printing to a redirected stdout from a process that was called with "2>&1 > /dev/null &"

Chris Lambacher chris at kateandchris.net
Tue Jan 30 16:39:19 CET 2007


On Mon, Jan 29, 2007 at 03:12:37PM -0800, Pappy wrote:
> SHORT VERSION:
> Python File B changes sys.stdout to a file so all 'prints' are written 
> to the file.  Python file A launches python file B with os.popen("./B 
> 2>&^1 >dev/null &").  Python B's output disappears into never-never 
> land.
> 
> LONG VERSION:
> I am working on a site that can kick off large-scale simulations.  It 
> will write the output to an html file and a link will be emailed to 
> the user.  Also, the site will continue to display "Loading..." if the 
> user wants to stick around.
> 
> The simulation is legacy, and it basically writes its output to stdout 
> (via simple print statements).  In order to avoid changing all these 
> prints, I simply change sys.stdout before calling the output 
> functions.  That works fine.  The whole thing writes to an html file 
> all spiffy-like.
> 
> On the cgi end, all I want my (python) cgi script to do is check for 
> form errors, make sure the server isn't crushed, run the simulation 
> and redirect to a loading page (in detail, I write a constantly 
> updating page to the location of the final output file.  When the 
> simulation is done, the constantly updating file will be magically 
> replaced).  The root problem is that the popen mechanism described 
> above is the only way I've found to truly 'Detach' my simulation 
> process.  With anything else, Apache (in a *nix environment) sits and 
> spins until my simulation is done.  Bah.
> 
> Any ideas?
The subprocess module is probably a good starting point:
http://docs.python.org/dev/lib/module-subprocess.html

It will allow you greater control over what happens with the output of a
process that you start.  Specifically look at the the stdout and stderr
arguments to Popen.  You can provide these with an open file descriptor or a
file object and it will dump the output into there.

-Chris
> 
> _jason
> 
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