123 lines
2.2 KiB
Markdown
123 lines
2.2 KiB
Markdown
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### 概览
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- if 结构
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- if else 结构
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- if elif else 结构
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- 层层嵌套
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### if 结构
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```
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if [ condition ];then
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command
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command
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fi
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if test condition;then
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command
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fi
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if [[ condition ]];then
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command
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fi
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# 等价于
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[ condition ] && command
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```
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### if else 结构
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```
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if [ condition ];then
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command1
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else
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command2
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fi
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# 等价于
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[ condition ] && command1 || command2
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```
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```
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me@me-EQ59:~/shell_demo/scripts/shell01$ cat if1.sh
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#! /bin/env bash
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# S1 输入参数的第一个
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if [ "$1" = "hello" ];then
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echo $1
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else
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echo "world"
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fi
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me@me-EQ59:~/shell_demo/scripts/shell01$ ./if1.sh
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world
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me@me-EQ59:~/shell_demo/scripts/shell01$ ./if1.sh hello
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hello
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me@me-EQ59:~/shell_demo/scripts/shell01$
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```
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```
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me@me-EQ59:~/shell_demo/scripts/shell01$ ./if_read.sh
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请输入hello:hello
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world
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me@me-EQ59:~/shell_demo/scripts/shell01$ ./if_read.sh
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请输入hello:world
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再次运行后，请输入hello
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me@me-EQ59:~/shell_demo/scripts/shell01$ cat if_read.sh
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#! /bin/env bash
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read -p "请输入hello:" str
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if [ "$str" = "hello" ];then
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echo "world"
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else
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echo "再次运行后，请输入hello"
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fi
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me@me-EQ59:~/shell_demo/scripts/shell01$
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```
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```
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me@me-EQ59:~/shell_demo/scripts/shell01$ ./if_read_oneline.sh
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please input hello:hello
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hello
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me@me-EQ59:~/shell_demo/scripts/shell01$ ./if_read_oneline.sh
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please input hello:world
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please input hello!
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me@me-EQ59:~/shell_demo/scripts/shell01$ cat if_read_oneline.sh
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#! /bin/env bash
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read -p "please input hello:" str; [ "$str" = "hello" ] && echo "hello" || echo "please input hello!"
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me@me-EQ59:~/shell_demo/scripts/shell01$
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```
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### if elif else结构
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```
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if [ condition1 ];then
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command1
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elif [ condition2 ];then
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command2
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elif [ condition3 ];then
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else
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command3
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fi
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```
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```
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me@me-EQ59:~/shell_demo/scripts/shell01$ cat if_elif.sh
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#! /bin/env bash
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read -p "请输入hello" str
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if [ "$str" = "hello" ];then
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echo "hello"
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elif [ "$str" = "hello_elif" ];then
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echo "hello_elif"
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else
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echo "world"
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fi
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me@me-EQ59:~/shell_demo/scripts/shell01$ ./if_elif.sh
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请输入hellohello
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hello
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me@me-EQ59:~/shell_demo/scripts/shell01$ ./if_elif.sh
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请输入hellohello_elif
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hello_elif
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me@me-EQ59:~/shell_demo/scripts/shell01$ ./if_elif.sh
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请输入hellonothing
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world
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me@me-EQ59:~/shell_demo/scripts/shell01$
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```
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