bash shell script two variables in for loop

Question

I am new to shell scripting. so kindly bear with me if my doubt is too silly.

I have png images in 2 different directories and an executable which takes an images f

Answer 1

If you are depending on the two directories to match up based on a locale sorted order (like your attempt), then an array should work.

im1_files=(~/prev1/*.png)
im2_files=(~/prev3/*.png)

for ((i=0;i<=${#im1_files[@]};i++)); do
   run_black.sh "${im1_files[i]}" "${im2_files[i]}"
done