"S" mode for file permission?

[Peter Flynn]

On 12/12/2020 17:41, Gary Aitken wrote:
> I'm having to fix some permissions in some ruby file hierarchies, and
> came across one with a 'S' for group (note, not 's').  man chmod doesn't
> mention 'S', although it does mention 'X'.  Can someone tell me what 'S'
> represents, and why an attempt to chmod g+s results in a 'S'?  Also, why
> it would be set that way for a particular directory when all others above
> it have 's'?

S(s) sets the Sticky Bit. https://en.wikipedia.org/wiki/Sticky_bit

Peter