How can I replace an application icon programmatically?

Question

With the transition to Big Sur, the standard icon style for applications has changed to the rounded-square motif. Not a fan, I like to replace a few applications' icons with their older variants. I've typically done this with the "Get Info / Drag Icon" method (call it GIDI) similar to what is described in this question and answer. However, I'd like to automate the process instead of the manual one.

I've noticed that after the GIDI method, there is a file Icon^M in the application's root folder. I can't determine what kind of file it is since file doesn't recognize it and it has zero size; otherwise, I'd presume it to be a copy of the .icns file. Looking in the Application's Resources folder, I see that the GIDI method doesn't replace the application's standard icon, so something else is happening.

So that being said, what is the Icon^M file and is there a way I can do this replacement programmatically?

Note: I know that Apple's own applications that come with the macOS are protected under SIP and thus I would have to disable that for those icons. Those are out of scope for this; I'm only interested in 3rd party applications which do not require SIP to be disabled.

Answer 1

I found a tool that makes this scriptable: fileicon.

In short, if you have a .icns file you can do something like:

bash$ fileicon set /Applications/Microsoft\ Word.app icon_i_prefer.icns

...and it will do the resource fixing similar to what the 'Get Info / Drag Icon' routine I describe in the question does.

Problem solved!