You have to specify executable and arguments either as a list:
subprocess.Popen(['echo', '5'])
or as a string and set shell=True:
subprocess.Popen('echo 5', shell=True)
The first is recommended, as it handles needed escaping for you. The second simply passes the string to the shell. According to subprocess docs:
args is required for all calls and should be a string, or a sequence of program arguments. Providing a sequence of arguments is generally preferred, as it allows the module to take care of any required escaping and quoting of arguments (e.g. to permit spaces in file names). If passing a single string, either shell must be True (see below) or else the string must simply name the program to be executed without specifying any arguments.
