Find files and search for within tail

Just add the prefix in the -name argument.

find /Applications/MAMP/htdocs/css/ -type f -name "include_*.css" -exec cat {} + > /temporary/css_backend_merged.css

Something like this?

mkfifo child.fifo
tail -f child.out > child.fifo &
pid=$!
grep -q B child.fifo && kill $pid

in full:

#!/bin/sh    
rm -f child.out
./child.sh > child.out &
mkfifo child.fifo
tail -f child.out > child.fifo &
pid=$!
grep -q B child.fifo && kill $pid
rm child.fifo

Seems to run in 20 seconds.

$ time ./test2.sh

real    0m20.156s
user    0m0.033s
sys     0m0.058s

UPDATE

This way seems to work too:

#!/bin/sh
rm -f child.out
./child.sh > child.out &
(tail -f child.out | grep -q B child.out)

And if you're seeing it exit immediately sometimes, try adding a sleep 1, i.e.

#!/bin/sh
rm -f child.out
./child.sh > child.out &
sleep 1
(tail -f child.out | grep -q B child.out)