Shell() は整数に対する数学演算を実装しています。

[jb51.net]$ echo $((1+1))
2
[jb51.net]$ echo $((6-3))
3
[jb51.net]$ i=5
[jb51.net]$ ((i=i*2)) # can be abbreviated to ((i*=2)).
[jb51.net]$ echo $i # Add $ when using echo to output the result of a variable.
10

[jb51.net]$ ((a=1+2**3-4%3))
[c.biancheng.net]$ echo $a
8
[jb51.net]$ b=$((1+2**3-4%3)) # The result is assigned to the variable after the operation, and the variable is placed outside the parentheses.
[jb51.net]$ echo $b
8
[jb51.net]$ echo $((1+2**3-4%3)) # You can also output the result of the expression directly, being careful not to drop the $ sign.
8
[jb51.net]$ a=$((100*(100+1)/2)) # Use the formula to calculate the sum of 1+2+3+... +100 the sum.
[jb51.net]$ echo $a
5050
[jb51.net]$ echo $((100*(100+1)/2)) # You can also output the result of the expression directly.
5050

[jb51.net]$ echo $((3<8)) #3<8 holds, therefore, the output is 1, 1 means true
1
[jb51.net]$ echo $((8<3)) The result of #8<3 is not valid, therefore, 0 is output, 0 means false.
0
[jb51.net]$ echo $((8==8)) # Determine if equal.
1
[jb51.net]$ if ((8>7&&5==5))
> then
> echo yes
> fi
yes

[jb51.net]$ a=10
[jb51.net]$ echo $((a++)) # If ++ is after a, then the value of a is output when the whole expression is output, because a is 10, so the value of the expression is 10.
10
[jb51.net]$ echo $a # After executing the above expression, because there is a++, a will increment itself by 1, so the value of a will be output as 11.
11
[jb51.net]$ a=11
[jb51.net]$ echo $((a--)) # If -- is after a, then when the whole expression is output, the value of a is output because a is 11, so the value of the expression is 11.
11
[jb51.net]$ echo $a # After executing the above expression, because there is a --, a will be automatically subtracted by 1, so a is 10.
10
[jb51.net]$ a=10
[jb51.net]$ echo $((--a)) # If -- precedes a, then the whole expression is output with a self-incrementing or self-subtracting calculation first, because a is 10 and is to be self-subtracting, so the value of the expression is 9.
9
[jb51.net]$ echo $a #After executing the above expression, a is self-subtracting by 1, so a is 9.
9
[jb51.net]$ echo $((++a)) # If ++ is in front of a, when outputting the whole expression, first do the self-increment or self-subtraction calculation, because a is 9 and to be self-incremented by 1, so output 10.
10
[jb51.net]$ echo $a #After executing the above expression, a is self-incrementing by 1, so a is 10.
10

[jb51.net]$ ((a=3+5, b=a+10)) # Calculate the first expression first, then the second expression
[jb51.net]$ echo $a $b
8 18
[jb51.net]$ c=$((4+8, a+b)) # use the result of the last expression as the result of the entire (()) command
[jb51.net]$ echo $c
26