Move every first row down to the second row - awk

Question

I have the following sample text:
a b c
x_y_
d e f
x_y_
g h i
x_y_
k l m
x_y_
I need it to be formatted as follows:
x_y_ a b c
x_y_ d e f
x_y_ g h i
x_y_ k l m
Using sed, awk or something else in bash, how do we accomplish this?

Answer 1

Another awk:
$ awk 'NR%2==0{print $0,p}{p=$0}' file
Output:
x_y_ a b c
x_y_ d e f
x_y_ g h i
x_y_ k l m
Explained:
$ awk '
NR%2==0 { # on every even numbered record
print $0,p # output current record and previous
}{
p=$0 # buffer record for next round
}' file
Update:
In case of odd number of records (mostly due to the peer pressure :), you need to deal with the left-over x y z:
$ awk 'NR%2==0{print $0,p}{p=$0}END{if(NR%2)print}' file
Output:
...
x_y_ g h i
x_y_ k l m
x y z

Answer 2

With sed:
sed -E 'N;s/(.*)\n(.*)/\2 \1/g' sample.txt

Answer 3

a short pipeline:
tac file | paste -d ' ' - - | tac

Answer 4

$ awk 'NR%2{s=$0; next} {print $0, s}' file
x_y_ a b c
x_y_ d e f
x_y_ g h i
x_y_ k l m

Answer 5

1st solution: Could you please try following, tested and created with GNU awk.
awk -v RS="" -v FS="\n" '{for(i=2;i<=NF;i+=2){printf("%s\n",$i OFS $(i-1))}}' Input_file
OR(with print):
awk -v RS="" -v FS="\n" '{for(i=2;i<=NF;i+=2){print $i,$(i-1)}}' Input_file
2nd solution: By checking if a line number is completely divided by 2 then print previous and current lines values. It also checks if total number of lines are ODD in Input_file then it prints last remaining line too(by checking a flag(variable)'s status).
awk 'prev && FNR%2==0{print $0 OFS prev;prev="";next} {prev=$0} END{if(prev){print prev}}' Input_file
Output will be as follows.
x_y_ a b c
x_y_ d e f
x_y_ g h i
x_y_ k l m

Answer 6

This might work for you (GNU sed):
sed '1~2{h;d};G;s/\n/ /' file
Save odd line numbered lines in the hold space and append them to even numbered lines and replace the newline with a space.
Another variation:
sed -n 'h;n;G;s/\n/ /p' file
There are many more ways to achieve this, as can be seen by answers above.
How about this:
parallel -N2 echo "{2} {1}" :::: file
See here for parallel.