python中默认情况下是等待子进程完成之后，一下读取子进程的输出，要想逐行读取，需要自己做一点小技巧。

import sys,os,socket
import datetime
import fcntl
import subprocess
from threading import Thread

def log_worker(stdout):
    ''' needs to be in a thread so we can read the stdout w/o blocking '''
    username, hostname = os.environ.get('USER'), socket.gethostname()
    log_file = '/var/log/mysql-%s.log' % username
    log = open(log_file, 'a')
    while True:
        output = non_block_read(stdout).strip()
        if output:
            ''' [Tue Oct 30 22:13:13 2012 cseibert@host1]> '''
            prompt = '[%(timestamp)s %(username)s@%(host)s]> \n' % dict(
                    timestamp=datetime.datetime.now().strftime('%a %b %d %H:%M:%S %Y'),
                    username=username,
                    host=hostname)
            print prompt + output
            log.write(prompt + output + '\n')
    log.close()

def non_block_read(output):
    ''' even in a thread, a normal read with block until the buffer is full '''
    fd = output.fileno()
    fl = fcntl.fcntl(fd, fcntl.F_GETFL)
    fcntl.fcntl(fd, fcntl.F_SETFL, fl | os.O_NONBLOCK)
    try:
        return output.read()
    except:
        return ''

if __name__ == '__main__':
    sub_process = subprocess.Popen(
        ['sh', '/root/test/dummy.sh'],
        stdin=sys.stdin,
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE)

    thread = Thread(target=log_worker, args=[sub_process.stdout])
    thread.daemon = True
    thread.start()

    sub_process.wait()
    thread.join(timeout=1)