SHELL脚本编程的运算符

 　　　　　　　　SHELL脚本编程的运算符

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　作者：尹正杰

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一.算数运算

1>.bash中的算数运算

[root@node101.yinzhengjie.org.cn ~]# help let
let: let arg [arg ...]
    Evaluate arithmetic expressions.
    
    Evaluate each ARG as an arithmetic expression.  Evaluation is done in
    fixed-width integers with no check for overflow, though division by 0
    is trapped and flagged as an error.  The following list of operators is
    grouped into levels of equal-precedence operators.  The levels are listed
    in order of decreasing precedence.
    
        id++, id--    variable post-increment, post-decrement
        ++id, --id    variable pre-increment, pre-decrement
        -, +        unary minus, plus
        !, ~        logical and bitwise negation
        **        exponentiation
        *, /, %        multiplication, division, remainder
        +, -        addition, subtraction
        <<, >>        left and right bitwise shifts
        <=, >=, <, >    comparison
        ==, !=        equality, inequality
        &        bitwise AND
        ^        bitwise XOR
        |        bitwise OR
        &&        logical AND
        ||        logical OR
        expr ? expr : expr
                conditional operator
        =, *=, /=, %=,
        +=, -=, <<=, >>=,
        &=, ^=, |=    assignment
    
    Shell variables are allowed as operands.  The name of the variable
    is replaced by its value (coerced to a fixed-width integer) within
    an expression.  The variable need not have its integer attribute
    turned on to be used in an expression.
    
    Operators are evaluated in order of precedence.  Sub-expressions in
    parentheses are evaluated first and may override the precedence
    rules above.
    
    Exit Status:
    If the last ARG evaluates to 0, let returns 1; let returns 0 otherwise.
[root@node101.yinzhengjie.org.cn ~]# 

[root@node101.yinzhengjie.org.cn ~]# help let

+, -, *, /, %取模（取余）, **（乘方），乘法符号有些场景中需要转义。

实现算术运算：
　　(1) let var=算术表达式
　　(2) var=$[算术表达式]
　　(3) var=$((算术表达式))
　　(4) var=$(expr arg1 arg2 arg3 ...)
　　(5) declare –i var = 数值
　　(6) echo ‘算术表达式’ | bc

增强型赋值：
　　+=, -=, *=, /=, %=

let varOPERvalue
　　例如:let count+=3
　　自加3后自赋值

自增，自减：
　　let var+=1
　　let var++
　　let var-=1
　　let var--