Writing shell script to scan a list of folders

Answer 1

one way

while read -r folders
do
  # add -maxdepth 1 if recursive traversal is not required
  find "$folders" -type f -iname "*.txt" | while read -r FILE
  do
      echo "do something with $FILE"
  done
done <"file"

or

folders=$(<file)
find $folders -type f -iname "*.txt" | while read -r FILE
do
    echo "do something with $FILE"
done

Bash 4.0 (if recursive find is required)

shopt -s globstar
folders=$(<file)
for d in $folders
do
  for file in $d/**/*.txt
  do
    echo "do something with $file"
  done
done

Answer 2

Given the post is simply asking for a list of files, it's quite simple:

tmp=$IFS
IFS=$(echo -en "\n\b") 
for i in `cat folders.txt` ; do
    ls -l "$i/*.txt"
done
IFS=$tmp

Answer 3

Simply do it on command line:

xargs ls -l < folders.txt | grep '.txt$'