supertee

#!/usr/bin/env bash

NLINES=2
IFS='' # force `read line` to take whole lines
LOG_PREFIX=$1

if test X$LOG_PREFIX = X; then
    echo usage: $0 "<log-prefix>"
    exit 1
fi

I=0
N=0

cat | grep foo | while read line; do
    if test $I = $NLINES; then
        mv $LOG_PREFIX$N.working.log $LOG_PREFIX$N.log
        I=0
        N=$(expr $N + 1)
    fi
    echo $line >> $LOG_PREFIX$N.working.log
    I=$(expr $I + 1)
done