bc
SYNTAX bc [ hlwsqv ] [longoptions] [ file ... ]
VERSION This man page documents GNU bc version 1.06.
DESCRIPTION bc is a language that supports arbitrary precision numbers
with interactive 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 proces
sor, bc will never read from the standard input.)
This version of bc contains several extensions beyond tra
ditional bc implementations and the POSIX draft standard.
Command line options can cause these extensions to print a
warning or to be rejected. This document 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 precision numbers. This precision is both in
the integer part and the fractional part. All numbers are
any number of letters, digits and underscores. All let
ters must be lower case. (Full alphanumeric 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 example, 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 com
ment has been added as an extension. A single line com
ment 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, state
ments 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 con
stants 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 con
tain the characters 09 and AF. (Note: They must be capi
tals. Lower case letters are variable names.) Single
digit numbers always have the value of the digit regard
less of the value of ibase. (i.e. A = 10.) For multi
digit numbers, bc changes all input digits greater or
refers to a complete expression and "var" refers to a sim
ple 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 expressions.
expr / expr
The result of the expression is the quotient of the
two expressions. 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 computed in the following way. To compute
a%b, first a/b is computed 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 inte
gers this expression is the integer remainder func
tion.
expr ^ expr
( expr )
This alters the standard precedence to force the
evaluation of the expression.
var = expr
The variable is assigned the value of the expres
sion.
var <op>= expr
This is equivalent to "var = var <op> 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 statements
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 opera
tions 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
^ operator, right associative
unary  operator, nonassociative
++ and  operators, nonassociative
This precedence was chosen so that POSIX compliant bc pro
grams will run correctly. This will cause the use of the
relational and logical operators to have some unusual
behavior when used with assignment expressions. 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(parame
ters)". 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 program in the standard
input. The best use for this function is in a pre
viously 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 conver
sion 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 expression. If the expression is negative,
a run time error is generated.
STATEMENTS
Statements (as in most algebraic languages) provide the
the statement.)
expression
This statement does one of two things. If the
expression starts with "<variable> <assignment>
...", it is considered to be an assignment state
ment. If the expression is not an assignment
statement, the expression is evaluated and printed
to the output. After the number is printed, a new
line is printed. For example, "a=1" is an assign
ment 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 multicharacter digit method of printing the
numbers where each higher base digit is printed as
a base 10 number. The multicharacter digits are
separated by spaces. Each digit contains the num
ber of characters required to represent the base
ten value of "obase1". Since numbers are of arbi
trary precision, 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 charac
ters printed per line is 70. Due to the interac
tive 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
number 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 double quote character and contain all char
acters until the next double quote character. All
characters are take literally, including any new
line. 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 separated by commas. Each string
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 exe
cutes statement1 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 expression is nonzero. It evaluates the
expression before each execution of the statement.
Termination of the loop is caused by a zero expres
sion 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 evaluated before each execu
tion of the statement. If it is nonzero, the
statement is evaluated. If it is zero, the loop is
terminated. After each execution of the statement,
expression3 is evaluated before the reevaluation of
expression2. If expression1 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 expres
sion2. (The optional expressions are an extension.
POSIX bc requires all three expressions.) The fol
lowing 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)
is performed at "compile" time.
limits Print the local limits enforced by the local ver
sion of bc. This is an extension.
quit When the quit statement is read, the bc processor
is terminated, regardless of where the quit state
ment is found. For example, "if (0 == 1) quit"
will cause bc to terminate.
warranty
Print a longer warranty notice. This is an exten
sion.
FUNCTIONS
Functions provide a method of defining a computation that
can be executed 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 def
inition 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 definition, zero or more parameters are defined
by listing their names separated by commas. Numbers are
only call by value parameters. Arrays are only call by
variable. Arrays are specified in the parameter defini
tion by the notation "name[]". In the function call,
actual parameters are full expressions for number parame
ters. 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 definitions are
dynamic, parameter numbers and types are checked when a
function 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 variable. Arrays may be spec
ified 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 ini
tialized to zero and used throughout the execution of the
Return statements cause the termination of a function and
the return of a value. There are two versions of the
return statement. The first form, "return", returns the
value 0 to the calling expression. The second form,
"return ( expression )", computes the value of the expres
sion and returns that value to the calling expression.
There is an implied "return (0)" at the end of every func
tion. This allows a function to terminate and return 0
without 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 fol
lowing 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.
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 implement 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
define f (x) {
if (x <= 1) return (1);
return (f(x1) * 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 number 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
feature. 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 implemen
tations. 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 "undocumented" 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 fea
ture is extended to add more functionality and additions,
where new features are added. The following is the list
of differences and extensions.
LANG This version does not conform to the POSIX standard
in the processing 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 multicharacter names that
start with a letter and may contain letters, num
bers and the underscore character.
Strings
Strings are not allowed to contain NUL characters.
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 parame
ters 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" assign
ment operators to be defined. This version may
allow these "old style" assignments. Use the lim
its statement to see if the installed version sup
ports 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 num
bers. For example, "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.
ments. 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 execution of the current execution block. A runtime
warning will not terminate the current execution block.
Interrupts
During an interactive session, the SIGINT signal
(usually generated by the controlC 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 nonauto variables are the value
at the point of interruption. All auto variables
and function parameters are removed during the
clean up process. During a noninteractive ses
sion, 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
BC_ENV_ARGS
This is another mechanism to get arguments to bc.
The format is the same as the command line argu
ments. These arguments are processed first, so any
files listed in the environent arguments are pro
cessed 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 environment variables would
typically contain function definitions for func
tions 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 newline 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
selfexplanatory.
BUGS Error recovery is not very good yet.
Email bug reports to bugbc@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.Som
mars@att.com) for his extensive help in testing the imple
mentation. Many great suggestions were given. This is a
much better product due to his involvement.
. bc(1)
