How to get stderr return from a command executed on remote machine using Paramiko?

Question

Please check the below code (I think it's self-explanatory).

Code :

import sys
import paramiko

def execute_command_on_remote_machine(ip_addr, command):
    try:
        client = paramiko.SSHClient()
        client.set_missing_host_key_policy(paramiko.AutoAddPolicy())

        client.connect(str(ip_addr), username='root', password='****')
        chan = client.get_transport().open_session()
        #chan.get_pty()

        stdin, stderr, stdout = [], [], []
        stdin, stdout, stderr = client.exec_command(command, get_pty=True)

        err_list = [line for line in stderr.read().splitlines()]
        out_list = [line for line in stdout.read().splitlines()]

        client.close()
        return out_list, err_list
    except Exception, e:
        print str(e)
        sys.exit(1)


output, error = execute_command_on_remote_machine("*.*.*.*", "dinesh")

print "Output is - ",output
print "Error is - ",error

Output :

D:\dsp_jetbrains\AWS_Persistence>python chk.py
Output is -  ['bash: dinesh: command not found']
Error is -  []

Problem:

As an input I have passed an incorrect command and I expected "dinesh:command not found" will be printed with "Error is". However, it is coming with "Output is".

Question:

How do the output and error for exec_command in paramiko work?

Additional Test Case:

I also tried with command which is present but passed wrong input to that command. Example - lsof -f dinesh

But result is same.

D:\dsp_jetbrains\AWS_Persistence>python chk.py
Output is -  ['lsof: unknown file struct option: d', 'lsof: unknown file struct
option: i', 'lsof: unknown file struct option: n', 'lsof: unknown file struct op
tion: e', 'lsof: unknown file struct option: s', 'lsof: unknown file struct opti
on: h', 'lsof 4.87', ' latest revision: ftp://lsof.itap.purdue.edu/pub/tools/uni
x/lsof/', ' latest FAQ: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/FAQ', ' l
atest man page: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/lsof_man', ' usag
e: [-?abhKlnNoOPRtUvVX] [+|-c c] [+|-d s] [+D D] [+|-f[gG]] [+|-e s]', ' [-F [f]
] [-g [s]] [-i [i]] [+|-L [l]] [+m [m]] [+|-M] [-o [o]] [-p s]', '[+|-r [t]] [-s
 [p:s]] [-S [t]] [-T [t]] [-u s] [+|-w] [-x [fl]] [-Z [Z]] [--] [names]', "Use t
he ``-h'' option to get more help information."]
Error is -  []

Answer 1

You likely should not use get_pty unless you know you need it. Consider removing:

chan.get_pty()

and changing:

stdin, stdout, stderr = client.exec_command(command, get_pty=True)

to:

stdin, stdout, stderr = client.exec_command(command)

From the DOCS:

Request a pseudo-terminal from the server. This is usually used right after creating a client channel, to ask the server to provide some basic terminal semantics for a shell invoked with invoke_shell. It isn’t necessary (or desirable) to call this method if you’re going to exectue a single command with exec_command.

A pseudo-terminal is used if you want your ssh connection to look like a user is connected. It is uncommon to need this. One contrived example would be if you wanted to exercise VIM at the far end by sending and receiving ansi-terminal sequences.

Without get_pty, the remotely executed command will simply have a stdin/stdout/stderr trio connected just like it would if it were in a piepline.

The rarity of the need for get_pty is likely why it is not the default.