It has always irked me that OS X doesn’t have the seq command (I am easily irked). Brian Peterson’s old e-mail Re: seq from core utils has it, but the link to sh-utils doesn’t work any more, since the project has been archived. Here’s the new link: http://ftp.gnu.org/old-gnu/sh-utils/. Compile it as Brian suggested, and all will be well.
$ seq 1 12 1 2 3 4 5 6 7 8 9 10 11 12
(at least 99% of you will be mystified why anyone would want this.)