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permission bit. This bit usually defaults to being disabled, and must be explicitly enabled by the user for each file
that needs it. In a working copy, new files are being created all the time as new versions of existing files are received
during an update. This means that you might enable the execute bit on a file, then update your working copy, and if
that file was changed as part of the update, its execute bit might get disabled. So, Subversion provides the
svn:executable property as a way to keep the execute bit enabled.
This property has no effect on filesystems that have no concept of an executable permission bit, such as FAT32 and
NTFS. 28 Also, although it has no defined values, Subversion will force its value to * when setting this property. Finally, this property is valid only on files, not on directories.
svn:mime-type
The svn:mime-type property serves many purposes in Subversion. Besides being a general-purpose storage location for a file's Multipurpose Internet Mail Extensions (MIME) classification, the value of this property determines
some behavioral characteristics of Subversion itself.
For example, if a file's svn:mime-type property is set to a non-text MIME type (generally, something that doesn't
begin with text/, though there are exceptions), Subversion will assume that the file contains binary—that is, not
human-readable—data. 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 believed to contain binary
data, there is no concept of a “line”. So, for those files, 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 renamed with a .orig extension, and then Subversion stores a new working copy file that contains the
changes received during the update, but not your own local modifications, at the original filename. 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.
Also, if the svn:mime-type property is set, then the Subversion Apache module will use its value to populate the
Content-type: HTTP header when responding to GET requests. This gives a crucial clue about how to display a
file when perusing your repository with a web browser.
svn:ignore
The svn:ignore property contains a list of file patterns which certain Subversion operations will ignore. Perhaps
the most commonly used special property, it works in conjunction with the global-ignores run-time configuration option (see Section , “Config”) to filter unversioned files and directories out of commands like svn status, svn
add, and svn import.
The rationale behind the svn:ignore property is easily explained. Subversion does not assume that every file or
subdirectory in a working copy directory is intended for version control. Resources must be explicitly placed under
Subversion's management using the svn add or svn import commands. As a result, there are often many resources
in a working copy that are not versioned.
Now, the svn status command displays as part of its output every unversioned file or subdirectory in a working
copy that is not already filtered out by the global-ignores option (or its built-in default value). This is done so
that users can see if perhaps they've forgotten to add a resource to version control.
But Subversion cannot possibly guess the names of every resource that should be ignored. Also, quite often there are
things that should be ignored in every working copy of a particular repository. To force every user of that repository
to add patterns for those resources to their run-time configuration areas would be not just a burden, but has the potential to clash with the configuration needs of other working copies that the user has checked out.
The solution is to store ignore patterns that are unique to the resources likely to appear in a given directory with the
directory itself. Common examples of unversioned resources that are basically unique to a directory, yet likely to appear there, include output from program compilations. Or—to use an example more appropriate to this book—the
HTML, PDF, or PostScript files generated as the result of a conversion of some source DocBook XML files to a
more legible output format.
28The Windows filesystems use file extensions (such as .EXE, .BAT, and .COM) to denote executable files.
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