Renaming files in a folder to sequential numbers

Try to use a loop, let, and printf for the padding:

a=1
for i in *.jpg; do
  new=$(printf "%04d.jpg" "$a") #04 pad to length of 4
  mv -i -- "$i" "$new"
  let a=a+1
done

using the -i flag prevents automatically overwriting existing files, and using -- prevents mv from interpreting filenames with dashes as options.

Beauty in one line:

ls -v | cat -n | while read n f; do mv -n "$f" "$n.ext"; done 

You can change .ext with .png, .jpg, etc.

I like gauteh's solution for its simplicity, but it has an important drawback. When running on thousands of files, you can get "argument list too long" message (more on this), and second, the script can get really slow. In my case, running it on roughly 36.000 files, script moved approx. one item per second! I'm not really sure why this happens, but the rule I got from colleagues was "find is your friend".

find -name '*.jpg' | # find jpegs
gawk 'BEGIN{ a=1 }{ printf "mv %s %04d.jpg\n", $0, a++ }' | # build mv command
bash # run that command

To count items and build command, gawk was used. Note the main difference, though. By default find searches for files in current directory and its subdirectories, so be sure to limit the search on current directory only, if necessary (use man find to see how).