bc(1) bc(1)
NAME
bc - An arbitrary precision calculator language
SYNTAX
bc [ -hlwsqv ] [long-options] [ file ... ]
VERSION
This man page documents GNU bc version 1.06.
DESCRIPTION
bc is a language that supports arbitrary precision numbers with inter
active execution of statements. There are some similarities in the
syntax to the C programming language. A standard math library is
available by command line option. If requested, the math library is
defined before processing any files. bc starts by processing code from
all the files listed on the command line in the order listed. After
all files have been processed, bc reads from the standard input. All
code is executed as it is read. (If a file contains a command to halt
the processor, bc will never read from the standard input.)
This version of bc contains several extensions beyond traditional bc
implementations and the POSIX draft standard. Command line options can
cause these extensions to print a warning or to be rejected. This doc
ument describes the language accepted by this processor. Extensions
will be identified as such.
OPTIONS
-h, --help
Print the usage and exit.
-i, --interactive
Force interactive mode.
-l, --mathlib
Define the standard math library.
-w, --warn
Give warnings for extensions to POSIX bc.
-s, --standard
Process exactly the POSIX bc language.
-q, --quiet
Do not print the normal GNU bc welcome.
-v, --version
Print the version number and copyright and quit.
NUMBERS
The most basic element in bc is the number. Numbers are arbitrary pre
cision numbers. This precision is both in the integer part and the
fractional part. All numbers are represented internally in decimal and
all computation is done in decimal. (This version truncates results
from divide and multiply operations.) There are two attributes of num
bers, the length and the scale. The length is the total number of sig
nificant decimal digits in a number and the scale is the total number
of decimal digits after the decimal point. For example:
.000001 has a length of 6 and scale of 6.
1935.000 has a length of 7 and a scale of 3.
VARIABLES
Numbers are stored in two types of variables, simple variables and
arrays. Both simple variables and array variables are named. Names
begin with a letter followed by any number of letters, digits and
underscores. All letters must be lower case. (Full alpha-numeric
names are an extension. In POSIX bc all names are a single lower case
letter.) The type of variable is clear by the context because all
array variable names will be followed by brackets ([]).
There are four special variables, scale, ibase, obase, and last. scale
defines how some operations use digits after the decimal point. The
default value of scale is 0. ibase and obase define the conversion base
for input and output numbers. The default for both input and output is
base 10. last (an extension) is a variable that has the value of the
last printed number. These will be discussed in further detail where
appropriate. All of these variables may have values assigned to them
as well as used in expressions.
COMMENTS
Comments in bc start with the characters /* and end with the characters
*/. Comments may start anywhere and appear as a single space in the
input. (This causes comments to delimit other input items. For exam
ple, a comment can not be found in the middle of a variable name.)
Comments include any newlines (end of line) between the start and the
end of the comment.
To support the use of scripts for bc, a single line comment has been
added as an extension. A single line comment starts at a # character
and continues to the next end of the line. The end of line character
is not part of the comment and is processed normally.
EXPRESSIONS
The numbers are manipulated by expressions and statements. Since the
language was designed to be interactive, statements and expressions are
executed as soon as possible. There is no "main" program. Instead,
code is executed as it is encountered. (Functions, discussed in detail
later, are defined when encountered.)
A simple expression is just a constant. bc converts constants into
internal decimal numbers using the current input base, specified by the
variable ibase. (There is an exception in functions.) The legal values
of ibase are 2 through 16. Assigning a value outside this range to
ibase will result in a value of 2 or 16. Input numbers may contain the
characters 0-9 and A-F. (Note: They must be capitals. Lower case let
ters are variable names.) Single digit numbers always have the value
of the digit regardless of the value of ibase. (i.e. A = 10.) For
multi-digit numbers, bc changes all input digits greater or equal to
ibase to the value of ibase-1. This makes the number FFF always be the
largest 3 digit number of the input base.
Full expressions are similar to many other high level languages. Since
there is only one kind of number, there are no rules for mixing types.
Instead, there are rules on the scale of expressions. Every expression
has a scale. This is derived from the scale of original numbers, the
operation performed and in many cases, the value of the variable scale.
Legal values of the variable scale are 0 to the maximum number repre
sentable by a C integer.
