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From: Kushal Kumaran <kushal.kumaran+python@gmail.com>
Date: Fri, 1 Apr 2011 11:03:31 +0530
Subject: Re: Behaviour of subprocess.Popen, ssh and nohup I don't understand
To: Adriaan Renting <renting@astron.nl>
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On Fri, Apr 1, 2011 at 4:31 AM, Adriaan Renting <renting@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):
> =C2=A0command =3D ["ssh", "-T", "localhost", "uptime"]
> =C2=A0comm =3D subprocess.Popen(command, shell=3DFalse, stdin=3DNone,
> stdout=3Dsubprocess.PIPE, stderr=3Dsubprocess.STDOUT, close_fds=3DTrue)
> =C2=A0print =C2=A0'1'
> =C2=A0if comm.returncode:
> =C2=A0 =C2=A0print "error: %i" % (comm.return_code)
> =C2=A0else:
> =C2=A0 =C2=A0print =C2=A0'2'
> =C2=A0 =C2=A0(output, output2) =3D comm.communicate(input=3DNone)
> =C2=A0 =C2=A0print output
> =C2=A0 =C2=A0print output2
> =C2=A0print =C2=A0'3'
> =C2=A0time.sleep(3)
>
> (renting)myhost> python --version
> Python 2.5.2
>
> (renting)myhost> nohup ./test.py -O2 &
> [1] 15679
>
> (renting)myhost> 1
> 2
> =C2=A022:40:30 up 24 days, =C2=A07:32, =C2=A01 user, =C2=A0load average: =
0.00, 0.00, 0.00
>
> None
> 3
> 1
> 2
>
> [1] =C2=A0+ Suspended (tty input) =C2=A0 =C2=A0 =C2=A0 =C2=A0 ./test.py -=
O2
> (renting)myhost> fg
> ./test.py -O2
>
> =C2=A022:40:35 up 24 days, =C2=A07:32, =C2=A01 user, =C2=A0load average: =
0.00, 0.00, 0.00
>
> None
> 3
> 1
> 2
> =C2=A022:40:56 up 24 days, =C2=A07:32, =C2=A01 user, =C2=A0load 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=3Dopen(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.

--=20
regards,
kushal