[syslog-ng] Using macro substring as another macro

[Jakub Jankowski]

Friday 08 of April 2011 00:51:23 Balazs Scheidler napisał(a):
> On Thu, 2011-04-07 at 15:01 +0200, Jakub Jankowski wrote:
> > I'd like to implement simple hashing in file() destination, mainly to
> > avoid ext3 32k directories limit, but also for convenience. Is it
> > possible to do something simliar what this shell snippet does:
> > $ NAME=value
> > $ echo ${NAME[*]:0:1}
> > v
> > $
> > but for macros (especially those coming from parsers)?
[...]
> hmm.... thinking out loud. there are basically two syntactic options:
>
> 1) to use the new template function syntax

I had a look at the code yesterday, using Gergely's pointers from the other 
mail (thanks for that!), but one thing stopped me from doing some pointer 
arithmetics there myself - I wasn't sure how it'd work for unicode (i.e. 
multibyte) strings. Also pity that glib doesn't have a g_substr() :)

> 2) to continue extending our bash-like expansion syntax.
>
> The 1) option would be something like:
>
> $(cut -c 1-10 $NAME)
>
> or
>
> $(substr $NAME 1 10)

As for the syntax - I'd stick with substr <string> <offset> <length>, just 
like Perl, PHP, or MySQL do.

> The alternative would be to add really simple functions as an addition
> to the basic template syntax. We currently have the "default" values
> like this way:
>
> ${NAME:-default}
>
> this results in "default" if $NAME is not-defined or empty.
>
> The template function stuff is easier to write (as you can write plugins
> for that, and there's a whole framework to support it). The bash
> syntax-like stuff is more difficult to write as the template parser is
> manually coded, and each expansion mode has to be open-coded, although
> it has better performance (since we can avoid a copy of $NAME into a
> temporary buffer).

Efficiency is one of the factors I need to consider, so I'd love to see that 
${macro:offset:len} working eventually, but for a starter, having it even as 
a (slower) template function would be neat too. Do you think syslog-ng could 
have both implemented $someday?

> I'm somewhat undecided. What do others think? Can you also propose a
> syntax for 1) above? I wouldn't copy the bash case exactly, as that is
> an array operator if I'm right, and syslog-ng doesn't (yet) support
> arrays in name-value pairs.

It works with simple syntax as well ${foo:0:1}; in fact - ${foo[*]:x:y} 
doesn't work in bash < 3.1. I wouldn't mind syslog-ng using the simpler 
form :)


Regards,

-- 
Jakub Jankowski|shasta at toxcorp.com|http://toxcorp.com/
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