S9fES  (system-command string)  ==>  integer

Pass the given STRING to the operating system for execution and
return the exit code returned by the system. On Unix this typically
means passing the command to /bin/sh and on Plan 9 commands will
be passed to /bin/rc.

An exit code of zero indicates success and a non-zero exit code
signals failure.

(system-command "ls")              ==>  0
(system-command "ls nonexistent")  ==>  127
