When must a project that uses GPL licensed projects use a GPL license?

Question

i have a project that needs to use tools licensed under the GNU/GPL license.
my question is:
must a project that a uses GPL licensed tool like netdiscover be licensed as a GPL project?
if my project contains a line that contains:

def discoverAllNetworks():
  # execute a terminal command and get it's output
  os.popen("arp-scan > output.txt").read()
  # open the file created by command
  file = open("output.txt", "rw")
  fileContents = file.read()
  newFileContents = fileContents.find_ip_addresses()
  file.write(newFileContents)
  args = file.read().split("\n")
  # feed modified output of the modified "gpl_licensed_program"
  # to another gpl licensed program
  output = os.system("nmap" + args).read()
  flash(output)
  redirect('/output_url')

does this require that my project use the GPL license?
as far as i know if i copy GPL code my project needs to use the same license.
but i am not copying the code; i am only using the project.
distribution
i will not bundle the tools this project uses, as most of them are available on most linux package managers.
hopefully will post the project on github when it's mature enough

Answer 1

IANAL/IANYL. In answer to your specific question, it depends on how tightly the two programs are coupled. According to the FSF's GPL FAQ:

A main program that uses simple fork and exec to invoke plug-ins and does not establish intimate communication between them results in the plug-ins being a separate program.

which means that your program doesn't need to be GPLed also. But:

If the main program uses fork and exec to invoke plug-ins, and they establish intimate communication by sharing complex data structures or shipping complex data structures back and forth, [then] that can make them one single combined program.

And then your code would need to be distributed under the GPL also (edits in the above are mine).

Edit: if I read your code correctly (which isn't a given!) you're forking tools to build a list of local IP addresses, forking nmap to run a defaults-only scan of those addresses, and simply displaying the results. My take is this doesn't constitute intimate communication, and thus the works remain at arm's-length.

You say you're not distributing nmap etc., so you won't incur any licence obligations with respect to that GPL code. I can't see that your code would also be required to be GPL. As I said, though, IANAL/IANYL, so if you're betting a business on this, get professional legal advice.