SED one liners

Replace all tabs in a file with space

#Replace all tabs in a file with 4 space(s)
sed -i 's/\t/    /g' temp.txt

#Add a string to start of every line in a file
sed -i 's/^/String/g' temp.txt

#Replace a string1 with string2 only if string1 is at the start of the line
sed -i's/^string1/string2/g' temp.txt