Appending a line to a file only if it does not already exist

I need to add the following line to the end of a config file:

include "/configs/projectname.conf"

to a file called lighttpd.conf

I am looking into using sed to do this, but I can't work out how.

How would I only insert it if the line doesn't already exist?

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.

Try this:

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