Insert lines from an input file every n lines in awk

Question

With input files A and B as such:

File A

1 1 1
2 2 2
3 3 3

File B

10 10 10
20 20 20
30 30 30
40 40 40
50 50 50
60 60 60 
70 70 70

I would like to insert one line of file A every 3 lines of file B, starting with the first line being the first line of file A so that the output would like so:

1 1 1
10 10 10
20 20 20
30 30 30
2 2 2
40 40 40
50 50 50
60 60 60 
3 3 3 
70 70 70

My original attempt at this involved creating separate files for every three lines, but my dataset is extremely large so I'm interested in saving hard drive space.

Answer 1

If you don't need to use awk, paste could work:

$ paste -d '\n' <fileB fileA - - - | tr -s '\n' >fileC

The tr part isn't strictly necessary, so you can remove it if you don't mind extra newlines.

Answer 2

Sounds like a good case for getline:

awk '(NR-1)%3 == 0 {getline line < "FileA"; print line} 1' FileB
1 1 1
10 10 10
20 20 20
30 30 30
2 2 2
40 40 40
50 50 50
60 60 60 
3 3 3
70 70 70

Answer 3

With GNU sed’s R command:

sed -e $'R fileb\nR fileb\nR fileb' filea > output