Rename all files in directory through terminal

Question

Say, I have several files in a directory.

How do I rename all of them to "001", "002", etc., saving their current file extensions?

Answer 1

• Terminal method
  cd (change directory) to the folder.
  run command:

• Command 1

  rename -n -v 'our $n; my $zn=sprintf("%03d", ++$n); s/.*\./$zn./' *.*
  

This will display the changes. Remove -n for actual rename.
Answer from @Kusalananda

• Command 2

  a=1; for i in *.*; do new=$(printf "%03d" "$a"); mv -i -- "$i" "$new.${i##*.}"; let a=a+1; done
  

• GUI method
  Select all files to be renamed.
  Right click and select rename.
  Delete [Original file name] from the option area. Click on "Add" and select 001, 002, 003, 004.
  Click on "Rename".