Substitute in a file according to a pattern and save it back to that file, while at the same time making a backup called %file%.back

change_in_place.sh

#!/usr/bin/zsh
#You don't need to use / as a delimiter, especially not when it's not convenient.

sudo sed -i.back 's#<img src="#<img src="http://mitpress.mit.edu/sicp/full-text/book/#g' book.html