code: def lookup(command):
      
      for path in os.environ.get('PATH', '').split(':'):
          fname = os.path.join(path, command)
          if os.path.exists(fname):
              return fname
      raise ValueError("Invalid command")
docs: Lookup a command in PATH. For example::
  
          >>> lookup('ls')
          '/usr/bin/ls'
          >>> lookup('usermod')
          '/usr/sbin/usermod'
          >>> lookup('foobar')
          Traceback (most recent call last):
            File "<stdin>", line 1, in <module>
          ValueError: Invalid command
  
      This function is incredibly dumb, and does not search for an executable
      file.