Syntax knowledge of shell scripts--Numerical calculation

①$[]

　　$[$a operator $b]

Example: a=2;b=3;

echo $[$a + $b] # 5

echo $[$a-$b] #-3

echo $[++ $a-$b] #0

c=$[$a + $b] #√c=6

D=$[A+B] # 6

　　Note: Except that the self-add-decrement operator is supported; only integer operations can be performed; + + does not error or self-add in front of the operand; $[] outputs the result of the operation

②let

Let A=b+c

Example: a=1;b=2;

Let C=a+b;echo $c # 3

Let C=++a+b;echo $c # 4

C=let a+b #xlet does not output calculation results

Echo ' Let A+b ' #x

　　Note: Almost all operators are supported; the variable in the expression does not add $; In general, the expression is not enclosed in double quotes, but if the expression has the Bash keyword, it must be added; only integer operations are allowed; Let does not output the result of the operation, and the assignment needs to be done in the expression; bash built-in command

③[$] (())

Cases:

A=3;

((b=a+2)); Echo $b # 5

Echo ((a+5)) or c= ((a+3)) #-bash:syntax error near unexpected token ' ('

echo $ ((a+5)) or c=$ ((a+3)); Echo $c # 8 or 6

　　Note: The usage is exactly the same as let, if the $ symbol is preceded, the result is output, otherwise the result is not output

④bc

Cases:

F= ' echo 3+1|BC '; Echo $f # 4

Touch F;echo 3+4 > F;cat F |BC # 7

　　Note: Supports floating-point calculation; supports all operations except bitwise operations; precision setting by scale

⑤expr

Cases:

A=3;

b= ' expr $a + 3 '; Echo $b # 6

　　Note: Some operators need to be escaped; the operator and operand are separated by a space; only integer operations are supported

Syntax knowledge of shell scripts--Numerical calculation