Mac Geekery

Unless I’m missing something, the treatment of calendar based events in launchd (once working) isn’t much of a replacement for cron. The launchd.plist manpage indicates that StartCalendarInterval is a dictionary of integers (Minute, Hour, Day, etc.) where “The semantics are much like crontab(5)”. Well in a crontab, these parameters are not just integers. Any idea how to do the equivalent of this cron command:

50 8-20/2 * * * root chmod -R 775 /Users/Shared/*

i.e. run said command every two hours each day from 8:50am to 8:50pm

?