How to merge every two lines into one from the command line?

Question

I have a text file with the following format. The first line is the \"KEY\" and the second line is the \"VALUE\".

KEY 4048:1736 string
3
KEY 0:1772 string
1         


        

Answer 1

Here is another way with awk:

awk 'ORS=NR%2?FS:RS' file

$ cat file
KEY 4048:1736 string
3
KEY 0:1772 string
1
KEY 4192:1349 string
1
KEY 7329:2407 string
2
KEY 0:1774 string
1

$ awk 'ORS=NR%2?FS:RS' file
KEY 4048:1736 string 3
KEY 0:1772 string 1
KEY 4192:1349 string 1
KEY 7329:2407 string 2
KEY 0:1774 string 1

As indicated by Ed Morton in the comments, it is better to add braces for safety and parens for portability.

awk '{ ORS = (NR%2 ? FS : RS) } 1' file

ORS stands for Output Record Separator. What we are doing here is testing a condition using the NR which stores the line number. If the modulo of NR is a true value (>0) then we set the Output Field Separator to the value of FS (Field Separator) which by default is space, else we assign the value of RS (Record Separator) which is newline.

If you wish to add , as the separator then use the following:

awk '{ ORS = (NR%2 ? "," : RS) } 1' file