Linux shell script to add leading zeros to file names

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 filenames are four digits long.

I.e., 23.txt becomes 0023.txt.

What is a shell script that will do this? Or a related question: How do I use grep to only match lines that contain \d.txt (i.e., one digit, then a period, then the letters txt)?

Here's what I have so far:

for a in [command i need help with]
do
  mv $a 000$a
done

Basically, run that three times, with commands there to find one digit, two digits, and three digit filenames (with the number of initial zeros changed).

Try:

for a in [0-9]*.txt; do
    mv $a `printf %04d.%s ${a%.*} ${a##*.}`
done

Change the filename pattern ([0-9]*.txt) as necessary.

---

A general-purpose enumerated rename that makes no assumptions about the initial set of filenames:

X=1;
for i in *.txt; do
  mv $i $(printf %04d.%s ${X%.*} ${i##*.})
  let X="$X+1"
done

---

On the same topic:

• Bash script to pad file names
• Extract filename and extension in bash

Using the rename (prename in some cases) script that is sometimes installed with Perl, you can use Perl expressions to do the renaming. The script skips renaming if there's a name collision.

The command below renames only files that have four or fewer digits followed by a ".txt" extension. It does not rename files that do not strictly conform to that pattern. It does not truncate names that consist of more than four digits.

rename 'unless (/0+[0-9]{4}.txt/) {s/^([0-9]{1,3}\.txt)$/000$1/g;s/0*([0-9]{4}\..*)/$1/}' *

A few examples:

Original    Becomes
1.txt       0001.txt
02.txt      0002.txt
123.txt     0123.txt
00000.txt   00000.txt
1.23.txt    1.23.txt

Other answers given so far will attempt to rename files that don't conform to the pattern, produce errors for filenames that contain non-digit characters, perform renames that produce name collisions, try and fail to rename files that have spaces in their names and possibly other problems.

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

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$'

Let's assume you have files with datatype .dat in your folder. Just copy this code to a file named run.sh, make it executable by running chmode +x run.sh and then execute using ./run.sh:

#!/bin/bash
num=0
for i in *.dat
do

  a=`printf "%05d" $num`
  mv "$i" "filename_$a.dat"
  let "num = $(($num + 1))"
done

This will convert all files in your folder to filename_00000.dat, filename_00001.dat, etc.

This version also supports handling strings before(after) the number. But basically you can do any regex matching+printf as long as your awk supports it. And it supports whitespace characters (except newlines) in filenames too.

for f in *.txt ;do
    mv "$f" "$( 
        awk -v f="$f" '{
            if ( match(f, /^([a-zA-Z_-]*)([0-9]+)(\..+)/, a)) {
                printf("%s%04d%s", a[1], a[2], a[3])
            } else {
                print(f)
            }
        }' <<<''
    )"
done

To only match single digit text files, you can do...

$ ls | grep '[0-9]\.txt'

One-liner hint:

while [ -f ./result/result`printf "%03d" $a`.txt ]; do a=$((a+1));done
RESULT=result/result`printf "%03d" $a`.txt

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

There is a rename.ul command installed from util-linux package (at least in Ubuntu) by default installed.

It's use is (do a man rename.ul): > rename [options] expression replacement file...

The command will replace the first occurrence of expression with the given replacement for the provided files.

While forming the command you can use:

rename.ul -nv replace-me with-this in-all?-these-files*

for not doing any changes but reading what changes that command would make. When sure just reexecute the command without the -v (verbose) and -n (no-act) options

for your case the commands are:

rename.ul "" 000 ?.txt
rename.ul "" 00 ??.txt
rename.ul "" 0 ???.txt