In the following descriptions of legal expressions, "expr" refers to a
complete expression and "var" refers to a simple or an array variable.
A simple variable is just a
name
and an array variable is specified as
name[expr]
Unless specifically mentioned the scale of the result is the maximum
scale of the expressions involved.
- expr The result is the negation of the expression.
++ var The variable is incremented by one and the new value is the
result of the expression.
-- var The variable is decremented by one and the new value is the
result of the expression.
var ++ The result of the expression is the value of the variable and
then the variable is incremented by one.
var -- The result of the expression is the value of the variable and
then the variable is decremented by one.
expr + expr
The result of the expression is the sum of the two expressions.
expr - expr
The result of the expression is the difference of the two
expressions.
expr * expr
The result of the expression is the product of the two expres
sions.
expr / expr
The result of the expression is the quotient of the two expres
sions. The scale of the result is the value of the variable
scale.
expr % expr
The result of the expression is the "remainder" and it is com
puted in the following way. To compute a%b, first a/b is com
puted to scale digits. That result is used to compute a-(a/b)*b
to the scale of the maximum of scale+scale(b) and scale(a). If
scale is set to zero and both expressions are integers this
expression is the integer remainder function.
expr ^ expr
The result of the expression is the value of the first raised to
the second. The second expression must be an integer. (If the
second expression is not an integer, a warning is generated and
the expression is truncated to get an integer value.) The scale
of the result is scale if the exponent is negative. If the
exponent is positive the scale of the result is the minimum of
the scale of the first expression times the value of the expo
nent and the maximum of scale and the scale of the first expres
sion. (e.g. scale(a^b) = min(scale(a)*b, max( scale,
scale(a))).) It should be noted that expr^0 will always return
the value of 1.
( expr )
This alters the standard precedence to force the evaluation of
the expression.
var = expr
The variable is assigned the value of the expression.
var = expr
This is equivalent to "var = var expr" with the exception
that the "var" part is evaluated only once. This can make a
difference if "var" is an array.
Relational expressions are a special kind of expression that always
evaluate to 0 or 1, 0 if the relation is false and 1 if the relation is
true. These may appear in any legal expression. (POSIX bc requires
that relational expressions are used only in if, while, and for state
ments and that only one relational test may be done in them.) The
relational operators are
expr1 < expr2
The result is 1 if expr1 is strictly less than expr2.
expr1 <= expr2
The result is 1 if expr1 is less than or equal to expr2.
expr1 > expr2
The result is 1 if expr1 is strictly greater than expr2.
expr1 >= expr2
The result is 1 if expr1 is greater than or equal to expr2.
expr1 == expr2
The result is 1 if expr1 is equal to expr2.
expr1 != expr2
The result is 1 if expr1 is not equal to expr2.
Boolean operations are also legal. (POSIX bc does NOT have boolean
operations). The result of all boolean operations are 0 and 1 (for
false and true) as in relational expressions. The boolean operators
are:
!expr The result is 1 if expr is 0.
expr && expr
The result is 1 if both expressions are non-zero.
expr || expr
The result is 1 if either expression is non-zero.
The expression precedence is as follows: (lowest to highest)
|| operator, left associative
&& operator, left associative
! operator, nonassociative
Relational operators, left associative
Assignment operator, right associative
+ and - operators, left associative
*, / and % operators, left associative
^ operator, right associative
unary - operator, nonassociative
++ and -- operators, nonassociative
This precedence was chosen so that POSIX compliant bc programs will run
correctly. This will cause the use of the relational and logical opera
tors to have some unusual behavior when used with assignment expres
sions. Consider the expression:
a = 3 < 5
Most C programmers would assume this would assign the result of "3 < 5"
(the value 1) to the variable "a". What this does in bc is assign the
value 3 to the variable "a" and then compare 3 to 5. It is best to use
parenthesis when using relational and logical operators with the
assignment operators.
There are a few more special expressions that are provided in bc.
These have to do with user defined functions and standard functions.
They all appear as "name(parameters)". See the section on functions
for user defined functions. The standard functions are:
length ( expression )
The value of the length function is the number of significant
digits in the expression.
read ( )
The read function (an extension) will read a number from the
standard input, regardless of where the function occurs.
