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

Question

At the Linux/Unix command line, I want to turn this:

A
B
C
A
B
C
.
.
.

into this:

A,B,C
A,B,C
...

Is there a simple built-in command to do this, or does this require diving into Awk, Perl, etc.?

Answer 1

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

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

Answer 2

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 ' '