numeric
The name of the operating system on which the system is currently running. This is useful within macros, when os-dependent commands are required.
Some values are:
aix - IBM.
hpux - Hewlett Packard.
linux - Linux.
solaris - Solaris on Sparc/Intel.
windows - Windows 9x.
windows_nt - Windows NT/XP.
This identifier is a constant and cannot be redefined.