Min Jun Shik, South Korean businessman, and his friends handle the tension of life in Seoul these days with flashes of gallows humor.

Since the latest upheavel, Wednesday's coup within the military, Min says they no longer greet each other each day with usual, "Good morning."

"Now we say 'are you still alive?'" he explains.

Such grim jokes underscore the anxieties of daily life in Seoul and illuminate the sense of insecurity and uncertainty that seems to grip the capital.

Too much has happened too fast. In the past seven weeks a president has been assassinated, his power assumed by a civilian caretaker government guided by a military commander. Now that commander has been arrested, in a lightning stroke of tanks and gun battles, and a new military leadership has taken control.

There is a widespread feeling that more is to come. Rumors persist of a new countercoup in making. Columns of tanks move through the city's empty streets after curfew on unexplained missions. The sonic boom of a jet fighter, a common sound ignored in normal times, now startles people.

For most of two days, the interim president, Choi Kyu Hah, seemed to have disappeared. Public statements are mistrusted widely. Two violent gun battles Wednesday night and Thursday morning are described in the censored press as, repsectively, a "minor" clash and a "misunderstanding."

THE AVERAGE South Korean knows far less of what has happened than the rest of the world. What he first knew of the internal coup in the armed forces was merely that the military guards at key buildings Thursday morning were from different units than those who were there on Wednesday morning.

As cryptically described in the press, the chain of events is bewildering. Six weeks ago, a week after the assassination of president Park Chung Hee, the citizens were informed that a famous general, Chung Sung Wha, would rule as martial-law commander. He promised a thorough investigation. The investigation revealed a narrow, hasty plot directed by a single man, who was put on trial. Suddenly Gen. Chung was arrested for suspected complicity in the plot.

Little else is explained publicly. The insurgent generals based their arrest of Chung solely on the claim that he may have participated with the accused assassin, Kim Jae Kyu. But Saturday, according to sources, Kim blurted out in his court-martial the news that Chung was not involved. The press could not report it.

Citizens learned from newspapers that President Choi had approved the arrest of Chung. Actually, Choi approved it only hours after it was over. According to the press, former defense minister Ro Jae Hyun announced the arrest. In fact, the statement was merely telephoned to the newspapers in his name. The public still has not been given the name of Gen. Chon Doo Whan, the general who directed his chief's arrest and who some observors believe is now the most powerful man in South Korea.

THE SUDDEN UPHEAVALS produce a sense of despair because they violated the cardinal rule most South Koreans have grown up with: Stability and unity must be maintained at all costs.

President Park justified much of his harsh rule by appealing to that sentiment. Dissent was intolerable, he preached, because it would encourage the North Koreans to attack. Most people accepted that view of national life, believing that their existence depended on it.

Now that stability has been broken not by political dissenters but by the single most prestigious institution in South Korea, the Army. The only undisputed fact seems to be that one Army clique has replaced another Army clique. Everything else is speculation.

ONE FOREIGNER here, an Asian who has lived in Seoul many years, says his South Korean friends speak in tones of despair. The country, they tell him, is becoming a "banana republic." The worst thing, he says, is the sense of incompetence: "They are ashamed."

The sense of helpesness is deepened by the total ignorance of what the new military leaders have in mind. A shopkeeper, Kim Soon Ja, says, "I worry most because I don't know what's really going on. We want things to be settled down as soon as possible. No one wants this uneasy feeling to go on for so long a time."

Politicians with better access to information are equally in doubt Spokesmen for both national parties expressed confidence this week that the new generals in power would not interfere in politics. A former Cabinet minister, however, said he was not optimistic. "I am worried that they will go in the wrong direction," he said. None of those interviewed had any facts to support their conflicting views.

THE SUDDEN POLITICAL changes erupted precisely at a time when South Korea was undergoing some unaccustomed economic difficulties after years of spectacular growth.

The annual rate of inflation rose to 24 percent and the trade deficit may grow to a peak of $4.5 billion this year, an alarming figure for a country that built its prosperity on exports.

There are now signs that the instability may be affecting business. One businessman describes the case of a foreign investor who suddenly postponed a contract here with his South Korean partner because of the recent troubles. a

"I have heard of many cases like this," the businessman said. "It is hard for businessmen to overcome this kind of situation.We need stabilization fast or we cannot do business at all."