From: Jeremy Bopp <jeremy@...>
Date: 2012-02-07T05:14:59+09:00
Subject: Re: Make a file read-only?

On 02/06/2012 01:38 PM, Intransition wrote:
> Hi, I can make a file writeable easy enough,
> 
>     mode = File.stat(file).mode
>     File.chmod(mode | 0000200, file)
> 
> But how do I make a file read-only?
> 
>     mode = File.stat(file).mode
>     File.chmod(mode - 0000222, file)
> 
> Subtraction doesn't work. And I've tried `&`, `|` and `^`. Do I have to
> do some sort compliments operation here? What's the trick to this?
> 
> FYI, on the command line it's as easy as:
> 
>   $ chmod a-w file

More specifically, then, you want to remove the write permission from
the file, leaving it whatever other permissions were already set
(possibly none!).  In other words, you want to /mask out/ the write bits
from whatever the current file mode is.  Try the following:

mode = 0100664
mask = mode ^ 0222
new_mode = mode & mask

-Jeremy