How to read 1 line from 2 files sequentially?

Question

How do I read from 2 files 1 line at a time? Say if I have file1 and file2 with following content:

file1:

line1.a    
line2.a   
line3.a   

file2:

line1.b   
line2.b   
line3.b   

How do I get output like this -

line1.a   
line1.b   
line2.a   
line2.b   
line3.a   
line3.b   
...
...

Answer 1

You can do it either via a pure bash way or by using a tool called paste:

Your files:

[jaypal:~/Temp] cat file1
line1.a
line2.a
line3.a
line4.a

[jaypal:~/Temp] cat file2
line1.b
line2.b
line3.b
line4.b

Pure Bash Solution using file descriptors:

<&3 tells bash to read a file at descriptor 3. You would be aware that 0, 1 and 2 descriptors are used by Stdin, Stdout and Stderr. So we should avoid using those. Also, descriptors after 9 are used by bash internally so you can use any one from 3 to 9.

[jaypal:~/Temp] while read -r a && read -r b <&3; do
> echo -e "$a\n$b";
> done < file1 3<file2
line1.a
line1.b
line2.a
line2.b
line3.a
line3.b
line4.a
line4.b

Paste Utility:

[jaypal:~/Temp] paste -d"\n" file1 file2
line1.a
line1.b
line2.a
line2.b
line3.a
line3.b
line4.a
line4.b

Answer 2

This might work for you (GNU sed though):

sed 'R file2' file1