Re: unusual uniq results

[Simon Valiquette <v.simon@ieee.org>]

William Stanard a écrit :
> I'm confused. To me the > has always meant to take what is to the left of
> the sign and redirect its output to whatever is to the right.

   That's basically what it does.  Actually, it also create an empty 
file or erase it if it already exist.

> In my case
> this would mean taking the output of uniq dup_num and redirecting it. Can
> you set me straight?
> -- Bill
>

   It's the opposite.  It creates the redirection first, even before 
knowing if you ran a valid command.

   What you don't seems to understand, is that the ">" ask the shell to 
create an empty file _before_ executing uniq.  It means that the data in 
"dup_num" are already overwritten when uniq is executed, and thus uniq 
is opening an empty file.


Simon Valiquette
http://gulus.USherbrooke.ca



> Stone <xstonedogx@gmail.com> writes:
> 
>>The redirection takes place before your command.  So basically what
>>you told the system to do was:
>>
>>1.  Create the file dup_num.  If the file exists overwrite it.
>>2.  Redirect stdout of "uniq dup_num" into dup_num.
>>
>>Since you'd already overwritten the data in dup_num, there was nothing
>>for uniq to process.
>>
>>On Tue, 28 Sep 2004 11:09:12 -0400, William Stanard
>><wstanard@palmertrinity.org> wrote:
>>
>>>In doing a demo before a class (Linux Red-hat 2.4.18-14), I used the
>>
>>uniq
>>
>>>command on a file (dup_nums) that consisted of twelve lines, each line
>>>containing a number, from one to 9. I repeated the numbers, 6, 8, and 9.
>>>The std output showed the expected list of numbers, all duplicates
>>>removed. At a student's suggestion, I ran uniq again, but this time
>>>directed the output to the file itself....
>>>
>>>uniq dup_num > dup_num
> 
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