Beware, this can cause problems with the mixing of data and pro
gram in the standard input. The best use for this function is
in a previously written program that needs input from the user,
but never allows program code to be input from the user. The
value of the read function is the number read from the standard
input using the current value of the variable ibase for the con
version base.
scale ( expression )
The value of the scale function is the number of digits after
the decimal point in the expression.
sqrt ( expression )
The value of the sqrt function is the square root of the expres
sion. If the expression is negative, a run time error is gener
ated.
STATEMENTS
Statements (as in most algebraic languages) provide the sequencing of
expression evaluation. In bc statements are executed "as soon as pos
sible." Execution happens when a newline in encountered and there is
one or more complete statements. Due to this immediate execution, new
lines are very important in bc. In fact, both a semicolon and a newline
are used as statement separators. An improperly placed newline will
cause a syntax error. Because newlines are statement separators, it is
possible to hide a newline by using the backslash character. The
sequence "\", where is the newline appears to bc as whitespace
instead of a newline. A statement list is a series of statements sepa
rated by semicolons and newlines. The following is a list of bc state
ments and what they do: (Things enclosed in brackets ([]) are optional
parts of the statement.)
expression
This statement does one of two things. If the expression starts
with " ...", it is considered to be an
assignment statement. If the expression is not an assignment
statement, the expression is evaluated and printed to the out
put. After the number is printed, a newline is printed. For
example, "a=1" is an assignment statement and "(a=1)" is an
expression that has an embedded assignment. All numbers that
are printed are printed in the base specified by the variable
obase. The legal values for obase are 2 through BC_BASE_MAX.
(See the section LIMITS.) For bases 2 through 16, the usual
method of writing numbers is used. For bases greater than 16,
bc uses a multi-character digit method of printing the numbers
where each higher base digit is printed as a base 10 number.
The multi-character digits are separated by spaces. Each digit
contains the number of characters required to represent the base
ten value of "obase-1". Since numbers are of arbitrary preci
sion, some numbers may not be printable on a single output line.
These long numbers will be split across lines using the "\" as
the last character on a line. The maximum number of characters
printed per line is 70. Due to the interactive nature of bc,
printing a number causes the side effect of assigning the
printed value to the special variable last. This allows the user
to recover the last value printed without having to retype the
expression that printed the number. Assigning to last is legal
and will overwrite the last printed value with the assigned
value. The newly assigned value will remain until the next num
ber is printed or another value is assigned to last. (Some
installations may allow the use of a single period (.) which is
not part of a number as a short hand notation for for last.)
string The string is printed to the output. Strings start with a dou
ble quote character and contain all characters until the next
double quote character. All characters are take literally,
including any newline. No newline character is printed after
the string.
print list
The print statement (an extension) provides another method of
output. The "list" is a list of strings and expressions sepa
rated by commas. Each string or expression is printed in the
order of the list. No terminating newline is printed.
Expressions are evaluated and their value is printed and
assigned to the variable last. Strings in the print statement
are printed to the output and may contain special characters.
Special characters start with the backslash character (\). The
special characters recognized by bc are "a" (alert or bell), "b"
(backspace), "f" (form feed), "n" (newline), "r" (carriage
return), "q" (double quote), "t" (tab), and "\" (backslash).
Any other character following the backslash will be ignored.
{ statement_list }
This is the compound statement. It allows multiple statements
to be grouped together for execution.
if ( expression ) statement1 [else statement2]
The if statement evaluates the expression and executes state
ment1 or statement2 depending on the value of the expression.
If the expression is non-zero, statement1 is executed. If
statement2 is present and the value of the expression is 0, then
statement2 is executed. (The else clause is an extension.)
while ( expression ) statement
The while statement will execute the statement while the expres
sion is non-zero. It evaluates the expression before each exe
cution of the statement. Termination of the loop is caused by
a zero expression value or the execution of a break statement.
for ( [expression1] ; [expression2] ; [expression3] ) statement
The for statement controls repeated execution of the statement.
