+++ /dev/null
-{% extends "templates/_base.html" %}
-
-{% block file_metadata %}
- <meta name="title" content="Version Control With Subversion" />
- <meta name="status" content="Ready for comment" />
-{% 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>
- Wolfman and Dracula have been hired by Universal Missions
- (a space services spinoff from Euphoric State University)
- to figure out where the company should send its next planetary lander.
- 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>
- It keeps a record of who made what changes when,
- so that if people have questions later on,
- they know who to ask
- (or blame).
- </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>
- The rest of this chapter will explore how to use
- a popular open source version control system called Subversion.
- It does not have all the features of some newer systems,
- such as <a href="git.html">Git</a>,
- but it is still widely used,
- and is simpler to pick up than those more advanced alternatives.
- No matter which system you use,
- the most important thing to learn is not the details of their more obscure commands,
- but the workflow that they encourage.
-</p>
-
-<div class="guide">
- <h2>For Instructors</h2>
-
- <p>
- Version control is the most important practical skill we introduce.
- As the last paragraph of the introduction above says,
- the workflow matters more than the ins and outs of any particular tool.
- By the end of 90 minutes,
- the instructor should be able to get learners to chant,
- "Update, edit, merge, commit," in unison,
- and have them understand what those terms mean
- and why that's a good way to structure their working day.
- </p>
-
- <p>
- Provided there aren't network problems,
- this entire lesson can be covered in <span class="duration">90 minutes</span>.
- The example at the end
- showing how to use Subversion keywords to track provenance
- is the "ah ha!" moment for many learners.
- If time is short,
- skip the material on recovering old versions of files
- in order to get to this section instead.
- (The fact that provenance is harder in Git,
- both mechanically and conceptually,
- is one reason to keep teaching Subversion.)
- </p>
-
- <div class="prereq">
- <h3>Prerequisites</h3>
- <p>
- Basic shell concepts and skills
- (<code>ls</code>, <code>cd</code>, <code>mkdir</code>,
- editing files);
- basic shell scripting
- (for the discussion of <a href="#s:provenance">provenance</a>).
- </p>
- </div>
-
- <div class="notes">
- <h3>Teaching Notes</h3>
- <ul>
- <li>
- Make sure the network is working <em>before</em> starting this lesson.
- </li>
- <li>
- Give learners a ten-minute overview of what version control does for them
- before diving into the watch-and-do practicals.
- Most of them will have tried to co-author papers by emailing files back and forth,
- or will have biked into the office
- only to realize that the USB key with last night's work
- is still on the kitchen table.
- Instructors can also make jokes about directories with names like
- "final version",
- "final version revised",
- "final version with reviewer three's corrections",
- "really final version",
- and,
- "come on this really has to be the last version"
- to motivate version control as a better way to collaborate
- and as a better way to back work up.
- </li>
- <li>
- Version control is typically taught after the shell,
- so collect learners' names during that session
- and create a repository for them to share
- with their names as both their IDs and their passwords.
- The easiest way to create the repository is to use
- a server managed by an ISP such as Dreamhost,
- or on SourceForge, Google Code, or some other "forge" site,
- all of which provide web interfaces for repository creation and management.
- If your learners are advanced enough to be using SSH,
- you can instead create it on any server they can access,
- and connect with the <code>svn+ssh</code> protocol instead of HTTPS.
- </li>
- <li>
- Be very clear what files learners are to edit
- and what user IDs they are to use
- when giving instructions.
- It is common for them to edit the instructor's biography,
- or to use the instructor's user ID and password when committing.
- Be equally clear <em>when</em> they are to edit things:
- it's also common for someone to edit the file the instructor is editing
- and commit changes while the instructor is explaining what's going on,
- so that a conflict occurs when the instructor comes to commit the file.
- </li>
- <li>
- Learners could do most exercises with repositories on their own machines,
- but it's hard for them to see how version control helps collaboration
- unless they're sharing a repository with other learners.
