Run a bash script on parallel N-defined processes

bash_parallel.sh

#!/bin/bash

#Write your function here, will pass arguments: $1, $2, ..
getalphabet() {
  > "$1"_alphabet.txt awk 'BEGIN{FS=""} {for(i=1;i<=NF;i++){chars[$(i)]=$(i);}} END{for(c in chars){print c;} }' $1 | sort >/dev/null
}

N=8;i=0
for f in `ls ar_*`; do
  ((i=i%N)); ((i++==0)) && wait
  getalphabet $f &
done