Linux shell script to add leading zeros to file names

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