hpr-knowledge-base/hpr_transcripts/hpr1202.txt

Episode: 1202
Title: HPR1202: LiTS 025: bc
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr1202/hpr1202.mp3
Transcribed: 2025-10-17 21:33:05

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Welcome to Linux in the Shell Episode 25, VC. My name is Dan Washco, I'll be your host
today, and before I get started I'd like to thank Hacker Public Radio for hosting
the website and the audio files. Remember, head on over to Hacker Public Radio for some
great content every day of the work week. Not on Saturday and Sundays, but some really great stuff
from geeky people all over the world. And if you're inclined to and have a subject in mind,
consider doing an episode of Hacker Public Radio. They're always looking for volunteer,
so all that information can be found at hackerpublicradio.org. All right, let's talk about BC,
but before we get into that, why would you want to use it? Well, let's see you want to do some
simple math, find just, you know, multiplication, addition, division, anything. And you don't want
to fire up a calculator, go find a calculator. Even though there's a ton of them handy,
that, you know, chances are they're installed on your system and your menu, but you're down on
the shell and you want to do some simple math. Well, how can you do that in a shell? You can use
the echo command, and there'd be echo, space, dollar sign, parentheses, parentheses, some statement,
parentheses, parentheses, hit enter. For instance, echo, space, dollar sign, parentheses, parentheses,
two, asteris, three, close the parentheses, both of them, and hit enter, and you're going to get six.
So you would enclose the statement within a dollar sign and two parentheses.
That's great, but it only does simple, very, very simple math, and it will not particularly
end a vision, or any other statements, work with decimals. For instance, if you were to do echo,
dollar sign, parentheses, parentheses, four divided by three, close parentheses, and you're going
to get a value of one. When the value should actually be 1.33333 to infinity, because it's repeating.
You will not get that from just using echo commands. Similarly, you can use the expression command
EXPR, and do some simple mathematics, for instance, like EXPR, space, five, space, plus, space,
five, and when you hit enter, it's going to show 10. You've got to have spaces between everything,
or else you're just going to have an echo back to you, five plus five, if you didn't put any spaces in
there. But expression can be used for simple mathematics, and again, like echo, it doesn't recognize
decimals. If you want to get a little more advanced, do a little more equations, functions, and
stuff like that, there's the BC command. Well, it's a tool, actually. BC itself is a mathematical
language. It's an arbitrary precision calculator language, is what it's called. And when you run BC,
if you just type in BC, you're going to be put into what's called interactive mode,
and interactive mode is going to show you what version it is. It's going to show you some copyright
information, and tell you that, you know, for warranty, or details, see the warranty.
It's going to put you out of a command prompt, so you're in the BC tool. That's the same thing as
echoing, or it's not echoing, but using the dash i or dash dash interactive switch to BC,
which puts you into that tool. Now, there's another switch similar to that relevant
to dash q, quiet. So when you start up BC with dash q, it doesn't show you the warranty
information to the version information. You just go right into the command, the shell, BC tool itself,
the interactive mode. Now, you can type in your statement, for instance, we just did four divided
by three, which would be four slash three, and hit enter. And you'll notice, if you do this,
you're following along, it's going to return one. You might say, well, damn, I thought you were
going to give me some of tools I could use. It's going to have decimals. Well, BC does have decimals,
and to explain why you only see one value there. You only see one as opposed to one with some decimals.
If you type in the word scale, SCALE, and hit enter, it's going to return the value of zero.
Now, what scale is? There's four special variables here that you have available to you.
An addition to scale, but scale is the number of decimal digits,
number of digits after the decimal point. So if we were to type in scale equals three,
hit enter, and do four divided by three again, you're going to see one dot three three three.
Scale, when we set scale equal to three, that means three decimal digits. So there's going to be three
up to three digits after the decimal point available. So there we have our decimals back. So
they're not gone. It's not that it doesn't recognize decimals or allow you to do decimals.
It does, but by default, the scale is set to zero. Also by default, there's not a lot of other
functions loaded into it. So there's another switch that you might want to consider using
when you start up BC, and that's the dash L. So if you want to get into more complex functions
of mathematics, if you use the dash L, it loads the standard math library when it's executed.
