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texinode(Arithmetic Evaluation)(Conditional Expressions)(Jobs & Signals)(Top)
chapter(Arithmetic Evaluation)
ifzman(\
sect(Arithmetic Evaluation)
)\
cindex(arithmetic evaluation)
cindex(evaluation, arithmetic)
findex(let, use of)
The shell can perform integer and floating point arithmetic, either using
the builtin tt(let), or via a substitution of the form tt($((...))). For
integers, the shell is usually compiled to use 8-byte precision where this
is available, otherwise precision is 4 bytes. This can be tested, for
example, by giving the command `tt(print - $(( 12345678901 )))'; if the
number appears unchanged, the precision is at least 8 bytes. Floating
point arithmetic is always double precision.
The tt(let) builtin command takes arithmetic expressions as arguments; each
is evaluated separately. Since many of the arithmetic operators, as well
as spaces, require quoting, an alternative form is provided: for any
command which begins with a `tt(LPAR()LPAR())', all the characters until a
matching `tt(RPAR()RPAR())' are treated as a quoted expression and
arithmetic expansion performed as for an argument of tt(let). More
precisely, `tt(LPAR()LPAR())var(...)tt(RPAR()RPAR())' is equivalent to
`tt(let ")var(...)tt(")'. For example, the following statement
example((( val = 2 + 1 )))
is equivalent to
example(let "val = 2 + 1")
both assigning the value 3 to the shell variable tt(foo) and returning a
zero status.
cindex(bases, in arithmetic)
Integers can be in bases other than 10.
A leading `tt(0x)' or `tt(0X)' denotes hexadecimal.
Integers may also be of the form `var(base)tt(#)var(n)',
where var(base) is a decimal number between two and thirty-six
representing the arithmetic base and var(n)
is a number in that base (for example, `tt(16#ff)' is 255 in hexadecimal).
The var(base)tt(#) may also be omitted, in which case
base 10 is used. For backwards compatibility the form
`tt([)var(base)tt(])var(n)' is also accepted.
Floating point constants are recognized by the presence of a decimal point
or an exponent. The decimal point may be the first character of the
constant, but the exponent character tt(e) or tt(E) may not, as it will be
taken for a parameter name.
cindex(arithmetic operators)
cindex(operators, arithmetic)
An arithmetic expression uses nearly the same syntax, precedence, and
associativity of expressions in C.
The following operators are supported (listed in decreasing order
of precedence):
startsitem()
sitem(tt(PLUS() - ! ~ PLUS()PLUS() --))(unary plus/minus, logical NOT, complement, {pre,post}{in,de}crement)
sitem(tt(<< >>))(bitwise shift left, right)
sitem(tt(&))(bitwise AND)
sitem(tt(^))(bitwise XOR)
sitem(tt(|))(bitwise OR)
sitem(tt(**))(exponentiation)
sitem(tt(* / %))(multiplication, division, modulus (remainder))
sitem(tt(PLUS() -))(addition, subtraction)
sitem(tt(< > <= >=))(comparison)
sitem(tt(== !=))(equality and inequality)
sitem(tt(&&))(logical AND)
sitem(tt(|| ^^))(logical OR, XOR)
sitem(tt(? :))(ternary operator)
sitem(tt(= PLUS()= -= *= /= %= &= ^= |= <<= >>= &&= ||= ^^= **=))(assignment)
sitem(tt(,))(comma operator)
endsitem()
The operators `tt(&&)', `tt(||)', `tt(&&=)', and `tt(||=)' are
short-circuiting, and only one of the latter two expressions in a ternary
operator is evaluated. Note the precedence of the bitwise AND, OR,
and XOR operators.
An expression of the form `tt(#\)var(x)' where var(x) is any character
sequence such as `tt(a)', `tt(^A)', or `tt(\M-\C-x)' gives the ascii
value of this character and an expression of the form `tt(#)var(foo)'
gives the ascii value of the first character of the value of the
parameter var(foo). Note that this is different from the expression
`tt($#)var(foo)', a standard parameter substitution which gives the
length of the parameter var(foo).
Named parameters and subscripted arrays can be referenced by name within an
arithmetic expression without using the parameter expansion syntax. For
example,
example(((val2 = val1 * 2)))
assigns twice the value of tt($val1) to the parameter named tt(val2).
An internal integer representation of a named parameter
can be specified with the tt(integer) builtin.
cindex(parameters, integer)
cindex(integer parameters)
findex(integer, use of)
Arithmetic evaluation is performed on the value of each
assignment to a named parameter declared integer
in this manner. Assigning a floating point number to an integer results in
rounding down to the next integer.
cindex(parameters, floating point)
cindex(floating point parameters)
findex(float, use of)
Likewise, floating point numbers can be declared with the tt(float)
builtin; there are two types, differing only in their output format, as
described for the tt(typeset) builtin. The output format can be bypassed
by using arithmetic substitution instead of the parameter substitution,
i.e. `tt(${)var(float)tt(})' uses the defined format, but
`tt($LPAR()LPAR())var(float)tt(RPAR()RPAR())' uses a generic floating point
format.
Promotion of integer to floating point values is performed where
necessary. In addition, if any operator which requires an integer
(`tt(~)', `tt(&)', `tt(|)', `tt(^)', `tt(%)', `tt(<<)', `tt(>>)' and their
equivalents with assignment) is given a floating point argument, it will be
silently rounded down to the next integer.
Scalar variables can hold integer or floating point values at different
times; there is no memory of the numeric type in this case.
If a variable is first assigned in a numeric context without previously
being declared, it will be implicitly typed as tt(integer) or tt(float) and
retain that type either until the type is explicitly changed or until the
end of the scope. This can have unforeseen consequences. For example, in
the loop
example(for (( f = 0; f < 1; f += 0.1 )); do;
# use $f
done)
if tt(f) has not already been declared, the first assignment will cause it
to be created as an integer, and consequently the operation `tt(f += 0.1)'
will always cause the result to be truncated to zero, so that the loop will
fail. A simple fix would be to turn the initialization into `tt(f = 0.0)'.
It is therefore best to declare numeric variables with explicit types.
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