How can I add a comma at the end of every line except the last line?

The following sed script has an address expression $! which matches lines which are not the last, and a substituton action s/$/,/ which adds a comma next to the end of line.

sed '$!s/$/,/' file

(The $ in the address refers to the last line while the $ in the regular expression in the substitution refers to the last character position on every line. They are unrelated, though in some sense similar.)

This prints the modified contents of file to standard output; redirect to a different file to save them to a file, or use sed -i if your sed supports that. (Some variants require an empty argument to the -i option, notably *BSD / OSX sed.)

If your task is to generate valid JSON from something which is not, I'm skeptical of this approach. Structured formats should be manipulated with tools which understand structured formats, not basic line-oriented tools; but if this helps you transform line-oriented data towards proper JSON, that's probably a valid use.

... As an afterthought, maybe you want to wrap the output in a set of square brackets, too. Then it's actually technically valid JSON (assuming each line is a valid JSON fragment on its own).

sed '1s/^/[/;$!s/$/,/;$s/$/]/' file

On the first line (address expression 1) substitute in an opening square bracket at beginning of line (s/^/[/). On lines which are not the last, add a trailing comma, as above. On the line which is the last (address expression $) add a closing square bracket at the end of line (s/$/]/). If you prefer newlines to semicolons, that's fine; many sed dialects also allow you to split this into multiple -e arguments.

Keep it simple, just use awk:

$ awk '{printf "%s%s",sep,$0; sep=",\n"} END{print ""}' file
{...},
{...},
{...},
{...}

vim <file-name>

:%s/$/,/g

To remove

:%s/,//g