Sometimes it is useful to construct a working copy that is made out of a number of different checkouts. For example, you may want different subdirectories to come from different locations in a repository or perhaps from different repositories altogether. You could certainly set up such a scenario by hand—using svn checkout to create the sort of nested working copy structure you are trying to achieve. But if this layout is important for everyone who uses your repository, every other user will need to perform the same checkout operations that you did.

Fortunately, Subversion provides support for externals definitions. An externals definition is a mapping of a local directory to the URL—and ideally a particular revision—of a versioned directory. In Subversion, you declare externals definitions in groups using the svn:externals property. You can create or modify this property using svn propset or svn propedit (see the section called “Manipulating Properties”). It can be set on any versioned directory, and its value describes both the external repository location and the client-side directory to which that location should be checked out.

The convenience of the svn:externals property is that once it is set on a versioned directory, everyone who checks out a working copy with that directory also gets the benefit of the externals definition. In other words, once one person has made the effort to define the nested working copy structure, no one else has to bother—Subversion will, after checking out the original working copy, automatically also check out the external working copies.

	Warning
	The relative target subdirectories of externals definitions must not already exist on your or other users' systems—Subversion will create them when it checks out the external working copy.

You also get in the externals definition design all the regular benefits of Subversion properties. The definitions are versioned. If you need to change an externals definition, you can do so using the regular property modification subcommands. When you commit a change to the svn:externals property, Subversion will synchronize the checked-out items against the changed externals definition when you next run svn update. The same thing will happen when others update their working copies and receive your changes to the externals definition.

	Tip
	Because the svn:externals property has a multiline value, we strongly recommend that you use svn propedit instead of svn propset.

Subversion releases prior to 1.5 honor an externals definition format that is a multiline table of subdirectories (relative to the versioned directory on which the property is set), optional revision flags, and fully qualified, absolute Subversion repository URLs. An example of this might look as follows:

$ svn propget svn:externals calc
# Resources versioned elsewhere.
third-party/sounds             http://svn.example.com/repos/sounds
third-party/skins -r148        http://svn.example.com/skinproj
third-party/skins/toolkit -r21 http://svn.example.com/skin-maker
$

In the previous example, three externals have been defined. The first is “unpinned”—it effectively refers to the HEAD revision of its target. The other two have specific revisions declared. Also demonstrated is the fact that lines that begin with a hash (#) are treated as comments and ignored by the externals definitions parsing logic.

When someone checks out a working copy of the calc directory referred to in the previous example, Subversion also continues to check out the items found in its externals definition.

$ svn checkout http://svn.example.com/repos/calc
A    calc
A    calc/Makefile
A    calc/integer.c
A    calc/button.c
Checked out revision 148.

Fetching external item into calc/third-party/sounds
A    calc/third-party/sounds/ding.ogg
A    calc/third-party/sounds/dong.ogg
A    calc/third-party/sounds/clang.ogg
…
A    calc/third-party/sounds/bang.ogg
A    calc/third-party/sounds/twang.ogg
Checked out revision 14.

Fetching external item into calc/third-party/skins
…

As of Subversion 1.5, though, a new format of the svn:externals property is supported. Externals definitions are still multiline, but the order and format of the various pieces of information have changed. The new syntax more closely mimics the order of arguments you might pass to svn checkout: the optional revision flags come first, then the external Subversion repository URL, and finally the relative local subdirectory. Notice, though, that this time we didn't say “fully qualified, absolute Subversion repository URLs.” That's because the new format supports relative URLs and URLs that carry peg revisions. The previous example of an externals definition might, in Subversion 1.5, look like the following:

$ svn propget svn:externals calc
# Resources versioned elsewhere.
      http://svn.example.com/repos/sounds third-party/sounds
-r148 http://svn.example.com/skinproj third-party/skins
-r21  http://svn.example.com/skin-maker third-party/skins/toolkit
$

Or, making use of the peg revision syntax (which we describe in detail in the section called “Peg and Operative Revisions”), it might appear as:

$ svn propget svn:externals calc
# Resources versioned elsewhere.
http://svn.example.com/repos/sounds third-party/sounds
http://svn.example.com/skinproj@148 third-party/skins
http://svn.example.com/skin-maker@21 third-party/skins/toolkit
$

Tip

You should seriously consider using explicit revision numbers in all of your externals definitions. Doing so means that you get to decide when to pull down a different snapshot of external information, and exactly which snapshot to pull. Besides avoiding the surprise of getting changes to third-party repositories that you might not have any control over, using explicit revision numbers also means that as you backdate your working copy to a previous revision, your externals definitions will also revert to the way they looked in that previous revision, which in turn means that the external working copies will be updated to match the way they looked back when your repository was at that previous revision. For software projects, this could be the difference between a successful and a failed build of an older snapshot of your complex codebase.

