Math with the shell

The Bash shell performs basic arithmetic operations using the let, (( )), and [] commands. The expr and bc utilities are used to perform advanced operations.

How to do it...

1. A numeric value is assigned to a variable the same way strings are assigned. The value will be treated as a number by the methods that access it:

        #!/bin/bash
        no1=4;
        no2=5;

2. The let command is used to perform basic operations directly. Within a let command, we use variable names without the $ prefix. Consider this example:

        let result=no1+no2
        echo $result

                  Other uses of let command are as follows:

• Use this for increment: 

                $ let no1++

• For decrement, use this:

                $ let no1--

• Use these for shorthands:

                let no+=6
                let no-=6

                These are equal to let no=no+6 and let no=no-6, respectively.

• Alternate methods are as follows:

              The [] operator is used in the same way as the let command:

                    result=$[ no1 + no2 ]

              Using the $ prefix inside the [] operator is legal; consider this example:

                    result=$[ $no1 + 5 ]

      The (( )) operator can also be used. The prefix variable names                        with a $ within the (( )) operator:

                    result=$(( no1 + 50 ))

             The expr expression can be used for basic operations:

                    result=`expr 3 + 4`
                    result=$(expr $no1 + 5)

      The preceding methods do not support floating point numbers,
      and operate on integers only.

3. The bc application, the precision calculator, is an advanced utility for mathematical operations. It has a wide range of options. We can perform floating point arithmetic and use advanced functions:

        echo "4 * 0.56" | bc
        2.24
        no=54;
        result=`echo "$no * 1.5" | bc`
        echo $result
        81.0

The bc application accepts prefixes to control the operation. These are separated from each other with a semicolon.

• Decimal places scale with bc: In the following example, the scale=2 parameter sets the number of decimal places to 2. Hence, the output of bc will contain a number with two decimal places:

                echo "scale=2;22/7" | bc
                3.14

• Base conversion with bc: We can convert from one base number system to another one. This code converts numbers from decimal to binary and binary to decimal:

                #!/bin/bash
                Desc: Number conversion
                no=100
                echo "obase=2;$no" | bc 
                1100100
                no=1100100
                echo "obase=10;ibase=2;$no" | bc
                100

• The following examples demonstrate calculating squares and square roots:

                echo "sqrt(100)" | bc #Square root
                echo "10^10" | bc #Square