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

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 lik

Answer 1

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/'