This series is dedicated to exploring little-known—and occasionally useful—trinkets lurking in the dusty corners of UNIX-like operating systems. Arithmetic is something that one would normally expect computers to be able to do. With UNIX, one could of course always write a program to perform a calculation, but for people like me who are bad at programming, it would be nice to have a tool that makes things a bit easier. First Edition UNIX in 1971 included such a tool, called dc , which stands for "desk calculator" 1. This utility performed integer arithmetic (though later versions can handle real numbers) using reverse Polish notation. To divide 11 by 4 with this method, instead of entering "11 ÷ 4 =", you would key in "11 ENTER 4 ENTER ÷". The very first calculator I ever used, one made by Hewlett-Packard that my father brought home from work a few times, employed reverse Polish notation but I have never gotten used to it. All the calculators I have made significant use of and bought for myself used the more common infix notation "11 ÷ 4"—I also prefer computer utilities following that pattern, so I almost never use dc. For reasons explained in the rationale for the bc utility 2, dc has not been standardized in POSIX despite its long tenure. However, while it doesn't seem to get a lot of attention, I still would not consider dc to be a UNIX Curio. Today, it is possible to do integer arithmetic in a standard POSIX shell without any outside utilities. This is called "arithmetic expansion" and is described in references or manual pages for many shells. It wasn't always this way, however. Before a shell was available that supported arithmetic expansion, you needed to call another utility for your calculations, and expr was one of those 3. That program is the UNIX Curio for this episode. Some people might pronounce this name, but I find it awkward to say, so I just spell out expr the same as I would do with dc. Its name is an abbreviation of "expression", and it takes arguments representing an expression. An expression is formed by combining integers or strings with zero or more operator symbols. There is quite a variety of operators—some are mathematical, some perform comparisons, one matches a regular expression, and others are used for grouping or logical tests. Since we started this episode talking about arithmetic, let's tackle those first. The "+", "-", "*", and "/" symbols are for performing addition, subtraction, multiplication, and division respectively, as is common in many programming and scripting languages. The "%" produces the remainder of integer division. So, expr 11 / 4 would output 2; it only does integer calculations. The command expr 11 % 4 would output 3—in this case, the remainder left after "4" is removed from "11" twice. Take note that