It would depend on the desert area, but there was little control over sparsely populated areas, especially in hostile environments. With Persia, for example, there were fairly well demarcated borders (as well as fighting over them), but between, say, the Hijaz and the interior of Arabia, or between Egypt and Libya, there wouldn't have been a border so much as a "this is our last garrison town, and beyond this, the territory no longer concerns us."

I don't honestly think I've seen a study that addresses how those kinds of borders worked in the premodern era, but to answer the question posed, no, they wouldn't have been that well defined. Most of those lines only exist in the imaginations of modern cartographers!

It is hard to actually say how much influence the Ottoman government had outside of its major cities and towns. A myriad of nomadic and semi-nomadic peoples would have lived throughout the empire. The government's rule over them was tenuous at best. So, I'd argue that instead of the coast, it's generally the further away from the established towns, routes, and cities that Ottoman rule weakened.

Introduction

For this answer, I'm mainly going to look at Bedouins from 1517 to about 1610. "Bedouins" is more of a catch-all term for a series of varied nomadic and sometimes semi-nomadic, tribal peoples who lived throughout regions in the Maghreb, Egypt, the Levant, the Arabian Peninsula, and Iraq. So, we should be careful to not look at them as a single, monolithic group, but instead as many different, independent tribes that all had their own agendas and relationships with the Ottoman government. Overall, their relationship with the state government was complex and varied significantly by time and space.

Governance of Bedouins

We actually have a lot of archived evidence of Bedouin legal issues. Land disputes, the rustling of animals, and violence between the expanding settled peoples and nomadic peoples was common. These disputes would often regard the rights to pastures, which the nomads' animals would graze. In many cases, the Ottoman government sided with the Bedouins, as these were often regarded as ancestral pastures, which the nomads had used for generations.

As an example of one land dispute between tribal and settled people, the Ottoman government ordered the following:

"When this command arrives personally go to the location in question. Bring the litigants together and that way investigate according to law. Let it be known where the aforementioned tribe has traditionally pastured its animals and what are the limits of the land where the above mentioned villagers have cultivated their fields. Apportion the traditional boundary in between and do not let the aforementioned tribe transgress from their pastures..."

(White's The Climate of Rebellion in the Early Modern Ottoman Empire, cited from MD 6/300)

Here we can see that the process - at least in this case - was actually rather methodical and doesn't seem to favor either party. There is, once again, this distinction of rights to the land. Traditionally held land of tribes would generally be protected by the government. In one dispute, the Bedouins claimed "We have come here from ancient times for pasture, and the places where we wandered were previously empty lands. Now villages have been built.”

But, of course, the struggle for the rights to land was continuous and often worked on a much smaller scale than involving the state government. State rulings often were temporary and disputes would quickly resume. And despite their attempts to mediate between the two parties and find a middle path, over time the settlers eventually prevailed and we see more and more nomadic groups being pushed out of their traditional lands. The failure of the state to uphold these rulings shows that governance in these regions was somewhat limited.

Besides legal disputes, Bedouins would generally be outside of the Ottoman governance while in remote, sparsely populated regions.

Bedouins Within the Ottoman System

We should be careful to not look at all Bedouins as somehow being outside of or against the Ottoman state. Bedouins would have had a multitude of different roles within the empire's system. For example: some Bedouins played a vital role in protecting the pilgrimage routes and guiding pilgrims; some supported the movement of Ottoman armies through the terrain, which they knew incredibly well; many raised livestock, which supported the growing needs of the cities through meat and the army through horses and camels; and some were tasked with protecting sparsely populated areas from bandits.

In Egypt, some Bedouins shaykhs even found themselves in important administrative roles for the empire. Upper Egypt, al-Minufiyya, al-Buhayra, and al-Jiza would have traditionally had Bedouins shaykhs as provisional governors. (Winter's Egyptian Society under Ottoman Rule, 1517-1798) These shaykhs would have collected taxes and grain from the tribes; ensured the protection of the province; ensured that harvest was successful; ensured that the irrigation and dam system was working smoothly; and more. Some Bedouins in Egypt were more sedentary and agricultural than their non-Egyptian counterparts.

However, within this system, many Bedouins faced a series of challenges. "[the Ottoman government]...assigned specific winter and summer pastures and migration routes, punishing violations by sharp fines and the threat of military action." (White's The Climate of Rebellion in the Early Modern Ottoman Empire) Here, we see that the Ottoman government tried to restrict the free movement of the Bedouins and were prepared to use force if necessary. Military attacks and other retaliation against tribes for perceived wrong-doings also occurred. Additionally, the nomads continued to lose land as towns and cities grew, demanding more and more resources.