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

Perl:

cat data.txt | perl -pe 'if(!eof){chomp;$_.=","}'

or yet shorter and faster, surprisingly:

cat data.txt | perl -pe 'if(!eof){s/\n/,/}'

or, if you want:

cat data.txt | perl -pe 's/\n/,/ unless eof'