(Update: Late Friday night, lawmakers reached an agreement that keeps the federal government up and running. Crain's story below was posted Friday morning.)

(Crain's) — Chicago's largest employer is likely to be taking at least Monday off, but few will likely notice the no-show.

A shutdown of the federal government appears imminent as chances dim for a last-minute extension of funding that by law expires at midnight Friday.

With Republicans and Democrats still far apart on how much to cut, some 800,000 federal workers nationwide will be furloughed, including many of the nearly 50,000 federal workers in the Chicago region, but a short-term shutdown will be barely noticeable to the public and most businesses.

Mail will still be delivered, as the Post Office depends on postage, not appropriations. Social Security checks will still go out, too. Using funds on hand, Argonne National Laboratory and Fermi National Accelerator Laboratory will remain open, worried far more about long-term budget cuts than a short-term shutdown.

Planes will still stream in and out of Chicago's airports, as air traffic controllers are considered essential personnel who must remain on the job. Customs will clear passengers and cargo as usual, although most support staff will stay home.

“The public will not see that,” a Customs Bureau spokesman said.

While the closing of Smithsonian museums and attractions such as the Lincoln Memorial at the height of tourist season will produce howls in Washington, D.C., what's affected in Chicago will be far less visible, annoying only those with non-critical needs for government services.

For instance, a government shutdown would not only close local Small Business Administration offices but halt any entrepreneur's efforts in obtaining a SBA loan.

"The resolution that gives (small-business borrowers) money expires at the end of Friday," an SBA spokesman said.

Commercial lenders, which make roughly one-third of all SBA-approved loans, can still process paperwork, but the deals can't be completed without a federal guarantee. That stamp of approval won't happen if the agency is closed.

The SBA processes and approves roughly 1,000 loans a week at an average of $400,000 per borrower. The SBA spokesman said he is unaware of an increase in requests in anticipation of a shutdown, but he won't know for sure until next week, when he hopes to be still at work.

"We are still confident there will be an agreement by the end of the day Friday," the SBA spokesman said.

The Chicago office of Score, a small-business counseling agency, received notice from its Virginia headquarters that volunteers will not be allowed to meet with entrepreneurs should a federal shutdown occur.

"We won't be able to see any clients," said Mitchell Morris, a Score counselor in the Chicago office. "All our offices will be closed."

Mr. Morris said he is unaware of any anxious small business owners concerned that a shutdown would hamper business operations or hamper plans to open.

This week, many federal agencies were unprepared or at least unwilling to say how they would be affected as they scrambled to make plans for a long-threatened shutdown that now seems likely.

Even a major defense contractor like Boeing Co., which has major production contracts and scores of employees providing services to NASA, the Federal Aviation Administration, the Pentagon and other agencies, couldn't say what would and would not continue.

The Chicago-based aircraft maker “is evaluating the potential impact of a government shutdown,” a spokesman said. “However, until a shutdown is declared, we won't speculate on any impact.”

At the Securities and Exchange Commission, computerized filing of financial reports will continue, but processing of stock offerings or regulatory changes will come to a halt.

The Commodity Futures Trading Commission, which regulates Chicago's futures industry, said Friday that in the event of a shutdown only 25 of its 675 staffers will be at their posts Monday, the “bare minimum” needed to keep the markets open. All meetings will be canceled, and the agency said “the vast bulk of the CFTC's oversight and surveillance functions will cease during a lapse of appropriations,” although some furloughed employees may be called back to work if necessary.

But traders will have to do without the market news and statistical reports that stream constantly out of the U.S. Department of Agriculture, and that may affect trading in farm commodities.

Essential USDA functions such as meat, poultry and egg inspections will continue, as will monitoring of imports and exports for pests. Benefits such as food stamps will continue, at least for a few weeks, until previously appropriated funds run out.

The myriad ways in which federal agencies are funded, even the same federal agency, make a federal shutdown a complex situation.

At the Railroad Retirement Board, the largest federal agency based in Chicago, special benefit programs are administered for railroad workers, running parallel to the unemployment insurance and Social Security benefits that most workers enjoy.

There are funds available for railroad unemployment insurance benefits, so about 100 of the agency's 900 workers who handle those benefits will remain on the job. Most of the remaining 800 will be furloughed. Like Social Security, the agency will continue to pay railroad retirement benefits, but it won't process any new applications, which come in at a pace of roughly 100 a day.

For most people, a short-term shutdown of the federal government is no big deal. But “if you're one of those 100 a day, it's important,” a spokesman for the board said.

(Crain's senior reporter Lynne Marek contributed to this report.)