ask his colleagues to send him their copies of the files
+ (which is almost as embarrassing as chasing the neighbor's cat when in wolf form).
+ </p>
+
+ <p>
+ Since he's using Subversion, though,
+ and hasn't committed his work to the repository,
+ all he has to do is <a href="glossary.html#revert">revert</a> his local changes.
+ <code>svn revert</code> simply throws away local changes to files
+ and puts things back the way they were before those changes were made.
+ This is a purely local operation:
+ since Subversion stores the history of the project inside every working copy,
+ Wolfman doesn't need to be connected to the network to do this.
+ </p>
+
+ <p>
+ To start,
+ Wolfman uses <code>svn diff</code> <em>without</em> the <code>-r HEAD</code> flag
+ to take a look at the differences between his file
+ and the master copy in the repository.
+ Since he doesn't want to keep his changes,
+ his next command is <code>svn revert moons.txt</code>.
+ </p>
+
+<pre>
+$ <span class="in">cd jupiter</span>
+$ <span class="in">svn revert moons.txt</span>
+<span class="out">Reverted moons.txt</span>
+</pre>
+
+ <p>
+ What if someone <em>has</em> committed their changes,
+ but still wants to undo them?
+ For example,
+ suppose Dracula decides that the numbers in <code>moons.txt</code> would look better with commas.
+ He edits the file to put them in:
+ </p>
+
+<pre src="svn/moons_commas.txt">
+Name Orbital Radius Orbital Period Mass Radius
+ (10^3 km) (days) (10^20 kg) (km)
+Amalthea 181.4 0.498179 0.075 131 x 73 x 67
+Io 421.6 1.769138 893.2 1<span class="highlight">,</span>821.6
+Europa 670.9 3.551181 480.0 1<span class="highlight">,</span>560.8
+Ganymede 1<span class="highlight">,</span>070.4 7.154553 1<span class="highlight">,</span>481.9 2<span class="highlight">,</span>631.2
+Callisto 1<span class="highlight">,</span>882.7 16.689018 1<span class="highlight">,</span>075.9 2<span class="highlight">,</span>410.3
+Himalia 11<span class="highlight">,</span>460 250.5662 0.095 85.0
+Elara 11<span class="highlight">,</span>740 259.6528 0.008 40.0
+</pre>
+
+ <p class="continue">
+ then commits his changes to create revision 13.
+ A little while later,
+ the Mummy sees the change and orders Dracula to put things back the way they were.
+ What should Dracula do?
+ </p>
+
+ <p>
+ We can draw the sequence of events leading up to revision 13
+ as shown in <a href="#f:before_undoing">Fixture XXX</a>:
+ </p>
+
+ <figure id="f:before_undoing">
+ <img src="svn/before_undoing.png" alt="Before Undoing" />
+ </figure>
+
+ <p class="continue">
+ Dracula wants to erase revision 13 from the repository,
+ but he can't actually do that:
+ once a change is in the repository,
+ it's there forever.
+ What he can do instead is merge the old revision with the current revision
+ to create a new revision
+ (<a href="#f:merging_history">Fixture XXX</a>).
+ </p>
+
+ <figure id="f:merging_history">
+ <img src="svn/merging_history.png" alt="Merging History" />
+ </figure>
+
+ <p class="continue">
+ This is exactly like merging changes made by two different people;
+ the only difference is that the "other person" is his past self.
+ </p>
+
+ <p>
+ To undo his commas,
+ Dracula must merge revision 12 (the one before his change)
+ with revision 13 (the current head revision)
+ using <code>svn merge</code>:
+ </p>
+
+<pre>
+$ <span class="in">svn merge -r HEAD:12 moons.txt</span>
+<span class="out">-- Reverse-merging r13 into 'moons.txt'
+U moons.txt</span>
+</pre>
+
+ <p class="continue">
+ The <code>-r</code> flag specifies the range of revisions to merge:
+ to undo the changes from revision 12 to revision 13,
+ he uses either <code>13:12</code> or <code>HEAD:12</code>
+ (since he is going backward in time from the most recent revision to revision 12).
+ This is called a <a href="glossary.html#reverse-merge">reverse</a> merge
+ because he's going backward in time.
+ </p>
+
+ <p>
+ After he runs this command,
+ he must run <code>svn commit</code> to save the changes to the repository.
