The following was posted as a comment on another thread but I wanted it to be a post. It was posted by "DAR"
I heard an interesting pshat today (from Rabbi Elefant's daf yomi shiur on ouradio.org) regarding tying garbage bags on shabbos. The Debetziner Rav says that since a "knot", by definition, is something which can be tied and untied, a knot on a garbage bag is not really a "knot" since it is never intended to be opened once you tie it. Thus, you can make such a tying on shabbos since a tying which you intend to last forever does not fall within the definition of a knot. The Maharil Diskin offers a similar explanation as to how a string was tied around the Seir Hamishtaleach on Yom Kippur. Why should this be allowed - isn't tying a knot assur? He explains that since the knot is intended to last forever - i.e. once the seir is pushed off the mountain, he is never heard from again - it is permissible to make such a knot.I find that to be a huge chiddush. I'd have thought that the definition of a kesher shel kayama (at least according to Rashi) is one that lasts forever. I thought the only question is what is the minimum shiur of "shel kayama" - is it a week, a month or what but to say that there is a maximum shiur is just unbelievable.
I just looked what the Kollel Iyun Hadaf had to say and this is what they said:
HALACHAH: The SHULCHAN ARUCH (OC 317:1) cites the opinion of the Rif and Rambam (c), that one is permitted to tie a knot only when it is not a professional knot and it is tied to last for a short time. The REMA cites the opinion of Rashi (a), that if it is a permanent knot, one is Chayav regardless of the expertise of the knot.
All of the abovementioned Rishonim (Rashi (a), Re'em (b), and the Rif and Rambam (c)) require that the knot be permanent in order for one to be Chayav for tying it. What is a permanent knot that one may not tie, and what is a temporary knot that one may tie?
From the words of Rashi and the Rif, it appears that a permanent knot is defined as a knot that one ties in order to leave it as a knot forever, with no intention to ever untie it.
A knot that is not permanent and that one may tie l'Chatchilah is defined by the REMA in the name of the TUR and MORDECHAI as a knot that one ties with intention to leave it for less than one week.
According to this tying a garbage bag would be an isur d'oraysa. Before tying one on Shabbos you should consult with your LOR.