popen(string, enum) starts a new process. The first argument to popen is a shell command line that will be executed. The second argument is either `readonly or `writeonly. It is not possible to use popen for read and write access at the same time. If the process is opened for reading, all data it write to its standard output will be readable through the file object. If the process is opened for writing, it will receive all data written to the file object on its standard input.