Gemara

Only Melachos are forbidden in Shemitah, but not exertion. On Chol ha'Mo'ed, even exertion is forbidden!

Mo'ed Katan 2a (Mishnah): One may water a Beis ha'Shelachin (a field that must be irrigated) on Chol ha'Mo'ed, either from a new or old spring. One may not draw water from a channel with a bucket to water it.

R. Elazar ben Azaryah forbids making a new irrigation channel;

Chachamim permit.

We may fix a ruined irrigation channel and fix the water supply in Reshus ha'Rabim and unclog it.

Question: Who is our Tana, who permits only to avoid a loss, but not in order to gain, and even to avoid a loss he forbids exertion?

Answer (Rav Huna): It is R. Eliezer ben Yakov, who permits drawing water from one tree to another

4b (R. Aba): If an irrigation channel is now only one Tefach deep, this is considered ruined. One may restore it to its initial depth of six Tefachim.

Clearly, if it was initially three Tefachim and now only half a Tefach (deep), one may not fix it. Since water did not flow properly before, and it will not flow properly after it is fixed, this is unnecessary exertion. Clearly, if it was initially 12 Tefachim and now it is only two Tefachim, one may not restore it to 12. (Since six Tefachim suffice,) this is unnecessary exertion.

Question: If it was initially seven Tefachim and now it is two, may one restore it to seven? Just like one may clear out five clogged Tefachim of a six Tefach channel, it is also permitted here. Or, perhaps it is forbidden, for the seventh Tefach is unnecessary exertion?

This question is not resolved.

10b (Rava): All business of any amount is forbidden on Chol ha'Mo'ed.

Pesachim 55b (Beraisa): One may not bring Kelim from a craftsman (on Chol ha'Mo'ed). If one fears lest they be stolen, he may clear them to another Chatzer (near the craftsman, but he may not toil to take them to his house).

Rishonim

The Rif and Rosh (Mo'ed Katan 2a and 1:6) bring the Heter to restore an irrigation channel from one Tefach to six Tefachim.

Nimukei Yosef (1a DH Gemara): If the channel is now less than a Tefach deep, it is considered sealed and one may not restore it. One may not restore a channel to seven Tefachim, for six is enough. Also, it is a great exertion to remove dirt that is so deep.

Nimukei Yosef (DH Mashkin): Even though a Beis ha'Ba'al would produce more if one watered it, a loss is only something that reached (produced) Peros, and then they will be lost. To increase on what is (already) seen is profit. We do not override Chol ha'Mo'ed for it.

Rambam (Hilchos Yom Tov 8:4): During the Mo'ed, one may not water crops that did not get water before the Mo'ed, for they need much water and one will come to excessive exertion.

Kesef Mishneh: The Ramban forbids excessive exertion even if there would be a loss.

Hagahos Maimoniyos (6:40): Avi ha'Ezri says that the Torah empowered Chachamim to decide laws of Chol ha'Mo'ed. They fixed the laws so that man will be happy, and able to learn and understand. They forbade excessive exertion, for this makes one sad, and permitted Devar ha'Avud and for one who lacks what to eat.

Magid Mishneh (7:1): The Ramban says that the Torah permits Devar ha'Avud, even a great exertion Lo l'Tzorech ha'Mo'ed (not for the need of the Mo'ed). Chachamim forbade certain Melachos.

Magid Mishneh (8:2): Whatever is l'Tzorech ha'Mo'ed, even a great exertion is permitted. For Davar ha'Avud (he will suffer a loss if he does not do it now) that is not l'Tzorech ha'Mo'ed, we permit a great exertion.

Rosh (1:23): Business is forbidden on Chol ha'Mo'ed due to exertion.

Rosh (2:4): The Ra'avad says that the Torah empowered Chachamim to decide what is forbidden on Chol ha'Mo'ed. They forbade excessive exertion even for Devar ha'Avud, due to honor of the Mo'ed.

Poskim

Shulchan Aruch (OC 537:1): One may do Devar ha'Eved on Chol ha'Mo'ed without a Shinuy.

Rema: Whatever he can do to minimize exertion, he should do.

