Re: [gentoo-amd64] How to use &&

[Martin Herrman]

On Sun, Jan 4, 2009 at 3:09 PM, Paul Stear <gentoo@××××××××××××.com> wrote:

> Thanks for your reply Martin.  I have looked at your scripts and I might be
> able to use then in the future.  My main problem is that my collection is so
> large, e.g 3104 files just in the directory beginning with "D" and that take
> about 4+ hours to run. So I could do with operating on 3  directories
> overnight.
> I must be able to add commands to be operated on in sequence.
>
> Any ideas?
>
> Paul

E.g. files are in /home/Fred Music/mp3; script is in /home/Fred
Music/script.sh; script is run from /home/Fred Music. The following
script would work (not tested!):

======= script 1 ========
export IFS=$'\n'

for file in `find "/home/Fred Music/mp3" -type f -iregex '.*\.mp3'`;
do
   mp3gain --auto "${file}"
done
======== script 1 ==========

But: mp3gain is CPU-bound and this script won't use your 2 or 4 cores (right?)

You want to solve this by using &&, but this doesn't seem to work. &&
is just 'AND'. So A && B is true if and only if A is true and B is
true. So is the first command returns true, the second has to be
executed as well to verify if A && B is true. But if A is false, we
don't need to execute B, because A && B can never be true (because A
was already false).

Maybe your A doesn't return true?

But after all: this won't make use of your dual or quad core machine's power.

If you only have a single core available: use above 'script 1'. If you
have more cores available, you might want to use the idea that is used
in ogg2mp3:

========= script 2 (pseudo code between ")  =======

export IFS=$'\n'

function Gain {
  "given left and right value, process(mp3gain) these elements of the
global array"
}

declare global array

for file in `find "/home/Fred Music/mp3" -type f -iregex '.*\.mp3'`;
do
   "put file in the global array"
done

"divide array length among available cores"

i =0
while i < numcores do
  "call function Gain" &
done

========= script 2 =======

The "&" in the function call will create a child process and make it
use all your cores.

HTH,

Martin