How to replace space with comma using sed?

Question

I would like to replace the empty space between each and every field with comma delimiter.Could someone let me know how can I do this.I tried the below command but it doesn\

Answer 1

If you want the output on terminal then,

$sed 's/ /,/g' filename.txt

But if you want to edit the file itself i.e. if you want to replace space with the comma in the file then,

$sed -i 's/ /,/g' filename.txt

Answer 2

I just confirmed that:

cat file.txt | sed "s/\s/,/g"

successfully replaces spaces with commas in Cygwin terminals (mintty 2.9.0). None of the other samples worked for me.