Subversion has a nice way of including the content of one repository into another repository. This is useful e.g. when you have a repository with common routines that you want to use in multiple projects but you don't want to duplicate the common code in multiple repositories (for obvious reasons).

Using svn:externals property you can tell subversion to fetch content of external repository in a given directory. However, the documentation of svn:externals is weak so this post hopefully will save someone time googling and figuring out how it works.

You set svn:externals property on an existing directory. The value of this property is a list of directory/repository path values, separated by spaces.

The best way to explain this is using an example. Usually you would use just one external repository, but I'll use two in the example, just to show how to create a list.

Let's assume that you have external subversion repositories http://svn.my.com/path/to/repo_one and http://svn.my.com/path/to/repo_two

You want repo_one in directory dir_of_repo_one and repo_two in dir_of_repo_two. Create a text file with the value of svn:externals property. Each list item is in it's own line, dir name is separated from repo path by whitespace e.g.:

$ cat >svn_ext_val.txt
dir_of_repo_one http://svn.my.com/path/to/repo_one
dir_of_repo_two http://svn.my.com/path/to/repo_two

Now set the property on any directory already in subversion. In the example it's the current directory:

svn propset svn:externals . -F svn_ext_val.txt

Now when you do svn update, dir_of_repo_one will be created with the content of repo_one and dir_of_repo_two with the content of repo_two.

Voila! Pretty simple but you wouldn't know that just by reading the docs.