Linux
and the remaining part of the string identifies the version.
Rexx code should not rely on the exact format returned.
-- CentOS 7
say SysVersion() -- Linux 3.10.0-957.10.1.el7.x86_64
-- Ubuntu 16.04
say SysVersion() -- Linux 4.4.0-154-generic
-- Raspbian
say SysVersion() -- Linux 4.14.70-v7+
-- SLES/390
say SysVersion() -- Linux 3.12.74-60.64.40-default