选择执行if语句

格式：

if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ]... [ else COMMANDS; ] fi

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1
    ......
else
    条件为假的分支代码2
    ......
fi

多分支

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

注：

• 多个条件时，逐个条件进行判断，第一次为“真”条件时，执行其分支，而后结束整个if语句
• if语句可嵌套

例1：

[root@CentOS7 ~]# cat yesorno_sh.sh 
#!/bin/bash
#
#*************************************************************************************
#Author:                      wanghaomiao
#QQ:                          1074035975
#Date:                        2022-01-08
#FileName                     yesorno.sh
#URL:                         www.wanghaomiao.com.cn
#Description:                 The test script
#Copyreght (C):               2022 All rightsreserved
#*************************************************************************************
read -p "Are you OK (yes/no)? " answer
if [[ $answer =~ ^[Yy]$|^[Yy][Ee][Ss]$ ]]; then
    echo "yes"
elif [[ $answer =~ ^[Nn]$|^[Nn][Oo]$ ]]; then
    echo "no"
else
    echo "Input false,please input yes or no!"
fi

例2：

[root@CentOS7 ~]# cat if_bmi.sh 
#!/bin/bash
#
#*************************************************************************************
#Author:                      wanghaomiao
#QQ:                          1074035975
#Date:                        2022-01-09
#FileName                     if_bmi.sh
#URL:                         www.wanghaomiao.com.cn
#Description:                 The test script
#Copyreght (C):               2022 All rightsreserved
#*************************************************************************************
COLOR="\E[$[RANDOM%7+31];1m"
COLOR_END="\E[0m"

echo -ne "${COLOR}"
read -p "Please enter your height(cm): " HIGH
echo -ne "${COLOR_END}"

if [[ ! "$HIGH" =~ ^[0-2]([0-9]{,2}?)$ ]];then
    echo -e ""$COLOR"Incorrectheight,please re-enter!$COLOR_END"
    exit 1
fi

echo -ne "${COLOR}"
read -p "Please enter your weight(KG): " WEIGHT
echo -ne "${color}"

if [[ ! "$WEIGHT" =~ ^[0-9]{1,3}\.[0-9]$ ]];then
    echo -e ""$COLOR"Incorrect weight,please re-enter!$COLOR_END"
    exit 1
fi

BMI=`echo $WEIGHT/$HIGH^2|bc`

if [ $BMI -le 18 ];then
    echo -e ""$COLOR"You're too skinny!$COLOR_END"
elif [ $BMI -lt 24 ];then
    echo -e ""$COLOR"good figure!$COLOR_END"
else
    echo -e ""$COLOR"You're too fat!$COLOR_END"
fi

条件判断case语句

格式：

case WORD in [PATTERN [| PATTERN]...) COMMANDS ;;]... esac

case 变量引用 in
PAT1)
    分支1
    ;;
PAT2)
    分支2
    ;;
PAT3)
    分支3
    ;;
...
*)
    默认分支
    ;;
csac

case支持glob风格的通配符：

*    任意长度的任意字符
？   任意单个字符
[]   指定范围内的任意单个字符
|    或者，如：a|b

例1：

[root@CentOS7 ~]# cat yesorno_case.sh 
#!/bin/bash
#
#*************************************************************************************
#Author:                      wanghaomiao
#QQ:                          1074035975
#Date:                        2022-01-08
#FileName                     yesorno.sh
#URL:                         www.wanghaomiao.com.cn
#Description:                 The test script
#Copyreght (C):               2022 All rightsreserved
#*************************************************************************************
read -p "Are you OK (yes/no)? " answer

case $answer in
[Yy]|[Yy][Ee][Ss])
    echo "yes"
    ;;
[Nn]|[Nn][Oo])
    echo "no"
    ;;
*)
    echo "Input false,please input yes or no!"
    ;;
esac

[root@CentOS7 ~]# cat yesorno_case1.sh 
#!/bin/bash
#
#*************************************************************************************
#Author:                      wanghaomiao
#QQ:                          1074035975
#Date:                        2022-01-08
#FileName                     yesorno.sh
#URL:                         www.wanghaomiao.com.cn
#Description:                 The test script
#Copyreght (C):               2022 All rightsreserved
#*************************************************************************************
read -p "Are you OK (yes/no)? " answer

answer=`echo $answer | tr 'A-Z' 'a-z'`

case $answer in
y|yes)
    echo "yes"
    ;;
n|no)
    echo "no"
    ;;
*)
    echo "Input false,please input yes or no!"
    ;;
esac

例2：

[root@CentOS7 ~]# cat work_menu.sh 
#!/bin/bash
#
#*************************************************************************************
#Author:                      wanghaomiao
#QQ:                          1074035975
#Date:                        2022-01-09
#FileName                     work_menu.sh
#URL:                         www.wanghaomiao.com.cn
#Description:                 The test script
#Copyreght (C):               2022 All rightsreserved
#*************************************************************************************
echo -ne "\E[$[RANDOM%7+31];1m"
cat <<EOF
请选择：
1)备份数据库
2)清理日志
3)软件升级
4)软件回滚
5)删库跑路
EOF
echo -ne '\E[0m'

read -p "请输入上面数字1-5: " MENU

case $MENU in
1)
    ./buckup.sh
    ;;
2)
    echo "清理日志"
    ;;
3)
    echo "软件升级"
    ;;
4)
    echo "软件回滚"
    ;;
5)
    echo "删库跑路"
    ;;
*)
    echo "INPUT FALSE!"
esac