shell - 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?

Example. Consider a text file, foo.txt, with three lines:

foo
bar
baz

The desired output is:

foo,bar,baz

The command I'm using now:

tr '
' ',' <foo.txt |sed 's/,$//g'

Ideally it would be something like this:

cat foo.txt |join ,

What's:

1. the most portable, concise, readable way.
2. the most concise way using non-standard unix tools.

Of course I could write something, or just use an alias. But I'm interested to know the options.

Answer 1

Perhaps a little surprisingly, paste is a good way to do this:

paste -s -d","

This won't deal with the empty lines you mentioned. For that, pipe your text through grep, first:

grep -v '^$' | paste -s -d"," -