- In particular,
- showing learners who changed what using <code>svn blame</code>
- is only compelling if a file has been edited by at least two people.
- </li>
- <li>
- If some learners are using Windows,
- there will inevitably be issues merging files with different line endings.
- <code>svn diff -x -w</code> is supposed to suppress differences in whitespace,
- but we have found that it doesn't always work as advertised.
- </li>
- </ul>
- </div>
-
-</div>
-
-<section id="s:basics">
- <h2>Basic Use</h2>
-
- <div class="understand">
- <h3>Learning Objectives</h3>
- <ul>
- <li>Draw a diagram showing the places version control stores information.</li>
- <li>Check out a working copy of a repository.</li>
- <li>View the history of changes to a project.</li>
- <li>Explain why working copies of different projects should not overlap.</li>
- <li>Add files to a project.</li>
- <li>Commit changes made to a working copy to a repository.</li>
- <li>Update a working copy to get changes from the repository.</li>
- <li>Compare the current state of a working copy to the last update from the repository, and to the current state of the repository.</li>
- <li>Explain what "version 123 of <code>xyz.txt</code>" actually means.</li>
- </ul>
- <p>
- <span class="duration">20 minutes</span>.
- </p>
- </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 1</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 machines.
- They can each edit their working copies whenever and however they want.
- </p>
-
- <figure id="f:repository">
- <img src="svn/repository.png" alt="Repositories and Working Copies" />
- <figcaption>Figure 1: Repositories and Working Copies</figcaption>
- </figure>
-
- <p id="a:commit">
- When Wolfman is ready to share his changes with Dracula,
- he <a href="glossary.html#commit">commits</a> his work to the repository
- (<a href="#f:workflow">Figure 2</a>).
- Dracula can then <a href="glossary.html#update">update</a> his working copy
- to get those changes when he's ready for them.
- And of course,
- when Dracula finishes working on something,
- he can commit and so that Wolfman can update.
- </p>
-
- <figure id="f:workflow">
- <img src="svn/workflow.png" alt="Sharing Files Through Version Control" />
- <figcaption>Figure 2: Sharing Files Through Version Control</figcaption>
- </figure>
-
- <p>
- If this is all there was to version control,
- it would be no better than FTP or Dropbox.
- But what if Dracula and Wolfman change their working copies at the same time?
- If Wolfman commits first,
- his changes are simply copied to the repository
- (<a href="#f:merge_first_commit">Figure 3</a>):
- </p>
-
- <figure id="f:merge_first_commit">
- <img src="svn/merge_first_commit.png" alt="Wolfman Commits First" />
- <figcaption>Figure 3: Wolfman Commits First</figcaption>
- </figure>
-
- <p class="continue">
- If Dracula now tries to commit something that would overwrite Wolfman's changes
- the version control system detects the <a href="glossary.html#conflict">conflict</a>,
- halts the commit,
- and tells Dracula that there's a problem
- (<a href="#f:merge_second_commit">Figure 4</a>):
- </p>
-
- <figure id="f:merge_second_commit">
- <img src="svn/merge_second_commit.png" alt="Dracula Has a Conflict" />
- <figcaption>Figure 4: Dracula Has a Conflict</figcaption>
- </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
- (<a href="#f:merge_resolve">Figure 5</a>).
- Once he has cleaned things up, he can go ahead and try committing again.
- If all of the conflicts have been resolved,
- the version control will accept it this time.
- </p>
-
- <figure id="f:merge_resolve">
- <img src="svn/merge_resolve.png" alt="Resolving the Conflict" />
- <figcaption>Figure 5: Resolving the Conflict</figcaption>
- </figure>
-
- <div class="box">
- <h3>Forgiveness vs. Permission</h3>
-
- <p>
- Old-fashioned version control systems prevented conflicts 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 editing files.
- </p>
-
- <p>
- Most of today's version control systems use
- an <a href="glossary.html#optimistic-concurrency">optimistic</a> strategy instead:
- people are always allowed to edit their working copies,
- and if a conflict occurs,
- the version control system helps them sort it out after the fact.
