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

I needed to accomplish something similar, printing a comma-separated list of fields from a file, and was happy with piping STDOUT to xargs and ruby, like so:

cat data.txt | cut -f 16 -d ' ' | grep -o "\d\+" | xargs ruby -e "puts ARGV.join(', ')"