Join several lines in a single one, inside a single file or from several files.
join is a utility in Unix systems that takes two files and a field number for each and outputs a file where the lines with an identical field in both are merged.
For example, given a file
a hey b how are you ? c ok
and a file
a hi b fine, thank you d er...
join -1 1 -2 1 a b outputs:
a hey hi b how are you ? fine, thank you
"Join" is also a common operation in text editors. Joining several lines means basically removing the end of lines between them (and optionally, replacing them with regular spaces), effectively making them a single line.