Paramiko / ssh / tail + grep hangs

Question

Situation: I want to tail remote logs over ssh with paramiko.

channel.exec_command('tail -f log.log') works fine

channel.exec_command('tail -f log.log | grep "filter" ') hangs

Cannot understand reason why tail with grep hangs.

Code example:

import paramiko
import select
import re

interesting_line_pattern = re.compile('xxx')

def do_tail():
    client = paramiko.SSHClient()
    client.load_system_host_keys()
    client.set_missing_host_key_policy(paramiko.AutoAddPolicy())
    from os.path import expanduser
    home = expanduser("~")
    client.connect('xxx',
                   username='xxx',
                   key_filename='%s/.ssh/id_rsa' % home)

    log_file = '/home/xxx/log.log'
    grep_pattern = "grep_filter"
    remote_command = 'tail -f %s | grep "%s" ' % (log_file, grep_pattern)
    print remote_command

    transport = client.get_transport()
    channel = transport.open_session()
    channel.exec_command(remote_command)

    while 1:
        try:
            rl, _, _ = select.select([channel], [], [], 0.0)
            if len(rl) > 0:
                print "ready to read"
                for line in linesplit(channel):
                    print line

        except (KeyboardInterrupt, SystemExit):
            print 'got ctrl+c'
            break

    client.close()
    print 'client closed'

def linesplit(socket):
    buffer_string = socket.recv(4048)
    done = False
    while not done:
        if "\n" in buffer_string:
            (line, buffer_string) = buffer_string.split("\n", 1)
            yield line + "\n"
        else:
            more = socket.recv(4048)
            if not more:
                done = True
            else:
                buffer_string = buffer_string + more
    if buffer_string:
        yield buffer_string


if __name__ == '__main__':

    do_tail()

Answer 1

Let me share linesplit function update for python >= 3.6

def linesplit(socket):
buffer_bytes = socket.recv(4048)
buffer_string = buffer_bytes.decode()
done = False
while not done:
    if "\n" in buffer_string:
        (line, buffer_string) = buffer_string.split("\n", 1)
        yield line + "\n"
    else:
        more = socket.recv(4048)
        if not more:
            done = True
        else:
            buffer_string = buffer_string + more.decode()
if buffer_string:
    yield buffer_string

Answer 2

grep thinks it is not writing to a terminal, so it is buffering its output. Not enough lines of its input are matching for it to write any of its output, so you think it has hung.

Try giving grep the --line-buffered option. e.g.

channel.exec_command('tail -f log.log | grep --line-buffered "filter" ')