Python Paramiko Client SSH very slow when read stdout

Question

I am using Paramiko to control a VM over SSH. When I send any command it executes almost instantly but when reading the output from stdout it take forever.

I get around 5 seconds for a ls to read:

Time to execute command: 0.1445319652557373

Time to read output: 5.382704973220825

Here is a snippit:

import time
import paramiko

ssh = paramiko.SSHClient()
ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
ssh.connect(hostname="192.168.0.123",
        port=1234,
        username="admin",
        password="admin")

t1 = time.time()
stdin, stdout, stderr = ssh.exec_command("powershell -command \"ls -recurse .\"")
t2 = time.time()

t3 = time.time()
print(stdout.readlines())
t4 = time.time()

print(t2 - t1)
print(t4 - t3)

Thank you!

The exec_command does not execute the command completely. It only starts the execution. It's actually the readlines that waits for the command to complete. See Paramiko with continuous stdout. Or do you have any proof that Paramiko is actually reading the output slower than other SSH clients? How long does it take to execute ssh [email protected] powershell -command "ls -recurse ."? — Martin Prikryl
– Martin Prikryl, Commented Jun 17, 2021 at 13:35

David Meu · Accepted Answer · 2021-06-17 12:58:38Z

1

stdout.readlines() Is reading all lines before your subprocess finishes and sends EOF to your main process. If you want to read line by line when available, then do for line in exp.stdout, the file object allows you to iterate over each line as its feed through the pipe. Otherwise it would probably be better to use stdout, stderr = Subprocess.communicate(None) as the communicate method uses a better system calls to read data from a Subprocess (like epoll, select, poll).

answered Jun 17, 2021 at 12:58

David Meu

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3 Comments

Martin Prikryl Over a year ago

How can you use Subprocess.communicate with SSH?

David Meu Over a year ago

see: stackoverflow.com/questions/26227791/ssh-with-subprocess-popen

Martin Prikryl Over a year ago

OK, but that's a completely different approach (and inferior to the one taken by OP).

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