And then that set scale automatically to 20. So you're going to get 20 decimal digits. In
addition to that, you're also going to get sine functionality, cosine functionality. Both of those
are shown in radians. You're going to get arc tangent and arc tangent returns in radians. You're
going to get the logarithm exponential functions and bezel function sine. If you want to do sine,
it's just sine. And then the value that you want to have to get the sine of in parentheses. So
sine parenthesis X closed parenthesis. Similar with cosine, it's C parenthesis X closed parenthesis.
And then arc tangent tangent is A parenthesis X parenthesis, closed parenthesis. Then you have
logarithm, which is L, and then the X parenthesis. X-ponential function, which is E, and then X-pon
parenthesis, and the bezel, which is J, and then in parentheses, you have the number comma X,
and of X right there. Now, if you're not familiar with those values are sine, cosine,
arc tangent, logarithm, exponential functions, bezel, head on over to the website. There is some
basic information I have in the bibliography that will explain some of those values.
I don't want to really go into too much detail here, but one of the things you might not recognize
X-ponential function is not exponents, or necessarily. The X-ponential function is a function
E of X, where E is the number approximately 2.718281828, and therefore such that the function of
E of X is his own derivative. So the value of what you'll be seeing there is 2.718281828 to the power
of whatever X value you put into the power of 2, 5, 10, whatever, but it's exponential function. It's
not, if you do E of X in there, you're not going to get X to the exponent that you put in 10 of,
so you might say, well, if it's exponents, how do I enter the second value? No, no, it's an
exponential function. It's not the actual exponent exponent. To do an exponential value of a number,
like if you wanted to do 5 to the 10th power, it would be 5 carat 10 to do exponents like that.
So its exponential function is not exponents. Now, as for what bezel function is, I'm not even
going to begin to try and explain that right now. It's way beyond the scope of this show, but
that function is in there, and I have a link to it in the bottom of the big biography.
Well, let's get back to BC. So now, if you run BC with the dash L library loaded, and you were to
type in for divided by 7, which is 4 slash 7, the value return is going to be 1.3, and you're going
to see 23s after this, because it's two 20 decimal spaces by default for when the math
lib library is loaded. You can see that by typing scale, and scale will return the value of 20.
Now, I said there are three other special functions in there, special variables. I'm going to talk
about those right now. First, most simple last. Last is the value of the last number printed. So,
if we were to type last right now, we're going to get 20, because the last thing we did was we said
scale in the variable return was 20. So, if you type last, you're going to get 20. If you put
in a statement like four times eight, and you hit enter, you're going to get a value of 32 back. If
you type last, that value is now 32. Very simple. It's a handy thing that you could use to continue
on with the return of a statement or a function that you put in the BC. To other values, I base
an O base. Now, by default, BC puts you in decimal, base decimal. You can change that. You can
change the input base, which is I base, or the O base, output base, to be any other base value.
Decimal by default, binary, hex, octal, you name it. So, you would just type in
I base equals 10 for decimal. And if I were to type in 15 times 1 and hit enter, that's going to
give me 15. But if I do O base, which is output, equals 2, puts me in binary, and I type 15 times
1. The result that gets returned is going to be not in decimal anymore, but it's going to be in
binary. So, I do 15 times 1. The value is going to be 111141. That's a value of 15. If I wanted to
do hex, I would just type O base equals 8. No, I'm sorry, that's octal. And when I do that,
if I do the same thing, 15 times 1, I get a value of 117 or 17. That gives me the value of 15
back, 117 and an octal. Same thing with hex. If I do hex, O base equals 16, 15 times 1, I get
the value F returned. So, you can switch between bases with the BC command by using I base or
O base. And then if you want to set it back, it's just I base equals or O base equals 10 for decimal,
or you can exit and restart BC. The BC command respects the standard order of operations and all
mathematical functions, that basic mathematical functions. Similarly, anything that you can do in
a calculator, you can do in pretty much in the BC command. Now, where one of the things that
makes says BC apart though, is because it's a language, you can actually kind of do some programming in it
with the BC language. For instance, you can create functions. And to create a function,
the format is just like any other programming language. You type in define, we're define,
and then the function name. For instance, let's create a simple function that just converts a value
into binary, octal and hex, and then puts you back into decimal. So you would define a function,
call it Litz converter. And then in parentheses, you say the variables that you accept be passed to
that function. In our case right here, we're just going to have one variable x. And then open
parenth, curly brackets, the syntax of your function in there, all your statements for the function,
and then you close it with curly brackets, closing curly brackets. So for instance, if our function
Litz converter, we would say O base equals 2, put a semicolon, you don't have to put a semicolon,
but it's probably a good idea. I've done it both ways. Hit enter, and then print, which is a value
that would print out onto the screen, binary value is, and then double quotes, close the quotes,
semicolon, a times one, semicolon, print, and double quotes, backslash, and for a new line.
