When I migrated to my new blog, I needed to re-write all the old WordPress URLs to point to the appropriate place. The problem was that standard rewrites aren’t made based on GET parameters, so I was stuck.

Luckily Curly060 and Seph in #nginx gave me some ideas, and together we ended up with this:

# Put this outside of server stanza
map $arg_cat $category {
  6 /blog/topic/security;
  11    /blog/topic/mac;
  8.*   /blog/topic/lisp;
  9.*   /blog/topic/code;
  3     /blog/topic/linux;
  14    /blog/topic/thoughts;
  10    /blog/topic/php;
  7     /blog/topic/security;
  12    /blog/topic/emacs;
  5     /blog/topic/projects;
  9     /blog/topic/code;
  13    /blog/topic/python;
  1     /blog;
  8     /blog/topic/lisp;

# Put this with other re-writes
if ($category) {
  rewrite ^ $category;

If you want to do a permanent redirect, you have to clear all the arguments to prevent a redirect loop. The last stanza would look something like this:

if ($category) {
  set $args "";
  rewrite ^ $category permanent;