uname -rsm

but better just:

cat /etc/issue

$ uname -rsm
SunOS 5.6 sun4u
Linux 2.2.16 i686

echo "  OS: `uname -s`, version `uname -r`"
[ "`uname -s`" = SunOS ] && echo " (CPU : `uname -p`)"

$ uname -a
SunOS zkpks001 5.6 Generic_105181-12 sun4u sparc
Linux sunshine 2.0.36 #2 Sun Feb 21 15:55:27 EST 1999 i586 unknown

# -v is not useful!:

tong@iitrc:~$ uname -v
Generic_105181-20

tong@sunny:~$ uname -v
#1 Tue Mar 7 21:07:39 EST 2000