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From: mop2 on 16 Feb 2010 10:30
On Tue, 16 Feb 2010 12:15:29 -0200, Hongyi Zhao
<hongyi.zhao(a)gmail.com> wrote:

> On Tue, 16 Feb 2010 05:25:55 -0800 (PST), Janis
> <janis_papanagnou(a)hotmail.com> wrote:
>
>> $ grep '[ \t]' <<< $'\v' | od -c
>> 0000000
>>
>> $ grep '[[:space:]]' <<< $'\v' | od -c
>> 0000000 \v \n
>> 0000002
>
> Excellent examples but obscure for newbie like me to figure out its
> meaning, especially the latter part of your codes: <<< $'\v' | od
> -c.
> Any hints?

Yes!
To shell $'\v' is a byte, as '\n' is.

Don't be root to play and shell can teach a lot with a relatively
low risk.

For example, try in the command line:

echo -e 'a\nb' # and see

Idem for:
echo -e 'a\vb'

"od" is to shell an extern command.
In the command line you can try:

od --help

Or/and:
man od

You can try previously commands with "echo" before it, to a preview.
For example:
for i in [??][??]*.pdf;do echo something $i;done

If you see what you want, then you can remove the 'echo' word
and the command will be executed.

From: Janis on 16 Feb 2010 11:34
On 16 Feb., 15:15, Hongyi Zhao <hongyi.z...(a)gmail.com> wrote:
> On Tue, 16 Feb 2010 05:25:55 -0800 (PST), Janis
>
> <janis_papanag...(a)hotmail.com> wrote:
> >$ grep '[ \t]' <<< $'\v' | od -c
> >0000000
>
> >$ grep '[[:space:]]' <<< $'\v' | od -c
> >0000000  \v  \n
> >0000002
>
> Excellent examples but obscure for newbie like me to figure out its
> meaning, especially the latter part of your codes: <<< $'\v' | od -c.

The part

CMD <<< STRING

is similar to the ancient but often used

echo STRING | CMD

The $'\CHAR' is used to express control characters \CHAR, like \n for
newline or \t for tab.

So what I do is to feed a control character \v into the grep command.
Why I do this is to show that \v is part of the [:space:] character
class.

Because the output of the grep command will contain a "non-printable"
control code, the \v character, I use the od(1) command to make it
visible.

Janis

> Any hints?
> --
> .: Hongyi Zhao [ hongyi.zhao AT gmail.com ] Free as in Freedom :.

From: Janis on 16 Feb 2010 08:25
On 16 Feb., 14:13, Hongyi Zhao <hongyi.z...(a)gmail.com> wrote:
> Hi all,
>
> What's the differences between [[:space:]] and [ \t ] when using both
> in regex pattern?
> --
> .: Hongyi Zhao [ hongyi.zhao AT gmail.com ] Free as in Freedom :.

$ grep '[ \t]' <<< $'\v' | od -c
0000000

$ grep '[[:space:]]' <<< $'\v' | od -c
0000000 \v \n
0000002

IOW; [:space:] defines a character class that may contain more
characters than explicitly listed in your character enumeration.

Janis
From: Ed Morton on 16 Feb 2010 08:31
On 2/16/2010 7:13 AM, Hongyi Zhao wrote:
> Hi all,
>
> What's the differences between [[:space:]] and [ \t ] when using both
> in regex pattern?

http://en.wikipedia.org/wiki/Regular_expression#POSIX_character_classes
From: Chris F.A. Johnson on 16 Feb 2010 11:47
On 2010-02-16, Janis wrote:
> On 16 Feb., 15:15, Hongyi Zhao <hongyi.z...(a)gmail.com> wrote:
>> On Tue, 16 Feb 2010 05:25:55 -0800 (PST), Janis
>>
>> <janis_papanag...(a)hotmail.com> wrote:
>> >$ grep '[ \t]' <<< $'\v' | od -c
>> >0000000
>>
>> >$ grep '[[:space:]]' <<< $'\v' | od -c
>> >0000000 ?\v ?\n
>> >0000002
>>
>> Excellent examples but obscure for newbie like me to figure out its
>> meaning, especially the latter part of your codes: <<< $'\v' | od -c.
>
> The part
>
> CMD <<< STRING
>
> is similar to the ancient but often used
>
> echo STRING | CMD

It's a non-portable equivalent of the standards-compliant here document:

CMD <<EOF
STRING
EOF

> The $'\CHAR' is used to express control characters \CHAR, like \n for
> newline or \t for tab.

But it's not standard.


--
Chris F.A. Johnson, author <http://shell.cfajohnson.com/>
===================================================================
Shell Scripting Recipes: A Problem-Solution Approach (2005, Apress)
Pro Bash Programming: Scripting the GNU/Linux Shell (2009, Apress)
===== My code in this post, if any, assumes the POSIX locale =====
===== and is released under the GNU General Public Licence =====
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