On my machine, I have some software which takes commands in the terminal and returns a list of values. To run it, I have to type something like: pdv -t filename I am trying to run it as part of a python programme. When I run the following: os.system('pdv -t %s' % (epoch_name)) then I get the values that I desire returned to my terminal (where epoch_name is the variable name for the filename). But when I try to write the result to a file: os.system('pdv -t %s % "(epoch_name)" > 123.txt') the file 123.txt is produced but it is empty. I know that I am misplacing the " and/or ' characters, but I can't figure out where they should go. Any help would be gratefully received!