Gemara

45a (Mishnah - R. Yochanan ben Nuri): If one slept on the road and did not realize that it was dark, he has 2000 Amos in every direction;

Chachamim say, he has only four Amos.

46a (R. Yakov bar Idi): The Halachah follows R. Yochanan ben Nuri.

38b - Question (Abaye): Rav Yehudah taught that if one was Me'arev with his legs on the first day, he may be Me'arev with his legs on the second day. If he was Me'arev with bread on the first day, he may be Me'arev with bread on the second day;

Yom Tov prepares for Shabbos!

Answer (Rabah): You assume that he goes and says that he is Me'arev for the second day. This is wrong. Rather, he goes and is silent.

Suggestion (Abaye): This is like R. Yochanan ben Nuri, who says that Hefker objects acquire Shevisah (no acquisition is needed)!

Rejection (Rabah): It is even like Chachamim. They argue about one who was sleeping, for he could not have spoken;

They agree about one who was awake. Since he could speak, even if he did not speak, it is as if he spoke!

49a (Mishnah): If one was traveling and it was because dark, and he saw a tree and said 'my Shevisah is at the trunk', he may walk 2000 Amos to the tree, and another 2000 Amos from the trunk to his house.

This is like it was taught, that an Oni [who lacks bread] can be Me'arev with his legs;

R. Meir says, only an Oni may do so;

R. Yehudah says, the same applies to an Ashir;

An Eruv with bread is a leniency for Ashirim, so they need not walk.

51a (Rav Nachman): They argue about one who said 'my Shevisah is in my place':

R. Meir holds that Eruv is primarily with bread. Chachamim were lenient only for Aniyim;

R. Yehudah holds that Eruv is primarily through walking, for Oniyim and Ashirim.

If he said 'my Shevisah is in that place', all agree that this works only for an Oni.

Pesachim 49a (Mishnah): If one was on his way to make Shevisah of Reshus, and remembered that he has Chametz, he must immediately return to eradicate it.

Rishonim

Rambam (Hilchos Eruvin 7:9): One who acquires Shevisah far away [from where he is] must be Machazik b'Derech (start going there). He need not go to the field. Even if he descended from the second story to go there, and before he left the Chatzer his friend returned him, this is called Machazik b'Derech, and he acquires Shevisah. One who acquires Shevisah far away need not say 'my Shevisah is in my place.' Once he resolved in his heart and was Machazik b'Derech any amount, he acquires Shevisah. We need not say that one who went and stood where he will acquire Shevisah need not say anything. Once he resolved in his heart, he acquired Shevisah.

Rosh (4:13): The Mishnah says 'he said my Shevisah is in my place.' Really, he need not say this. He acquires even without saying this, like it says above (38b). As long as he is awake, so he could say it if he wanted, it is as if he said it. Also on 45a, we say that one did not acquire Shevisah if he did not intend. If he intended, he acquired, like the case of R. Tarfon. The Mishnah mentions saying 'my Shevisah is in my place' due to the Seifa, to teach that [according to R. Meir] an Ashir cannot acquire Shevisah with his legs even if he said. We find in Bava Metzi'a (11a) 'he said 'my field should acquire for me.'' Really, he can acquire without saying, even regarding the decree that one acquires in his four Amos. There it mentions saying to teach that in the Seifa, regarding something not destined to remain in his field, he does not acquire even if he said.

Tosfos (Eruvin 49b DH v'Amar): Rabanan argue with R. Yochanan ben Nuri about one who was sleeping. This implies that all agree that if he was awake, he acquired Shevisah even without saying anything. On 45a, we say that one who was on the road and did not intend may not enter the city. He acquires Shevisah in his place.

Rashi (Pesachim 49a DH Lishbos): The Mishnah discusses one going to the end of the Techum at the end of Erev Yom Tov, to acquire Shevisah there, to be able to go to a matter of Reshus.

