Executes command as a shell statement ("/bin/sh -c
command" for Unix, "cmd /c command" for Windows),
waits until it has finished and returns the result as a string.
Open a "process" for reading or writing. The command is executed
as a shell statement ("/bin/sh -c command" for Unix,
"cmd /c command" for Windows). The parameter mode
should be one of the following letters:
"r"
Open for reading. Data written by the process to stdout
is available for read.
"w"
Open for writing. Data written to the file is available
to the process on stdin.