Linux obtiene parte de la cadena
b=${a:12:5}
Sparkling Skunk
b=${a:12:5}
INPUT='someletters_12345_moreleters.ext'
SUBSTRING=$(echo $INPUT| cut -d'_' -f 2)
echo $SUBSTRING
number=$(echo $filename | egrep -o '[[:digit:]]{5}' | head -n1)