Unix Tip: More on bc

by Sandra Henry-Stocker

March 20, 2008 —

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:

rivet> ./factorial
num> 111
176295255109024466387216104710707578876140953602656551604157406\
33473469550872483164365555745984623157731960476628379789131458474971\
99871623320096254145331200000000000000000000000000

The bc utility also includes a rudimentary form of test logic. If you type "12 > 6" in bc, for example, bc will respond with a "1", meaning "true"; Twelve is greater than 6. If you type "6 > 12" on the other hand, the response will be "0" for false. In a similar vein, you can use less than (<), less than or equal (<=), greater
than or equal (>=), equal (==) and not equal (!=). You can also calculate the reverse response, if for some reason you want to, by prepending a not (!) sign to your statement. For example:

!(6>12)
1

AND and OR operations are invoked with || and && operators as shown
in these examples:

(6>12)||(6>10)
0
(6>5)&&(6>3)
1

The bc utitity also has while and for statements. Here's a simple example of while statement:

while ( number > 0 ) {
  print number
  number=number-1
}
1110987654321

Here's a similar for statement:

for (number=11; number=number-1; number>0) {
    print number
    print "\n"
}
10
9
8
7
6
5
4
3
2
1

Some versions of bc also provide geometric functions, such as sine and cosine and natural logarithms.

There's a lot to this little language you are likely to find it handy whether you need to work in various numeric bases, handle extremely large numbers or do rapid fire numerical comparisons.

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