Batch rename files in terminal

The easy 1-line way:

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

You can also perform calculations:

ls | cat -n | while read n f; do mv "$f" "U+$(($n+61)).svg"; done

And another very simple bash one liner that keeps the original extensions, adds leading zeros, and also works in OSX:

num=0; for i in *; do mv "$i" "$(printf '%04d' $num).${i#*.}"; ((num++)); done

Happy hacking!