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:

a hey
b how are you ?
c ok

and a file b:

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 an 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.

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