+ This creates a new revision, number 14,
+ rather than erasing revision 13.
+ That way,
+ the changes he made to create revision 13 are still there
+ if he can ever convince the Mummy that numbers should have commas.
+ </p>
+
+ <p>
+ Merging can be used to recover older revisions of files,
+ not just the most recent,
+ and to recover many files or directories at a time.
+ The most frequent use, though,
+ is to manage parallel streams of development in large projects.
+ This is outside the scope of this chapter,
+ but the basic idea is simple.
+ </p>
+
+ <p>
+ Suppose that Universal Monsters has just released a new program for designing secret lairs.
+ Dracula and Wolfman are supposed to start adding a few features
+ that had to be left out of the first release because time ran short.
+ At the same time,
+ Frankenstein and the Mummy are doing technical support:
+ their job is to fix any bugs that users find.
+ All sorts of things could go wrong if both teams tried to work on the same code at the same time.
+ For example,
+ if Frankenstein fixed a bug and sent a new copy of the program to a user in Greenland,
+ it would be all too easy for him to accidentally include
+ the half-completed shark tank control feature that Wolfman was working on.
+ </p>
+
+ <p>
+ The usual way to handle this situation is
+ to create a <a href="glossary.html#branch">branch</a>
+ in the repository for each major sub-project
+ (<a href="#f:branch_merge">Figure XXX</a>).
+ While Wolfman and Dracula work on
+ the <a href="glossary.html#main-line">main line</a>,
+ Frankenstein and the Mummy create a branch,
+ which is just another copy of the repository's files and directories
+ that is also under version control.
+ They can work in their branch without disturbing Wolfman and Dracula and vice versa:
+ </p>
+
+ <figure id="f:branch_merge">
+ <img src="svn/branch_merge.png" alt="Branching and Merging" />
+ </figure>
+
+ <p>
+ Branches in version control repositories are often described as "parallel universes".
+ Each branch starts off as a clone of the project at some moment in time
+ (typically each time the software is released,
+ or whenever work starts on a major new feature).
+ Changes made to a branch only affect that branch,
+ just as changes made to the files in one directory don't affect files in other directories.
+ However,
+ the branch and the main line are both stored in the same repository,
+ so their revision numbers are always in step.
+ </p>
+
+ <p>
+ If someone decides that a bug fix in one branch should also be made in another,
+ all they have to do is merge the files in question.
+ This is exactly like merging an old version of a file with the current one,
+ but instead of going backward in time,
+ the change is brought sideways from one branch to another.
+ </p>
+
+ <p>
+ Branching helps projects scale up by letting sub-teams work independently,
+ but too many branches can cause as many problems as they solve.
+ Karl Fogel's excellent book
+ <a href="bib.html#fogel-producing-oss"><cite>Producing Open Source Software</cite></a>,
+ and Laura Wingerd and Christopher Seiwald's paper
+ "<a href="bib.html#wingerd-seiwald-scm">High-level Best Practices in Software Configuration Management</a>",
+ talk about branches in much more detail.
+ Projects usually don't need to do this until they have a dozen or more developers,
+ or until several versions of their software are in simultaneous use,
+ but using branches is a key part of switching from software carpentry to software engineering.
+ </p>
+
+ <div class="keypoints" id="k:rollback">
+ <h3>Summary</h3>
+ <ul>
+ <li>Old versions of files can be recovered by merging their old state with their current state.</li>
+ <li>Recovering an old version of a file does not erase the intervening changes.</li>
+ <li>Use branches to support parallel independent development.</li>
+ <li><code>svn merge</code> merges two revisions of a file.</li>
+ <li><code>svn revert</code> undoes local changes to files.</li>
+ </ul>
+ </div>
+
+ </section>
+
+ <section id="s:setup">
+
+ <h2>Setting up a Repository</h2>
+
+ <div class="understand" id="u:setup">
+ <h3>Understand:</h3>
+ <ul>
+ <li>How to create a repository.</li>
+ </ul>
+ </div>
+
+ <p>
+ It is finally time to see how to create a repository.
+ As a quick recap,
+ we will keep the master copy of our work in a repository
+ on a server that we can access from other machines on the internet.
+ That master copy consists of files and directories that no-one ever edits directly.
+ Instead, a copy of Subversion running on that machine
+ manages updates for us and watches for conflicts.
+ Our working copy is a mirror image of the master sitting on our computer.
