Linux Shell脚本编程提高(12)

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实际上Shell是一个命令解释器,它解释由用户输入的命令并且把它们送到内核,不仅如此,Shell有自己的编程语言用于对命令的编辑,它允许用户编写由shell命令组成的程序.Shel编程语言具有普通编程语言的很多特点,比如它也有循环结构和分支控制结构等,用这种编程语言编写的Shell程序与其他应用程序具有同样的效果,下面我们会介绍Shell-Script的编写.

Shell 条件判断

◆按文件类型判断◆

1.使用 [] 来执行判断,并使用echo $? 判断执行结果

[root@localhost ~]# ls
lyshark.log  passwd.awk

[root@localhost ~]# [ -e ./lyshark.log ]
[root@localhost ~]# echo $?              #返回0表示文件 ./lyshark.log 存在
0

[root@localhost ~]# [ -e ./shar ]        #返回1表示文件 ./shar 不存在
[root@localhost ~]# echo $?
1

2.我们可以结合之前所学的 && 和 || 实现判断后输出判断结果

[root@localhost ~]# ls
lyshark.log  passwd.awk

[root@localhost ~]# [ -e ./lyshark.log ] && echo "check ok" || echo "None"
check ok

[root@localhost ~]# [ -e ./shar ] && echo "check on" || echo "None"
None

◆按文件权限判断◆

1.使用 -r 判断文件是否可读,和可执行

[root@localhost ~]# ls
lyshark.log passwd.awk

[root@localhost ~]# [ -r ./lyshark.log ] && echo "check ok" || echo "None"
check ok

[root@localhost ~]# [ -x ./lyshark.log ] && echo "check ok" || echo "None"
None

[root@localhost ~]# [ -r ./lyshark.log ] && [ -x ./lyshark.log ] && echo "可读,可执行" || echo "可读,不可执行" || echo
"不可读,不可执行"

可读,不可执行

◆两文件之间的比较◆

1.通过 -ef 判断是否是相同的I节点

[root@localhost ~]# ls -li
total 0
8409155 -rw-r--r-- 2 root root 0 Sep 25 21:39 lyshark_ln.log
8409155 -rw-r--r-- 2 root root 0 Sep 25 21:39 lyshark.log

[root@localhost ~]# [ ./lyshark.log -ef lyshark_ln.log ] && echo "ok" || echo "no"
ok

◆数值之间的比较◆

1.比较两数字,大小关系

[root@localhost ~]# [ 23 -ge 24 ] && echo "23 >= 24 " || echo "23 <= 24"
23 <= 24

[root@localhost ~]# [ 30 -ge 24 ] && echo "30 >= 24 " || echo "30 <= 24"
30 >= 24

◆字符串之间的比较◆

1.字符串之间的比较例子,字符串是否为空

[root@localhost ~]# name=LyShark
[root@localhost ~]# age=""
[root@localhost ~]# unset sex

[root@localhost ~]# [ -z "$name" ] && echo "字符串空" || echo "不为空"
不为空

[root@localhost ~]# [ -z "$age" ] && echo "字符串空" || echo "不为空"
字符串空

[root@localhost ~]# [ -z "$sex" ] && echo "字符串空" || echo "不为空"
字符串空

2.两个字符串相等比较

[root@localhost ~]# x=100
[root@localhost ~]# y=200
[root@localhost ~]# z=100

[root@localhost ~]# [ "x" == "y" ] && echo "yes" || echo "no"
no
[root@localhost ~]# [ "$x" == "$y" ] && echo "yes" || echo "no"
no

[root@localhost ~]# [ "$x" == "$z" ] && echo "yes" || echo "no"
yes

◆多重条件判断◆

1.逻辑与：判断变量x不为空,并且x大于20

[root@localhost ~]# declare -i x=10
[root@localhost ~]# [ -n "$x" -a "$x" -gt 20 ] && echo "yes" || echo "no"
no
[root@localhost ~]# declare -i x=30
[root@localhost ~]# [ -n "$x" -a "$x" -gt 20 ] && echo "yes" || echo "no"
yes

2.逻辑非：判断x变量不为空,或！取反

[root@localhost ~]# declare -i x=30

[root@localhost ~]# [ -n "$x" ] && echo "yes" || echo "no"
yes

[root@localhost ~]# [ ! -n "$x" ] && echo "yes" || echo "no"
no

IF条件判断

if、then、else语句用于判断给定的条件是否满足,并根据测试条件的真假来选择相应的操作.if/else仅仅用于两分支判断,多分支的选择时需要用到if/else语句嵌套、if/elif/else和case多分支选择判断结构.

