How to read a line from each of several files in each iteration of a single loop?

Question

I need to read from multiple file in one loop. I have one file with X coords, one file with Y coords and one file with chars on those coords.

For now I use pas

Answer 1

You can call read thrice, each from a different file descriptor.

# Use && to stop reading once the shortest file is consumed
while read -u 3 -r X &&
      read -u 4 -r Y &&
      read -u 5 -r CHAR; do
    ...
done 3< X.txt 4< Y.txt 5< CHAR.txt 

(-u is a bash extension, used for clarity. For POSIX compatibility, each call would look something like read -r X <&3.)