Expression1 is evaluated before the loop. Expression2 is evalu
ated before each execution of the statement. If it is non-zero,
the statement is evaluated. If it is zero, the loop is termi
nated. After each execution of the statement, expression3 is
evaluated before the reevaluation of expression2. If expres
sion1 or expression3 are missing, nothing is evaluated at the
point they would be evaluated. If expression2 is missing, it is
the same as substituting the value 1 for expression2. (The
optional expressions are an extension. POSIX bc requires all
three expressions.) The following is equivalent code for the
for statement:
expression1;
while (expression2) {
statement;
expression3;
}
break This statement causes a forced exit of the most recent enclosing
while statement or for statement.
continue
The continue statement (an extension) causes the most recent
enclosing for statement to start the next iteration.
halt The halt statement (an extension) is an executed statement that
causes the bc processor to quit only when it is executed. For
example, "if (0 == 1) halt" will not cause bc to terminate
because the halt is not executed.
return Return the value 0 from a function. (See the section on func
tions.)
return ( expression )
Return the value of the expression from a function. (See the
section on functions.) As an extension, the parenthesis are not
required.
PSEUDO STATEMENTS
These statements are not statements in the traditional sense. They are
not executed statements. Their function is performed at "compile"
time.
limits Print the local limits enforced by the local version of bc.
This is an extension.
quit When the quit statement is read, the bc processor is terminated,
regardless of where the quit statement is found. For example,
"if (0 == 1) quit" will cause bc to terminate.
warranty
Print a longer warranty notice. This is an extension.
FUNCTIONS
Functions provide a method of defining a computation that can be exe
cuted later. Functions in bc always compute a value and return it to
the caller. Function definitions are "dynamic" in the sense that a
function is undefined until a definition is encountered in the input.
That definition is then used until another definition function for the
same name is encountered. The new definition then replaces the older
definition. A function is defined as follows:
define name ( parameters ) { newline
auto_list statement_list }
A function call is just an expression of the form "name(parameters)".
Parameters are numbers or arrays (an extension). In the function defi
nition, zero or more parameters are defined by listing their names sep
arated by commas. Numbers are only call by value parameters. Arrays
are only call by variable. Arrays are specified in the parameter defi
nition by the notation "name[]". In the function call, actual parame
ters are full expressions for number parameters. The same notation is
used for passing arrays as for defining array parameters. The named
array is passed by variable to the function. Since function defini
tions are dynamic, parameter numbers and types are checked when a func
tion is called. Any mismatch in number or types of parameters will
cause a runtime error. A runtime error will also occur for the call to
an undefined function.
The auto_list is an optional list of variables that are for "local"
use. The syntax of the auto list (if present) is "auto name, ... ;".
(The semicolon is optional.) Each name is the name of an auto vari
able. Arrays may be specified by using the same notation as used in
parameters. These variables have their values pushed onto a stack at
the start of the function. The variables are then initialized to zero
and used throughout the execution of the function. At function exit,
these variables are popped so that the original value (at the time of
the function call) of these variables are restored. The parameters are
really auto variables that are initialized to a value provided in the
function call. Auto variables are different than traditional local
variables because if function A calls function B, B may access function
As auto variables by just using the same name, unless function B has
called them auto variables. Due to the fact that auto variables and
parameters are pushed onto a stack, bc supports recursive functions.
The function body is a list of bc statements. Again, statements are
separated by semicolons or newlines. Return statements cause the ter
mination of a function and the return of a value. There are two ver
sions of the return statement. The first form, "return", returns the
value 0 to the calling expression. The second form, "return ( expres
sion )", computes the value of the expression and returns that value to
the calling expression. There is an implied "return (0)" at the end of
every function. This allows a function to terminate and return 0 with
out an explicit return statement.
Functions also change the usage of the variable ibase. All constants
in the function body will be converted using the value of ibase at the
time of the function call. Changes of ibase will be ignored during the
execution of the function except for the standard function read, which
will always use the current value of ibase for conversion of numbers.
As an extension, the format of the definition has been slightly
relaxed. The standard requires the opening brace be on the same line
as the define keyword and all other parts must be on following lines.
