running programs from within python

[M.N.A.Smadi]

hi;
in general: how can i pass a string containing a concatenated sequcne of 
command line arguments to a unix system CLI?

my specific problem:
i have an executable that is called xyz (written in C if that matters) 
that i cannot modify, which prints it is text output to the screen and 
takes an input as t.txt.
on a unix terminal, i do the ./xyz t.txt > tmp.txt
so that the tmp.txt will contain the output of the xyz executable

i tried the following but it did not work

import os
os.execl("./xyz","t.txt > tmp.txt") #the thing just hangs there
i aslo tried
os.execl("./xyz","t.txt") # but it does not output anything to the 
screen either, hangs there too

any suggestions?

thanks
m.smadi