Long-running ssh commands in python paramiko module (and how to end them)

Question

I want to run a tail -f logfile command on a remote machine using python's paramiko module. I've been attempting it so far in the following fashion:

interface = paramiko.SSHClient()
#snip the connection setup portion
stdin, stdout, stderr = interface.exec_command("tail -f logfile")
#snip into threaded loop
print stdout.readline()

I'd like the command to run as long as necessary, but I have 2 problems:

1. How do I stop this cleanly? I thought of making a Channel and then using the shutdown() command on the channel when I'm through with it- but that seems messy. Is it possible to do something like sent Ctrl-C to the channel's stdin?
2. readline() blocks, and I could avoid threads if I had a non-blocking method of getting output- any thoughts?

Answer 1

Instead of calling exec_command on the client, get hold of the transport and generate your own channel. The channel can be used to execute a command, and you can use it in a select statement to find out when data can be read:

#!/usr/bin/env python
import paramiko
import select
client = paramiko.SSHClient()
client.load_system_host_keys()
client.connect('host.example.com')
transport = client.get_transport()
channel = transport.open_session()
channel.exec_command("tail -f /var/log/everything/current")
while True:
  rl, wl, xl = select.select([channel],[],[],0.0)
  if len(rl) > 0:
      # Must be stdout
      print channel.recv(1024)

The channel object can be read from and written to, connecting with stdout and stdin of the remote command. You can get at stderr by calling channel.makefile_stderr(...).

I've set the timeout to 0.0 seconds because a non-blocking solution was requested. Depending on your needs, you might want to block with a non-zero timeout.

Answer 2

1) You can just close the client if you wish. The server on the other end will kill the tail process.

2) If you need to do this in a non-blocking way, you will have to use the channel object directly. You can then watch for both stdout and stderr with channel.recv_ready() and channel.recv_stderr_ready(), or use select.select.

Answer 3

Just a small update to the solution by Andrew Aylett. The following code actually breaks the loop and quits when the external process finishes:

import paramiko
import select

client = paramiko.SSHClient()
client.load_system_host_keys()
client.connect('host.example.com')
channel = client.get_transport().open_session()
channel.exec_command("tail -f /var/log/everything/current")
while True:
    if channel.exit_status_ready():
        break
    rl, wl, xl = select.select([channel], [], [], 0.0)
    if len(rl) > 0:
        print channel.recv(1024)

Answer 4

Just for information, there is a solution to do this using channel.get_pty(). Fore more details have a look at: https://stackoverflow.com/a/11190727/1480181

Answer 5

The way I've solved this is with a context manager. This will make sure my long running commands are aborted. The key logic is to wrap to mimic SSHClient.exec_command but capture the created channel and use a Timer that will close that channel if the command runs for too long.

import paramiko
import threading


class TimeoutChannel:

    def __init__(self, client: paramiko.SSHClient, timeout):
        self.expired = False
        self._channel: paramiko.channel = None
        self.client = client
        self.timeout = timeout

    def __enter__(self):
        self.timer = threading.Timer(self.timeout, self.kill_client)
        self.timer.start()

        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        print("Exited Timeout. Timed out:", self.expired)
        self.timer.cancel()

        if exc_val:
            return False  # Make sure the exceptions are re-raised

        if self.expired:
            raise TimeoutError("Command timed out")

    def kill_client(self):
        self.expired = True
        print("Should kill client")
        if self._channel:
            print("We have a channel")
            self._channel.close()

    def exec(self, command, bufsize=-1, timeout=None, get_pty=False, environment=None):
        self._channel = self.client.get_transport().open_session(timeout=timeout)
        if get_pty:
            self._channel.get_pty()
        self._channel.settimeout(timeout)
        if environment:
            self._channel.update_environment(environment)
        self._channel.exec_command(command)
        stdin = self._channel.makefile_stdin("wb", bufsize)
        stdout = self._channel.makefile("r", bufsize)
        stderr = self._channel.makefile_stderr("r", bufsize)
        return stdin, stdout, stderr

To use the code it's pretty simple now, the first example will throw a TimeoutError

ssh = paramiko.SSHClient()
ssh.connect('hostname', username='user', password='pass')

with TimeoutChannel(ssh, 3) as c:
    ssh_stdin, ssh_stdout, ssh_stderr = c.exec("cat")    # non-blocking
    exit_status = ssh_stdout.channel.recv_exit_status()  # block til done, will never complete because cat wants input

This code will work fine (unless the host is under insane load!)

ssh = paramiko.SSHClient()
ssh.connect('hostname', username='user', password='pass')

with TimeoutChannel(ssh, 3) as c:
    ssh_stdin, ssh_stdout, ssh_stderr = c.exec("uptime")    # non-blocking
    exit_status = ssh_stdout.channel.recv_exit_status()     # block til done, will complete quickly
    print(ssh_stdout.read().decode("utf8"))                 # Show results

Answer 6

To close the process simply run:

interface.close()

In terms of nonblocking, you can't get a non-blocking read. The best you would be able to to would be to parse over it one "block" at a time, "stdout.read(1)" will only block when there are no characters left in the buffer.