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

Question

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

include \"/configs/projectname.conf\"

to a file called lighttpd.conf

Answer 1

Here's a sed version:

sed -e '\|include "/configs/projectname.conf"|h; ${x;s/incl//;{g;t};a\' -e 'include "/configs/projectname.conf"' -e '}' file

If your string is in a variable:

string='include "/configs/projectname.conf"'
sed -e "\|$string|h; \${x;s|$string||;{g;t};a\\" -e "$string" -e "}" file