- </p>
- </div>
-
- <p>
- To see how this actually works,
- let's assume that the Mummy
- (Dracula and Wolfman's boss)
- has already put some notes in a version control repository
- whose URL is <code>https://universal.software-carpentry.org/explore</code>.
- Every repository has an address like this that uniquely identifies the location of the master copy.
- </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.
- Please see the <a href="ref.html#s:svn:gui">reference</a> for links.
- </p>
- </div>
-
- <p>
- It's Monday morning,
- and Dracula has just joined the project.
- 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>
-
- <p>
- While in his home directory,
- Dracula types the command:
- </p>
-
-<pre>
-$ <span class="in">svn checkout https://universal.software-carpentry.org/explore</span>
-</pre>
-
- <p class="continue">
- This creates a new directory called <code>explore</code>
- and fills it with a copy of the repository's contents
- (<a href="#f:example_repo">Figure 6</a>).
- </p>
-
-<pre>
-<span class="out">A explore/jupiter
-A explore/mars
-A explore/mars/mons-olympus.txt
-A explore/mars/cydonia.txt
-A explore/earth
-A explore/earth/himalayas.txt
-A explore/earth/antarctica.txt
-A explore/earth/carlsbad.txt
-Checked out revision 6.</span>
-</pre>
-
- <figure id="f:example_repo">
- <img src="svn/example_repo.png" alt="Example Repository" />
- <figcaption>Figure 6: Example Repository</figcaption>
- </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 explore</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/dracula/explore</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 to explore.
-------------------------------------------------------------------------
-r5 | mummy | 2010-07-26 09:19:39 -0400 (Mon, 26 Jul 2010) | 1 line
-
-The budget might not even stretch to the Arctic :-(
-------------------------------------------------------------------------
-r4 | mummy | 2010-07-26 09:17:46 -0400 (Mon, 26 Jul 2010) | 1 line
-
-Budget cuts may force us to do another dry run in the Arctic.
-------------------------------------------------------------------------
-r3 | mummy | 2010-07-26 09:14:14 -0400 (Mon, 26 Jul 2010) | 1 line
-
-Converting document to wiki-formatted text.
-------------------------------------------------------------------------
-r2 | mummy | 2010-07-26 09:11:55 -0400 (Mon, 26 Jul 2010) | 1 line
-
-Or put it down near the Face of Cydonia?
-------------------------------------------------------------------------
-r1 | mummy | 2010-07-26 09:08:23 -0400 (Mon, 26 Jul 2010) | 1 line
-
-Send the probe to Mons Olympus?
-------------------------------------------------------------------------</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>explore</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>
-
- <div class="box">
- <h3>Numbering Versions</h3>
-
- <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 7</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 Schemes" />
- <figcaption>Figure 7: Version Numbering Schemes</figcaption>
- </figure>
- </div>
-
- <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>
- When Dracula runs the <code>svn commit</code> 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 8</a>).
- </p>
-
- <figure id="f:updated_repo">
- <img src="svn/updated_repo.png" alt="Updated Repository" />
- <figcaption>Figure 8: Updated Repository</figcaption>
- </figure>
-
- <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 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 an interesting place to send a probe
- 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>
- Next,
- he 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">
- <h3>Which Editor?</h3>
- <p>
- If you don't have a default editor set up,
- Subversion will probably open an editor called Vi.
- If this happens,
- type escape-colon-w-q-! to exit
- and hope it never happens again.
- </p>
- </div>
-
- <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,
- Dracula wants to synchronize with Wolfman's work.
- Before updating his working copy with <code>svn update</code>,
- though,
- he checks to see if he has made any changes locally
- by running <code>svn diff</code>.
- Without arguments,
- it compares what's in his working copy to what he got the last time he updated.
- There are no differences,
- so there's no output:
- </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> is cryptic even by Unix standards.
