Parts of your function can be parallized and others cannot: E.g. you cannot append the images before you have downloaded them.
function DOWNLOAD_PAGE {
export PAGE=$1
for COLUMN in {0..1}
do
parallel wget -O "$PAGE"_"$COLUMN"_{}.jpg "http://www.webb$PAGE$COLUMN{}" ::: {0..2}
convert "$PAGE"_"$COLUMN"_*.jpg -append "$PAGE"__"$COLUMN".jpg
done
convert "$PAGE"__*.jpg +append "$PAGE"_done.jpg
}
export -f DOWNLOAD_PAGE
parallel -j10 DOWNLOAD_PAGE ::: {0041..0100}
A more parallelized version (but harder to read):
function DOWNLOAD_PAGE {
export PAGE=$1
parallel -I // --arg-sep /// parallel wget -O "$PAGE"_//_{}.jpg "http://www.webb$PAGE//{}"\; convert "$PAGE"_"//"_\*.jpg -append "$PAGE"__"//".jpg ::: {0..2} /// {0..1}
convert "$PAGE"__*.jpg +append "$PAGE"_done.jpg
}
export -f DOWNLOAD_PAGE
parallel -j10 DOWNLOAD_PAGE ::: {0041..0100}
Your understanding of what GNU Parallel does is somewhat misguided. Consider walking though the tutorial http://www.gnu.org/software/parallel/parallel_tutorial.html and then try to understand how the examples work: n1_argument_appending">http://www.gnu.org/software/parallel/man.html#example__working_as_xargs_n1_argument_appending