Shell script ------------ programming conditional statement (if, case)

Conditional test operation

File test

[Operator file or directory]

Commonly used test operators

-d: test whether it is a directory (Directory)
-e: Test whether the directory or file exists (Exist)
f: test whether it is a file (File)
-r: Test whether the current user has permission to read (Read)
-w: Test whether the current user has permission to write (Write)
-x: Test whether the current user has permission to execute (eXcute)

Note: The current user here refers to the user executing the command.

Example:

[ -d /media/cdrom ] && echo $？|| echo $?			###当等式成立输出“0”否则输出“1”

Notes:

"0": The equation holds
"1": The equation does not hold
"&&": logical and, the meaning of "and"

Integer value comparison

[Integer 1 Operator Integer 2]

Commonly used test operators

-eq: equal to (Equal)
-ne: Not Equal
-gt: Greater Than
-lt: Less than (Lesser Than)
-le: Less than or equal to (Lesser or Equal)
-ge: Greater or Equal

String comparison

Format 1: [String 1 = String 2]
[String 1! =String 2]
Format 2: [-z string]

Commonly used test operators

=: The string content is the same
! =: The string content is different,! Sign means the opposite
-z: The string content is empty

Logic test

Format 1: [Expression 1] Operator [Expression 2]…
Format 2: Command 1 Operator Command 2…

Commonly used test operators

-a or &&: logical and, meaning "and"
-o or ||: logical or, meaning "or"
! : Logical no

The structure of the if statement

Single branch structure

if 条件测试操作
	then 命令序列
fi

Example:

if [ ! -d $MOUNT_DIR ]
then
	mkdir -p $MOUNT_DIR
fi

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Double branch structure

if 条件测试操作
	then 命令序列1
	else 命令序列2
fi

Example:

ping -c 3 -i 0.2 -W 3 $1 &>/dev/null
if [ $? -eq 0 ]
then
	echo "Host $1 is up."
else
	echo "Host $1 is down."
fi

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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

Example

read -p "请输入你的分数(0-100):" A
if [ $A -ge 85 ] && [ $A -le 100 ]
then
   echo "$A 分,优秀,继续保持！！"
elif [ $A -ge 70 ] && [ $A -le 84 ]
then
   echo "$A 分,合格，仍需加油！“
elif [ $A -ge 0 ] && [ $A -le 69 ]
then
   echo "$A 分,不合格，需要加强啦！"
else
   echo "$A 分,再乱输,嘶啦嘶啦滴！！！"
fi

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case statement structure

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

Example:

read -p ”请输入一个字符，并按Enter键确认：“KEY
case ”$KEY“ in
[a-z]|[A-Z])
	echo "你输入的是字母"
	;;
[0-9])
	echo "你输入的是数字"
	;;
*)
	echo "你输入的是空格、功能键或其他控制字符"
esac

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to sum up:

Conditional test syntax

File test, integer value comparison, string comparison, logic test

Syntax of if conditional statement

Single branch, double branch, multiple branch

Syntax of case multi-branch statement