Ubuntu comes with a script called rename. It's just a little Perl script that features a number of powerful bulk-renaming features but the best (in this case) is the ability for it to run Perl during the replacement. The result is a truly compact solution:

rename 's/d+/sprintf("%05d", $&)/e' *.jpg

This is similar to the other printf-style answers here but it's all handled for us. The code above is for a 5-digit number (including a variable number of leading zeros).

It will search and replace the first number-string it finds with a zero-padded version and leave the rest of the filename alone. This means you don't have to worry too much about carrying any extension or prefix over.

Note: this is not completely portable. Many distributions use rename.ul from the util-linux package as their default rename binary. This is a significantly stunted alternative (see man rename.ul) which won't understand the above. If you'd like this on a platform that isn't using Perl's rename, find out how to install that first.

And here's a test harness:

$ touch {1..19}.jpg



$ ls

10.jpg  12.jpg  14.jpg  16.jpg  18.jpg  1.jpg  3.jpg  5.jpg  7.jpg  9.jpg

11.jpg  13.jpg  15.jpg  17.jpg  19.jpg  2.jpg  4.jpg  6.jpg  8.jpg



$ rename 's/d+/sprintf("%05d", $&)/e' *.jpg



$ ls

00001.jpg  00005.jpg  00009.jpg  00013.jpg  00017.jpg

00002.jpg  00006.jpg  00010.jpg  00014.jpg  00018.jpg

00003.jpg  00007.jpg  00011.jpg  00015.jpg  00019.jpg

00004.jpg  00008.jpg  00012.jpg  00016.jpg

And an example prefixes (we aren't doing anything different):

$ touch track_{9..11}.mp3 && ls

track_10.mp3  track_11.mp3  track_9.mp3



$ rename 's/d+/sprintf("%02d", $&)/e' *.mp3 && ls

track_09.mp3  track_10.mp3  track_11.mp3