The article I linked to has the details, but basically you just pass your shell script string to the command. Note that special characters such as quotes, backslashes, spaces in file names, etc, need to be escaped (the backslash is AppleScript's escape character).
For example, your file command (1 gig, really?) would be something like:
Code:
do shell script "cd /Users/you/Desktop; mkfile 1g test.abc"