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<p>This text is a work in progress—highly subject to
change—and may not accurately describe any released
version of the Apache™ Subversion® software.
Bookmarking or otherwise referring others to this page is
probably not such a smart idea. Please visit
<a href="http://www.svnbook.com/">http://www.svnbook.com/</a>
for stable versions of this book.</p>
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<th colspan="3" align="center">File Portability</th>
</tr>
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<th width="60%" align="center">Chapter 3. Advanced Topics</th>
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<div class="sect1">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="svn.advanced.props.file-portability"></a>File Portability</h2>
</div>
</div>
</div>
<p>Fortunately for Subversion users who routinely find
themselves on different computers with different operating
systems, Subversion's command-line program behaves almost
identically on all those systems. If you know how to wield
<span class="command"><strong>svn</strong></span> on one platform, you know how to wield it
everywhere.</p>
<p>However, the same is not always true of other general classes
of software or of the actual files you keep in Subversion. For
example, on a Windows machine, the definition of a <span class="quote">“<span class="quote">text
file</span>”</span> would be similar to that used on a Linux box, but
with a key difference—the character sequences used to mark
the ends of the lines of those files. There are other
differences, too. Unix platforms have (and Subversion supports)
symbolic links; Windows does not. Unix platforms use filesystem
permission to determine executability; Windows uses filename
extensions.</p>
<p>Because Subversion is in no position to unite the whole
world in common definitions and implementations of all of these
things, the best it can do is to try to help make your life
simpler when you need to work with your versioned files and
directories on multiple computers and operating systems. This
section describes some of the ways Subversion does this.</p>
<div class="sect2">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="svn.advanced.props.special.mime-type"></a>File Content Type</h3>
</div>
</div>
</div>
<p>Subversion joins the ranks of the many applications that
recognize and make use of Multipurpose Internet Mail
Extensions (MIME) content types. Besides being a
general-purpose storage location for a file's content type,
the value of the <code class="literal">svn:mime-type</code> file
property determines some behavioral characteristics of
Subversion itself.</p>
<div class="sidebar">
<div class="titlepage">
<div>
<div>
<p class="title">
<strong>Identifying File Types</strong>
</p>
</div>
</div>
</div>
<p>Various programs on most modern operating systems make
assumptions about the type and format of the contents of a
file by the file's name, specifically its file extension.
For example, files whose names end in
<code class="filename">.txt</code> are generally assumed to be
human-readable; that is, able to be understood by simple perusal
rather than requiring complex processing to decipher. Files
whose names end in <code class="filename">.png</code>, on the other
hand, are assumed to be of the Portable Network Graphics
type—not human-readable at all, and sensible only when
interpreted by software that understands the PNG format and
can render the information in that format as a raster
image.</p>
<p>Unfortunately, some of those extensions have changed
their meanings over time. When personal computers first appeared,
a file named <code class="filename">README.DOC</code> would have
almost certainly been a plain-text file, just like today's
<code class="filename">.txt</code> files. But by the mid-1990s, you
could almost bet that a file of that name would not be a
plain-text file at all, but instead a Microsoft Word
document in a proprietary, non-human-readable format. But
this change didn't occur overnight—there was certainly
a period of confusion for computer users over what exactly
they had in hand when they saw a <code class="filename">.DOC</code>
file.<a href="#ftn.idm2957" class="footnote" id="idm2957"><sup class="footnote">[19]</sup></a></p>
<p>The popularity of computer networking cast still more
doubt on the mapping between a file's name and its content.
