test line buffer

1 . run this
     watch -n 1 "date >> /tmp/test" &
2.TEST THIS:
2.1. tail -3f /tmp/test
     every 1 sec print output, OK
2.2. tail -3f /tmp/test | grep --line-buffered 20
    every 1 sec print date, "20" match with "2016"
2.3. tail -3f /tmp/test | grep --line-buffered 20 | sed  -e 's/20/18/'
    every 1 sec print date and replace 20 to 18, or 2016 to 1816
2.4. tail -3f /tmp/test | grep --line-buffered 20 | sed -u -e 's/20/18/' | xargs -I {} echo {}
    every 1 sec print date, similar command in 2.3