分支与循环结构

一. if条件语句
1.单分支结构

语法：
if [条件]
then
    指令
fi

或者

if [条件];then
    指令
file

#提示：分号相当于命令换行

例1：单分支if条件句整数比较大小

[root@localhost ~]# cat com_num.sh
#!/bin/bash
a=$1
b=$2

#read -p "Pls input two number:" a b
if [ $a -gt $b ];then
    echo "yes,$a > $b"
fi

if [ $a -le $b ];then
    echo "yes,$a <= $b"
fi

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例2：文件不存在，则创建

方式1：
[root@localhost test]# cat test.sh
#!/bin/bash
FILEPATH="/root/test"
if [ -e "$FILEPATH/f.sh" ];then
    echo "$FILEPATH/f.sh is exist."
fi

if [ ! -e "$FILEPATH/f.sh" ];then
    [ ! -d $FILEPATH ] && mkdir -p $FILEPATH
    [ -d $FILEPATH ] && {
      cd $FILEPATH
      touch f.sh
      echo "f.sh is touched!"
      }
fi


方式2：
[root@localhost test]# vim test.sh
#!/bin/bash
FILEPATH="/root/test"
[ ! -d $FILEPATH ] && mkdir -p $FILEPATH
cd $FILEPATH
if [ -f $FILEPATH/f.sh ];then
    echo "f.sh is exits"
    exit 0
else
    touch $FILEPATH/f.sh
    echo "$FILEPATH/f.sh make cpmplete."
fi

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例3：判断内存大小，低于1000报警

[root@localhost test]# vim mem.sh

#!/bin/bash
cur_free=`free -m|awk '/buffers\// {print $NF}'`
chars="current memory is $cur_free"
if [$cur_free -lt 1000 ];then
    echo $chars
    echo $chars|mail -s "chars" mail.com
fi

#!/bin/bash
gen_size=$(df / |awk '/\//{print $4}') #提取根分区剩余空间
mem_size=$(free |awk '/Mem/{print $4}') #提取内存剩余空间
while :
do
#注意内存和磁盘提取的空间大小都是以 Kb 为单位
if [ $gen_size -le 512000 -a $mem_size -le 1024000 ];then
mail -s Warning root <<EOF
Insufficient resources,资源不足
EOF
fi
done

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2.双分支结构

语法
if 条件
  then
     指令
elif 条件
   then
     指令
else
    指令
fi

例1：多分支判断大小

[root@localhost test]# cat num.sh
#!/bin/bash
read -p "Pls input two mumbers:" a b
if [ $a -gt $b ];then
    echo "yes,$a > $b"
elif [ $a -eq $b ];then
    echo "yes,$a = $b"
else
    echo "yes,$a <= $b"
fi

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例2:多分支if语句实现对传入的参数进行判断

[root@localhost test]# cat num.sh
#!/bin/bash
a=$1
b=$2

if [ $# -ne 2 ];then
    echo "Usage:bash $0 num1 num2"
    exit 1
fi

if [ $a -gt $b ];then
    echo "yes,$a > $b"
elif [ $a -eq $b ];then
    echo "yes,$a = $b"
else
    echo "yes,$a < $b"
fi

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例3：对参数个数以及类型严格判断#!/bin/bash

a=$1
b=$2

if [ $# -ne 2 ];then
    echo "Usage:bash $0 num1 num2"
    exit 1
fi

[ -n "`echo $1|sed 's/[0-9]//g'`" ] && echo "第一个参数为数字" && exit 1
[ -n "`echo $2|sed 's/[0-9]//g'`" ] && echo "第二个参数为数字" && exit 11

if [ $a -gt $b ];then
    echo "Yes,$a > $b"
elif [ $a -eq $b ];then
    echo "Yes,$a = $b"
else
    echo "Yes,$a < $b"
fi

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二.case结构条件句

#语法：
case "字符串变量" in
    值1) 指令...
;;
    值2) 指令...
;;
    *  ) 指令...
esac

例1：根据用户的输入判断是哪个数字

[root@localhost scripts]# cat num.sh
#!/bin/bash

read -p "Pls input a number:" num
case "$num" in
1)
    echo "The num you input is 1"
;;
2)
    echo "The num you input is 2"
;;
[3-9])
    echo "The num you input is $num"
;;
*)
    echo "The num input must be less 9"
    exit;
;;
esac

