How to concatenate multiple lines of output to one line?

Question

If I run the command cat file | grep pattern, I get many lines of output. How do you concatenate all lines into one line, effectively replacing each \"\\n

Answer 1

This is an example which produces output separate by commas. You can replace the comma by whatever separator you need.

cat < 1
> 2
> 3
> 4
> 5
> EOD

produces:

1,2,3,4,5