Vice President Hosni Mubarak, the former Air Force general who announced Anwar Sadat's death to the Egyptian people yesterday and is expected to inherit the presidency, has shown himself to be an energetic and practical leader, loyal to Sadat's foreign and domestic policies.

Since his appointment as vice president six years ago, it has been clear that Mubarak was being prepared by Sadat to assume the presidency. Unlike Sadat, a vice president virtually unknown outside Egypt when Gamal Abdel Nasser died in 1970, Mubarak was repeatedly thrust into sensitive international negotiations where he earned a reputation as a capable and accomplished diplomat.

Yesterday, the ruling National Democratic Party made it clear that it intends the presidency to pass to Mubarak, nominating him within hours of Sadat's death as the presidential candidate, subject to a national referendum confirming him within 60 days.

Announcing the president's assassination on Egyptian television in a strong, reassuring voice, Mubarak, 53, pledged to "follow his path," honoring all the treaty commitments made by Sadat and standing with Egyptians "as one solid front around all the banners Sadat has raised."

His adherence to Sadat's policies has forced Mubarak to make some of the same zig-zags the assassinated leader made in his decade in power.

Mubarak commanded Egypt's Air Force in the October 1973 war against Israel, and then he traveled to hostile Arab countries in an effort to explain why Egypt was entering a peace treaty with the Israelis.

He received much of his military training in the Soviet Union and was a frequent visitor there. Later he was in the forefront of Egyptian moves to expel Soviet technicians and put the Kremlin at arm's length.

To many who know him, Mubarak, at the time he was made vice president, brought an image of vigor and energy to a government dominated by older men. He plays squash, reads widely and makes a point of punctuality that is rare in Egypt's bureaucracy.

Former vice president Walter Mondale, who knows Mubarak well, described him yesterday as "a very moderate, impressive person" who, as the number two official in Egypt, was "not just a passive official but a direct participant, well involved and trying to solve problems."

Mondale said "it was clear" that other Egyptian officials "looked to him for leadership and respected him."

Diplomats in Cairo, however, say they have never been sure how much influence Mubarak actually exercised in the governing of the country. Some saw him as at his best in amiably greeting foreign dignitaries but others saw his seeming good-natured dullness as necessary discretion for a Sadat subordinate and considered him a powerful influence behind the ruler.

In the past 11 months Mubarak has visited the United States twice, most recently last week to request U.S. military aid to Sudan to help that Egyptian neighbor protect itself against possible harassment by Libya. Last December he spent 11 days in the United States, paying Egypt's farewells to the Carter administration and initiating contacts with the Reagan administration.

Just before coming to Washington, Mubarak, at the national congress of Egypt's ruling National Democratic Party, issued a strong warning clearly aimed at the Soviet Union, cautioning that any aggression by Kremlin surrogates against Egyptian allies Sudan, Somalia or Oman would be considered "a direct aggression on Egypt that must be confronted firmly."

Mubarak was born in Menufia, a village in Sadat's home region of the Nile Delta.

After Egyptian military training in the late 1940s and early 1950s, he went to the Soviet Union for the first of many visits, to take advanced flight training. He went back for bomber training and later took courses at the Frunze General Staff Academy, the top Soviet military training facility.

He held Air Force command positions in Egypt and from 1967 to 1969 he headed the country's Air Force Academy.

In 1972, Sadat appointed Mubarak commander-in-chief of the Air Force, replacing a general who had complained about the presence of Soviet military advisers in Egypt. Mubarak immediately was brought along with Sadat on a fence-mending and arms-shopping trip to Moscow. He returned to Moscow a few months later reportedly to consult about the tactics Egypt would later use to launch the 1973 war.

When Sadat promoted Mubarak to vice president in April 1975, the Air Force general had little political and diplomatic experience. Sadat promptly entrusted him with one sensitive chore after another to give him that experience.

In his first year as vice president Mubarak mediated the dispute among Morocco, Algeria and Mauritania over the future of the former Spanish Sahara, ending the shooting war if not achieving a political solution.

He also drew the difficult assignment of going to several Arab capitals to explain Egypt's decision to sign the Sinai disengagement agreement with Israel, and he was sent on a landmark mission to Peking where he successfully nailed down arms and trade agreements with Chairman Mao Tse-tung's government.

While he has a reputation for easy-going affability, Mubarak, in recent months, has shown, on issues that troubled Egypt, a hard edge that allowed Sadat to remain the detached statesman.

Last month Mubarak chaired the Cabinet meeting in which Egypt canceled the contracts of all the Soviet technicians working in Egypt and ordered the Soviet Embassy to cut its staff in half.

In July, Mubarak delivered the sternest Egyptian remonstrations to Israel's attacks into Lebanon, which he called "foolish actions."

"If some Israeli circles have failed to understand the meaning of peace and remained prisoner of the sorry past, we believe it is imperative for the Israeli people to realize the gravity of these actions and their destructive effect on the peace effort," he said.

But, he told Egyptians, "We should not be discouraged. . . . We have firm faith the next step should be in the direction of solving the Palestinian problem, which is the core of the Middle East conflict."