how to simulate tar filename substitution across piped subprocess.Popen() calls?

Mon Nov 12 11:35:43 EST 2012

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