How can I remove the last comma separator from the end of a file?

Using GNU sed:

sed -i '$s/,$//' file

That is, on the last line ($) substitute (s) the comma at the end of the line (,$) by nothing.

The change will be done in-place due to the -i flag.

With standard sed:

sed '$s/,$//' <file > &&
mv file

Note: Someone suggested an edit to change "on the last line" to "last on the line" (or something similar). This is wrong. When $ is used to specify an address (a line where to apply an editing command), then it refers to the last line of the stream or file. This is different from using $ in a regular expression.

For in-place editing you could use ed - conveniently, it sets the position to the last line by default on opening so you don't need to address the last line explicitly:

ed file

Or, as a one-liner

printf 's/,$//\nwq\n' | ed -s file

In general, I'd probably go with the straightforward sed solution. However, if your input files are huge, you might want a solution that doesn't take the time to read through the whole file just to edit the last couple of bytes.

If you are 100% sure your input file ends with a comma followed by a newline, you can use truncate to lop off these last two characters, and then re-append a final newline:

filesize=$(stat -L --format "%s" lastcomma.txt)
truncate --size $((filesize - 2)) lastcomma.txt 
echo >> lastcomma.txt 

This assumes a distro that includes GNU truncate or equivalent.

@StéphaneChazelas points out that GNU truncate now supports truncate -s -2 file to shorten the file by two bytes; if you have this version, the above simplifies to:

truncate --size -2 lastcomma.txt
echo >> lastcomma.txt