Shell script if statement and case statement in detail

The structure of the if statement

Single branch structure

• A corresponding fi must be inserted at the end of the if statement to represent the end of the statement.
• The judgment process of if single branch structure

if  条件测试操作
        then    命令序列
fi    #结尾语句

Double branch structure

• The judgment process of if double branch structure
  

if       条件测试操作
       then    命令序列1
       else     命令序列2
 fi    #结尾语句

Multi-branch structure

• The judgment process of if multi-branch structure

if      条件测试操作1
      then    命令序列1
elif 条件测试操作2
      then    命令序列2
else
      命令序列3
fi

case statement

case multi-branch structure

• The end of the case line must be the word "in". Each mode must end with a closing bracket ")".
• The double semicolon ";;" indicates the end of the command sequence.
• In the pattern string, square brackets can be used to indicate a continuous range, such as "[0-9]"; it can also be expressed with "|", such as "A|B"
• The last "*)" represents the default mode, and the asterisk is equivalent to a wildcard.
• Judgment process of case multi-branch structure

case 变量值 in
模式1）
      命令序列1
      ;;
模式2）
      命令序列2
      ;;
      ......
*)
     默认命令序列
esac