Linux5：Shell编程——流程控制

目录

前言

一、if-else

二、for循环

三、while语句、无限循环和until循环

1.while语句

2.无限循环

3.until循环

四、case ... esac

五、跳出循环

1.break

2.continue循环

总结

---

前言

        Shell流程控制包括：if-else语句、for循环、while语句、无限循环、until循环、case ... esac和跳出循环。

 

一、if-else

• if结构的几种写法

# if 控制语句
 
# if判断
# if command;
# then
#     code
# fi
 
 
# if else 语句 
# if command;
# then
#     code
# else
#     code
# fi
 
 
# if elif else 语句
# if command;
# then   
#     code
# elif command2;
# then
#     code
# *n
# else
#     code
# fi

• 举例：

#!/bin/sh
 
if [ `ps -ef | grep -c "ssh"` -gt 1 ];   # ps -ef 列出当前系统中所有的进程信息。
                                         # grep -c "ssh" 统计这些进程信息中包含字符串 "ssh" 的行数。
then
    echo "True"
fi
 
echo "-----------------"
if ((10>29));
then 
    echo "10 bigger then 20"
else
    echo "10 less then 20"
fi
 
echo "------------------"
# elif
a=10
b=20
if [ $a -eq $b ];
then 
    echo "a=b"
elif [ $a -gt $b ];
then
    echo  "a>b"
else [ $a -lt $b ];
    echo "a<b"
fi

• 输出：

True
-----------------
10 less then 20
------------------
a<b

 

二、for循环

• 结构：

# for循环
# for i in xxx;
# do 
#     code
# done

• 举例：

#!/bin/sh
 
for i in "I\'m good at coding";
do 
    echo $i
done
 
for i in 1 2 3 4 5 ;
do 
    echo the value is $i
done
 
for i in "$@";         # 文件传参 $@ 循环获取传入的参数
do 
    echo $i
done

• 输出：

[root@tokyo001 shell_code]# sh demo10.sh 2 3 4 5 5  # 文件传参写法
I\'m good at coding
the value is 1
the value is 2
the value is 3
the value is 4
the value is 5
2
3
4
5
5

 

三、while语句、无限循环和until循环

1.while语句

• 结构：

# while循环
# while codition
# do 
#     code
# done

• 举例：注意这里出现了shell编程里的自增语句 let "n++"

#!/bin/sh
 
a=1
while [ $a -le 5 ]
do 
    echo $a
    let "a++"   # 等价于 a=`expr $a + 1 `
done

• 输出：

1
2
3
4
5

 

2.无限循环

• 结构：

#无限循环语法格式
# while :   # 或者 while True
# do 
#     echo "im handsome"
# done
 
# for ((;;))

• 举例：

#!/bin/sh
 
# 死循环
echo "请按ctrl+c结束"
echo -n "请输入你喜欢的电影："     # -n 不要在输出末尾添加换行符
while read film
do 
    echo "${film}是一部好电影"
done

• 输出：程序无限循环 按ctrl + c可退出

请按ctrl+c结束
请输入你喜欢的电影：hhh
hhh是一部好电影
xxxx
xxxx是一部好电影
gggg
gggg是一部好电影

 

3.until循环

• 举例：

#!/bin/sh
 
# until循环 直到条件为真为止
a=0
until [ ! $a -lt 10 ]
do 
    echo $a
    let "a++"
done

• 输出：

0
1
2
3
4
5
6
7
8
9

 

四、case ... esac

• 举例：

#!/bin/sh
# case esac  模式匹配
 
 
echo "请输入1-4之间的数字"
read -p "请输入数字：" num
case $num in
    1)
        echo "you choose 1"
        ;;   # 表示匹配结束
    2)
        echo "you choose 2"
        ;;
    3)
        echo "you choose 3"
        ;;
    4)
        echo "you choose 4"
        ;;
    *)   # 8 通配其他结果
        echo "you choose others"
        ;;  
esac
 
echo "----------------"
a="hhh"
case $a in 
    "hhh")
        echo "you are hhhhhhhhh"
        ;;
    "xxx")
        echo "you are xxxxxxxxx"
        ;;
    "ggg")
        echo "you are gggggggggg"
        ;;
esac

• 输出：一种是获取用户输入的数据进行判断，一种是直接判断

请输入1-4之间的数字
请输入数字：2
you choose 2
----------------
you are hhhhhhhhh

 

五、跳出循环

1.break

• break 命令允许跳出所有循环（终止执行后面的所有循环）
• 举例：

#!/bin/sh
 
# break 跳出循环
echo "welcome to paris"
echo "plese tell me yuor age"
while :
do 
    read -p "plese tell me yuor age of ten " age
    case $age in
        1|2|3|4)
            echo "your are so yong"
            ;;
        *)
            echo "old man"
            break          # 跳出整个循环
            ;;
    esac
done

• 输出：

welcome to paris
plese tell me yuor age
plese tell me yuor age of ten 1
your are so yong
plese tell me yuor age of ten 1
your are so yong
plese tell me yuor age of ten 1
your are so yong
plese tell me yuor age of ten 5
old man

 

2.continue循环

• continue 命令，仅跳出当前循环。
• 举例：

#!/bin/sh
 
# continue 跳出循环
echo "welcome to paris"
echo "plese tell me yuor age"
while :
do 
    read -p "plese tell me yuor age of ten " age
    case $age in
        1|2|3|4)
            echo "your are so yong"
            ;;
        *)
            echo "old man"
            continue        # 跳出当前循环
            ;;
    esac  
done

• 输出：用户输入1,2,3,4以外的数字仍不会跳出整个循环，只跳出了这一次的循环

welcome to paris
plese tell me yuor age
plese tell me yuor age of ten 1
your are so yong
plese tell me yuor age of ten 2
your are so yong
plese tell me yuor age of ten 3
your are so yong
plese tell me yuor age of ten 1
your are so yong
plese tell me yuor age of ten 6
old man
plese tell me yuor age of ten 6
old man
plese tell me yuor age of ten 

 

总结

        Shell流程控制包括：if-else语句、for循环、while语句、无限循环、until循环、case ... esac和跳出循环。

        Shell流程控制包括：if-else语句、for循环、while语句、无限循环、until循环、case ... esac和跳出循环。

        重要的事情说三遍哈哈哈哈。