Shabbos 1:2-3

Shabbos 1:2

A man may not begin a haircut a half-hour before mincha time on Friday until he has davened (out of concern that he may be delayed). Similarly, one may not enter a bathhouse or a tannery at this time, nor start a meal or a court case. If they already started one of these activities, they need not interrupt them when this time arrives. These activities must be interrupted in order to recite the Shema but they need not be interrupted in order to daven Shemoneh Esrei.

Shabbos 1:3

A tailor should not go out with his needle late on Friday afternoon out of concern that he may come to carry it on Shabbos; the same applies to a scribe with his pen. On Shabbos, one may not search his clothes or read by the light of an oil lamp (which one habitually tends to adjust to improve the light). There is an ancient tradition that a teacher may use an oil lamp to find the places where children should read their lessons but he may not himself read. (The children are unlikely to adjust the lamp in their teacher’s presence, even absent-mindedly.) Another case where the Sages instituted a safeguard is that a zav and a zavah (a man and a woman, each of whom has experienced a particular type of emission) should not eat together because of force of habit.