Command Line Editing
Substitution changes any previous command:
host% !7:s/0/1
cat /home/ths/sunclass1/basics/history/congrats
History list should be similar to the following:
7 cat /home/ths/sunclass0/basics/history/congrats
8 cat /home/ths/sunclass1/basics/history/congrats
9 history
10 cat /home/ths/sunclass1/basics/history/congrats
11 history