转载：linux下使用bc在命令行中进行计算

原文：http://www.basicallytech.com/blog/index.php?/archives/23-command-line-calculations-using-bc.html

bc是一个在命令行使用的强大计算器，本文作者通过实例来介绍bc命令的使用，是你瞬间掌握bc的基本应用。

文末，作者还给出了通过bc计算5000位PI的命令来测定你的cpu性能

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This article shows how to quickly perform standard calculations on the command line including addition, subtraction, multiplication, division, square root, powers, conversion from decimal to hex, decimal to binary, hex to decimal, and binary to decimal. It also briefly introduces using?bc?in interactive mode and how to write files for use with?bc?for frequently repeated operations. There is a mention of using Google for performing calulations. It finishes with a little challenge to test the power of your CPU.

Other advantages of using?bc?include:

• bc?is included with (almost?) all Linux distros as standard, as well as (again, almost?) all Unices.
• Results from calculations in some proprietary flavours of?bchave up to 99 decimal digits before and after the decimal point. This limit has been greatly surpassed in GNU?bc. I don't know what that limit is, but it's at least many, many tens of thousands. Certainly it's more than any GUI-based calculators (I've used) could accomodate.
• You may also find yourself working in an environment where you simply don't have access to a GUI.
• The syntax for basic sums is almost identical to?Google's calculator function, so you can learn how to use two utilities in one go!

  bc?is a preprocessor for?dc. The useful thing about?bc?is that it accepts input from files and from standard input. This allows us to pipe data to it for quick calculations.

  • addition
  • subtraction
  • multiplication
  • scale
  • division
  • square root
  • power
  • parentheses
  • obase and ibase
  • convert from decimal to hexadecimal
  • convert from decimal to binary
  • convert from binary to decimal
  • convert from hexadecimal to decimal
  • a brief introduction to interactive mode
  • using bc with shell scripts
  • a brief introduction to using bc with files
  • a quick challenge for your PC (GNU bc only)

    Most of these examples follow a simple formula.

    addition

    $?echo '57+43' | bc

    100

    subtraction

    $?echo '57-43' | bc

    14

    multiplication

    $?echo '57*43' | bc

    2451

    scale

    The?scale?variable determines the number of digits which follow the decimal point in your result. By default, the value of the?scalevariable is zero. (Unless you use the?-l?option in which case it defaults to 20 decimal places. More about?-l?later.) This can be set by declaring?scale?before your calculation, as in the following division example:

    division

    $?echo 'scale=25;57/43' | bc

    1.3255813953488372093023255

    square root

    $?echo 'scale=30;sqrt(2)' | bc

    1.414213562373095048801688724209

    This beats?Google's calculator function?which only calculates the result to 8 decimal places!??Although Google's calculator function has this 8 decimal places limitation, it will allow?imaginary numbers?as answers.

    power

    $?echo '6^6' | bc

    46656

    parentheses

    If you have read Robert Heinlein's?The Number of the Beast, you may recall that the number of parallel universes in the story equals (six to the power of six) to the power of six. If you should try to calculate that like this:

    $?echo '6^6^6' | bc

    You will get a screen?full?of numbers (some 37374 digits), not the

    10314424798490535546171949056

    that you might expect.

    If you're running a non-GNU version of?bc, you'll most likely get something like:

    exp too bigempty stacksave:args

    The Google Calculator balks at '6^6^6' as well. Good ol' GNU.

    That's because you typed the wrong question. You need to type:

    $?echo '(6^6)^6' | bc

    Whereas what you did type was interpreted as:

    $?echo '6^(6^6)' | bc

    which is an entirely different number altogether. So the positioning of parentheses (brackets to you and me!) is very important. I use brackets to separate the different components of my sums whenever possible, just eliminate any possible doubt that I may get the wrong answer. Consider the following calculations:

    $?echo '7+(6*5)' | bc

    $?echo '7+6*5' | bc

    $?echo '6*5+7' | bc

    They all give the same answer, 37, but I would have typed the first calculation, unless of course, I meant:

    $?echo '(7+6)*5' | bc

    Or to put it another way:

    $?echo '13*5' | bc

    which is 65.

    obase and ibase

    obase?and?ibase?are special variables which define output and input base.

    Legitimate?obase?values range from 2 to 999, although anything beyond 16 is wasted on me!

    Legitimate?ibase?values range from 2 to 16.

