Appending a line to a file only if it does not 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.