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

One-liner:

ls | awk '/^([0-9]+)\.txt$/ { printf("%s %04d.txt\n", $0, $1) }' | xargs -n2 mv

How do I use grep to only match lines that contain \d.txt (IE 1 digit, then a period, then the letters txt)?

grep -E '^[0-9]\.txt$'