You need to loop on every line of your File.txt, and for each one describe and grep for CHECKEDOUT
.
#!/bin/sh
lines=$(cat Files.txt)
while read -r line; do
# do your descr there
cleartool descr -l ${line}|grep CHECKEDOUT
done <<< "${lines}
Another way is to look for all checkout out folders, and for each one grep it from the Files.txt
.
cleartool find . -type d -exec 'cleartool describe -fmt "%En %Rf\n" "$CLEARCASE_PN"' | grep "CHECKEDOUT"