If you plan to use rm -rf I suggest using the quoted form command to quote the path:

Example:
set myUserName to (do shell script "whoami")
do shell script "rm -r " & (quoted form of ("/Users/" & myUserName & "/Library/Cookies/" as string))

The shell script line ends up looking something like this: "rm -r '/Users/adminx/Library/Cookies/'"

Also the file at /System/Library/ScriptingAdditions/StandardAdditions.osax has more than a few useful commands you might want to check out.

EX: set myUserName to short user name of (system info)

This is not to say I am condoning using the unix layer, but merely pointing out another way, and other options.

Have fun-