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Or maybe I can put in a counter of some kind and flush the cache every Friday or something.




That is what the parms are for.
-f[:yyyy/mm/dd]

by default, they flush every 30 days.
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because groups by nature are ever changing



Ja, but not all groups have conditionals in the logon script. I would think the few changes that might affect one or two users could be managed manually rather than have every user suffer through longer logon times on every logon.
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