+ When our Subversion client needs to communicate with the master,
+ it exchanges data with the copy of Subversion running on the server.
+ </p>
+
+ <figure id="f:repo_four_things">
+ <img src="svn/repo_four_things.png" alt="What's Needed for a Repository" />
+ </figure>
+
+ <p>
+ To make this to work, we need four things
+ (<a href="#f:repo_four_things">Figure XXX</a>):
+ </p>
+
+ <ol>
+
+ <li>
+ The repository itself.
+ It's not enough to create an empty directory and start filling it with files:
+ Subversion needs to create a lot of other structure
+ in order to keep track of old revisions, who made what changes, and so on.
+ </li>
+
+ <li>
+ The full URL of the repository.
+ This includes the URL of the server
+ and the path to the repository on that machine.
+ (The second part is needed because a single server can,
+ and usually will,
+ host many repositories.)
+ </li>
+
+ <li>
+ Permission to read or write the master copy.
+ Many open source projects give the whole world permission to read from their repository,
+ but very few allow strangers to write to it:
+ there are just too many possibilities for abuse.
+ Somehow, we have to set up a password or something like it
+ so that users can prove who they are.
+ </li>
+
+ <li>
+ A working copy of the repository on our computer.
+ Once the first three things are in place,
+ this just means running the <code>checkout</code> command.
+ </li>
+
+ </ol>
+
+ <p>
+ To keep things simple,
+ we will start by creating a repository on the machine that we're working on.
+ This won't let us share our work with other people,
+ but it <em>will</em> allow us to save the history of our work as we go along.
+ </p>
+
+ <p>
+ The command to create a repository is <code>svnadmin create</code>,
+ followed by the path to the repository.
+ If we want to create a repository called <code>lair_repo</code>
+ directly under our home directory,
+ we just <code>cd</code> to get home
+ and run <code>svnadmin create lair_repo</code>.
+ This command creates a directory called <code>lair_repo</code> to hold our repository,
+ and fills it with various files that Subversion uses
+ to keep track of the project's history:
+ </p>
+
+<pre>
+$ <span class="in">cd</span>
+$ <span class="in">svnadmin create lair_repo</span>
+$ <span class="in">ls -F lair_repo</span>
+<span class="out">README.txt conf/ db/ format hooks/ locks/</span>
+</pre>
+
+ <p class="continue">
+ We should <em>never</em> edit anything in this repository directly.
+ Doing so probably won't shred our sanity and leave us gibbering in mindless horror,
+ but it will almost certainly make the repository unusable.
+ </p>
+
+ <p>
+ To get a working copy of this repository,
+ we use Subversion's <code>checkout</code> command.
+ If our home directory is <code>/users/mummy</code>,
+ then the full path to the repository we just created is <code>/users/mummy/lair_repo</code>,
+ so we run <code>svn checkout file:///users/mummy/lair lair_working</code>.
+ </p>
+
+ <p>
+ Working backward,
+ the second argument,
+ <code>lair_working</code>,
+ specifies where the working copy is to be put.
+ The first argument is the URL of our repository,
+ and it has two parts.
+ <code>/users/mummy/lair_repo</code> is the path to repository directory.
+ <code>file://</code> specifies the <a href="glossary.html#protocol">protocol</a>
+ that Subversion will use to communicate with the repository—in this case,
+ it says that the repository is part of the local machine's filesystem.
+ Notice that the protocol ends in two slashes,
+ while the absolute path to the repository starts with a slash,
+ making three in total.
+ A very common mistake is to type only two, since that's what web URLs normally have.
+ </p>
+
+ <p>
+ When we're doing a checkout,
+ it is <em>very</em> important that we provide the second argument,
+ which specifies the name of the directory we want the working copy to be put in.
+ Without it,
+ Subversion will try to use the name of the repository,
+ <code>lair_repo</code>,
+ as the name of the working copy.
+ Since we're in the directory that contains the repository,
+ this means that Subversion will try to overwrite the repository with a working copy.
+ Again,
+ there isn't much risk of our sanity being torn to shreds,
+ but this could ruin our repository.
+ </p>
+
+ <p>
+ To avoid this problem,
+ most people create a sub-directory in their account called something like <code>repos</code>,
+ and then create their repositories in that.
+ For example,
+ we could create our repository in <code>/users/mummy/repos/lair</code>,
+ then check out a working copy as <code>/users/mummy/lair</code>.
