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How to swap order of pairs of lines?

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# 1  
MySQL How to swap order of pairs of lines?

This seems to be a question whose answer uses sed or awk.

For a file like:

How to swap the order of the line pairs, to end up with:

All lines from the original file need to wind up in the output file.
# 2  
Hi Try this,

awk '{if(NR%2){a=$0}else{print $0"\n"a;a=""}} END {if(a!=""){print a}}' inputfile

# 3  
awk '{p=$0}getline;{print p}' infile

# 4  
 ruby -ne 'prv=$_;print gets;print prv'  file

# 5  
Shouldn't that be:
ruby -ne 'prv=$_;print if gets;print prv'  file

# 6  
it does not matter since we are iterating the file anyway.
# 7  
It seems to matters on the last line. Without the if statement I get:

If there is an odd number of lines..

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