    Some examples will explain all this better.

    convert from decimal to hexadecimal

    Here we're converting 255 from base 10 to base 16:

    $?echo 'obase=16;255' | bc

    FF

    convert from decimal to binary

    And here we're converting the number 12 from base 10 to base 2:

    $?echo 'obase=2;12' | bc

    1100

    Which reminds me of the old joke:

    There are only 10 types of people in the world -- those who understand binary, and those who don't.

    Which leads us neatly onto the next example:

    convert from binary to decimal

    Here we're converting the binary number 10 to a base 10 (decimal) number.

    $?echo 'ibase=2;obase=A;10' | bc

    2

    Note that the?obase?is "A" and?not?"10". Sorry, you've got to learn some hex. The reason for this is you've set the?ibase?to "2", so if you now had tried to use "10" as the value for the?obase, it would stay as "2", because "10" in base 2 is "2". So you need to use hex to "break out" of binary mode.

    Well, that was just to explain the joke; now something a bit more challenging:

    $?echo 'ibase=2;obase=A;10000001' | bc

    129

    convert from hexadecimal to decimal

    $?echo 'ibase=16;obase=A;FF' | bc

    255

    Again, note the use of "A" to denote base 10. That is because "10" in hex (base 16 - the?ibase?value) is 16.

    a brief introduction to interactive mode

    You can also run?bc?in interactive mode:

    $?bc

    If you're running GNU?bc, you should get the following notice:

    bc 1.06Copyright 1991-1994, 1997, 1998, 2000 Free Software Foundation, Inc.This is free software with ABSOLUTELY NO WARRANTY.For details type `warranty'.

    Followed by an uninviting blank prompt. Otherwise you'll just get an uninviting blank prompt.

    If you wish to get straight to the uninviting blank prompt, use the-q?option, which runs?bc?in quiet mode, preventing the normal GNU bc welcome from being printed:

    $?bc -q

    Using the basics we've been through from the examples above, enter a calculation:

    scale=5
    57/43
    1.32558

    Type?quit?to exit?bc?interactive mode.

    using bc with shell scripts

    You can use shell variables with?bc, which is very useful in shell scripts:

    $?FIVE=5 ; echo "$FIVE^2" | bc

    25

    Note the use of double-quotes to preserve the value of the variable$FIVE.

    a brief introduction to using bc with files

    Using?bc?with files allows complex calculations to be repeated, again and again, a bit like using a spreadsheet to run the same calculations on changing figures ... but faster.

    Here is a simple example:

    scale=2/* C-style comments   are allowed, as are spaces */print "\nConvert Fahrenheit to Celsius\n\n"print "Temperature in Fahrenheit: " ; fah = read()print "\n"print "Equivalent Temperature in Celsius is: "(fah - 32.0) * 5.0 / 9.0quit

    Create and save the file, then run it like this:

    $?bc -q?filename
    
    Convert Fahrenheit to Celsius
    
    Temperature in Fahrenheit:?61
    
    Equivalent Temperature in Celsius is: 16.11
    

    Note that this example has only been tested with GNU?bc. Other (proprietary) versions of?bc?may have more stringent syntax requirements. Some?bcs don't allow the use of?print?or?read, for example, so you have to edit your file before each calculation. Not very useful.

    a quick challenge for your PC (GNU bc only)

    If you wish to test the comparative speed of your PC, try this challenge: use?bc?to calculate Pi to 5000 decimal places. The idea for this challenge came from a great?article at Geekronomicon.

    If you really want to tie up your machine for an hour (or more), you could try the "Pi to 25000 decimal places" challenge from the aforementioned?Geekronomicon.?

    First, to put things in perspective, here is some information about my CPU:

    $?cat /proc/cpuinfo | egrep "model name|MHz"

    model name      : AMD Athlon(tm) 64 Processor 3500+cpu MHz         : 2211.346

    Note the use (below) of the command?bc -l -q.?
    -l?loads the math library which is required for the use of the "a" (arctangent) in the calculation for Pi. You can learn more about the math library functions in the?bc command manual.?
    I'm not sure what effect the?-q?option (quiet, no welcome message printed) has on our test, but I guess it can't harm.

    $?time echo "scale=5000; 4*a(1)" | bc -l -q

    3.141592653589793238462643383279502884197169399375105820974944592307\.........73774418426312986080998886874132604720real    0m44.164suser    0m44.099ssys     0m0.008s

    44.099 seconds! Not bad.??I imagine that some Gentoo folks may be interested to see what difference their compile-time optimisations make to the speed of?bc. FWIW, my distro of choice isArch Linux.

    useful links

    GNU bc manual

    Google's calculator function