Sun Microsystems seemed to care about Solaris versus SunOS branding...

Solaris also has a fork named Illumos, which is free (but not GPL-compatible)
software and which has behavior close to that of Solaris 11.
See <https://illumos.org>.

Here is a mapping table that maps the `uname -sr` values to the marketing names.

       SunOS 1.x       == SunOS 1.x
       SunOS 2.x       == SunOS 2.x
       SunOS 3.x       == SunOS 3.x
       SunOS 4.0       == SunOS 4.0
       SunOS 4.0.x     == SunOS 4.0.x

Starting with 4.1.1B of SunOS we have the 'Solaris 1.0' release.

       SunOS 4.1.1B    == Solaris 1.0
       SunOS 4.1.1.1   ==    "     "
       SunOS 4.1.1_U1  ==    "     "
       SunOS 4.1.2     == Solaris 1.0.1
       SunOS 4.1.3     == Solaris 1.1A
       SunOS 4.1.3B    == Solaris 1.1B
       SunOS 4.1.3C    == Solaris 1.1C
       SunOS 4.1.3_U1  == Solaris 1.1.1
       SunOS 4.1.3_U1B == Solaris 1.1.1B
       SunOS 4.1.4     == Solaris 1.1.2

The SunOS 5.x uname output maps to the Solaris 2.x numbers through 2.6.

       SunOS 5.0       == Solaris 2.0
       SunOS 5.1       == Solaris 2.1
       SunOS 5.2       == Solaris 2.2
       SunOS 5.3       == Solaris 2.3
       SunOS 5.4       == Solaris 2.4
       SunOS 5.5       == Solaris 2.5
       SunOS 5.5.1     == Solaris 2.5.1
       SunOS 5.6       == Solaris 2.6

After SunOS 5.6, they started numbering based on the second digit.

       SunOS 5.7       == Solaris 7
       SunOS 5.8       == Solaris 8
       SunOS 5.9       == Solaris 9
       SunOS 5.10      == Solaris 10
       SunOS 5.11      == Solaris 11 or Illumos

The common terminology is to use the name "SunOS ..." to designate SunOS 2.x,
3.x, 4.x, and "Solaris ..." or "Illumos" to designate SunOS 5.x.

Most of these operating systems are obsolete.  As of 2023-07-11 only
Solaris 10 (end-of-life January 2025) and Solaris 11 (end-of-life
November 2034) are supported by Oracle and are therefore of
practical porting concern for GNU applications.  For the current list, see
<https://en.wikipedia.org/wiki/Solaris_(operating_system)#Version_history>.

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