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Message from discussion how to simulate tar filename substitution across piped subprocess.Popen() calls?
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Hans Mulder  
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 More options Nov 12 2012, 11:36 am
Newsgroups: comp.lang.python
From: Hans Mulder <han...@xs4all.nl>
Date: Mon, 12 Nov 2012 17:35:43 +0100
Local: Mon, Nov 12 2012 11:35 am
Subject: Re: how to simulate tar filename substitution across piped subprocess.Popen() calls?
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On 12/11/12 16:36:58, jkn wrote:

> slight followup ...

> I have made some progress; for now I'm using subprocess.communicate to
> read the output from the first subprocess, then writing it into the
> secodn subprocess. This way I at least get to see what is
> happening ...

> The reason 'we' weren't seeing any output from the second call (the
> 'xargs') is that as mentioned I had simplified this. The actual shell
> command was more like (in python-speak):

> "xargs -I {} sh -c \"test -f %s/{} && md5sum %s/{}\"" % (mydir, mydir)

> ie. I am running md5sum on each tar-file entry which passes the 'is
> this a file' test.

> My next problem; how to translate the command-string clause

>     "test -f %s/{} && md5sum %s/{}" # ...

> into s parameter to subprocss.Popen(). I think it's the command
> chaining '&&' which is tripping me up...

It is not really necessary to translate the '&&': you can
just write:

    "test -f '%s/{}' && md5sum '%s/{}'" % (mydir, mydir)

, and xargs will pass that to the shell, and then the shell
will interpret the '&&' for you: you have shell=False in your
subprocess.Popen call, but the arguments to xargs are -I {}
sh -c "....", and this means that xargs ends up invoking the
shell (after replacing the {} with the name of a file).

Alternatively, you could translate it as:

    "if [ -f '%s/{}' ]; then md5sum '%s/{}'; fi" % (mydir, mydir)

; that might make the intent clearer to whoever gets to
maintain your code.

Hope this helps,

-- HansM


 
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