- 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:
- editors and other tools can use this information
- to replay a series of edits against a file.
- </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>
-
- <div class="box">
- <h3>Nothing's Perfekt</h3>
-
- <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."
- Reconciling those differences will probably require use of an auxiliary tool,
- such as an audio editor
- or Microsoft Word's "Compare and Merge" utility.
- </p>
- </div>
-
- <div class="box">
- <h3>Diffing Other Files</h3>
-
- <p>
- <code>svn diff</code> mimics the behavior of
- the Unix <code>diff</code> command,
- which can be used to compare any two files.
- Given these two files:
- </p>
-
- <table>
- <tr>
- <th><code>left.txt</code></th>
- <th><code>right.txt</code></th>
- </tr>
- <tr>
- <td valign="top">
-<pre>hydrogen
-lithium
-sodium
-magnesium
-rubidium</pre>
- </td>
- <td valign="top">
-<pre>hydrogen
-lithium
-beryllium
-sodium
-potassium
-strontium</pre>
- </td>
- </tr>
- </table>
-
- <p class="continue">
- <code>diff</code>'s output is:
- </p>
-<pre>
-$ <span class="in">diff left.txt right.txt</span>
-<span class="out">2a3
-> beryllium
-4,5c5,6
-< magnesium
-< rubidium
----
-> potassium
-> strontium</span>
-</pre>
- </div>
-
- <p>
- This is a very common workflow,
- and is the basic heartbeat of most developers' days.
- The steps are:
- </p>
-
- <ol>
-
- <li>
- Update our working copy
- so that we have any changes other people have committed.
- </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="box">
- <h3>Who Did What?</h3>
-
- <p>
- One other very useful command is <code>svn blame</code>,
- which shows when each line in the file was last changed
- and by whom:
- </p>
-
-<pre>
-$ <span class="in">svn blame moons.txt</span>
-<span class="out"> 14 dracula Name Orbital Radius Orbital Period Mass Radius
- 14 dracula (10**3 km) (days) (10**20 kg) (km)
- 14 dracula Amalthea 181.4 0.498179 0.075 131 x 73 x 67
- 9 mummy Io 421.6 1.769138 893.2 1821.6
- 9 mummy Europa 670.9 3.551181 480.0 1560.8
- 9 mummy Ganymede 1070.4 7.154553 1481.9 2631.2
- 14 dracula Callisto 1882.7 16.689018 1075.9 2410.3
- 14 dracula Himalia 11460 250.5662 0.095 85.0
- 14 dracula Elara 11740 259.6528 0.008 40.0</span>
-</pre>
-
- <p>
- If you are ever wondering who to talk to about a change,
- or why it was made,
- <code>svn blame</code> is a good place to start.
- </p>
- </div>
-
- <div class="keypoints">
- <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>The version control system prevents people from overwriting each other's work by forcing them to merge concurrent changes before committing.</li>
- <li>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>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>
-
- <div class="challenges">
- <h3>Challenges</h3>
-
- <ol>
-
- <li>
- Using the repository URL, user ID, and password provided by the instructor,
- perform the following actions:
- <ol>
- <li>
- Check out a working copy of the repository.
- </li>
- <li>
- Create a text file called <em>your_id</em>.txt
- (using your user ID instead of <em>your_id</em>)
- and write a three-line biography of yourself in it.
- </li>
- <li>
- Add this file to your working copy.
- </li>
- <li>
- Commit your changes to the repository.
- </li>
- <li>
- Update your working copy to get other people's biographies.
- </li>
- <li>
- Examine the change log to see
- the order in which people added their biographies
- to the repository.
- </li>
- </ol>
- </li>
-
- <li>
- What does the command <code>svn diff -r 14</code> do?
- What does it do if there have only been 10 changes to the repository?
- </li>
-
- <li>
- By default,
- Unix <code>diff</code> and <code>svn diff</code> compare files line by line.
- Why doesn't this work for MP3 audio files?
