I'm sorry, but if one just needs to add an entry to a text file I find using an editor to be a bit "overkill". In particular suggesting to install a GUI editor or using a file manager (like 'mc') for such a "trivial" task is IMHO "over the top" (and I'm saying this as a very long time user of 'mc').
What I'd suggest is to use echo path/to/file >> /opt/.filetool.lst As always for entrys to '/opt/.filetool.lst': please ensure to not use an absolute path (i.e. don't use '/path/to/file').
I dont' want to start a flamewar about which editor is to be preferred, but I think that using the "bigger guns" to solve a trivial task is not always appropriate.
BTW my suggestion for the task of the deletion of a entry would be sed -i '/path\/to\/file/d' /opt/.filetool.lst Now this time what appears to be a leading forward slash in the 'path/to/file' is actually required syntax for the "delete" command of 'sed' (i.e. '/RegularExpression/d'), and therefore the forward slashes "inside" of the regular expression will have to be "escaped" (with a back-slash for each of them).
Well, that's all pretty standard UNIX/Linux stuff, but if anyone still insist to install a couple of extenions to achieve the same I'm certainly not in a position to stop you ...