Stan Hoeppner wrote: > I'm not very skilled at writing shell scripts. > > #! /bin/sh > for k in $(ls *.JPG); do convert $k -resize 1024 $k; done
First off don't use ls to list files matching a pattern. Instead let
the shell match the pattern.
#! /bin/sh
for k in *.JPG; do convert $k -resize 1024 $k; done
I never like to resize in place. Because then if I mess things up I
can lose resolution. So I recommend doing it to a named resolution
file. Do anything you like but this would be the way I would go.
for k in *.JPG; do
convert $k -resize 1024 $(basename $k .JPG).1024.jpg
done
And not wanting to do the same work again and again:
for k in *.JPG; do
base=$(basename $k .JPG)
test -f $base.1024.jpg && continue # skip if already done
convert $k -resize 1024 $base.1024.jpg
done
> I use the above script to batch re-size digital camera photos after I
> dump them to my web server. It takes a very long time with lots of new
> photos as the server is fairly old, even though it is a 2-way SMP,
> because the script only runs one convert process at a time serially,
> only taking advantage of one CPU. The convert program is part of the
> imagemagick toolkit.
>
> How can I best modify this script so that it splits the overall job in
> half, running two simultaneous convert processes, one on each CPU?
> Having such a script should cut the total run time in half, or nearly
> so, which would really be great.
GNU xargs has an extension to run jobs in parallel. This is already
installed on your system. (But won't work on other Unix systems.)
for k in *.JPG; do echo $k; done | xargs -I{} -P4 echo convert {} -resize
1024 {}
Verify that does what you want and then remove the echo.
unfortunately that simple approach is harder to do with my renaming
scheme. So I would probably write a helper script that did the
options to convert and renamed the file and so forth.
for k in *.JPG; do
base=$(basename $k .JPG)
test -f $base.1024.jpg && continue # skip if already done
echo $k;
done | xargs -L1 -P4 echo my-convert-helper
And my-convert-helper could take the argument and apply the options in
the order needed and so forth.
Adjust 4 in the above to be the number of jobs you want to run on your
multicore system. Note that in Sid in the latest coreutils there is a
new command 'nproc' to print out the number of cores. Or you could
get it from grep.
grep -c ^processor /proc/cpuinfo
All of the above is off the top of my head and needs to be tested.
YMMV. But hopefully it will give you some ideas.
HTH,
Bob
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