sed: replacing newline at a given line (or last line)

Question

I have a file like the following:

a,b,c,d,e,f
1,2,3,4,5,6
7,8,9,0,1,2

I need to join the last two lines with a comma, like so:

a,b,c,d,e,f
1,2,3,4,5,6,7,8,9,0,1,2

The problem is that this is all inside a loop that will add two new lines to that file, say

a,b,c,d,e,f
1,2,3,4,5,6,7,8,9,0,1,2
0,9,8,7,6,5
4,3,2,1,0,9

And now I need to do the same to that last line for the file to end like

a,b,c,d,e,f
1,2,3,4,5,6,7,8,9,0,1,2
0,9,8,7,6,5,4,3,2,1,0,9

So, in only a few words, I need to replace the last new line of a text file with a comma, but only the last line, as lines are being added.

I've tried several things but they don't work. The last one was sed '$!N;s/\n/,/' dna.data, but it applies to all lines with a newline.

Any ideas?

Answer 1

If the file does not end with a newline, you just need to address the last substitute command (use the -i flag of sed to change the file inplace):

$ sed '$!N;$s/\n/,/' dna.data
a,b,c,d,e,f
1,2,3,4,5,6,7,8,9,0,1,2
0,9,8,7,6,5,4,3,2,1,0,9  

If the file ends with a newline, you can use the editor ex:

$ ex dna.data
$-1s/\n/,/
1,2,3,4,5,6,7,8,9,0,1,2
x

or in one line

echo -e '$-1s/\\n/,/\nx' | ex dna.data

Answer 2

This might work or for you (GNU sed):

sed -i '$!N;$s/\n/,/;P;D' file