14.3. Arithmetic

14.3.1 Integers (declare and let Commands)

The declare Command

Variables can be declared as integers with the declare –i command. If you attempt to assign any string value, bash assigns 0 to the variable. Arithmetic can be performed on variables that have been declared as integers. (If the variable has not been declared as an integer, the built-in let command allows arithmetic operations. See "The let Command" on page 873.) If you attempt to assign a floating-point number, bash reports a syntax error . Numbers can also be represented in different bases such as binary, octal, and hex.

Example 14.6.

1   $ declare –i num

2   $ num=hello

    $ echo $num

    0

3   $ num=5 + 5

    bash: +: command not found

4   $ num=5+5

    $ echo $num

    10

5   $ num=4*6

    $ echo $num

    24

6   $ num="4 * 6"

    $ echo $num

    24

7   $ num=6.5

    bash: num: 6.5: syntax error in expression (remainder of expression is ".5")

EXPLANATION

Listing Integers

The declare command with only the –i argument will list all preset integers and their values, as shown in the following display.

$ declare –i

declare -ir EU     # effective user id

declare -ir PP    # parent process id

declare -ir U      # user id

Representing and Using Different Bases

Numbers can be represented in decimal (base 10, the default), octal (base 8), hexadecimal (base 16), and a range from base 2 to 36.

FORMAT

variable=base#number-in-that-base

Example 14.7.

n=2#101     # Base is 2; number 101 is in base 2

Example 14.8.

(The Command Line)

1  $ declare -i x=017

   $ echo $x

   15

2  $ x=2#101

   $ echo $x

   5

3  $ x=8#17

   $ echo $x

   15

4  $ x=16#b

   $ echo $x

   11

EXPLANATION

The let Command

The let command is a bash built-in command that is used to perform integer arithmetic and numeric expression testing. To see what let operators your version of bash supports, type at the prompt:

help let

A list of the let operators is also found in Table 14.4 on page 884.

Example 14.9.

(The Command Line)

1   $ i=5  or let i=5

2   $ let i=i+1

    $ echo $i

    6

3   $ let "i = i + 2"

    $ echo $i

    8

4   $ let "i+=1"

    $ echo $i

    9

5   $ i=3

6   $ (( i+=4))

    $ echo $i

    7

7   $ (( i=i-2 ))

    $ echo $i

    5

Table 14.4. The let Command Operators^[a]

Operator	Meaning
–	Unary minus
+	Unary plus
!	Logical NOT
~	Bitwise NOT (negation)
*	Multiply
/	Divide
%	Remainder
+	Add
–	Subtract
let Operators Not Implemented Prior to bash 2.x
<<	Bitwise left shift
>>	Bitwise right shift
<= >= < >	Comparison operators
== !=	Equal to and not equal to
&	Bitwise AND
^	Bitwise exclusive OR
|	Bitwise OR
&&	Logical AND
||	Logical OR
= *= /= %= += –= <<= >>= &= ^= |=	Assignment and shortcut assignment

^[a] See Example 14.18.

EXPLANATION

14.3.2 Floating-Point Arithmetic

Bash supports only integer arithmetic, but the bc, awk, and nawk utilities are useful if you need to perform more complex calculations.

Example 14.10.

(The Command Line)

1   $ n=`echo "scale=3; 13 / 2" | bc`

    $ echo $n

    6.500

2   product=`gawk -v x=2.45 -v y=3.123 'BEGIN{printf "%.2f\n",x*y}'`

    $ echo $product

    7.65

EXPLANATION