Shell Programming and Scripting

use 
thread
when
posting
do
no

use thread
thread when
when posting
posting do
do no
use thread when
thread when posting
when posting do
posting do no

cat demo.txt | xargs -n 2

use thread
when posting
do no

$ perl -ne 'chomp; push @a, $_; if ($#a >= 1) {@a = @a[@a-2..$#a]; print "$a[0] $a[1]\n"}' join.txt 
use thread
thread when
when posting
posting do
do no

$ perl -ne 'chomp; push @a, $_; if ($#a >= 2) {@a = @a[@a-3..$#a]; print "$a[0] $a[1] $a[2]\n"}' join.txt 
use thread when
thread when posting
when posting do
posting do no

awk '{if(NR>1) print t,$0;t=$0}' join.txt
use thread
thread when
when posting
posting do
do no

awk '{if(NR>2) print s,t,$0;s=t;t=$0}' join.txt
use thread when
thread when posting
when posting do
posting do no

awk '{if(NR>3) print r,s,t,$0;r=s;s=t;t=$0}' join.txt
use thread when posting
thread when posting do
when posting do no