SHELL学习――判断

简单的if 结构

if expression

then

       command

       command

……

fi

测试条件后没有“；”，则 then 语句要换行。 if 和 then 处于同一行，必须用“；”来终止 if 语句

if expression ; then

       command

……

fi

举例说明：

#!/bin/bash

echo "Please input a integer: "

read integer1

if [ "$integer1" -lt 15 ] ; then

echo "The integer which you input is lower than 15."

fi

 

exit 命令

exit status    # status 用 0~255 之间的数字表示 ,exit 不能在终端运行。

举例说明 :

[root@localhost tmp]# cat 2.sh

#!/bin/bash

echo "Please input a string: "

read str1

if [ -z "$str1" ]

then

    echo "What you input is null!"

    exit 1

fi

[root@localhost tmp]# ./2.sh

Please input a string:

hello

[root@localhost tmp]# echo $?

0

[root@localhost tmp]# ./2.sh

Please input a string:

 

What you input is null!

[root@localhost tmp]# echo $?

1

[root@localhost tmp]#

 

if/else 结构

if expression

then

       command

       ……       

else

       command

       ……

fi

举例说明：（例一）

[root@localhost tmp]# cat 3.sh

#!/bin/bash

file=/tmp/3.sh

if [ ! -e "$file" ]

then

        echo "file $file do not exist ."

        exit

else

        echo "file $file exist ."

fi

[root@localhost tmp]# ./3.sh

file /tmp/3.sh exist .

[root@localhost tmp]#

举例说明：（例二）

#!/bin/bash

echo "Please input the file which you want to delete:"

read file

if rm -f "$file"

then

        echo "Delete the file $file sucessfully!"

else

        echo "Delete the file $file failed!"

fi

 

if/else 语句嵌套

if expression1

then

       if expression2

       then

              command

       else

              command

       fi

else

       if expression3

       then

              command

       else

              command

       fi

fi

举例说明：

#!/bin/bash

echo "Please input a file_name:"

read word

if [ "$word" ]

then

        echo "What you input is not null!"

        if [ -e "$word" ]

        then

                echo "The file $word is existence !"

        else

                echo "The file $word is not existence !"

        fi

else

        echo "what you input is null!"

fi

 

if/elif/else 结构

if expression1

then

command

elif expression2

then

       command

elif expressionN

       command

else

       command

fi

举例说明

#!/bin/bash

echo "Please Input a integer(0-100): "

read score

if [ "$score" -lt 0 -o "$score" -gt 100 ]

then

        echo "The score what you input is not integer or the score is not in (0-100)."

elif [ "$score" -ge 90 ]

then

        echo "The grade is A!"

elif [ "$score" -ge 80 ]

then

        echo "The grade is B!"

elif [ "$score" -ge 70 ]

then

        echo "The grade is C!"

elif [ "$score" -ge 60 ]

then

        echo "The grade is D!"

else

        echo "The grade is E!"

fi

        

case 结构

case variable in

value1)

       command

       ……

       command;;

Value2)

       command

       command;;

……

valueN)

       command

       ……

       command;;

*)

       command

       command;;

esac

举例说明 :

#!/bin/bash

echo "Please Input a month(0-12): "

read month

case "$month" in

1)

        echo "The month is January!";;

2)

        echo "The month is February!";;

3)

        echo "The month is March!";;

4)

        echo "The month is April!";;

5)

        echo "The month is May!";;

6)

        echo "The month is June!";;

7)

        echo "The month is  July!";;

8)

        echo "The month is August!";;

9)

        echo "the month is September!";;

10)

        echo "The month is October!";;

11)

        echo "The month is November!";;

12)

        echo "The month is December!";;

*)

        echo "The month is not in (0-12)";;

esac

echo "The 'case' command ends!"