A target to list categories for posts

Makefile

@@ -10,6 +10,10 @@ all : commands
 commands :
 	@grep -E '^##' Makefile | sed -e 's/## //g'
 
+## categories : show known categories
+categories :
+	@python bin/categories.py $(POSTS)
+
 ## check      : build locally into _site directory for checking
 check :
 	make OUT=$(PWD)/_site build

bin/categories.py

@@ -0,0 +1,20 @@
+#!/usr/bin/env python
+
+"""
+Display all the categories used in blog posts.
+"""
+
+import sys
+import re
+
+cat_p = re.compile('categories:\s*\[(.+)\]')
+
+categories = set()
+for filename in sys.argv[1:]:
+    with open(filename, 'r') as reader:
+        data = reader.read()
+    m = cat_p.search(data)
+    assert m, "No categories match for %s" % filename
+    categories.update([x.strip() for x in m.group(1).split(',')])
+for c in sorted(categories):
+    print c