For most repositories, these three ways of formatting the externals definitions have the same ultimate effect. They all bring the same benefits. Unfortunately, they all bring the same annoyances, too. Since the definitions shown use absolute URLs, moving or copying a directory to which they are attached will not affect what gets checked out as an external (though the relative local target subdirectory will, of course, move with the renamed directory). This can be confusing—even frustrating—in certain situations. For example, say you have a top-level directory named my-project, and you've created an externals definition on one of its subdirectories (my-project/some-dir) that tracks the latest revision of another of its subdirectories (my-project/external-dir).

$ svn checkout http://svn.example.com/projects .
A    my-project
A    my-project/some-dir
A    my-project/external-dir
…
Fetching external item into 'my-project/some-dir/subdir'
Checked out external at revision 11.

Checked out revision 11.
$ svn propget svn:externals my-project/some-dir
subdir http://svn.example.com/projects/my-project/external-dir

$

Now you use svn move to rename the my-project directory. At this point, your externals definition will still refer to a path under the my-project directory, even though that directory no longer exists.

$ svn move -q my-project renamed-project
$ svn commit -m "Rename my-project to renamed-project."
Deleting       my-project
Adding         renamed-project

Committed revision 12.
$ svn update
Updating '.':

svn: warning: W200000: Error handling externals definition for 'renamed-projec
t/some-dir/subdir':
svn: warning: W170000: URL 'http://svn.example.com/projects/my-project/externa
l-dir' at revision 12 doesn't exist
At revision 12.
svn: E205011: Failure occurred processing one or more externals definitions
$

Also, absolute URLs can cause problems with repositories that are available via multiple URL schemes. For example, if your Subversion server is configured to allow everyone to check out the repository over http:// or https://, but only allow commits to come in via https://, you have an interesting problem on your hands. If your externals definitions use the http:// form of the repository URLs, you won't be able to commit anything from the working copies created by those externals. On the other hand, if they use the https:// form of the URLs, anyone who might be checking out via http:// because his client doesn't support https:// will be unable to fetch the external items. Be aware, too, that if you need to reparent your working copy (using svn relocate), externals definitions will not also be reparented.

Subversion 1.5 takes a huge step in relieving these frustrations. As mentioned earlier, the URLs used in the new externals definition format can be relative, and Subversion provides syntax magic for specifying multiple flavors of URL relativity.

So, looking a fourth time at our previous externals definition example, and making use of the new absolute URL syntax in various ways, we might now see:

$ svn propget svn:externals calc
# Resources versioned elsewhere.
^/sounds third-party/sounds
/skinproj@148 third-party/skins
//svn.example.com/skin-maker@21 third-party/skins/toolkit
$

Subversion 1.6 brought two more improvements to externals definitions. First, it added a quoting and escape mechanism to the syntax so that the path of the external working copy may contain whitespace. This was previously problematic, of course, because whitespace is used to delimit the fields in an externals definition. Now you need only wrap such a path specification in double-quote (") characters or escape the problematic characters in the path with a backslash (\) character. Of course, if you have spaces in the URL portion of the external definition, you should use the standard URI-encoding mechanism to represent those.

$ svn propget svn:externals paint
http://svn.thirdparty.com/repos/My%20Project "My Project"
http://svn.thirdparty.com/repos/%22Quotes%20Too%22 \"Quotes\ Too\"
$

Subversion 1.6 also introduced support for external definitions for files. File externals are configured just like externals for directories and appear as a versioned file in the working copy.

For example, let's say you had the file /trunk/bikeshed/blue.html in your repository, and you wanted this file, as it appeared in revision 40, to appear in your working copy of /trunk/www/ as green.html.

The externals definition required to achieve this should look familiar by now:

$ svn propget svn:externals www/
^/trunk/bikeshed/blue.html@40 green.html
$ svn update
Updating '.':

Fetching external item into 'www'
E    www/green.html
Updated external to revision 40.

