There are four ways to do the Bash Shell's arithmetic operations.

No.1: Using expr
eg. R= ' Expr 4 + 5 '
Echo $r
Note : There should be a space between the operators and operands in the 4 + 5 expression

# finally found the reason, to force Ah.

In addition, for multiplication operator *, use expr needs to be added \ To escape that is r= ' expr 4 \* 5 '
PS: no power-* operation in expr expression

No.2: Use $ (())
eg. r=$ ((4 + 5))
Echo $r
Note: $ (there is no space between, (()) Two pairs of parentheses there is no space between

No.3: Using $[]
eg. R=$[4 + 5]
Echo $r
Note: There are no spaces between $[

No.4: Using Let
eg. Let A=4+5
Echo $a
Note: a=4+5 This expression is the same as a normal arithmetic expression, with no spaces

Examples:
Multiplication:
R= ' Expr 4 \* 5 '
Echo $r
r=$ ((4 * 5))
Echo $r
r=$[4 * 5]
Echo $r
Let R=4*5
Echo $r
4 outputs are: 20

Division:
r= ' Expr 40/5 '
Echo $r
r=$ ((40/5))
Echo $r
r=$[40/5]
Echo $r
Let R=40/5
Echo $r
4 Outputs are: 8

Minus method:
r= ' Expr 40-5 '
Echo $r
r=$ ((40-5))
Echo $r
r=$[40-5]
Echo $r
Let r=40-5
Echo $r
4 outputs are: 35

Take the mold:
r= ' Expr 100% 43 '
Echo $r
r=$ ((100% 43))
Echo $r
r=$[100% 43]
Echo $r
Let r=100%43
Echo $r
4 outputs are: 14

By 冪 (such as 2 of the 3-time Party):
r=$ ((2 * * 3))
Echo $r
r=$[2 * * 3]
Echo $r
Let R=2**3
Echo $r
4 Outputs are: 8
Note: Expr does not multiply 冪

Although the bash shell has several arithmetic operations, not every method can be cross-platform, so that the script can be used across platforms, preferably with ' expr expression ' or $ (()), the Linux design 3rd book recommends using $ (()).