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

I needed to edit a file with restricted write permissions so needed sudo. working from ghostdog74's answer and using a temp file:

awk 'FNR==NR && /configs.*projectname\.conf/{f=1;next}f==0;END{ if(!f) { print "your line"}} ' file > /tmp/file
sudo mv /tmp/file file