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

Question

paste is good for this job:

paste -d " "  - - < filename

awk:

awk 'NR%2{printf "%s ",$0;next;}1' yourFile

note, there is an empty line at the end of output.

sed:

sed 'N;s/\n/ /' yourFile

Alternative to sed, awk, grep:

xargs -n2 -d'\n'

This is best when you want to join N lines and you only need space delimited output.

My original answer was xargs -n2 which separates on words rather than lines. -d (GNU xargs option) can be used to split the input by any singular character.

There are more ways to kill a dog than hanging. ^[1]

awk '{key=$0; getline; print key ", " $0;}'

Put whatever delimiter you like inside the quotes.

---

References:

1. Originally "Plenty of ways to skin the cat", reverted to an older, potentially originating expression that also has nothing to do with pets.

Here is my solution in bash:

while read line1; do read line2; echo "$line1, $line2"; done < data.txt

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