How do I rename a file, or a group of files?

The first thing to remember is that it is the Finder is the application that handles the naming of files. Suppose you have a file called “Monterey” on your desktop and you want to rename it “Eden” via AppleScript. The following would give you an error:

set the name of file "Monterey" to "Eden"

This is because setting a file name is a Finder task. Putting the same line inside a Finder tell-block fixes the problem:

tell application "Finder"
	set the name of file "Monterey" to "Eden"
end tell

Another example: using AppleScript’s Text Item Delimiters you could rename a file with any extension (.tif, .tiff, .psd, etc) to its corresponding .jpg filename using an approach like this:

tell application "Finder"
	set theFile to file "Disneyland.tiff"
	
	set oldDelims to AppleScript's text item delimiters
	set AppleScript's text item delimiters to {"."}
	
	try
		set fileName to name of theFile --> Disneyland.tiff
		set nameWithoutExtension to first text item of fileName --> Disneyland
		set newName to nameWithoutExtension & ".jpg " --> Disneyland.jpg
		set name of theFile to newName --> rename the file
		set AppleScript's text item delimiters to oldDelims
	on error
		set AppleScript's text item delimiters to oldDelims
	end try
end tell

You could use a repeat loop to rename a folder of files in this way. Always remember to put the naming of files in a Finder tell-block!

For more advanced file-renaming routines, you may consider learn a bit of shell scripting and use of the do shell script command, which is fastest than the Finder.