Gra (1): The Ra'avad asked that in Perek 2 we permit Devar ha'Avud, even if there is exertion, but in Perek 1 (4a) we forbid even a small exertion, e.g. to bring water from the bottom half of a patch to the top half. Shmuel and Rav Dimi argue about whether we are concerned for exertion. He answered that the Torah empowered Chachamim to decide laws of Chol ha'Mo'ed, and they allowed exertion based on the size of the loss.

Beis Yosef (DH Hilkach): The Gemara asked 'who is our Tana, who forbids exertion, i.e. excessive exertion to avoid a loss?' The Tana permits (a small exertion) in a Beis ha'Shelachin, for if one does not water it there will be a loss, but not in Beis ha'Ba'al (a field that need not be irrigated), for one waters it merely for profit.

Chazon Ish (134:14 DH v'Shoresh): The Ritva says that even according to the opinion that forbids Melachah on Chol ha'Mo'ed mid'Oraisa, the Isur of exertion for Devar ha'Avud is only mid'Rabanan, therefore, one may be lenient about a Safek.

Shulchan Aruch (534:3): One may bring Kelim from a craftsman l'Tzorech ha'Mo'ed, e.g. pillows and blankets and plates. One may not bring Kelim Lo l'Tzorech ha'Mo'ed, such as a plow or wool from a dyer.

Mishnah Berurah (16): This is due to exertion. Some say that it is lest people think that he gave them to him to fix during the Mo'ed and he fixed them during the Mo'ed. If so, it is Asur even within the same Chatzer, when there is no exertion. The Pri Megadim permits to bring from a Nochri craftsman for the sake of a Mitzvah, even if it is not l'Tzorech ha'Mo'ed.

Shulchan Aruch (539:1): All business of any amount is forbidden.

Shulchan Aruch (5): Something that is not always found after the Mo'ed, e.g. if ships or caravans arrived or they are about to leave, and they are selling cheaply or buying for a high price, one may buy or sell, even just for profit. The same applies to periodic fairs (when prices are better than usual).

Magen Avraham (6): We permit profit if it is no exertion.

Rema: One may even travel to another city for a fair.

Bi'ur Halachah (DH v'Afilu): Exertion is forbidden for the sake of profit (Siman 537). We must say that here we discuss one whose livelihood is from fairs. For him it is Devar ha'Avud.

Shulchan Aruch (540:2): One may remove a mound in the house.

Beis Yosef (DH Mutar): Piskei Tosfos, Semag, Semak, and Hagahos Maimoniyos permit. Even though it is a proper Melachah and it is not Devar ha'Avud, it is permitted on Chol ha'Mo'ed because it is not a Melachah of exertion.

Mishnah Berurah (7): Normally, needs of the Mo'ed that are not Devar ha'Avud are permitted only through a Shinuy. Here is different, for it is not an exertion.

Kaf ha'Chayim (15): This is a need of the Mo'ed, so one will not trip over it. Needs of the Mo'ed other than food preparation require a Shinuy; this is only if they are an exertion.

Shulchan Aruch (545:7): Whatever one may write on Chol ha'Mo'ed is permitted without a Shinuy. What is forbidden is forbidden even with a Shinuy.

Beis Yosef (DH Yesh): Tosfos (Mo'ed Katan 19a DH v'Toveh) says that there is no Heter to write in a circle, or sideways. Even if it is not forbidden due to Melachah, it is Asur due to exertion. However, people are lenient and say that the Mishnah does not discuss this. Some make a break in every letter; one cannot do so for 'Yud' so it is written sideways.

Gra: Because the Isur is due to exertion, it makes no difference how one writes.

Mishnah Berurah (43): Holding the pen differently, e.g. between the thumb and index finger, or making the lines circular or slanted, does not permit. If the letters themselves are different, e.g. they are cut and broken in the middle, it is permitted.

Sha'ar ha'Tziyon (49): The Terumas ha'Deshen cites a Gadol who permits through holding the pen differently, but we do not rely on this, for the primary Isur on Chol ha'Mo'ed is due to exertion, and it is more exertion to write this way.