Shell 学习15 – Shell if else 语句
分类：Shell………….
linuxshell
if 语句通过关系运算符判断表达式的真假来决定执行哪个分支。Shell 有三种 if … else 语句：if … fi 语句；if … else … fi 语句；if … elif … else … fi 语句。1) if … else 语句if … else 语句的语法：if [ expression ]thenStatement(s) to be executed if expression is truefi如果 expression 返回 true，then 后边的语句将会被执行；如果返回 false，不会执行任何语句。最后必须以 fi 来结尾闭合 if，fi 就是 if 倒过来拼写，后面也会遇见。注意：expression 和方括号([ ])之间必须有空格，否则会有语法错误。举个例子：#!/bin/sha=10b=20if [ $a == $b ]thenecho "a is equal to b"fiif [ $a != $b ]thenecho "a is not equal to b"fi运行结果：a is not equal to b2) if … else … fi 语句if … else … fi 语句的语法：if [ expression ]thenStatement(s) to be executed if expression is trueelseStatement(s) to be executed if expression is not truefi如果 expression 返回 true，，那么 then 后边的语句将会被执行；否则，执行 else 后边的语句。举个例子：#!/bin/sha=10b=20if [ $a == $b ]thenecho "a is equal to b"elseecho "a is not equal to b"fi执行结果：a is not equal to b3) if … elif … fi 语句if … elif … fi 语句可以对多个条件进行判断，语法为：if [ expression 1 ]thenStatement(s) to be executed if expression 1 is trueelif [ expression 2 ]thenStatement(s) to be executed if expression 2 is trueelif [ expression 3 ]thenStatement(s) to be executed if expression 3 is trueelseStatement(s) to be executed if no expression is truefi哪一个 expression 的值为 true，就执行哪个 expression 后面的语句；如果都为 false，那么不执行任何语句。举个例子：#!/bin/sha=10b=20if [ $a == $b ]thenecho "a is equal to b"elif [ $a -gt $b ]thenecho "a is greater than b"elif [ $a -lt $b ]thenecho "a is less than b"elseecho "None of the condition met"fi运行结果：a is less than bif … else 语句也可以写成一行，以命令的方式来运行，像这样：if test $[2*3] -eq $[1+5]; then echo 'The two numbers are equal!'; fi;if … else 语句也经常与 test 命令结合使用，如下所示：num1=$[2*3]num2=$[1+5]if test $[num1] -eq $[num2]thenecho 'The two numbers are equal!'elseecho 'The two numbers are not equal!'fi输出：The two numbers are equal!test 命令用于检查某个条件是否成立，与方括号([ ])类似。
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