gNU Bourne-Again SHell (GNU Bourne-Again SHell)
ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated, under
certain circumstances (see the let and declare builtin commands,
the (( compound command, and Arithmetic Expansion). Evaluation
is done in fixed-width integers with no check for overflow,
though division by 0 is trapped and flagged as an error. The
operators and their precedence, associativity, and values are the
same as in the C language. The following list of operators is
grouped into levels of equal-precedence operators. The levels
are listed in order of decreasing precedence.
id++ id--
variable post-increment and post-decrement
- + unary minus and plus
++id --id
variable pre-increment and pre-decrement
! ~ logical and bitwise negation
** exponentiation
* / % multiplication, division, remainder
+ - addition, subtraction
<< >> left and right bitwise shifts
<= >= < >
comparison
== != equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
expr?expr:expr
conditional operator
= *= /= %= += -= <<= >>= &= ^= |=
assignment
expr1 , expr2
comma
Shell variables are allowed as operands; parameter expansion is
performed before the expression is evaluated. Within an
expression, shell variables may also be referenced by name
without using the parameter expansion syntax. A shell variable
that is null or unset evaluates to 0 when referenced by name
without using the parameter expansion syntax. The value of a
variable is evaluated as an arithmetic expression when it is
referenced, or when a variable which has been given the integer
attribute using declare -i is assigned a value. A null value
evaluates to 0. A shell variable need not have its integer
attribute turned on to be used in an expression.
Integer constants follow the C language definition, without
suffixes or character constants. Constants with a leading 0 are
interpreted as octal numbers. A leading 0x or 0X denotes
hexadecimal. Otherwise, numbers take the form [base#]n, where
the optional base is a decimal number between 2 and 64
representing the arithmetic base, and n is a number in that base.
If base# is omitted, then base 10 is used. When specifying n, if
a non-digit is required, the digits greater than 9 are
represented by the lowercase letters, the uppercase letters, @,
and _, in that order. If base is less than or equal to 36,
lowercase and uppercase letters may be used interchangeably to
represent numbers between 10 and 35.
Operators are evaluated in order of precedence. Sub-expressions
in parentheses are evaluated first and may override the
precedence rules above.
