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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

Beginner
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  • 2
    A for loop is not the right solution for this, is this a homework question? – jesse_b Jun 15 '22 at 17:58
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    You have two nested loops, so of course you get all pairings. You'd need to read once from both files on each iteration. Or just use paste a.txt b.txt – ilkkachu Jun 15 '22 at 18:01
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    Related: Why is using a shell loop to process text considered bad practice? – Stéphane Chazelas Jun 15 '22 at 18:16
  • @jesse_b is not a homework. I'm doing script for trunk port creation. For that I needed. I should take port id and host name to create the subport – Beginner Jun 15 '22 at 20:14
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    @Beginner In the comments, you mention that your data is different from what you show in the question. Could you please update the question with the actual data and expected result? If paste -d ' ' a.txt b.txt is "the wrong answer" for you, then please also explain why. There is absolutely no need for any for loop to solve this. – Kusalananda Jun 15 '22 at 20:39
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    Are these the same two files you talk about in your other recent question? If so, the two files may be of different lengths, and you should mention this in the question and incorporate this fact in the test data and the expected output. I also still lack an explanation as to why the obvious solution with paste is not acceptable to you. – Kusalananda Jun 16 '22 at 07:38
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    It would be helpful (to you) if you could answer my simple questions so that I understand your requirements and reasoning. Flagging any comment that says anything about paste or not using for loops or asking for further clarifications as "not needed" is not an optimal communication method. If we understood why you needed a for loop, then we would be able to formulate an answer that is ultimately helpful to you. As it stands right now, you have an answer that shows no fewer than 3 different ways to solve your issue (with variations), one of which employs a perfectly good for loop. – Kusalananda Jun 16 '22 at 09:28

1 Answers1

3

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
Kusalananda
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glenn jackman
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