Assuming your RUNDATE var has the properly-formatted date you want in the RUN_DATE key's Value field, you're halfway there.
Try this in your linux job's process tab:
:SET &VALUE# = GET_VAR("RUNDATE","RUN_DATE")
uc4date = &VALUE#
echo "The UC4 run_date value is $uc4date."
Passing UC4 variables into your scripts is easy; it's passing values back from your batch script into UC4 that's more complicated.