On 03/28/2012 05:00 AM, Karel Zak wrote:
> On Tue, Mar 27, 2012 at 11:43:45AM -0700, Ted Ts'o wrote:
>> On Tue, Mar 27, 2012 at 01:52:51PM -0400, Michael Conrad wrote:
>>> I see how that could be useful if and only if you use exactly one
>>> "-s", but if you requested more than one "-s" they still come out in
>>> non-stable order.  It's like running a SQL query and having all the
>>> columns shifted because the first one was a NULL.  
>> Actually, it's more like running an SQL query without a SORTED BY and
>> assuming the order of the rows was stable.
Only if the data weren't in any normal form.  I say it's like columns
getting shifted because if you ask for 2 values you expect the field
offset to imply the type of the data.  If one field is NULL, the rows
shift upward, resulting in a value interpreted as the wrong type.

This is why I'm saying the "-o values" is only usable if you use exactly
one "-s", because any other scenario means every value returned has an
indeterminate type, which is garbage data.  IMO a tool should never
output garbage.  It should prevent or warn about illegitimate usage. 
Since the tool has been out there and in use for quite a while, I would
suggest a warning.

(patch attached)

>>> At the very least, I would appreciate it if the man page mentioned
>>> that "-o value" should only ever be used with exactly one "-s".
>> What I would suggest is that blkid explicitly state that the order of
>> fields or file systems (if more than one file system is selected) is
>> not defined, since that's something which is true in general, and not
>> just for "-o value".
>>
>> It might also be useful to have an EXAMPLES section which gives some
>> sample ways that blkid can be used.
>  Good idea. I'll update the man page.
>
>     Karel

Thanks!