- </li>
-
- </ol>
- </div>
-
-</section>
-
-<section id="s:merge">
- <h2>Merging Conflicts</h2>
-
- <div class="understand">
- <h3>Learning Objectives</h3>
- <ul>
- <li>Explain what causes conflicts to occur and how to tell when one has occurred.</li>
- <li>Resolve a conflict.</li>
- <li>Identify the auxiliary files created when a conflict occurs.</li>
- </ul>
- <p>
- <span class="duration">20 minutes</span>.
- </p>
- </div>
-
- <p>
- Dracula and Wolfman have both synchronized their working copies of <code>explore</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 9</a>).
- </p>
-
- <figure id="f:after_dracula_commits">
- <img src="svn/after_dracula_commits.png" alt="After Dracula Commits" />
- <figcaption>Figure 9: After Dracula Commits</figcaption>
- </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
- (<a href="#f:merge_without_conflict">Figure 10</a>).
- </p>
-
- <figure id="f:merge_without_conflict">
- <img src="svn/merge_without_conflict.png" alt="Merging Without Conflict" />
- <figcaption>Figure 10: Merging Without Conflict</figcaption>
- </figure>
-
- <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
- (<a href="#f:merge_with_conflict">Figure 11</a>).
- </p>
-
- <figure id="f:merge_with_conflict">
- <img src="svn/merge_with_conflict.png" alt="Merge With Conflict" />
- <figcaption>Figure 11: Merge With Conflict</figcaption>
- </figure>
-
- <p>
- 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 12</a>):
- </p>
-
- <figure id="f:diff_viewer">
- <img src="svn/diff_viewer.png" alt="A Difference Viewer" />
- <figcaption>Figure 12: A Difference Viewer</figcaption>
- </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">
- <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>
-
- <div class="challenges">
- <h3>Challenges</h3>
-
- <p>
- If you are working in a group,
- partner with someone who has also wrote a biography for themselves
- for the previous section's challenges.
- </p>
-
- <ol>
- <li>
- Both partners use <code>svn update</code>
- to make sure their working copies are up to date
- and that there are no local changes.
- </li>
- <li>
- The first partner edits her biography and commits the changes.
- </li>
- <li>
- The second partner edits her copy of the file
- (<em>without</em> having updated to get the first partner's changes),
- then tries to <code>svn commit</code>.
- </li>
- <li>
- Once the second partner has resolved the conflict,
- she commits her changes.
- </li>
- <li>
- Repeat these four steps with roles reversed.
- </li>
- </ol>
-
- <p>
- If you are working on your own,
- you can simulate the steps above
- by checking out a second copy of the project into a new directory.
- (Remember,
- this cannot overlap any existing checked-out copies.)
- Edit your biography in one copy and commit those changes,
- then switch to the other copy and edit the same file
- before updating.
- </p>
- </div>
-
-</section>
-
-<section id="s:rollback">
- <h2>Recovering Old Versions</h2>
-
- <div class="understand">
- <h3>Learning Objectives</h3>
- <ul>
- <li>Discard changes made to a working copy.</li>
- <li>Recover an old version of a file.</li>
- <li>Explain what branches are and when they are used.</li>
- </ul>
- <p>
- <span class="duration">20 minutes</span>.
- </p>
- </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>explore</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">Figure 13</a>:
- </p>
-
- <figure id="f:before_undoing">
- <img src="svn/before_undoing.png" alt="Before Undoing" />
- <figcaption>Figure 13: Before Undoing</figcaption>
- </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">Figure 14</a>).
- </p>
-
- <figure id="f:merging_history">
- <img src="svn/merging_history.png" alt="Merging History" />
- <figcaption>Figure 14: Merging History</figcaption>
- </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>
-
- <div class="box">
- <h3>Another Way to Do It</h3>
-
- <p>
- Another way to recover a particular version of a particular file
- is to use the <code>svn copy</code> command.
