if,case

if

条件选择，选择执行，可嵌套

单分支
      if 判断条件；then
                条件为真的分支代码
      fi

双分支
      if 判断条件；then
                条件为真的分支代码
      else
                条件为假的分支代码 
      fi

多分支
      if 判断条件1；then
                条件为真的分支代码
      elif 判断条件2；then
                条件为真的分支代码
      elif 判断条件3；then
                条件为真的分支代码
      else
                以上条件都为假的分支代码
      fi

逐条件进行判断，第一次遇为“真”条件时，执行其分支，而后结束整个if语句

例子：根据命令的退出状态来执行命令
if ping -c1 -W2 station1 &> /dev/null; then
echo 'Station1 is UP'
elif grep "station1" ~/maintenance.txt &> /dev/null then
echo 'Station1 is undergoing maintenance‘
else
echo 'Station1 is unexpectedly DOWN!' exit 1
fi

[root@centos SC]#vim age.sh
#!/bin/bash
######输入一个数
read -p "please input your age: " age
######判断输入的值是否为数字，如有不是，提示输入数字并退出
[[ ! "$age" =~ ^[[:digit:]]+$ ]] && echo please input digital && exit 100
######对输入的数进行判断，从而执行相应命令
if [ "$age" -le 18 ];then
        echo "you are baby" 
elif [ "$age" -gt 18 -a "$age" -le 60 ];then
        echo you need work hard
elif [ "$age" -le 80 ];then
        echo "you can enjoy life"
else
        echo "you will be lucky"
fi
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"age.sh" 12L, 331C written                                                
[root@centos SC]#chmod +x age.sh
######测试结果
[root@centos SC]#./age.sh       
please input your age: 2
you are baby
[root@centos SC]#./age.sh
please input your age: 18
you are baby
[root@centos SC]#./age.sh
please input your age: 19
you need work hard
[root@centos SC]#./age.sh
please input your age: 65
you can enjoy life
[root@centos SC]#./age.sh
please input your age: 85
you will be lucky

case

case语句：条件判断

case  变量引用 in
PATH1）
                分支1
                ;;  
PATH2)
                分支2
                ;;
...
*)
                默认分支
                ;;
esac

case支持glob风格的通配符：

     *：任意长度任意字符
     ?：任意单个字符
     []：指定范围内的任意单个字符
     a|b：a或b

例子：

[root@centos SC]#vim yesorno.sh 
#!/bin/bash
#####提示输入，读取输入的值
read -p "please input yes or no: " ans
#####将输入转换为小写输出
ans=`echo $ans |tr '[:upper:]' '[:lower:]'`
#####进行判断，执行相应命令
case $ans in
y|ye|yes)
    echo this is yes
    ;;
n|no)
    echo this is no
    ;;
*)
    echo This is other information , please input yes or no
esac
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######测试结果                                                                                         
[root@centos SC]#./yesorno.sh     
please input yes or no: y
this is yes
[root@centos SC]#./yesorno.sh 
please input yes or no: Ye
this is yes
[root@centos SC]#./yesorno.sh 
please input yes or no: yeS
this is yes
[root@centos SC]#./yesorno.sh 
please input yes or no: n
this is no
[root@centos SC]#./yesorno.sh 
please input yes or no: No
this is no
[root@centos SC]#./yesorno.sh 
please input yes or no: nO
this is no
[root@centos SC]#./yesorno.sh 
please input yes or no: P
This is other information , please input yes or no