ricardo@kebek:~ $ echo "pepe,papo" > test
ricardo@kebek:~ $ sed "s/,/\n/g"

ricardo@kebek:~ $ sed "s/,/\n/g" test
pepe
papo
ricardo@kebek:~ $ sed "s/,/\n/g" test > test.out
ricardo@kebek:~ $ cat test.out
pepe
papo
ricardo@kebek:~ $