And I do, just remember that whole statement right there, print binary value equals,
and then a times n, and then another print statement. So then after that line, we do O base equals
8 to switch to octal, do the same thing, print, octal value is, and double quotes, and then
semicolon, a times one, semicolon, print, new line, and double quotes. Now we go O base equals 16,
semicolon, let me do print, and double quotes, hex value equals semicolon, put that in double
quotes, and semicolon in 8 times one, semicolon, print, new line, let me do O base equals 10,
and then print, and double quotes, all finished, and close it off, hit enter, or closing curly bracket.
Now when we run, let's converter two, you see the value of two, returned in binary,
octal, and hex, which are also going to see a value, the zero return at the end.
And you're going to say, why is the zero return? Well,
ignore the zero returns at the end of every function if return is not used. So
we're not returning anything in the function, so it returns a value of zero by default.
If we wanted to avoid the zero, we would return, and some value have the value of that be returned,
but because we're not using the return statement, it automatically returns a zero.
So that's BC in a nutshell. If you quit BC, you lose the value of your
defined function. So just be aware of that. You're not able to save it. It doesn't retain the
function, so you kind of lost there. Now, if you wanted to get some statement, return, some
mathematics done, you don't have to run BC in interactive mode. You can pass something
to BC with the echo command and pipe it to BC. So I echo in double quotes two times two
asterisk five, close to double quotes, pipe BC. That's going to pass two times five to BC,
and it's just going to spit out the value of 10. It doesn't put you in interactive mode,
but it allows you to pass or do simple mathematics, pass it to the BC command, simple statements.
Or actually any statement, you can pass to it. In fact, if you want to pass a whole
string of statements to BC, like with the echo command, just separate each statement with a
semicolon, like echo, double quotes, O base equals two, semicolon five times five, semicolon,
O base equals 10, semicolon last. What that's going to do is it's going to pass all that to BC,
so BC is going to process it one thing at a time. It's going to set the output base to binary.
It's going to process five times five, so you're going to get 25. It's going to show you the value
of 25 in binary. Then it's going to set the O base back output base back to decimal, but equals 10.
And then with the last command, it's going to show you the value of the last number that's put out,
because now it's in decimal, it's going to show you what it would be in decimal 25. So there we go.
That is a way BC command is a way to do mathematical function statements from the command line,
very easy or very complex, however deep you want to get. It's a great handy tool if you're
down there and you need to figure out some kind of value. A good example is I wanted to find something
in, convert something from megabytes to bytes. Instead of having to whip out a calculator,
I could just type in the BC command, the value of that multiply it by 1024 multiply again by 1024
to go from megabytes down to kilobytes down to bytes and get that number there. So I can use it
in the split command for whatever I wanted to, because this version of split I was using at the time,
only except in input values of bytes, not in kilobytes or megabytes that I wanted to,
it was all the version of split. Anyway, that is BC in a nutshell. You might be saying, Dan,
is there an easier way to get that value right there, convert something from one format to another?
I'm going to leave you with that because there is and that is foreshadowing my friend of an
upcoming show. This has been Linux in the show episode 25 talking about BC and some mathematics
on the command line. I hope that you will join me next week and two weeks, I'm going to
fortnight to continue on some other really handy mathematical tools on the command line. Thanks a
lot and have a great day.
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