Computation system cmd returns the exit code produced when the
operating system runs the shell command cmd.
This computation may fail with one of the following IOErrorType exceptions:
PermissionDeniedThe process has insufficient privileges to perform the operation.
ResourceExhaustedInsufficient resources are available to perform the operation.
UnsupportedOperationThe implementation does not support system calls.
On Windows, system passes the command to the Windows command
interpreter (CMD.EXE or COMMAND.COM), hence Unixy shell tricks
will not work.
On Unix systems, see waitForProcess for the meaning of exit codes when the process died as the result of a signal.