Update to revision 103.
$ svn status
    X   www/green.html
$

As you can see in the previous output, Subversion denotes file externals with the letter E when they are fetched into the working copy, and with the letter X when showing the working copy status.

	Warning
	While directory externals can place the external directory at any depth, and any missing intermediate directories will be created, file externals must be placed into a working copy that is already checked out.

When examining the file external with svn info, you can see the URL and revision the external is coming from:

$ svn info www/green.html 
Path: www/green.html
Name: green.html
Working Copy Root Path: /home/harry/projects/my-project
URL: http://svn.example.com/projects/my-project/trunk/bikeshed/blue.html
Relative URL: ^/trunk/bikeshed/blue.html
Repository Root: http://svn.example.com/projects/my-project
Repository UUID: b2a368dc-7564-11de-bb2b-113435390e17
Revision: 40
Node kind: file
Schedule: normal
Last Changed Author: harry
Last Changed Rev: 40
Last Changed Date: 2009-07-20 20:38:20 +0100 (Mon, 20 Jul 2009)
Text Last Updated: 2009-07-20 23:22:36 +0100 (Mon, 20 Jul 2009)
Checksum: 01a58b04617b92492d99662c3837b33b
$

Because file externals appear in the working copy as versioned files, they can be modified and even committed if they reference a file at the HEAD revision. The committed changes will then appear in the external as well as the file referenced by the external. However, in our example, we pinned the external to an older revision, so attempting to commit the external fails:

$ svn status
M   X   www/green.html
$ svn commit -m "change the color" www/green.html
Sending        www/green.html
svn: E155011: Commit failed (details follow):
svn: E155011: File '/trunk/bikeshed/blue.html' is out of date
$

Keep this in mind when defining file externals. If you need the external to refer to a certain revision of a file you will not be able to modify the external. If you want to be able to modify the external, you cannot specify a revision other than the HEAD revision, which is implied if no revision is specified.

Unfortunately, the support which exists for externals definitions in Subversion remains less than ideal. Both file and directory externals have shortcomings. For either type of external, the local subdirectory part of the definition cannot contain .. parent directory indicators (such as ../../skins/myskin). File externals cannot refer to files from other repositories. A file external's URL must always be in the same repository as the URL that the file external will be inserted into. Also, file externals cannot be moved or deleted. The svn:externals property must be modified instead. However, file externals can be copied.

Perhaps most disappointingly, the working copies created via the externals definition support are still disconnected from the primary working copy (on whose versioned directories the svn:externals property was actually set). And Subversion still truly operates only on nondisjoint working copies. So, for example, if you want to commit changes that you've made in one or more of those external working copies, you must run svn commit explicitly on those working copies—committing on the primary working copy will not recurse into any external ones.

We've already mentioned some of the additional shortcomings of the old svn:externals format and how the newer Subversion 1.5 format improves upon it. But be careful when making use of the new format that you don't inadvertently introduce new problems. For example, while the latest clients will continue to recognize and support the original externals definition format, pre-1.5 clients will not be able to correctly parse the new format. If you change all your externals definitions to the newer format, you effectively force everyone who uses those externals to upgrade their Subversion clients to a version that can parse them. Also, be careful to avoid naively relocating the -rNNN portion of the definition—the older format uses that revision as a peg revision, but the newer format uses it as an operative revision (with a peg revision of HEAD unless otherwise specified; see the section called “Peg and Operative Revisions” for a full explanation of the distinction here).

Warning

External working copies are still completely self-sufficient working copies. You can operate directly on them as you would any other working copy. This can be a handy feature, allowing you to examine an external working copy independently of any primary working copy whose svn:externals property caused its instantiation. Be careful, though, that you don't inadvertently modify your external working copy in subtle ways that cause problems. For example, while an externals definition might specify that the external working copy should be held at a particular revision number, if you run svn update directly on the external working copy, Subversion will oblige, and now your external working copy is out of sync with its declaration in the primary working copy. Using svn switch to directly switch the external working copy (or some portion thereof) to another URL could cause similar problems if the contents of the primary working copy are expecting particular contents in the external content.

Besides the svn checkout, svn update, svn switch, and svn export commands which actually manage the disjoint (or disconnected) subdirectories into which externals are checked out, the svn status command also recognizes externals definitions. It displays a status code of X for the disjoint external subdirectories, and then recurses into those subdirectories to display the status of the external items themselves. You can pass the --ignore-externals option to any of these subcommands to disable externals definition processing.