GNU bug report logs - #6393
The meaning of --key of sort command?

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Peng Yu <pengyu.ut <at> gmail.com>
To: bug-coreutils <at> gnu.org
Subject: The meaning of --key of sort command?
Date: Thu, 10 Jun 2010 11:39:11 -0500

Hi,

I don't think that I completely understand what key means. In the
following example, I thought that --key=2 should order the lines by
the 2nd letter in each line without reordering the lines with the same
2nd letter. But it turns out my understanding is not correct. For
example, "u a" was before "a a" sorting, but "u a" is after "a a"
after sorting.

According to the man page, I'd think the sorting is based on the 2nd
column only but the 1st column. Why the 1st column matters in this
case?

-k, --key=POS1[,POS2]
              start a key at POS1 (origin 1), end it at POS2 (default  end  of
              line)


$ cat input.txt
u a
c c
a a
e e
p a
m e
a a
l e
a a
$ sort --key=2 input.txt
a a
a a
a a
p a
u a
c c
e e
l e
m e


-- 
Regards,
Peng

Message #8 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Peng Yu <pengyu.ut <at> gmail.com>
To: bug-coreutils <at> gnu.org
Subject: Re: The meaning of --key of sort command?
Date: Thu, 10 Jun 2010 11:45:36 -0500

On Thu, Jun 10, 2010 at 11:39 AM, Peng Yu <pengyu.ut <at> gmail.com> wrote:
> Hi,
>
> I don't think that I completely understand what key means. In the
> following example, I thought that --key=2 should order the lines by
> the 2nd letter in each line without reordering the lines with the same
> 2nd letter. But it turns out my understanding is not correct. For
> example, "u a" was before "a a" sorting, but "u a" is after "a a"
> after sorting.
>
> According to the man page, I'd think the sorting is based on the 2nd
> column only but the 1st column. Why the 1st column matters in this
> case?
>
> -k, --key=POS1[,POS2]
>              start a key at POS1 (origin 1), end it at POS2 (default  end  of
>              line)
>
>
> $ cat input.txt
> u a
> c c
> a a
> e e
> p a
> m e
> a a
> l e
> a a
> $ sort --key=2 input.txt
> a a
> a a
> a a
> p a
> u a
> c c
> e e
> l e
> m e

I forget to mention the following command gives me the same thing
(sorting on the 2nd fieled then the 1st field). But according to info
sort, it  should not the case.

   Example:  To sort on the second field, use `--key=2,2' (`-k 2,2').
     See below for more examples.


$ sort --key=2,2 input.txt
a a
a a
a a
p a
u a
c c
e e
l e
m e



-- 
Regards,
Peng

Message #11 received at 6393 <at> debbugs.gnu.org (full text, mbox):

From: Eric Blake <eblake <at> redhat.com>
To: Peng Yu <pengyu.ut <at> gmail.com>
Cc: 6393 <at> debbugs.gnu.org
Subject: Re: bug#6393: The meaning of --key of sort command?
Date: Thu, 10 Jun 2010 11:01:51 -0600

[Message part 1 (text/plain, inline)]

On 06/10/2010 10:39 AM, Peng Yu wrote:
> Hi,
> 
> I don't think that I completely understand what key means. In the
> following example, I thought that --key=2 should order the lines by
> the 2nd letter in each line without reordering the lines with the same
> 2nd letter. But it turns out my understanding is not correct. For
> example, "u a" was before "a a" sorting, but "u a" is after "a a"
> after sorting.

'--key=2' implies a primary sort key from the second field (not second
letter), through the end of the line, with a secondary sort key of the
entire line.  '--key=2,2' implies a primary sort key of just the second
field, with a secondary sort key of the entire line.  You want
'--key=2,2 --stable', which disables the secondary sort over the entire
line.

-- 
Eric Blake   eblake <at> redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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