This version of bc will allow any number of newlines before and after
the opening brace of the function. For example, the following defini
tions are legal.
define d (n) { return (2*n); }
define d (n)
{ return (2*n); }
MATH LIBRARY
If bc is invoked with the -l option, a math library is preloaded and
the default scale is set to 20. The math functions will calculate
their results to the scale set at the time of their call. The math
library defines the following functions:
s (x) The sine of x, x is in radians.
c (x) The cosine of x, x is in radians.
a (x) The arctangent of x, arctangent returns radians.
l (x) The natural logarithm of x.
e (x) The exponential function of raising e to the value x.
j (n,x)
The bessel function of integer order n of x.
EXAMPLES
In /bin/sh, the following will assign the value of "pi" to the shell
variable pi.
pi=$(echo "scale=10; 4*a(1)" | bc -l)
The following is the definition of the exponential function used in the
math library. This function is written in POSIX bc.
scale = 20
/* Uses the fact that e^x = (e^(x/2))^2
When x is small enough, we use the series:
e^x = 1 + x + x^2/2! + x^3/3! + ...
*/
define e(x) {
auto a, d, e, f, i, m, v, z
/* Check the sign of x. */
if (x<0) {
m = 1
x = -x
}
/* Precondition x. */
z = scale;
scale = 4 + z + .44*x;
while (x > 1) {
f += 1;
x /= 2;
}
/* Initialize the variables. */
v = 1+x
a = x
d = 1
for (i=2; 1; i++) {
e = (a *= x) / (d *= i)
if (e == 0) {
if (f>0) while (f--) v = v*v;
scale = z
if (m) return (1/v);
return (v/1);
}
v += e
}
}
The following is code that uses the extended features of bc to imple
ment a simple program for calculating checkbook balances. This program
is best kept in a file so that it can be used many times without having
to retype it at every use.
scale=2
print "\nCheck book program!\n"
print " Remember, deposits are negative transactions.\n"
print " Exit by a 0 transaction.\n\n"
print "Initial balance? "; bal = read()
bal /= 1
print "\n"
while (1) {
"current balance = "; bal
"transaction? "; trans = read()
if (trans == 0) break;
bal -= trans
bal /= 1
}
quit
The following is the definition of the recursive factorial function.
define f (x) {
if (x <= 1) return (1);
return (f(x-1) * x);
}
READLINE AND LIBEDIT OPTIONS
GNU bc can be compiled (via a configure option) to use the GNU readline
input editor library or the BSD libedit library. This allows the user
to do editing of lines before sending them to bc. It also allows for a
history of previous lines typed. When this option is selected, bc has
one more special variable. This special variable, history is the num
ber of lines of history retained. For readline, a value of -1 means
that an unlimited number of history lines are retained. Setting the
value of history to a positive number restricts the number of history
lines to the number given. The value of 0 disables the history fea
ture. The default value is 100. For more information, read the user
manuals for the GNU readline, history and BSD libedit libraries. One
can not enable both readline and libedit at the same time.
DIFFERENCES
This version of bc was implemented from the POSIX P1003.2/D11 draft and
contains several differences and extensions relative to the draft and
traditional implementations. It is not implemented in the traditional
way using dc(1). This version is a single process which parses and
runs a byte code translation of the program. There is an "undocu
mented" option (-c) that causes the program to output the byte code to
the standard output instead of running it. It was mainly used for
debugging the parser and preparing the math library.
A major source of differences is extensions, where a feature is
extended to add more functionality and additions, where new features
are added. The following is the list of differences and extensions.
LANG environment
This version does not conform to the POSIX standard in the pro
cessing of the LANG environment variable and all environment
variables starting with LC_.
names Traditional and POSIX bc have single letter names for functions,
variables and arrays. They have been extended to be multi-char
acter names that start with a letter and may contain letters,
numbers and the underscore character.
Strings
Strings are not allowed to contain NUL characters. POSIX says
all characters must be included in strings.
last POSIX bc does not have a last variable. Some implementations of
bc use the period (.) in a similar way.
comparisons
POSIX bc allows comparisons only in the if statement, the while
statement, and the second expression of the for statement.
Also, only one relational operation is allowed in each of those
statements.
if statement, else clause
POSIX bc does not have an else clause.
for statement
POSIX bc requires all expressions to be present in the for
statement.
&&, ||, !
