Change file names

Using Perl's rename utility:

prename 's/(\d+)\./sprintf "%04d.", $1/e' A*.txt

prename is sometimes available under the name rename. There is, however, another unrelated and incompatible utility called rename that is installed by default on some distributions.

Debian-like distributions, among others, have Perl's rename/prename installed by default. If your distribution does not, instructions for installing it can be found here

(Hat tip to Steeldriver for improved version of the command.)

Example

Consider a directory with these files:

$ ls
A12345.txt  A123.txt  A12.txt  A1.txt

Now, let's run prename:

$ prename 's/(\d+)\./sprintf "%04d.", $1/e' A*.txt
$ ls
A0001.txt  A0012.txt  A0123.txt  A12345.txt

How it works

A single substitute command is used:

s/(\d+)\./sprintf "%04d.", $1/e

• (\d+)\. matches one or more digits followed by a period. The digits are saved in group 1.

• The final e tells perl to evaluate the command sprintf "%04d.", $1. This command returns the digits of group 1 in format %04d meaning zero-filled four digits, followed by a period.

If you don't have the prename option suggested by @John1024, you should be able to use the printf (a built-in in bash, but also available from GUN coreutils) to format the decimal digits with the desired field width and padding - unfortunately I don't think there's a one-step shell expansion to extract the digits, the closest I can get is

for file in A*.txt
do
  base="${file%.*}"
  printf -v newfile "A%04d.txt" "${base#A}"
  mv -- "$file" "$newfile"
done