Linux shell script to add leading zeros to file names

Question

I have a folder with about 1,700 files. They are all named like 1.txt or 1497.txt, etc. I would like to rename all the files so that all the filena

Answer 1

for a in *.txt; do
  b=$(printf %04d.txt ${a%.txt})
  if [ $a != $b ]; then
    mv $a $b
  fi
done