With information being served across networks and generated
dynamically by server-side scripts, there was often no real
file per se, and therefore no filename. Web
servers, for example, needed some other way to tell browsers
what they were downloading so that the browser could do something
intelligent with that information, whether that was to
display the data using a program registered to handle that
datatype or to prompt the user for where on the client
machine to store the downloaded data.</p>
<p>Eventually, a standard emerged for, among other things,
describing the contents of a data stream. In 1996, RFC 2045
was published. It was the first of five RFCs describing
MIME. It describes the concept of media types and subtypes
and recommends a syntax for the representation of those
types. Today, MIME media types—or <span class="quote">“<span class="quote">MIME
types</span>”</span>—are used almost universally across
email applications, web servers, and other software as the
de facto mechanism for clearing up the file content
confusion.</p>
</div>
<p>For example, one of the benefits that Subversion typically
provides is contextual, line-based merging of changes received
from the server during an update into your working file. But
for files containing nontextual data, there is often no
concept of a <span class="quote">“<span class="quote">line.</span>”</span> So, for versioned files
whose <code class="literal">svn:mime-type</code> property is set to a
nontextual MIME type (generally, something that doesn't begin
with <code class="literal">text/</code>, though there are exceptions),
Subversion does not attempt to perform contextual merges
during updates. Instead, any time you have locally modified a
binary working copy file that is also being updated, your file
is left untouched and Subversion creates two new files. One
file has a <code class="filename">.oldrev</code> extension and contains
the BASE revision of the file. The other file has a
<code class="filename">.newrev</code> extension and contains the
contents of the updated revision of the file. This behavior
is really for the protection of the user against failed
attempts at performing contextual merges on files that simply
cannot be contextually merged.</p>
<p>Additionally, since the acts of displaying line-based
differences and line-based change attribution are, rather
obviously, dependent on there being a meaningful definition
of <span class="quote">“<span class="quote">line</span>”</span> for a given file, files with nontextual
MIME types will by default trigger errors when used as the
targets of <span class="command"><strong>svn diff</strong></span> and <span class="command"><strong>svn
annotate</strong></span> operations. This can be especially
frustrating for users with XML files whose
<code class="literal">svn:mime-type</code> property is set to something
such as <code class="literal">application/xml</code> which is not
unambiguously human-readable and as such is treated as
nontextual by Subversion. Fortunately, those subcommands
offer a <code class="option">--force</code> option for forcing
Subversion to attempt the operations in spite of the apparent
non-human-readability of the files.</p>
<div class="warning" style="margin-left: 0.5in; margin-right: 0.5in;">
<table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25">
<img alt="[Warning]" src="images/warning.png" />
</td>
<th align="left">Warning</th>
</tr>
<tr>
<td align="left" valign="top">
<p>The <code class="literal">svn:mime-type</code> property, when set
to a value that does not indicate textual file contents, can
cause some unexpected behaviors with respect to other
properties. For example, since the idea of line endings
(and therefore, line-ending conversion) makes no sense when
applied to nontextual files, Subversion will prevent you
from setting the <code class="literal">svn:eol-style</code> property
on such files. This is obvious when attempted on a single
file target—<span class="command"><strong>svn propset</strong></span> will error
out. But it might not be as clear if you perform a
recursive property set, where Subversion will silently skip
over files that it deems unsuitable for a given
property.</p>
</td>
</tr>
</table>
</div>
<p>Subversion provides a number of mechanisms by which to
automatically set the <code class="literal">svn:mime-type</code>
property on a versioned file. See
<a class="xref" href="svn.advanced.props.html#svn.advanced.props.auto" title="Automatic Property Setting">the section called “Automatic Property Setting”</a> for details.</p>
<p>Also, if the <code class="literal">svn:mime-type</code> property is
set, then the Subversion Apache module will use its value to
populate the <code class="literal">Content-type:</code> HTTP header when
responding to GET requests. This gives your web browser a
crucial clue about how to display a file when you use it to
peruse your Subversion repository's contents.</p>
</div>
<div class="sect2">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="svn.advanced.props.special.executable"></a>File Executability</h3>
</div>
</div>
</div>
<p>On many operating systems, the ability to execute a file
as a command is governed by the presence of an execute
permission bit. This bit usually defaults to being disabled,
and must be explicitly enabled by the user for each file that
needs it. But it would be a monumental hassle to have to
remember exactly which files in a freshly checked-out working
copy were supposed to have their executable bits toggled on,
and then to have to do that toggling. So, Subversion provides
the <code class="literal">svn:executable</code> property as a way to
specify that the executable bit for the file on which that
property is set should be enabled, and Subversion honors that
request when populating working copies with such files.</p>
<p>This property has no effect on filesystems that have no
concept of an executable permission bit, such as FAT32 and
NTFS.<a href="#ftn.idm2992" class="footnote" id="idm2992"><sup class="footnote">[20]</sup></a> Also, although it has no defined
values, Subversion will force its value
to <code class="literal">*</code> when setting this property. Finally,
this property is valid only on files, not on
directories.</p>
</div>
<div class="sect2">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="svn.advanced.props.special.eol-style"></a>End-of-Line Character Sequences</h3>
</div>
</div>
</div>
<p>Unless otherwise noted using a versioned file's
<code class="literal">svn:mime-type</code> property, Subversion
assumes the file contains human-readable data. Generally
speaking, Subversion uses this knowledge only to determine
whether contextual difference reports for that file are
possible. Otherwise, to Subversion, bytes are bytes.</p>
<p>
<a id="idm3003" class="indexterm"></a>
<a id="idm3005" class="indexterm"></a>This means that by default, Subversion doesn't pay
any attention to the type of <em class="firstterm">end-of-line (EOL)
markers</em> used in your files. Unfortunately,
different operating systems have different conventions about
which character sequences represent the end of a line of text
in a file. For example, the usual line-ending token used by
software on the Windows platform is a pair of ASCII control
characters—a carriage return (<code class="literal">CR</code>)
followed by a line feed (<code class="literal">LF</code>). Unix
software, however, just uses the <code class="literal">LF</code>
character to denote the end of a line.</p>
<p>
<a id="idm3013" class="indexterm"></a>Not all of the various tools on these operating
systems understand files that contain line endings in a format
that differs from the <em class="firstterm">native line-ending
style</em> of the operating system on which they are
running. So, typically, Unix programs treat the
<code class="literal">CR</code> character present in Windows files as a
regular character (usually rendered as <code class="literal">^M</code>),
and Windows programs combine all of the lines of a Unix file
into one giant line because no <code class="literal">CR</code>
characters are found to denote the ends of the lines.</p>
<p>This sensitivity to foreign EOL markers can be
frustrating for folks who share a file across different
operating systems. For example, consider a source code
file, and developers who edit this file on both Windows and
Unix systems. If all the developers always use tools that
preserve the line-ending style of the file, no problems
occur.</p>
<p>But in practice, many common tools either fail to
properly read a file with foreign EOL markers, or
convert the file's line endings to the native style when the
file is saved. If the former is true for a developer, he
has to use an external conversion utility (such as
<span class="command"><strong>dos2unix</strong></span> or its companion,
<span class="command"><strong>unix2dos</strong></span>) to prepare the file for
editing. The latter case requires no extra preparation.
