Modulo in bash

The command below return the modulo 15 of variable $i:

t=$(expr $i % 15)

For example:

i=0
for x in ????_protein.pdb; do
    my_command &
    ((i++))
    t=$(expr $i % 15 + 2)
    sleep $t
done

allows to increment the sleep by $i % 15 + 2 which can be convenient to wait for a long process in background and to avoid overloading of the machine.