[X-Unix] So what is a dmg file?

[David Ledger]

>Date: Thu, 22 Jan 2004 10:02:34 -0500
>From: Victor Eijkhout <eijkhout at cs.utk.edu>
>Subject: Re: [X-Unix] So what is a dmg file?
>
>At 8:50 +0100 2004/01/22, Kirk McElhearn wrote:
>>I would suspect because the file command doesn't have file info for OS X?
>
>  From the man page it looks like it has a database. How hard would it
>be for Apple to update that?

It's not really a database.  It's a set of rules.  "look at the bits 
x into the file and if it reads 'abc' chances are it's a pqr file".
The rules are run against the target file in turn until a match is 
found.  There doesn't *have* to be a rule that matches a file type. 
Or more likely, the rule that matches for a type has to come after 
some other rule that also matches, and which rule that is, 
'depends'(tm).

Unix people defining a new file format are likely to want that format 
to be recognisable and include some identifier at the beginning as 
(from /usr/share/file/magic)
OBJ_ 12 bytes in means AOF object
LIB_ 12 bytes in means ALF library
As the Mac has two other ways of recognising file types, a magic way 
wasn't included for .dmg (deduced from the fact that it's not 
reliably recognised).


>From: "William H. Magill" <magill at mcgillsociety.org>
>... . Rememember, the 3.5 inch hard floppy was
>introduced in 82 or 83, only 10 years ago!

Wishful thinking I'm afraid William  :-)

David

-- 
David Ledger - Freelance Unix Sysadmin in the UK.
Chair of HPUX SysAdmin SIG of hpUG technical user group (www.hpug.org.uk)
dledger at ivdcs.co.uk (also dledger at ivdcs.demon.co.uk)
www.ivdcs.co.uk