IF结构例子: 一个就简单的单分支结构.测试条件后如果没有";"则then语句要换行.

#!/bin/sh
echo "Please input a Number:"
read Num

if [ "$Num" -lt 15 ]
then
	echo "$Num <= 15"
fi

IF/IF-ELSE结构: 一个双分支结构,如下执行删除文件,成功返回then是否返回else.

#!/bin/sh

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嵌套: 可同时判断三个或三个以上条件,但要注意if与else配对关系,else语句总是与它上面最近的未配对的if配对.

#!/bin/bash
#提示用户输入分数（0-100）
echo "Please Input a integer(0-100): "
read score

#判断学生的分数类别
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)."
else
     if [ "$score" -ge 90 ]
     then
         echo "The grade is A!"
     else
          if [ "$score" -ge 80 ]
          then
              echo "The grade is B!"
          else
               if [ "$score" -ge 70 ]
              then
                   echo "The grade is C!"
              else
                   if [ "$score" -ge 60 ]
                   then
                        echo "The grade is D!"
                   else
                        echo "The grade is E!"
                   fi
              fi
         fi
    fi
fi

IF/ELIF/ELSE结构: if/else嵌套在编程中很容易漏掉then或fi产生错误,而且可读性很差,因此引入if/elif/else结构针对某一事件的多种情况进行处理,fi只出现一次,可读性也提高了.

#!/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

实例1: 通过单分支判断磁盘是否已满.

[root@localhost ~]# cat if.sh

#!/bin/bash

ret=$( df -h | grep "/dev/sda1" | awk '{printf $5}' | cut -d "%" -f 1 )
if [ $ret -ge 80 ]
then
        echo -e　"/dev/sda1 is full !!"
fi

实例2: 通过多分支判断磁盘是否已满.

[root@localhost ~]# cat if.sh
#!/bin/bash

ret=$( df -h | grep "/dev/sda1" | awk '{printf $5}' | cut -d "%" -f 1 )
if [ $ret -ge 80 ]
then
        echo -e "/dev/sda1 is full !"
else
        echo -e "/dev/sda1 is not full !"
fi

实例3: 多分支判断小例子.

[root@localhost ~]# cat if.sh
#!/bin/bash

read -p "输入一个数字:" num

if [ "$num" -gt "100" ]
then
        echo -e "这个数大于100"
elif [ "$num" -lt "100" ]
then
        echo -e "这个数小于100"
elif [ "$num" -eq "100" ]
then
        echo -e "这个数等于100"
fi

实例4: 判断输入的年份是否是润年(润年条件：1、能被4整除,但不能被100整除的年份.2、能被100整除,又能被400整除的年份)

#!/bin/sh

echo "Please Input a year: "
read year

let "n1=$year % 4"
let "n2=$year % 100"
let "n3=$year % 400"
if [ "$n1" -ne 0 ]
then
    leap=0
elif [ "$n2" -ne 0 ]
then
    leap=1
elif [ "$n3" -ne 0 ]
then
    leap=0
else
    leap=1
fi

CASE分支结构

case结构变量值依次比较,遇到双分号则跳到esac后的语句执行,没有匹配则脚本将执行默认值*"后的命令,直到"';;"为止.case的匹配值必须是常量或正则表达式.

#!/bin/bash

read -p "输入一个符号:" temp

case $temp in

        "-print")
                echo -e "您执行了print函数"
        ;;
        "-save")
                echo -e "您执行了save函数"
        ;;
        "-help")
                echo -e "您执行了help函数"
        ;;
        *)
                echo -e "您执行了其他操作"
        ;;
esac

FOR循环结构

Shell编程中循环命令用于特定条件下决定某些语句重复执行的控制方式,有三种常用的循环语句：for、while和until.while循环和for循环属于"当型循环",而until属于"直到型循环",循环控制符：break和continue控制流程转向.