POSIX bc does not have the logical operators.
read function
POSIX bc does not have a read function.
print statement
POSIX bc does not have a print statement .
continue statement
POSIX bc does not have a continue statement.
return statement
POSIX bc requires parentheses around the return expression.
array parameters
POSIX bc does not (currently) support array parameters in full.
The POSIX grammar allows for arrays in function definitions, but
does not provide a method to specify an array as an actual
parameter. (This is most likely an oversight in the grammar.)
Traditional implementations of bc have only call by value array
parameters.
function format
POSIX bc requires the opening brace on the same line as the
define key word and the auto statement on the next line.
=+, =-, =*, =/, =%, =^
POSIX bc does not require these "old style" assignment operators
to be defined. This version may allow these "old style" assign
ments. Use the limits statement to see if the installed version
supports them. If it does support the "old style" assignment
operators, the statement "a =- 1" will decrement a by 1 instead
of setting a to the value -1.
spaces in numbers
Other implementations of bc allow spaces in numbers. For exam
ple, "x=1 3" would assign the value 13 to the variable x. The
same statement would cause a syntax error in this version of bc.
errors and execution
This implementation varies from other implementations in terms
of what code will be executed when syntax and other errors are
found in the program. If a syntax error is found in a function
definition, error recovery tries to find the beginning of a
statement and continue to parse the function. Once a syntax
error is found in the function, the function will not be
callable and becomes undefined. Syntax errors in the interac
tive execution code will invalidate the current execution block.
The execution block is terminated by an end of line that appears
after a complete sequence of statements. For example,
a = 1
b = 2
has two execution blocks and
{ a = 1
b = 2 }
has one execution block. Any runtime error will terminate the execu
tion of the current execution block. A runtime warning will not termi
nate the current execution block.
Interrupts
During an interactive session, the SIGINT signal (usually gener
ated by the control-C character from the terminal) will cause
execution of the current execution block to be interrupted. It
will display a "runtime" error indicating which function was
interrupted. After all runtime structures have been cleaned up,
a message will be printed to notify the user that bc is ready
for more input. All previously defined functions remain defined
and the value of all non-auto variables are the value at the
point of interruption. All auto variables and function parame
ters are removed during the clean up process. During a non-
interactive session, the SIGINT signal will terminate the entire
run of bc.
LIMITS
The following are the limits currently in place for this bc processor.
Some of them may have been changed by an installation. Use the limits
statement to see the actual values.
BC_BASE_MAX
The maximum output base is currently set at 999. The maximum
input base is 16.
BC_DIM_MAX
This is currently an arbitrary limit of 65535 as distributed.
Your installation may be different.
BC_SCALE_MAX
The number of digits after the decimal point is limited to
INT_MAX digits. Also, the number of digits before the decimal
point is limited to INT_MAX digits.
BC_STRING_MAX
The limit on the number of characters in a string is INT_MAX
characters.
exponent
The value of the exponent in the raise operation (^) is limited
to LONG_MAX.
variable names
The current limit on the number of unique names is 32767 for
each of simple variables, arrays and functions.
ENVIRONMENT VARIABLES
The following environment variables are processed by bc:
POSIXLY_CORRECT
This is the same as the -s option.
BC_ENV_ARGS
This is another mechanism to get arguments to bc. The format is
the same as the command line arguments. These arguments are
processed first, so any files listed in the environent arguments
are processed before any command line argument files. This
allows the user to set up "standard" options and files to be
processed at every invocation of bc. The files in the environ
ment variables would typically contain function definitions for
functions the user wants defined every time bc is run.
BC_LINE_LENGTH
This should be an integer specifing the number of characters in
an output line for numbers. This includes the backslash and new
line characters for long numbers.
DIAGNOSTICS
If any file on the command line can not be opened, bc will report that
the file is unavailable and terminate. Also, there are compile and run
time diagnostics that should be self-explanatory.
BUGS
Error recovery is not very good yet.
Email bug reports to bug-bc@gnu.org. Be sure to include the word
bc somewhere in the Subject: field.
AUTHOR
Philip A. Nelson
philnelson@acm.org
ACKNOWLEDGEMENTS
The author would like to thank Steve Sommars (Steve.Sommars@att.com)
for his extensive help in testing the implementation. Many great sug
gestions were given. This is a much better product due to his involve
ment.
. bc(1)
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