- If the URL of our repository is
- <code>https://universal.software-carpentry.org/explore</code>,
- then the command:
- </p>
-
-<pre>
-$ <span class="in">svn copy https://universal.software-carpentry.org/explore/mission.txt@120 ./mission.txt</span>
-</pre>
-
- <p class="continue">
- copies the file <code>mission.txt</code> as it was in revision 120
- into our working directory
- (overwriting whatever <code>mission.txt</code> file we currently have,
- if any).
- What's more,
- using <code>svn copy</code> brings along the file's history as well,
- so that future <code>svn log</code> operations will show
- how <code>mission.txt</code> was resurrected.
- </p>
- </div>
-
- <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 Missions has just released a new program
- for designing interplanetary voyages.
- Dracula and Wolfman are supposed to add some features
- that were 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.
- </p>
-
- <p>
- All sorts of things could go wrong
- if both teams tried to work on the same code at the same time.
- In particular,
- Dracula and Wolfman might want to make large changes
- to the structure of the code
- in order to make it easier to add new features,
- while Frankenstein and the Mummy want to make as few changes as possible
- so as not to introduce new bugs while fixing old ones.
- </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 15</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" />
- <figcaption>Figure 15: Branching and Merging</figcaption>
- </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">
- <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 revert</code> undoes local changes to files.</li>
- <li><code>svn merge</code> merges two revisions of a file.</li>
- </ul>
- </div>
-
- <div class="challenges">
- <h3>Challenges</h3>
-
- <ol>
- <li>
- Explain what the command:
-<pre>
-svn diff -r 240:261 fish.dat
-</pre>
- does, and when you might want to run it.
- </li>
-
- <li>
- Suppose that a file called <code>mission.txt</code>
- existed in revision 90 of a repository,
- but had been deleted in revision 91.
- What two commands could we use to recover it?
- </li>
-
- </ol>
- </div>
-
-</section>
-
-<section id="s:setup">
- <h2>Setting Up a Repository</h2>
-
- <div class="understand">
- <h3>Learning Objectives</h3>
- <ul>
- <li>How to create a repository.</li>
- </ul>
- <p>
- <span class="duration">25 minutes</span>
- (mostly discussion about where to host repositories).
- </p>
- </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>
-
- <p>
- To make this to work, we need four things:
- </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>missions_repo</code>
- directly under our home directory,
- we just <code>cd</code> to get home
- and run <code>svnadmin create missions_repo</code>.
- This command creates a directory called <code>missions_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 missions_repo</span>
-$ <span class="in">ls -F missions_repo</span>
-<span class="out">README.txt conf/ db/ format hooks/ locks/</span>
-</pre>
-
- <p class="continue">
- We should <em>never</em> edit any of this directly,
- since it will almost certainly make the repository unusable.
- Instead,
- we should use <code>svn checkout</code>
- to get a working copy of this repository.
- If our home directory is <code>/users/mummy</code>,
- then the full path to the repository we just created is <code>/users/mummy/missions_repo</code>,
- so we run <code>svn checkout file:///users/mummy/missions missions_working</code>.
- </p>
-
- <p>
- Working backward,
- the second argument,
- <code>missions_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/missions_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>missions_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/missions</code>,
- then check out a working copy as <code>/users/mummy/missions</code>.
- This practice makes both names easier to read.
- </p>
-
- <p>
- The obvious next step is to put our repository on a server,
- rather than on our personal machine.
- In fact,
- we should <em>always</em> do this
- so that we don't lose the history of our project
- if our laptop is damaged or stolen.
- A departmental server is also much more likely to be backed up regularly
- than our personal machine…
- </p>
-
- <p>
- Creating a repository on a server is simple:
- just log in and go through the steps described above.
- Accessing that repository from another machine
- is also straightforward.
