[Tutor] Remote access from Windows PC to a Linux box

[Yashwin Kanchan]

Hi Mike

have you tried running the tshark process in the background...

*CMD_LIST=[PLINK,sess_name,'/usr/bin/sudo /usr/sbin/tshark &', '-a',
'duration:10', '-i', 'wlan0', '-T', 'text', '-V','>', 'out.txt'];*

Or if you are using NOHUP try redirecting this way...

*CMD_LIST=[PLINK,sess_name,'/usr/bin/nohup','/usr/bin/sudo /usr/sbin/tshark >
out.txt 2> out.err < /dev/null ', '-a', 'duration:10', '-i', 'wlan0', '-T',
'text','&'];*

Regards
Yashwin Kanchan


On 21 April 2010 19:15, Mike Baker <mibaker88 at gmail.com> wrote:

> Thanks Tim,
>
> Your subprocess examples got me started in the right direction.  I've moved
> on to a slightly more advanced problem that I need help with.
>
> I want to remotely start a Tshark packet capture session on one of our
> Linux machines in the lab.  I want to start the session from my Windows
> machine running Python 2.5.  The output capture file needs to be saved on
> the remote Linux machine.
>
> The example below nearly does what I want.  It starts Tshark via Putty,
> runs for 10 seconds then writes the capture file (out.txt) to a remote Linux
> machine.  The problem is that the putty session hangs open while Tshark is
> running. So, I can't execute additional Python commands until the Tshark
> capture finishes.
>
> I've experimented with the Unix nohup command, but it does not seem to work
> as expected with Tshark.  If you call my function below with
> >>> test_subp(alt_cmd=1)
> then the nohup command is added to the subprocess command list (along
> with a trailing '&' to send the command to background).  This should work.
> Using this alternate command, out.txt gets created, but is always empty.
>
>
> Here is my code:
> **********************************************
> def test_subp(alt_cmd=0):
>     '''Establish a Putty connection with one of our Linux machines in the
> lab.
>     Send Tshark command to start a data collection session over Putty.
>     '''
>     PLINK = 'C:\\Progra~1\\putty\\plink'
>     sess_name='LabComp1'
>     if alt_cmd:
>         '''This command does not work as expected.  The tshark output file
> (out.txt)is created,
>         but there is nothing in it '''
>         CMD_LIST=[PLINK,sess_name,'/usr/bin/nohup','/usr/bin/sudo
> /usr/sbin/tshark', '-a', 'duration:10', '-i',  'wlan0', '-T', 'text',
> '-V','>', 'out.txt','&'];
>     else:
>         'This command works great, writing tshark output to out.txt on the
> remote machine.'
>         'Unfortunately, this command hangs the putty session until the
> tshark capture ends'
>         CMD_LIST=[PLINK,sess_name,'/usr/bin/sudo /usr/sbin/tshark', '-a',
> 'duration:10', '-i',  'wlan0', '-T', 'text', '-V','>', 'out.txt'];
>     print "The command list you are sending to the subprocess is: \n",
> "\t", CMD_LIST
>
>     PIPE = subprocess.PIPE
>     p = subprocess.Popen(CMD_LIST, stdout=PIPE, stderr=PIPE)
>     stdout, stderr = p.communicate ()
>     print 'stdout = ', stdout
>     print 'stderr = ', stderr
> *********************************************************************
>
> For both runs (atl_cmd=0 or alt_cmd=1), the stdout and stderr printouts at
> the end of the script are empty.
>
> Any suggestions would be appreciated.
>
> Thanks,
>
>     Mike
>
> *******************************************************************************************************
>
> On Wed, Mar 31, 2010 at 7:42 AM, Tim Golden <mail at timgolden.me.uk> wrote:
>
>> On 30/03/2010 17:29, Mike Baker wrote:
>>
>>> I'm trying to connect to a Linux box from my Windows machine and execute
>>> a
>>> series of commands
>>>
>>> I want a script to always
>>> execute the same series of commands without having to do so manually.   I
>>> also have code that will execute a single command like cat a file and
>>> write
>>> the ouput to a new file. However, when I try to use the communicate
>>> object
>>> in subprocess, my window hangs.
>>>
>>
>>
>> This works for me:
>>
>> <code>
>> import os, sys
>> import subprocess
>>
>> PLINK = "plink"
>> REMOTE_USER = "tgolden at web30.webfaction.com"
>> PIPE = subprocess.PIPE
>>
>> p = subprocess.Popen ([PLINK, REMOTE_USER, "ls"], stdout=PIPE)
>> stdout, stderr = p.communicate ()
>> print "#1:", stdout.splitlines ()[0]
>>
>> with open ("out.txt", "w") as f:
>>  p = subprocess.Popen ([PLINK, REMOTE_USER, "cat .bashrc"], stdout=f)
>>  p.communicate ()
>> print "#2:", open ("out.txt").read ().splitlines ()[0]
>>
>> p = subprocess.Popen ([PLINK, REMOTE_USER], stdin=PIPE, stdout=PIPE)
>> stdout, stderr = p.communicate ("ls\nexit\n")
>> print "#3", stdout
>>
>> p = subprocess.Popen ([PLINK, REMOTE_USER], stdin=PIPE, stdout=PIPE)
>> p.stdin.write ("ls\nexit\n")
>> stdout, stderr = p.communicate ()
>> print "#4", stdout
>>
>> </code>
>>
>> A few things to note, none of which I believe to be germane to the
>> issues you're experiencing:
>>
>> * You almost never need to use shell=True on a Windows call to subprocess.
>>  If in doubt, don't use it.
>>
>> * Definitely better to pass the list-of-params style as the first param
>>  of subprocess.Popen; it sorts out issues with embedded spaces etc.
>>
>> * The open ("...", "w") in your second example *may* be closing the
>>  file immediately. I doubt it, since you'd expect Popen to hold a
>>  reference, but I haven't checked the implementation.
>>
>> TJG
>>
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