SVN - How to Export Just a Revision
You can specify the folder/files that you want to export only, like
such as using wp svn
svn export -r 16873 http://core.svn.wordpress.org/branches/2.8/wp-admin/cssIf just few files but located on different directories
mkdir {css,image}svn export -r 16873 http://core.svn.wordpress.org/branches/2.8/wp-admin/css/install.css css/install.csssvn export -r 16873 http://core.svn.wordpress.org/branches/2.8/wp-admin/images/menu-arrows.gif images/menu-arrows.gif
You shouldn't use svn export if you want to update the project, you should use svn commit that sends changes from your working copy to the repository.
I was wondering Why are you using the -r 31 option this stands for revisions and the amount of them which are 31. Probably the option you want to invoke is the -R option which stands for recursing the directories.