Using 2 For loop print the output of 2 answers from two different files

Question

I have 2 files with some content/lines.

Example :

cat a.txt
  1
  2
  3
  4

cat b.txt
  a
  b
  c
  d

Need to get below output using for loop

 1 a
 2 b
 3 c
 4 d

I have tried the below script but it gives the wrong output

for i in `cat a.txt`
 do 
  for j in `cat b.txt`
   do 
     echo $i and $j
   done
 done

1 and a
1 and b
1 and c
1 and d
2 and a
2 and b
2 and c
2 and d
3 and a
3 and b
3 and c
3 and d
4 and a
4 and b
4 and c
4 and d

Kindly help me to do with For loop. Thanks in Advance

Answer 1

As @ilkkachu comments, paste is the best answer:

paste a.txt b.txt

or, with a single space as the delimiter in place of tab,

paste -d ' ' a.txt b.txt

For a shell-only answer, first don't read lines with for and Why is using a shell loop to process text considered bad practice?

You can use a single while loop to read lines from 2 files simultaneously

while IFS= read -r lineA <&3; IFS= read -r lineB <&4; do
    echo "$lineA $lineB"
done 3<a.txt 4<b.txt

if you like more whitespace for readability:

while IFS= read -r lineA <&3
      IFS= read -r lineB <&4
do
    echo "$lineA $lineB"
done 3<a.txt 4<b.txt

Another alternative: read each file into an array, then iterate over the array indices.

mapfile -t a < a.txt
mapfile -t b < b.txt
for idx in "${!a[@]}"; do printf '%s %s\n' "${a[idx]}" "${b[idx]}"; done