Behaviour of subprocess.Popen, ssh and nohup I don't understand

Adriaan Renting renting at astron.nl
Wed Apr 6 09:47:21 CEST 2011



>>> On 4/1/2011 at 07:33 AM, Kushal Kumaran
<kushal.kumaran+python at gmail.com>
wrote: 
> On Fri, Apr 1, 2011 at 4:31 AM, Adriaan Renting <renting at astron.nl>
wrote:
>> L.S.
>>
>> I have a problem that a background process that I'm trying to start
with
>> subprocess.Popen gets interrupted and starts waiting for input no
matter
>> what I try to do to have it continue to run. It happens when I run
it
>> with nohup in the background.
>> I've tried to find a solution searching the internet, but found
none.
>> I've written a small test script that reproduces the problem and
hope
>> maybe here there is someone who can tell me what's going wrong. Any
>> suggestions are welcome.
>>
>> (renting)myhost> cat test.py
>> #!/usr/bin/python
>> # script to test subprocess problem
>> import subprocess, sys, time
>>
>> for f in range(3):
>>  command = ["ssh", "-T", "localhost", "uptime"]
>>  comm = subprocess.Popen(command, shell=False, stdin=None,
>> stdout=subprocess.PIPE, stderr=subprocess.STDOUT, close_fds=True)
>>  print  '1'
>>  if comm.returncode:
>>    print "error: %i" % (comm.return_code)
>>  else:
>>    print  '2'
>>    (output, output2) = comm.communicate(input=None)
>>    print output
>>    print output2
>>  print  '3'
>>  time.sleep(3)
>>
>> (renting)myhost> python --version
>> Python 2.5.2
>>
>> (renting)myhost> nohup ./test.py -O2 &
>> [1] 15679
>>
>> (renting)myhost> 1
>> 2
>>  22:40:30 up 24 days,  7:32,  1 user,  load average: 0.00, 0.00,
0.00
>>
>> None
>> 3
>> 1
>> 2
>>
>> [1]  + Suspended (tty input)         ./test.py -O2
>> (renting)myhost> fg
>> ./test.py -O2
>>
>>  22:40:35 up 24 days,  7:32,  1 user,  load average: 0.00, 0.00,
0.00
>>
>> None
>> 3
>> 1
>> 2
>>  22:40:56 up 24 days,  7:32,  1 user,  load average: 0.00, 0.00,
0.00
>>
>> None
>> 3
>>
>> (renting)myhost>
>>
>> Now as you can see, it suspends on the second time through the for
loop,
>> until I bring it to the foreground and hit .
>> What you don't see, is that I make it do this by pushing the arrow
keys
>> a couple of times. The same happens when I would exit the shell,
despite
>> it running with nohup.
>> I don't need to exit to make it suspend, any combination of a few
random
>> keystrokes makes it do this. It seems depending on the timing
though,
>> during the sleep(3) it seems to ignore me, only when subprocess is
>> actually running will it suspend if I generate keystrokes.
>> If the ssh command is executed without -T option it suspends
directly,
>> so I think it's related to the ssh command. I log in with a
>> public/private key pair to avoid having to enter a password.
>>
>> Any suggestions are welcome,
>>
> 
> What operating system is this?  Try with stdin=open(os.devnull,
'rb')
> to the Popen call instead.  Also, this seems to be similar:
>
http://stackoverflow.com/questions/1365653/calling-fgets-on-popen-of-ssh-is-
> flushing-the-beginning-of-stdin-of-the-cal
> 
> The "Suspended (tty input)" message means that a background process
> tried to read from stdin, so got suspended.  This is part of the job
> control mechanism.
> 
> -- 
> regards,
> kushal

This solves the problem using stdin=open(os.devnull, 'rb') instead of
stdin=None makes it run even if there is input from stdin in the
foreground process.

The operating system is Ubuntu 8.04
I understood what Suspended (tty input) means. I don't understand why
it waits for input if stdin=None.

Thank you for your help.

Adriaan Renting.



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