#!/bin/bash
tr -d [:lower:]

while read FILE;
do
    echo "${FILE}" >> file.txt
done
tr -d 'a-z' < file.txt

while read input; do 
    echo $input | tr -d "[a-z]"
done

#!/bin/bash
while IFS= read -r line || [ -n "$line" ]; do
  echo "$line" | tr -d [:lower:]
done