gistfile1.txt

# Find all minutes during which there was at least one "502" status code:
grep ' 502 ' http.log | awk '{print $4}'  | cut -d: -f1-3 | uniq

# Find all minutes during which there were valid requests (excluding 502 and 400 status codes):
grep -v ' 502 ' http.log | grep -v ' 400 ' | awk '{print $4}'  | cut -d: -f1-3 | uniq

# If you want hours instead of minutes, replace f1-3 by f1-2.