[newbie] problem with if then

[Jean Dubois]

I'm writing some code to check whether an url is available or not,
therefore I make use of a wget-command in Linux and then check whether
this is successful (returning a 0) or not returning an 8
However the if then statement seems to give the same result in both
cases:
Here is my code:

#!/usr/bin/env python
import sys
import os
from datetime import datetime, timedelta
today=datetime.now()
yesterday= datetime.now() - timedelta(days=1)
daybeforeyesterday= datetime.now() - timedelta(days=2)
collection = [daybeforeyesterday,yesterday,today]
for thisday in collection:
     checkavailablestring='wget -q -O -
http://www.deredactie.be/cm/vrtnieuws/videozone/programmas/journaal/EP_'+thisday.strftime("%y%m%d")+'_JO7
>/dev/null ; echo $?'
     if os.system(checkavailablestring)==0:
          print thisday, 'stream is available'
     else:
          print thisday, 'stream is not available'

Can anyone here tell me what I'm doing wrong.
Thanks in advance
jean