1- In one line :
ls | cat -n | while read n f; do mv "$f" "$n.extension"; done
you can also use ls -uT
to sort files by creation date
2- With the 'rename' command
rename -N 0001 -X 's/.*/$N/' *.jpg
3- With a script
a=1
for i in *.jpg; do
new=$(printf "%04d.jpg" "$a") #04 pad to length of 4
mv -i -- "$i" "$new"
let a=a+1
done