I am trying to extract a particular line from a.log which keeps appending every sec and output that into a newfile b.log which should append itself with filtered data received from a.log

I tried

tail -f a.log |grep fail| tee -a b.log

nothing in b.log

tail -f a.log |grep fail >>b.log

nothin shows up in b.log

what am i missing?

Is there an option similar to tail -f with awk?