How to switch/rotate every two lines with sed/awk? - regex

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

For the first part of the question, here is a one way to swap every other line with each other in sed without using regular expressions:
sed -n 'h;n;p;g;p'
The -n command line suppresses the automatic printing. Command h puts copies the current line from the pattern space to the hold space, n reads in the next line to the pattern space and p prints it; g copies the first line from the hold space back to the pattern space, bringing the first line back into the pattern space, and p prints it.

Answer 2

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.

Answer 3

First question:
awk '{x = $0; getline; print; print x}' filename
next question: sort by 2nd line
paste - - < filename | sort -f -t $'\t' -k 2 | tr '\t' '\n'
which outputs:
dobroye utro!
Good morning!
annyonghaseyo
hello
watashi
me

Answer 4

Assuming an input file like this:
A sentence X
Z matching sentence Y
A sentence Z
B matching sentence N
A sentence Z
M matching sentence N
You could do both exchange and sort with Perl:
perl -lne'
$_{ $_ } = $v unless $. % 2;
$v = $_;
END {
print $_, $/, $_{ $_ }
for sort keys %_;
}' infile
The output I get is:
% perl -lne'
$_{ $_ } = $v unless $. % 2;
$v = $_;
END {
print $_, $/, $_{ $_ }
for sort keys %_;
}' infile
B matching sentence N
A sentence Z
M matching sentence N
A sentence Z
Z matching sentence Y
A sentence X
If you want to order by the first line (before the exchange):
perl -lne'
$_{ $_ } = $v unless $. % 2;
$v = $_;
END {
print $_, $/, $_{ $_ }
for sort {
$_{ $a } cmp $_{ $b }
} keys %_;
}' infile
So, if the original file looks like this:
% cat infile1
me
watashi
hello
annyonghaseyo
Good morning!
dobroye utro!
The output should look like this:
% perl -lne'
$_{ $_ } = $v unless $. % 2;
$v = $_;
END {
print $_, $/, $_{ $_ }
for sort {
$_{ $a } cmp $_{ $b }
} keys %_;
}' infile1
dobroye utro!
Good morning!
annyonghaseyo
hello
watashi
me
This version should handle duplicate records correctly:
perl -lne'
$_{ $_, $. } = $v unless $. % 2;
$v = $_;
END {
print substr( $_, 0, length() - 1) , $/, $_{ $_ }
for sort {
$_{ $a } cmp $_{ $b }
} keys %_;
}' infile
And another version, inspired by the solution posted by Glenn (record exchange included and assuming the pattern _ZZ_ is not present in the text file):
sed 'N;
s/\(.*\)\n\(.*\)/\1_ZZ_\2/' infile |
sort |
sed 's/\(.*\)_ZZ_\(.*\)/\2\
\1/'