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

How about to use xargs?

for your case

$ cat foo.txt | sed 's/$/, /' | xargs

Be careful about the limit length of input of xargs command. (This means very long input file cannot be handled by this.)