Wikipedia claims so... [LWN.net]

Wikipedia claims so...

Posted May 24, 2009 2:38 UTC (Sun) by khim (subscriber, #9252)
In reply to: Nope - they needed something workable by xilun
Parent article: Ex-Microsoftie: Free Software Will Kill Redmond (ComputerWorld)

It has POSIX compatibility and access to a command line interface through Bash, although internally it is not a Unix-derived operating system.

I'm not sure it ever passed certification tests, but real programs worked fine. Compare with Windows NT (which passed certification tests but was mostly unusable as POSIX system).