- If the machine's address is <code>serv.euphoric.edu</code>,
- and our user ID is <code>dracula</code>,
- the URL of the repository will be something like:
- </p>
-
-<pre>
-svn+ssh://dracula@serv.euphoric.edu/home/dracula/repos/missions
-</pre>
-
- <p>
- Reading from left to right:
- </p>
-
- <ul>
- <li>
- <code>svn+ssh</code> is the protocol that Subversion uses to connect to the server
- (in this case,
- a combination of Subversion's own protocol
- and <a href="shell.html#s:ssh">SSH</a>);
- </li>
- <li>
- <code>dracula@serv.euphoric.edu</code> identifies the server and who we are
- (just like an email address);
- and
- </li>
- <li>
- <code>/home/dracula/repos/missions</code> is the absolutely path of the repository
- on the server.
- </li>
- </ul>
-
- <p id="a:only_user">
- That's fine if you are the only person using the repository,
- but if you want to share it with others,
- you need to worry about security.
- As we discuss in the lesson on <a href="web.html">web programming</a>,
- as soon as you provide a service on the internet,
- there's the possibility that someone may try to attack your system through it.
- Rather than trying to learn enough system administration skills
- to set things up safely,
- it is usually easier to:
- </p>
-
- <ul>
-
- <li>
- ask your department's system administrator to set it up for you;
- </li>
-
- <li>
- use a 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
- 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">
- <h3>Summary</h3>
- <ul>
- <li><code>svnadmin create <em>name</em></code> creates a new repository.</li>
- <li>Repositories can be hosted locally, on local (departmental) servers, on hosting services, or on their owners' own domains.</li>
- </ul>
- </div>
-
- <div class="challenges">
- <h3>Challenges</h3>
-
- <ol>
-
- <li>
- Create a Subversion repository called <code>trials_repo</code>
- in your home directory.
- Check out a working copy in a directory called <code>trials_working</code>
- (also in your home directory).
- Add a couple of text files,
- commit the changes,
- and then use <code>svn info trials_working</code>
- to see what Subversion tells you about your working copy.
- </li>
-
- <li>
- We said <a href="#a:only_user">above</a> that
- you might be the only person using a particular repository.
- When and why is version control worth using
- if no-one else is working on a project with you?
- </li>
-
- <li>
- There are many ways to organize repositories.
- Some of the most common are to create one repository for:
- <ul>
- <li>each person</li>
- <li>each paper</li>
- <li>all the work done on one grant</li>
- <li>all the work done on one project</li>
- <li>the entire lab (which is shared by everyone in the lab)</li>
- <li>the entire department (typically with a top-level directory for each person or project in the department)</li>
- </ul>
- What activities does each one make easy or hard?
- Which of these would you prefer, and why?
- </li>
-
- </ol>
- </div>
-
-</section>
-
-<section id="s:provenance">
- <h2>Provenance</h2>
-
- <div class="understand">
- <h3>Learning Objectives</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>
- <p>
- <span class="duration">20 minutes</span>
- (without a practical exercise).
- </p>
- </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 big benefits of using version control is that
- it lets us track the provenance of scientific data automatically.
- 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 class="continue">
- This does nothing by itself,
- but now open the file in an editor
- and add the following line somewhere near the top:
- </p>
-
-<pre>
-$Revision:$
-</pre>
-
- <p>
- The <code>$Revision:$</code> string 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., it has inserted the version number
- after the colon and before the closing <code>$</code>.
- If we edit the file again—e.g., add a couple of lines with random numbers—and
- commit once more,
- the line is updated again to:
- </p>
-
-<pre>
-$Revision: 144$
-</pre>
-
- <p>
- Here's what just happened.
- First, Subversion allows uss to add
- <a href="glossary.html#property-subversion">properties</a>
- to files and and directories.
- These properties aren't stored in the files or directories themselves,
- but 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.
- We 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 <a href="python.html">later</a>.
- </p>
-
- <p>
- We can use this trick with shell scripts too,
- or with almost any other kind of program.
- Let's go back to Nelle Nemo's data processing from
- the lesson on the <a href="shell.html">shell</a>.
- Suppose she writes a shell script called <code>gooclean</code>
- to tidy up data files.
