Is there's a way to do line by line combination of two files using bash shell [duplicate]

Question

Please see example: Let's say I have File1.txt with three lines(a,b,c); File 2 also has 3 lines(1,2,3).

File1

a
b
c

File 2

1
2
3
...

I want to get A File3 like following:

File 3

a1
a2
a3
b1
b2
b3
c1
c2
c3
...

Many thanks!

Answer 1

Assuming bash 4.x:

#!/usr/bin/env bash
#              ^^^^-- NOT /bin/sh

readarray -t a <file1   # read each line of file1 into an element of the array "a"
readarray -t b <file2   # read each line of file2 into an element of the array "b"
for itemA in "${a[@]}"; do
  for itemB in "${b[@]}"; do
    printf '%s%s\n' "$itemA" "$itemB"
  done
done

On an older (pre-4.0) release of bash, you can replace readarray -t a <file1 and readarray -t b <file2 with:

IFS=$'\n' read -r -d '' -a a < <(cat -- file1 && printf '\0')
IFS=$'\n' read -r -d '' -a b < <(cat -- file2 && printf '\0')