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例2：根据用户的选择输入判断是哪种水果并加上不同颜色

[root@localhost scripts]# cat fruit.sh
#!/bin/bash

RED_COLOR='\E[1;31m'
GREEN_COLOR='\E[1;32m'
YELLOW_COLOR='\E[1;33m'
BLUE_COLOR='\E[1;34m'
RES='\E[0m'

read -p "Pls input the fruit name you like:" arg
case "$arg" in
apple|APPLE)
    echo -e "The fruit name you like is ${RED_COLOR}"$arg."${RES} "
;;
banana|BANANA)
    echo -e "The fruit name you like is ${YELLOW_COLOR}"$arg."${RES} "
;;
pear|PEAR)
    echo -e "The fruit name you like is ${GREEN_COLOR}"$arg."${RES} "
;;
*)
    echo -e "Here is not the fruit name you like-- ${BLUE_COLOR}"$arg."${RES} "
    exit;
esac

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例3：传两个参数，给指定内容(第一个参数)加指定颜色(第二个参数)

[root@localhost scripts]# cat test.sh
#!/bin/bash
RED_COLOR='\E[1;31m'
GREEN_COLOR='\E[1;32m'
YELLOW_COLOR='\E[1;33m'
BLUE_COLOR='\E[1;34m'
PINK_COLOR='\E[1;35m'
RES='\E[0m'

if [ $# -ne 2 ];then
    echo "Usage $0 content {red|yellow|blue|green}"
    exit
fi

case "$2" in
  red|RED)
         echo -e "${RED_COLOR}$1${RES}"
         ;;
  yellow|YELLOW)
    echo -e "${YELLOW_COLOR}$1${RES}"
        ;;
  green|GREEN)
        echo -e "${BLUE_COLOR}$1${RES}"
        ;;
  blue|BLUE)
        echo -e "${BLUE_COLOR}$1${RES}"
        ;;
  pink|PINK)
        echo -e "${PINK_COLOR}$1${RES}"
        ;;
  *)
        echo -e "${PINK_COLOR}$1${RES}"
esac

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例4：给指定内容添加颜色

[root@localhost scripts]# cat test.sh
#!/bin/bash
color_func(){
RED_COLOR='\E[1;31m'
GREEN_COLOR='\E[1;32m'
YELLOW_COLOR='\E[1;33m'
BLUE_COLOR='\E[1;34m'
PINK_COLOR='\E[1;35m'
RES='\E[0m'

if [ $# -ne 2 ];then
    echo "Usage $0 content {red|yellow|blue|green}"
    exit
fi

case "$2" in
  red|RED)
         echo -e "${RED_COLOR}$1${RES}"
         ;;
  yellow|YELLOW)
    echo -e "${YELLOW_COLOR}$1${RES}"
        ;;
  green|GREEN)
        echo -e "${BLUE_COLOR}$1${RES}"
        ;;
  blue|BLUE)
        echo -e "${BLUE_COLOR}$1${RES}"
        ;;
  pink|PINK)
        echo -e "${PINK_COLOR}$1${RES}"
        ;;
  *)
        echo -e "${PINK_COLOR}$1${RES}"
esac
}

color_func hello yellow
color_func world blue

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三.while循环和until循环

当型和直到循环(while循环和until循环)

1.while条件句

语法：
while 条件
do
    指令
done

2.until语句

until 条件
do
    指令
done

#until 了解

例1.屏幕输出

[root@localhost scripts]# cat test.sh
#!/bin/bash
while true
do
    uptime
    sleep 2
done

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例2.追加到log里，使用微秒单位

[root@localhost scripts]# cat t.sh
#!/bin/bash
while [ 1 ]   
do
    uptime >>./uptime.log
    usleep 1000000      #以微妙为单位 1000000=1秒
done

[root@localhost scripts]# bash t.sh &    #后台运行(提示：在后台永久执行，称为守护进程模式)
[root@localhost scripts]# fg  #调前台
bash t.sh

[root@localhost scripts]# jobs
[1]+  Running                 bash t.sh &
[root@localhost scripts]# fg 1
bash t.sh

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Linux 技巧：让进程在后台可靠运行的几种方法
https://www.ibm.com/developerworks/cn/linux/l-cn-nohup/

例3：
通过while语句计算从1加到100之和
1.while 计算

[root@localhost scripts]# cat sum.sh
#!/bin/bash

i=1
sum=0
while ((i <= 100))
do
    ((sum=sum+i))
    ((i++))
done
echo "sum=$sum"

#[ -n "$sum" ] && printf "totalsum is: $sum\n"   #使用 printf 输出

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2.通过数学公式

[root@localhost scripts]# cat sum.sh
#!/bin/bash

i=100
((sum=i*(i+1)/2))
echo $sum

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例4：使用while循环打印10，9，8...1
1.双小括号和双中括号

[root@localhost scripts]# cat pri.sh
#!/bin/bash
i=10
while ((i>0))   #双小括号
do
    echo $i
    ((i--))
done