- Her first version looks like this:
- </p>
-
-<pre>
-# gooclean: clean up a single data file
-goonorm -b 0 100 < $1 | goofilter -x --enlarge 2.0 > cleaned-$1
-</pre>
-
- <p class="continue">
- i.e.,
- it runs <code>goonorm</code> and then <code>goofilter</code> with some fixed parameters
- and creates an output file called <code>cleaned-something.dat</code>
- (if the input file's name was <code>something.dat</code>).
- Assuming that '#' is the comment character for her output files,
- she could instead write:
- </p>
-
-<pre>
-# gooclean: clean up a single data file
-<span class="highlight">echo "# gooclean $Revision:$" > cleaned-$1</span>
-goonorm -b 0 100 < $1 | goofilter -x --enlarge 2.0 <span class="highlight">>></span> cleaned-$1
-</pre>
-
- <p class="continue">
- then set the <code>svn:keywords</code> property
- and commit the file to insert the revision number,
- making it:
- </p>
-
-<pre>
-# gooclean: clean up a single data file
-<span class="highlight">echo "# gooclean $Revision: 487$" > cleaned-$1</span>
-goonorm -b 0 100 < $1 | goofilter -x --enlarge 2.0 <span class="highlight">>></span> cleaned-$1
-</pre>
-
- <p>
- Now,
- each time this script is run it will:
- </p>
-
- <ul>
- <li>
- put the line
-<pre>
-# gooclean $Revision: 487$
-</pre>
- in the output file,
- then
- </li>
- <li>
- append whatever the pipline containing <code>goonorm</code> and <code>goofilter</code>
- would have put in the file originally.
- (The double redirection <code>>></code> means "append to" rather than "overwrite".)
- </li>
- </ul>
-
- <p class="continue">
- In other words,
- the output of this shell script will always record
- exactly what version of the script produced it.
- This isn't enough to reproduce the output—we would need to record
- the version numbers of the input files and the <code>goonorm</code> and <code>goofilter</code> programs,
- and the values of the parameters those programs used
- in order to do that—but it's an important and useful first step.
- </p>
-
- <div class="keypoints">
- <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>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>
-
- <div class="challenges">
- <h3>Challenges</h3>
-
- <ol>
-
- <li>
- Add <code>$Id:$</code> to a file,
- use <code>svn propset</code> to set the corresponding property,
- and then commit a change to the file.
- What value does Subversion fill in for this keyword?
- When would you use this rather than <code>Revision</code> or <code>Author</code>?
- </li>
-
- <li>
- What does the <code>svn:ignore</code> property do when applied to a directory?
- When would you use it?
- </li>
-
- </ol>
-
- </div>
-
-</section>
-
-<section id="s:summary">
- <h2>Summing Up</h2>
-
- <p>
- In 2006,
- <a href="bib.html#mccullough-reproducibility">McCullough, McGeary, and Harrison</a>
- analyzed several years of
- the data and code archive of <cite>Journal of Money, Credit, and Banking</cite>,
- a prestigious journal with a mandatory archiving policy.
- Of 266 articles published during that time,
- 193 were empirical and should have had data and code deposited in the archive.
- Of those,
- only 69 actually had anything in the archive;
- Excluding eleven articles that only had data,
- and seven that required software or other resources they did not have,
- McCullough et al. were only able to replicate 14 of the remaining 186 articles.
- This doesn't mean that the other 92% were wrong,
- but it does mean there is no practical way to tell.
- </p>
-
- <p>
- By itself,
- version control doesn't making computational research reproducible.
- It <em>does</em> help,
- though,
- and also eliminates the frustration and wasted time caused by
- trying to figure out which emailed copy of a file,
- or which of a dozen directories or USB drives,
- is the most recent.
- And while correlation doesn't imply causality,
- there is certainly a strong correlation between
- knowing enough about good computational practices to use version control
- and knowing how to do other things right as well.
- </p>
-
-</section>
-{% endblock content %}
projects / swc-workshop.git / commitdiff