tr ' ' ',' <input >output

tr -s 't' <input | tr 't' ',' >output

awk '{$1=$1}1' OFS=","

sed 's/[:space:]+/,/g' orig.txt > modified.txt

sed 's/[:blank:]+/,/g' orig.txt > modified.txt

sed 's/[t ]+/,/g' orig.txt > modified.txt

cat texte.txt | sed -e 's/s/,/g' > texte-new.txt

sed -e 's/s/,/g' texte.txt > texte-new.txt

$ cat texte.txt
this is a text
in which I want to replace
spaces by commas

$ cat texte-new.txt
this,is,a,text
in,which,I,want,to,replace
spaces,by,commas

sed "s/s/,/g" < infile.txt > outfile.txt

sed 's/[t ]/,/g' input.file

sed -i 's/[t ]/,/g' input.file