How to merge every two lines into one from the command line?

There are more ways to kill a dog than hanging. ^[1]

awk '{key=$0; getline; print key ", " $0;}'

Put whatever delimiter you like inside the quotes.

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References:

1. Originally "Plenty of ways to skin the cat", reverted to an older, potentially originating expression that also has nothing to do with pets.

awk:

awk 'NR%2{printf "%s ",$0;next;}1' yourFile

note, there is an empty line at the end of output.

sed:

sed 'N;s/\n/ /' yourFile

Alternative to sed, awk, grep:

xargs -n2 -d'\n'

This is best when you want to join N lines and you only need space delimited output.

My original answer was xargs -n2 which separates on words rather than lines. -d (GNU xargs option) can be used to split the input by any singular character.

paste is good for this job:

paste -d " "  - - < filename