Question

Can any one help me in sorting this out in sed/awk/perl

Input file

Start variable 1
End   variable 2
A
Z
Next 

Start variable b
End   variable a 
X
Y
Next 
...
...
...

Start variable m
End   variable n
M
N
Next

If a pattern match End found, swap with its consecutive line

Output file :

Start variable 1
A
End variable 2 
Z
Next

Start variable b
X
End variable a
Y
Next
...
...
...

Start variable m
M
End variable n
N
Next

Answer 1

Simple Perl solution. $n stands for “now”, $b for “before”, i.e. “the previous line was End”.

perl -ne 'print unless $n = /^End$/; print "End\n" if $b; $b = $n'

For the updated version of your question, just add $l to remember the line to be printed:

perl -ne 'print unless $n = /^End /; $l = $_ if $n ; print $l if $b; $b = $n'

Or, equivalen but shorter:

perl -ne '$n = /^End / and $l = $_ or print; $b and print $l; $b = $n'