How to add a line in sed if not match is found

Try this:

grep -q '^option' file && sed -i 's/^option.*/option=value/' file || echo 'option=value' >> file

Just keep it simple :)

grep + echo should suffice:

grep -qxF 'include "/configs/projectname.conf"' foo.bar || echo 'include "/configs/projectname.conf"' >> foo.bar

• -q be quiet
• -x match the whole line
• -F pattern is a plain string
• https://linux.die.net/man/1/grep

Edit: incorporated @cerin and @thijs-wouters suggestions.