Question (Tosfos 49a DH Lishbos): If so, our Mishnah is difficult for the opinion that permits Eruv Techumim only for a Mitzvah! Rather, he was going to rejoice on Pesach in a friend's house.

Poskim

Shulchan Aruch (OC 409:7): How does one make an Eruv [Techumim]? If he wants to go to the end of the Techum or inside the Techum and stay until dark, this is the primary Mitzvah.

Beis Yosef (DH Keitzad): R. Meir holds that the primary Eruv is with bread. R. Yehudah holds that the primary Eruv is with his legs. The Halachah follows R. Yehudah. The Rambam and Rosh teach that he need not speak. We learn from the case of one who was sleeping.

Magen Avraham (10): This is if he intended to acquire Shevisah there. If not, he acquired Shevisah in his house.

Mishnah Berurah (26): He must be there all of Bein ha'Shemashos, until it is Vadai dark. This is clear from 415:2.

Mishnah Berurah (28): It is also l'Chatchilah to be Me'arev with bread, just the primary enactment was with his legs. It is a leniency to allow with bread, so he need not stay until dark, or he can send it through a Shali'ach.

Shulchan Aruch (ibid.): Even if he did not say 'my Shevisah is in my place', rather, he stayed until dark and was silent, whether he left his house to go there, or he was on the road when it got dark, he acquires 2000 Amos without speaking.

Beis Yosef (DH u'Mah she'Chasav Lo): The Tur brings two opinions about whether or not one who was Machazik b'Derech must say where he wants to acquire Shevisah. Even according to Rashi, who says that he must say, when he goes to where he acquires Shevisah, all agree that he need not say. One who left his house to stay until dark at the end of the Techum must intend, but one who was on the road when it got dark need not intend.

Beis Yosef (DH Liknos): If one who left his house to stay until dark at the end of the Techum did not intend, he acquired Shevisah in his house. One who was on the road when it got dark has 2000 Amos from his feet in every direction, even if he was sleeping. The Rosh proves this from 38b.

Gra (DH v'Afilu): See Tosfos 49b DH v'Amar. The Shulchan Aruch says 'whether he left his house to go there, or he was on the road when it got dark', for Tosfos distinguishes these.

Mishnah Berurah (29): In both cases, he need not speak. However, when he leaves his house, it must be with intent to stay until dark to acquire Shevisah there. If he left the city Stam, and he was in the Techum when night came, and he did not intend to acquire Shevisah there, he acquired Shevisah in his house, since he is near his house. If he was on the road when night came, even if he did not intend to acquire Shevisah there, automatically he acquired Shevisah there and he has 2000 Amos in every direction, even if he was sleeping.

Shulchan Aruch (ibid.): If he does not want to stay until dark there, he goes during the day and leaves food for two meals.

Bi'ur Halachah (DH v'Im): One cannot be Me'arev with his legs through a Shali'ach.

Beis Yosef (415 DH Kasav): The Rashba says that Rishonim argue about an Eruv with bread for Reshus. If he was Me'arev with his legs, all agree that he acquired Shevisah even for Reshus. I say that one may do so l'Chatchilah. The Isur to be Me'arev for Reshus applies only to bread. Letter of the law, bread should not help. It is a mere leniency for rich people (49b). Chachamim were lenient only for the sake of a Mitzvah.

Gra (DH v'Im): This is like Rashi (Pesachim 49a DH Lishbos). See Tosfos (49a DH Lishbos, who disagrees).

Magen Avraham (415:2): One who is Me'arev with his legs may be Me'arev for Reshus (409:7).

Mishnah Berurah (1): Some permit to be Me'arev with his legs l'Chatchilah even for Reshus. Some do not distinguish an Eruv through bread or through his legs. It is permitted l'Chatchilah only for a Mitzvah. When needed, one may rely on the first opinion.

Kaf ha'Chayim (1): One may be lenient only for a great need.