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

To provide a solution that's cautiously written to be correct even in the presence of filenames with spaces:

#!/usr/bin/env bash

pattern='%04d'  # pad with four digits: change this to taste

# enable extglob syntax: +([[:digit:]]) means "one or more digits"
# enable the nullglob flag: If no matches exist, a glob returns nothing (not itself).
shopt -s extglob nullglob

for f in [[:digit:]]*; do               # iterate over filenames that start with digits
  suffix=${f##+([[:digit:]])}           # find the suffix (everything after the last digit)
  number=${f%"$suffix"}                 # find the number (everything before the suffix)
  printf -v new "$pattern" "$number" "$suffix"  # pad the number, then append the suffix
  if [[ $f != "$new" ]]; then                   # if the result differs from the old name
    mv -- "$f" "$new"                           # ...then rename the file.
  fi
done