How to set the command(s) that get run before every single prompt?

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http://unix.stackexchange.com – I used to know this but I forgot: I want to execute a command or a couple of commands before every single prompt. When I am in our PROD box, I want a big fat reminder above every prompt that says * YOU ARE IN PROD * and maybe another thing or two. Currently, I think that is how mail is checked but I can't remember how that configuration is set. Using ksh and Solaris 5.10 Any idea? Thanks (HowTos)