Linux/Unix command to join N lines of input with delimiters?

Don't know such built-in. Using Bash you can:

while read a && read b && read c ; do echo $a,$b,$c ; done < file

That is horrible, surely something like

tr \\n ' '

would do the same thing?

So for e.g. to join lines of the file file_lines use

cat file_lines | tr \\n ' '

POSIX solution with pr:

pr -3 -a -t -s, file

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/pr.html

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