+ This practice makes both names easier to read.
+ </p>
+
+ <p>
+ The obvious next steps are
+ to put our repository on a server,
+ rather than on our personal machine,
+ and to give other people access to the repository we have just created
+ so that they can work with us.
+ We'll discuss the first in <a href="web.html#s:svn">a later chapter</a>,
+ but unfortunately,
+ the second really does require things that we are not going to cover in this course.
+ If you want to do this, you can:
+ </p>
+
+ <ul>
+
+ <li>
+ ask your system administrator to set it up for you;
+ </li>
+
+ <li>
+ use an open source hosting service like <a href="http://www.sf.net">SourceForge</a>,
+ <a href="http://code.google.com">Google Code</a>,
+ <a href="https://github.com/">GitHub</a>,
+ or <a href="https://bitbucket.org/">BitBucket</a>; or
+ </li>
+
+ <li>
+ spend a few dollars a month on a commercial hosting service like <a href="http://dreamhost.com">DreamHost</a>
+ that provides web-based GUIs for creating and managing repositories.
+ </li>
+
+ </ul>
+
+ <p>
+ If you choose the second or third option,
+ please check with whoever handles intellectual property at your institution
+ to make sure that putting your work on a commercially-operated machine
+ that is probably in some other legal jurisdiction
+ isn't going to cause trouble.
+ Many people assume that it's "just OK",
+ while others act as if not having asked will be an acceptable defence later on.
+ Unfortunately,
+ neither is true…
+ </p>
+
+ <div class="keypoints" id="k:setup">
+ <h3>Summary</h3>
+ <ul>
+ <li>Repositories can be hosted locally, on local (departmental) servers, on hosting services, or on their owners' own domains.</li>
+ <li><code>svnadmin create <em>name</em></code> creates a new repository.</li>
+ </ul>
+ </div>
+
+ </section>
+
+ <section id="s:provenance">
+
+ <h2>Provenance</h2>
+
+ <div class="understand" id="u:provenance">
+ <h3>Understand:</h3>
+ <ul>
+ <li>What data provenance is.</li>
+ <li>How to embed version numbers and other information in files managed by version control.</li>
+ <li>How to record version information about a program in its output.</li>
+ </ul>
+ </div>
+
+ <p>
+ In art,
+ the <a href="glossary.html#provenance">provenance</a> of a work
+ is the history of who owned it, when, and where.
+ In science,
+ it's the record of how a particular result came to be:
+ what raw data was processed by what version of what program to create which intermediate files,
+ what was used to turn those files into which figures of which papers,
+ and so on.
+ </p>
+
+ <p>
+ One of the central ideas of this course is that
+ wen can automatically track the provenance of scientific data.
+ To start,
+ suppose we have a text file <code>combustion.dat</code> in a Subversion repository.
+ Run the following two commands:
+ </p>
+
+<pre>
+$ svn propset svn:keywords Revision combustion.dat
+$ svn commit -m "Turning on the 'Revision' keyword" combustion.dat
+</pre>
+
+ <p>
+ Now open the file in an editor
+ and add the following line somewhere near the top:
+ </p>
+
+<pre>
+# $Revision:$
+</pre>
+
+ <p>
+ The '#' sign isn't important:
+ it's just what <code>.dat</code> files use to show comments.
+ The <code>$Revision:$</code> string,
+ on the other hand,
+ means something special to Subversion.
+ Save the file, and commit the change:
+ </p>
+
+<pre>
+$ svn commit -m "Inserting the 'Revision' keyword" combustion.dat
+</pre>
+
+ <p>
+ When we open the file again,
+ we'll see that Subversion has changed that line to something like:
+ </p>
+
+<pre>
+# $Revision: 143$
+</pre>
+
+ <p class="continue">
+ i.e., Subversion has inserted the version number
+ after the colon and before the closing <code>$</code>.
+ </p>
+
+ <p>
+ Here's what just happened.
+ First, Subversion allows you to set
+ <a href="glossary.html#property-subversion">properties</a>
+ for files and and directories.
+ These properties aren't in the files or directories themselves,
+ but live in Subversion's database.
+ One of those properties,
+ <code>svn:keywords</code>,
+ tells Subversion to look in files that are being changed
+ for strings of the form <code>$propertyname: …$</code>,
+ where <code>propertyname</code> is a string like <code>Revision</code> or <code>Author</code>.
