#!/bin/bash
LIMITE=99
for a in `seq 0 $LIMITE`
do
        if  [ $a -gt 10  ]
        then 
                composite -blend $a -gravity South logo.jpg black.jpg -matte  logo00$a.jpg
                composite -blend 100 -gravity South logo.jpg black.jpg -matte  logo10$a.jpg
                composite -blend `expr(100 - a)` -gravity South logo.jpg black.jpg -matte  logo20$a.jpg
        fi
done

sh ./script.sh
./script.sh: 1: Syntax error: word unexpected (expecting ")")