From: Jeremy Bopp <jeremy@...>
Date: 2012-02-08T15:46:39+09:00
Subject: Re: Make a file read-only?

On 02/07/2012 04:20 PM, Robert Klemme wrote:
> On Mon, Feb 6, 2012 at 9:14 PM, Jeremy Bopp <jeremy@bopp.net> wrote:
>> On 02/06/2012 01:38 PM, Intransition wrote:
>> 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
> 
> Interesting approach.  I usually do it like this:
> 
> irb(main):038:0> mode = 0100664
> => 33204
> irb(main):039:0> mask = 0222
> => 146
> irb(main):040:0> new_mode = (mode | mask) ^ mask
> => 33060
> irb(main):041:0> "%07o" % new_mode
> => "0100444"

As far as I can tell, these approaches are functionally equivalent in
both result and efficiency.  Folding the method I used all together
using your settings for mode and mask makes mine as follows:

new_mode = mode & (mode ^ mask)

I suspect that someone who more recently completed their college
philosophy of logic class than I might even remember how to derive one
of our solutions from the other using tidy boolean algebra.  I'm too
rusty for it at the moment unfortunately, but I did give it a try. :-(

Maybe I'll pick at it more tomorrow...

-Jeremy