Not arguing about the merits of capabilities and the open interface, but file handles are often cited as capability-like, and the ...at calls are in POSIX; but

Not arguing about the merits of capabilities and the open interface, but file handles are often cited as capability-like, and the ...at calls are in POSIX; but there's the global namespace. (Security in Plan 9 does say their capability system could be implemented for Unix, but I don't know if that's realistic and whether it provides proper capability security.)