列表FOR循环: 循环打印数据分别从1遍历到5.

do和done之间的命令称为循环体,执行次数和list列表中常数或字符串的个数相同.for循环,首先将in后list列表的第一个常数或字符串赋值给循环变量,然后执行循环体,以此执行list,最后执行done命令后的命令序列.

#!/bin/bash

for tmp in {1..5}
do
     echo "Hello, Welcome $tmp times"
done

FOR循环累加: 通过FOR循环累加打印数据.

通过i的按步数2不断递增,计算sum值为2500.同样可以使用seq命令实现按2递增来计算1～100内的所有奇数之和,for i in $(seq 1 2 100),seq表示起始数为1,跳跃的步数为2,结束条件值为100.

#!/bin/bash
sum=0

for i in {1..100..2}
do
    let "sum+=i"
done

echo "sum=$sum"

FOR循环遍历目录: 通过for循环显示当前目录下所有的文件.

#!/bin/bash

for file in $( ls )
#for file in *
do
   echo "file: $file"
done

不带列表FOR: 由用户制定参数和参数的个数,与上述的for循环列表参数功能相同.

#!/bin/bash

echo "number of arguments is $#"
echo "What you input is: "

for argument
do
    echo "$argument"
done

C风格FOR循环: 也被称为计次循环.

#!/bin/bash
for((integer = 1; integer <= 5; integer++))
do
    echo "$integer"
done

实例1: 通过for循环打印1-10.

[root@localhost ~]# cat for.sh
#!/bin/bash

for temp in `seq 1 10`
do
        echo -e "打印数据: $temp"
done

实例2: 通过for循环计算1-100的累加和.

[root@localhost ~]# cat for.sh
#!/bin/bash

declare -i sum=0

for temp in `seq 1 100`
do
        sum=$sum+$temp
done

echo -e "从1+到100的结果是: $sum"

实例3: 从列表中选择数据.

[root@localhost ~]# cat for.sh
#!/bin/bash

for temp in 1 3 5 7 9
do
        echo -e "打印: $temp"
done

[root@localhost ~]# bash for.sh
打印: 1
打印: 3
打印: 5
打印: 7
打印: 9

实例4: 读取文件内容并打印.

[root@localhost ~]# cat for.sh
#!/bin/bash

world=`cat /etc/passwd`
for temp in $world
do
        echo -e "打印: $temp"
done

WHILE 循环

也称为前测试循环语句,重复次数是利用一个条件来控制是否继续重复执行这个语句.为了避免死循环,必须保证循环体中包含循环出口条件即表达式存在退出状态为非0的情况.

计数控制: 指定了循环的次数500,初始化计数器值为1,不断测试循环条件i是否小于等于100.在循环条件中设置了计数器加2来计算1～100内所有的奇数之和.

#!/bin/bash

sum=0

i=1

while(( i <= 100 ))
do
     let "sum+=i"
     let "i += 2"
done

echo "sum=$sum"

结束标记控制的while循环: 设置一个特殊的数据值(结束标记)来结束while循环.

#!/bin/bash

echo "Please input the num(1-10) "
read num

while [[ "$num" != 4 ]]
do
   if [ "$num" -lt 4 ]
   then
        echo "Too small. Try again!"
        read num
   elif [ "$num" -gt 4 ]
   then
         echo "To high. Try again"
         read num
   else
       exit 0
    fi
done 

echo "Congratulation, you are right! "

标志控制的while循环: 使用用户输入的标志值来控制循环的结束(避免不知道循环结束标志的条件).

#!/bin/bash

echo "Please input the num "
read num

sum=0
i=1

signal=0

while [[ "$signal" -ne 1 ]]
do
    if [ "$i" -eq "$num" ]
    then
       let "signal=1"
       let "sum+=i"
       echo "1+2+...+$num=$sum"
    else
       let "sum=sum+i"
       let "i++"
    fi
done

命令行控制的while循环: 使用命令行来指定输出参数和参数个数,通常与shift结合使用,shift命令使位置变量下移一位($2代替$1、$3代替$2,并使$#变量递减),当最后一个参数显示给用户,$#会等于0,$*也等于空.

