Updated: October 6th, 2009
Every time I have to deal with svn:externals in SVN, I forget the command line syntax. Every single damn time. Normally, I use SVN GUI clients, such as SmartSVN, which make it very simple to add an svn:externals property. But for command line, it always takes looking at 25 different sites on google, which are all incredibly unhelpful for this question for some reason. Trying "svn help propset" on the command line was bloated and equally useless.
So this time I needed to write it down and make sure everyone who needed help with svn:externals would find exactly what they need here. I hope this page will soon come up on top of all the unhelpful results on google for "propset svn:externals" and other related queries.
I want to set a simple svn:externals property in one of my project's directories, lets say 'plugins' (talking about WordPress here). The outcome would be a directory called 'akismet' within 'plugins' that points to a remote svn url.
Various combinations of trying to do it produced pathetic results, like
svn propset svn:externals akismet <a href="http://plugins.svn.wordpress.org/akismet/trunk">http://plugins.svn.wordpress.org/akismet/trunk</a> svn: Setting property on non-local target 'http://plugins.svn.wordpress.org/akismet/trunk' needs a base revision
svn propset svn:externals . akismet http://plugins.svn.wordpress.org/akismet/trunk svn: Error parsing svn:externals property on 'akismet': '.'
svn propset svn:externals akismet http://plugins.svn.wordpress.org/akismet/trunk akismet svn: Setting property on non-local target 'http://plugins.svn.wordpress.org/akismet/trunk' needs a base revision
Finally, thanks to this post, I found the right command:
svn propset svn:externals 'akismet http://plugins.svn.wordpress.org/akismet/trunk' . property 'svn:externals' set on '.'
Note that dot at the end of the command and the quotes around the directory name and url.
Now commit via
svn up Fetching external item into 'akismet' A    akismet/akismet.gif A    akismet/akismet.php A    akismet/readme.txt Updated external to revision 127962. Updated to revision 16.
There, was it that hard, forum gurus and blog fiends?
Edit: in order to set multiple directory/url pairs in a single svn:externals property, you should put the individual dir/url pairs into a file (let's call it 'svn.externals'), like so
akismet http://svn.wp-plugins.org/akismet/trunk all-in-one-seo-pack http://svn.wp-plugins.org/all-in-one-seo-pack/trunk
and then apply the property using
svn propset svn:externals -F svn.externals .
You should also just check in 'svn.externals' to easily keep track of it.
In the meantime, if you found this article useful, feel free to buy me a cup of coffee below.