Replace all white spaces with commas in a text file

With sed:

sed -e 's/\s\+/,/g' orig.txt > modified.txt

Or with perl:

perl -pne 's/\s+/,/g' < orig.txt > modified.txt

Edit: To exclude newlines in perl you could use a double negative 's/[^\S\n]+/,/g' or match against just the white space characters of your choice 's/[ \t\r\f]+/,/g'.

Using tr:

tr -s '[:blank:]' ',' <file

This will replace any horizontal whitespace with a comma. Any repeated whitespace will only be replaced with a single comma.