How to replace space with comma using sed?

Inside vim, you want to type when in normal (command) mode:

:%s/ /,/g

On the terminal prompt, you can use sed to perform this on a file:

sed -i 's/\ /,/g' input_file

Note: the -i option to sed means "in-place edit", as in that it will modify the input file.

If you are talking about sed, this works:

sed -e "s/ /,/g" < a.txt

In vim, use same regex to replace:

s/ /,/g