Calculate modulo in sh script

If your sh really is sh and not just bash being run as sh then this will work just fine

if [ `expr $n % 5` -eq 0 ]
then
    # do something
fi

If your sh is really bash then put your test in (( )) like so

if (( $n % 5 == 0 ))
then
     # do something
fi

You should use bc when doing math in shell

if [ `echo "3 % 2" | bc` -eq 0 ]