grep -qxF 'your text "additional text"' yourfile.txt || echo 'your text "additional text"' >> yourfile.txt
Related
See also
ack, ag, grep, egrep, fgrep, agrep, ngrep, pgrep, awk, sed, strings, tr, tail, mtail, git grep, wc, uniq, LogQL, findstr (Windows), rg, git-grep, cut
