Summary of mathematical calculations in shell _linux Shell

1. Examples of error methods
A
Var=1+1
Echo $var
The result of the output is 1+1, tragedy, hehe

b
Var=1
var= $var +1
Echo $var
The output result is 1+1, still tragic, hehe

2. The correct method
1) using Let
Var=1
Let "Var+=1"
Echo $var
The output is 2, no tragedy this time.
Attention:
A I tested let almost support all operators, see an article on the internet that "let does not support + + 、--and commas, (,)", but I tested the self, the self reduction, and the parentheses are well supported by the priority
b) The square power operation should use "* *"
c) parameter is accessed directly in an expression without having to add $
d) in general, arithmetic expressions can be without double quotes, but if you have a keyword in bash in the expression, you need to add
E-let expression can only be integer operation

2) use (())
Var=1
((var+=1))
Echo $var
Output is 2
Attention:
(()) is used in exactly the same way as let

3) using $[]
Var=1
var=$[$var +1]
Echo $var
Output bit 2
Attention:
A) $[] the expression within the brackets as a mathematical operation to calculate the results before outputting
b) To access the variables in $[] before you need to add $
c) $[] supports the same operators as let, but only integer operations are supported

4) Using expr
Var=1
Var= ' expr $var + 1 '
Echo $var
Output is 2
Attention:
A the expression symbol after expr needs to be separated by a space
b The operators supported by expr are: |, &, <, <=, =,!=, >=, >, + 、-、 *,/,%
c) The operations supported by expr are used with \ to escape: |, &, <, <=, >=, >, *
e) expr also supports only integer operations

5 use BC (can be used for floating-point calculation)
Var=1
Var= ' echo ' $var +1 "|BC"
Echo $var
Output is 2
Introduced:
BC is a simple calculator under Linux, support floating-point calculation, the command line to enter the BC is entered into the calculator program, and we want to directly in the program floating-point calculation, the use of a simple pipe can solve the problem.
Attention:
1 by my test BC supports all operators except the bitwise operator.
2 BC to use scale for precision setting
3 example of floating point count
var=3.14
Var= ' echo ' scale=2; $var *3 "|BC"
Echo $var
Output is 9.42

6 use awk (floating point count is available)
Var=1
Var= ' echo ' $var 1 "|awk ' {printf ("%g ", $1*$2)} '
Echo $var
Output is 2
Introduced:
Awk is a text processing tool as well as a programming language, and as a programming language, AWK supports multiple operations, and we can use awk to compute floating-point numbers, just like the BC above, through a simple pipe, where we can call awk directly in the program for floating-point counting.
Attention:
1 awk supports all operators except the micro-operation operator
2 awk built-in log, sqr, cos, sin, and so on functions
3 example of floating point count
var=3.14
Var= ' echo ' $var 2 "|awk ' {printf ("%g ", Sin ($1/$2))} '
Echo $var
Output is 1
3, Summary
Finally put all the data can be collected to read, and finally summed up a set of their own things, and then encountered similar problems will have seen, haha ~

Yorking Alan