But both cases result in a file that differs from the
original quite literally on every line! Prior to committing
his changes, the user has two choices. Either he can use a
conversion utility to restore the modified file to the same
line-ending style that it was in before his edits were made,
or he can simply commit the file—new EOL markers and
all.</p>
<p>The result of scenarios like these include wasted time
and unnecessary modifications to committed files. Wasted
time is painful enough. But when commits change every line
in a file, this complicates the job of determining which of
those lines were changed in a nontrivial way. Where was
that bug really fixed? On what line was a syntax error
introduced?</p>
<p>The solution to this problem is the
<code class="literal">svn:eol-style</code> property. When this
property is set to a valid value, Subversion uses it to
determine what special processing to perform on the file so
that the file's line-ending style isn't flip-flopping with
every commit that comes from a different operating
system. The valid values are:</p>
<div class="variablelist">
<dl class="variablelist">
<dt>
<span class="term">
<code class="literal">native</code>
</span>
</dt>
<dd>
<p>This causes the file to contain the EOL markers
that are native to the operating system on which
Subversion was run. In other words, if a user on a
Windows machine checks out a working copy that
contains a file with an
<code class="literal">svn:eol-style</code> property set to
<code class="literal">native</code>, that file will contain
<code class="literal">CRLF</code> EOL markers. A Unix user
checking out a working copy that contains the same
file will see <code class="literal">LF</code> EOL markers in his
copy of the file.</p>
<p>Note that Subversion will actually store the file
in the repository using normalized
<code class="literal">LF</code> EOL markers regardless of the
operating system. This is basically transparent to
the user, though.</p>
</dd>
<dt>
<span class="term">
<code class="literal">CRLF</code>
</span>
</dt>
<dd>
<p>This causes the file to contain
<code class="literal">CRLF</code> sequences for EOL markers,
regardless of the operating system in use.</p>
</dd>
<dt>
<span class="term">
<code class="literal">LF</code>
</span>
</dt>
<dd>
<p>This causes the file to contain
<code class="literal">LF</code> characters for EOL markers,
regardless of the operating system in use.</p>
</dd>
<dt>
<span class="term">
<code class="literal">CR</code>
</span>
</dt>
<dd>
<p>This causes the file to contain
<code class="literal">CR</code> characters for EOL markers,
regardless of the operating system in use. This
line-ending style is not very common.</p>
</dd>
</dl>
</div>
</div>
<div class="footnotes">
<br />
<hr style="width:100; text-align:left;margin-left: 0" />
<div id="ftn.idm2957" class="footnote">
<p><a href="#idm2957" class="para"><sup class="para">[19] </sup></a>You think that was rough? During that
same era, WordPerfect also used <code class="filename">.DOC</code>
for their proprietary file format's preferred
extension!</p>
</div>
<div id="ftn.idm2992" class="footnote">
<p><a href="#idm2992" class="para"><sup class="para">[20] </sup></a>The Windows filesystems use file
extensions (such
as <code class="filename">.EXE</code>, <code class="filename">.BAT</code>, and
<code class="filename">.COM</code>) to denote executable
files.</p>
</div>
</div>
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<p>You are reading <em>Version Control with Subversion</em> (for
Subversion 1.8), by Ben Collins-Sussman, Brian W. Fitzpatrick,
and C. Michael Pilato.</p>
<p>This work is licensed under
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