Unix Tip: More on bc

In a previous column, we looked at bc, the surprisingly high precision and big value capable Unix tool that you will find on most versions of Unix today. The bc utility is not just a command line tool, however. You can also write numerically intensive scripts in bc. The tool incorporates enough syntax to assign values to variables, write and call functions, collect responses from users and print annotated results.

To start a bc script, you include the path to bc in your
shebang line as you would for any shell.

#!/usr/bin/bc

Let's look at a script for computing factorials. The mainpart of this script (the "f" function) comes straight from the man page. I've added a -q to the shebang line so that the script doesn't print the traditional bc welcome line, a prompt to nudge the user into providing some input, a read statement to collect the user's response, a call to the f function, a command to print the result and a quit command (otherwise, you will still be running bc when the script has ended.

#!/usr/bin/bc -q

/* factorial */
define f (x) {
  if (x <= 1) return (1);
  return (f(x-1) * x);
}

print "num> "
num=read()
ans=f(num)
print ans
print "\n"
quit

Notice that function f is recursive. Based on our understanding of factorials, each time the function calls itself, it does so with a decremented argument. If the user enters 3, for example, the function is called from the main body of the script with the argument 3 and the function then calls itself twice -- once with the number 2 and once with the number 1.

Notice the C-like comment on line 3 and the print newline statement on line 13. Like C and Perl, you need to request a newline if you want one. Even if you are using bc interactively, you will not get a newline unless you ask for one. Type "print 11" and your cursor will be sitting to the right of the printed number.

Probably the hardest thing to get used to is the lack of a prompt.You type "bc" on the command line and you will see something like this:

rivet> bc
bc 1.05
Copyright 1991, 1992, 1993, 1994, 1997, 1998 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.

Your cursor then sits on a blank line while bc waits for your request.

If we follow the math through the script, we see that f(3) becomes f(2) * 3 which becomes f(1) * 2 * 3 which finally becomes 1 * 2 * 3 or 6.

If the user types in a large number, say 111 or 1234, the result is going to span multiple lines just as did many of the answers we looked at last week:

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