Add line

grep -qxF 'your text "additional text"' yourfile.txt || echo 'your text "additional text"' >> yourfile.txt

LINE='include "/configs/projectname.conf"'
FILE='lighttpd.conf'
grep -qF -- "$LINE" "$FILE" || echo "$LINE" >> "$FILE"

Related

• append
• sed -i
• Ansible: lineinfile

See also

• line, newline, characters, wc, column, Add line, Concatenate lines, Ansible: lineinfile, rpl, sed -i