05. Arithmetic expansions and tests

Arithmetic expansions

There are two ways we can have the shell evaluate an expression. Either using $((expression)) syntax or the expr command.

The former does not require proper spacing, while the latter does.

$ echo $((2+3))
5
$ expr 2 + 3
5

Arithmetic expasion only supports integer values and no decimals.

$ expr 2.3 + 2
expr: not a decimal number: '2.3'

Operators

Here are a list of arithmetic operators.

++ --
Increment, Decrement
+ -
Unary plus and minus
* / %
Multiplication, division and remainder
== !=
Equal and not equal
! && ||
Logical negation, AND, and OR
<< >>
Bit-shift left and right
~ & ^ |
Bitwise negation, AND, exclusive OR, and OR
\= += -= *= /= %=
Assignment operators
&= ^- <<= >>= |=
Bitwise assignment operators

Arithmetic test

It's important to avoid using the = symbol when comparing numerals. The = looks for string equality, not numeric equality. Thus, when working with arithmetic equations, be sure to use -eq instead.

$ [ 1 = 1 ]; echo $?
0
$ [ 092 = 92 ]; echo $?
1
$ [ 092 -eq 92 ]; echo $?
0

Here is a list of more comparison tests we can use to compare two numbers. Be sure to not use canonical notations such as > or <!

-eq
Equal to
-ne
Not equal to
-lt
Less than
-gt
Greater than
-le
Less than or equal to
-ge
Greater than or equal to
$ [ 92 > 32 ]; echo $?
0      # Looks good...(but wrong)
$ [ 92 < 32 ]; echo $?
0      # Wrong
$ [ 92 -gt 32 ]; echo $?
0      # There we go!

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