sed

sed

sed – UNIX stream editor

changing points to commas
1 file dots2commas_01.data have 3 strings with text and numbers with decimal separator period (point)
2 sed '/[0-9]\./s/\./,/g' dots2commas_01.data > dots2commas_01_modified.data
3 check file dots2commas_01_modified.data for result.

change tabs with spaces
sed 's/\t/ /g' tab-file.txt > no-tab-file.txt

2 adding a string at the beginning of line
sed -i s/^/"1 "/ test.file