Concise and portable “join” on the Unix command-line

Question

How can I join multiple lines into one line, with a separator where the new-line characters were, and avoiding a trailing separator and, optionally, ignoring empty lines?

Answer 1

Simple way to join the lines with space in-place using ex (also ignoring blank lines), use:

ex +%j -cwq foo.txt

If you want to print the results to the standard output, try:

ex +%j +%p -scq! foo.txt

To join lines without spaces, use +%j! instead of +%j.

To use different delimiter, it's a bit more tricky:

ex +"g/^$/d" +"%s/\n/_/e" +%p -scq! foo.txt

where g/^$/d (or v/\S/d) removes blank lines and s/\n/_/ is substitution which basically works the same as using sed, but for all lines (%). When parsing is done, print the buffer (%p). And finally -cq! executing vi q! command, which basically quits without saving (-s is to silence the output).

Please note that ex is equivalent to vi -e.

This method is quite portable as most of the Linux/Unix are shipped with ex/vi by default. And it's more compatible than using sed where in-place parameter (-i) is not standard extension and utility it-self is more stream oriented, therefore it's not so portable.