[root@localhost scripts]# cat pri.sh
#!/bin/bash
i=10
while [[ $i > 0 ]]  #双中括号
do
    echo $i
    ((i--))
done

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2.脚本传参

root@localhost scripts]# cat  arg.sh
#!/bin/bash
i="$1"
while [ $i -gt 0 ]
do
    echo $i
    ((i--))
done


#while是条件满足执行，until是条件满足退出
[root@localhost scripts]# cat until.sh
#!/bin/bash
i=1
until ((i>100))
do
    ((sum=sum+i))
    ((i++))
done
[ -n "$sum" ] && echo $sum

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四. for 循环结构
语法：

for 变量名 in 变量取值列表
do
    指令
done

提示：在此结构中"in 变量取值列表"可省略，省略是相当于in"$@",使用
for i 就相当于使用 for i in "$@"

例1：打印5-1

[root@localhost scripts]# cat f.sh
#!/bin/bash
for i in {5..1}
do
    echo $i
done


#使用seq
[root@localhost scripts]# vim f.sh

#!/bin/bash
for i in `seq -s " " 5 -1 1`
do
    echo $i
done

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例2：打印目录

[root@localhost ~]# cat dir.sh
#!/bin/bash
for i in `ls -F|grep /`
do
    echo $i
done

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[root@localhost scripts]# ls
1.txt 2.txt 3.txt 4.txt

1.手动测试
[root@localhost scripts]# a=4.txt
[root@localhost scripts]# mv $a `echo $a|cut -d . -f1`.JPG
[root@localhost scripts]# ls
1.txt  2.txt  3.txt  4.JPG  

2.脚本处理
[root@localhost scripts]# cat cf.sh
#!/bin/bash
for i in `ls *.txt`
do
    mv $i `echo $i|cut -d . -f1`.gif
done
[root@localhost scripts]# bash cf.sh
[root@localhost scripts]# ls
1.gif  2.gif  3.gif  4.JPG  

3，使用rename
[root@localhost scripts]# ls
1.gif  2.gif  3.gif  4.TXT  
[root@localhost scripts]# cat cf.sh
#!/bin/bash
for i in `ls *.gif`
do
    rename .gif .HTML $i
done

[root@localhost scripts]# bash cf.sh
[root@localhost scripts]# ls
1.HTML  2.HTML  3.HTML  4.TXT  cf.sh

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例3：ls和awk

[root@localhost scripts]# ls *.jpg
stu_102999_1_finished.jpg  stu_102999_3_finished.jpg
stu_102999_2_finished.jpg  stu_102999_4_finished.jpg


[root@localhost scripts]# ls *.jpg|awk -F '_finished' '{print "mv " $0" "$1".jpg "}'
mv stu_102999_1_finished.jpg stu_102999_1.jpg
mv stu_102999_2_finished.jpg stu_102999_2.jpg
mv stu_102999_3_finished.jpg stu_102999_3.jpg
mv stu_102999_4_finished.jpg stu_102999_4.jpg
[root@localhost scripts]#
[root@localhost scripts]# ls *.jpg|awk -F '_finished' '{print "mv " $0" "$1".jpg "}' |bash
[root@localhost scripts]# ls *.jpg
stu_102999_1.jpg  stu_102999_3.jpg
stu_102999_2.jpg  stu_102999_4.jpg

#[root@localhost scripts]# ls *.jpg |awk -F '_finished' '{print "mv "$0" "$1$2" "}' |bash  #另一个写法

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例4：打印9*9

1.双for循环
[root@localhost scripts]# cat f9.sh
#!/bin/bash
for a in `seq 1 9`
do
    for b in `seq 1 9`
    do
        if [ $a -ge $b ];then
            echo -en "$a x $b = $(expr $a \* $b) "
        fi
    done
echo " "
done

2.简化写法

[root@localhost scripts]# cat f9.sh
#!/bin/bash
for a in `seq 9`
do
    for b in `seq 9`
    do
        [ $a -ge $b ] && echo -en "$a x $b = $(expr $a \* $b) "   
 done
echo " "
done

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例5：计算从1加到100

1.for
[root@localhost scripts]# cat sum.sh
#!/bin/bash
for((i=0;i<=100;i++))
do
   ((j=j+i))
done
echo $j

2.while
[root@localhost scripts]# cat sum.sh
#!/bin/bash
i=0
while((i<=100))
do
    ((j=j+i))
    ((i++))
done
echo $j

3.使用公式
root@localhost scripts]# cat sum.sh
#!/bin/bash
i=100
((j=i*(i+1)/2))
echo $j

提示：使用循环效率不佳，此类计算，使用数学公式会更快

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posted @ 2018-07-19 23:53 shadow3 阅读(364) 评论(0) 收藏举报

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分支与循环结构

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