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

Kushal Kumaran kushal.kumaran+python at gmail.com
Fri Apr 1 07:33:31 CEST 2011


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



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