Apenas $ {i}
./processing < base.ppm > picture${i}.ppm
Exemplo
$ cat foo
#!/bin/bash
for i in {1..5}
do
echo ./processing base.ppm picture${i}.ppm
done
$ ./foo
./processing base.ppm picture1.ppm
./processing base.ppm picture2.ppm
./processing base.ppm picture3.ppm
./processing base.ppm picture4.ppm
./processing base.ppm picture5.ppm
$