How to search and replace the specific line number string

Question

line_index=\"2d\";

file=\"./Desktop/books.sh\";

sed -i.bak -e $line_index $file

Will delete the entire line that $line_index is pointing to

Answer 1

To restrict substitution to a line number, add it before the command:

sed -i "2s/harry/potter/" $file

Answer 2

Sure, "addresses" and "commands" can be freely combined:

sed '2s/harry/potter/g'