#!/bin/bash

echo "number of arguments is $#"
echo "What you input is: "

while [[ "$*" != "" ]]
do
    echo "$1"
    shift
done

while计算: 通过while循环,计算1-100的累加和.

[root@localhost ~]# cat while.sh
#!/bin/bash

declare -i x=0
declare -i num=0

while [ "$x" -lt "100" ]
do
        x=$x+1
        num=$num+$x
done

echo -e "从1-100结果是: $num"

[root@localhost ~]# bash while.sh
从1-100结果是: 5050

UNTIL 循环

until命令和while命令类似,while能实现的脚本until同样也可以实现,但区别是until循环的退出状态是不为0,退出状态是为0(与while刚好相反),即whie循环在条件为真时继续执行循环而until则在条件为假时执行循环.

#!/bin/bash

i=0

until [[ "$i" -gt 5 ]]    #大于5
do
    let "square=i*i"
    echo "$i * $i = $square"
    let "i++"
done

#!/bin/bash

for (( i = 1; i <=9; i++ ))
do

    for (( j=1; j <= i; j++ ))
    do
        let "temp = i * j"
        echo -n "$i*$j=$temp  "
     done 

     echo ""   #output newline
done

#!/bin/bash

i=0

until [[ "$i" -gt 5 ]]    #大于5

do

    let "square=i*i"

    echo "$i * $i = $square"

    let "i++"

done

跳出语句(break,continue)

break: 在for、while和until循环中break可强行退出循环,break语句仅能退出当前的循环,如果是两层循环嵌套,则需要在外层循环中使用break.

#!/bin/bash

sum=0
for (( i=1; i <= 100; i++))
do
    let "sum+=i"

    if [ "$sum" -gt 1000 ]
    then
        echo "1+2+...+$i=$sum"
        break
    fi
done

continue: 在for、while和until中用于让脚本跳过其后面的语句,执行下一次循环.continue用于显示100内能被7整除的数.

#!/bin/bash

m=1
for (( i=1; i < 100; i++ ))
do
    let "temp1=i%7"         #被7整除

    if [ "$temp1" -ne 0 ]
    then
        continue
    fi

    echo -n "$i  "

    let "temp2=m%7"          #7个数字换一行

    if  [ "$temp2" -eq 0 ]
    then
        echo ""
    fi

    let "m++"
done

SELECT 语句

select结构从技术角度看不能算是循环结构,只是相似而已,它是bash的扩展结构用于交互式菜单显示,功能类似于case结构比case的交互性要好.

[root@localhost ~]# cat select.sh
#!/bin/bash

echo -e "请选择系统类型?"

select var in "Linux" "GNU HURD" "FreeBSD" "Other";
do
        if [ $var == "Linux" ]
        then
                echo -e "您的系统是Linux"
        elif [ $var == "FreeBSD" ]
        then
                echo -e "您的系统是FreeBSD"
        fi
break
done
[root@localhost ~]# bash select.sh
请选择系统类型?
1) Linux
2) GNU HURD
3) FreeBSD
4) Other

#? 1
您的系统是Linux

#!/bin/bash

declare -a serial
serial=(1 2 3 4)
PS3="Enter a number: "

select var in "a" "b" "c" "d"
do
    if ! echo ${serial[@]} | grep -q $REPLY
        then
                echo "please enter [1-4]."
                continue
    fi

        echo "your anwser is: $var"
        break
done

[root@localhost ~]# bash select.sh
1) a
2) b
3) c
4) d
Enter a number: 1
your anwser is: a

FUNCTION 函数

[root@localhost ~]# cat function.sh
#!/bin/bash

function my_print()
{
        echo -e "------------------------"
        echo -e "hello world"
        echo -e "------------------------"
        return "127"
}

my_print
echo $?

[root@localhost ~]# bash function.sh
------------------------
hello world
------------------------
127

参考文献：Linux鸟哥私房菜，Linux运维之道