Question

I have been doing this by hand and I just can’t do it anymore– I have thousands of lines and I think this is a job for sed or awk.

Essentially, we have a file like this:

A sentence X
A matching sentence Y
A sentence Z
A matching sentence N

This pattern continues for the entire file. I want to flip every sentence and matching sentence so the entire file will end up like:

A matching sentence Y
A sentence X
A matching sentence N
A sentence Z

Any tips?

edit: extending the initial problem

Dimitre Radoulov provided a great answer for the initial problem. This is an extension of the main problem– some more details:

Let’s say we have an organized file (due to the sed line Dimitre gave, the file is organized). However, now I want to organize the file alphabetically but only using the language (English) of the second line.

watashi 
me
annyonghaseyo
hello
dobroye utro!
Good morning!

I would like to organize alphabetically via the English sentences (every 2nd sentence). Given the above input, this should be the output:

dobroye utro!
Good morning!
annyonghaseyo
hello
watashi
me 

Answer 1

sed 'N; 
s/\(.*\)\n\(.*\)/\2\
\1/' infile

N – append the next line of input into the pattern space
\(.*\)\n\(.*\) – save the matching parts of the pattern space
the one before and the one after the newline.
\2\\
\1 – exchange the two lines (\1 is the first saved part,
\2 the second). Use escaped literal newline for portability

With some sed implementations you could use the escape sequence
\n: \2\n\1 instead.