Answer by pLumo for Convert .xls/.xlsx spreadsheets to multiple .csv's based on a list

You can just put a loop inside another loop.

To avoid errors, don't use for with find results.

while IFS= read -r file; do
    while IFS= read -r sheet; do
        in2csv --sheet "$sheet" "$file" > "${file%.*}-${sheet}.csv"
    done < <(in2csv -n "$file")
done < <(find . -name '*.xls' -o -name '*.xlsx')