+ (About half a dozen such strings are supported.)
+ </p>
+
+ <p>
+ If it sees such a string,
+ Subversion rewrites it as the commit is taking place to replace <code>…</code>
+ with the current version number,
+ the name of the person making the change,
+ or whatever else the property's name tells it to do.
+ You only have to add the string to the file once;
+ after that,
+ Subversion updates it for you every time the file changes.
+ </p>
+
+ <p>
+ Putting the version number in the file this way can be pretty handy.
+ If you copy the file to another machine,
+ for example,
+ it carries its version number with it,
+ so you can tell which version you have even if it's outside version control.
+ We'll see some more useful things we can do with this information in
+ <a href="python.html">the next chapter</a>.
+ </p>
+
+ <div class="box">
+
+ <h3>When <em>Not</em> to Use Version Control</h3>
+
+ <p>
+ Despite the rapidly decreasing cost of storage,
+ it is still possible to run out of disk space.
+ In some labs,
+ people can easy go through 2 TB/month if they're not careful.
+ Since version control tools usually store revisions in terms of lines,
+ with binary data files,
+ they end up essentially storing every revision separately.
+ This isn't that bad
+ (it's what we'd be doing anyway),
+ but it means version control isn't doing what it likes to do,
+ and the repository can get very large very quickly.
+ Another concern is that if very old data will no longer be used,
+ it can be nice to archive or delete old data files.
+ This is not possible if our data is version controlled:
+ information can only be added to a repository,
+ so it can only ever increase in size.
+ </p>
+
+ </div>
+
+ <p>
+ We can use this trick with shell scripts too,
+ or with almost any other kind of program.
+ Going back to Nelle Nemo's data processing from the previous chapter,
+ for example,
+ suppose she writes a shell script that uses <code>gooclean</code>
+ to tidy up data files.
+ Her first version looks like this:
+ </p>
+
+<pre>
+for filename in $*
+do
+ gooclean -b 0 100 < $filename > cleaned-$filename
+done
+</pre>
+
+ <p class="continue">
+ i.e., it runs <code>gooclean</code> with bounding values of 0 and 100
+ for each specified file,
+ putting the result in a temporary file with a well-defined name.
+ Assuming that '#' is the comment character for those kinds of data files,
+ she could instead write:
+ </p>
+
+<pre>
+for filename in $*
+do
+ <span class="highlight">echo "gooclean $Revision: 901$ -b 0 100" > $filename</span>
+ gooclean -b 0 100 < $filename <span class="highlight">>></span> cleaned-$filename
+done
+</pre>
+
+ <p>
+ The first change puts a line in the output file
+ that describes how that file was created.
+ The second change is to use <code>>></code> instead of <code>></code>
+ to redirect <code>gooclean</code>'s output to the file.
+ <code>>></code> means "append to":
+ instead of overwriting whatever is in the file,
+ it adds more content to it.
+ This ensures that the first line of the file is the provenance record,
+ with the actual output of <code>gooclean</code> after it.
+ </p>
+
+ <div class="keypoints" id="k:provenance">
+ <h3>Summary</h3>
+ <ul>
+ <li><code>$Keyword:$</code> in a file can be filled in with a property value each time the file is committed.</li>
+ <li idea="paranoia">Put version numbers in programs' output to establish provenance for data.</li>
+ <li><code>svn propset svn:keywords <em>property</em> <em>files</em></code> tells Subversion to start filling in property values.</li>
+ </ul>
+ </div>
+
+ </section>
+
+ <section id="s:summary">
+
+ <h2>Summing Up</h2>
+
+ <p>
+ Correlation does not imply causality,
+ but there is a very strong correlation between
+ using version control
+ and doing good computational science.
+ There's an equally strong correlation
+ between <em>not</em> using it and wasting effort,
+ so today (the middle of 2012),
+ I will not review a paper if the software used in it
+ is not under version control.
+ Its authors' work might be interesting,
+ but without the kind of record-keeping that version control provides,
+ there's no way to know exactly what they did and when.
+ Just as importantly,
+ if someone doesn't know enough about computing to use version control,
+ the odds are good that they don't know enough
+ to do the programming right either.
+ </p>
+
+ </section>
+{% endblock content %}
projects / swc-workshop.git / commitdiff