Convert .txt to .csv in shell

Only sed and nothing else

sed 's/ \+/,/g' ifile.txt > ofile.csv

cat ofile.csv

1,4,22.0,3.3,2.3
2,2,34.1,5.4,2.3
3,2,33.0,34.0,2.3
4,12,3.0,43.0,4.4

awk may be a bit of an overkill here. IMHO, using tr for straight-forward substitutions like this is much simpler:

$ cat ifile.txt | tr -s '[:blank:]' ',' > ofile.txt