shell里的` ` $( ) ${ } expr $(( )) (

快来打我* 2022-11-18 01:37 165阅读 0赞

` `官方解释：
    命令替换.`command` 结构使字符(`)[译者注：这个字符不是单引号，而是在标准美国键盘上的ESC键下面，在字符1左边，在TAB键上面的那个键，要特别留心]引住的命令（command）执行结果能赋值给一个变量。它也被称为后引号（backquotes）或是斜引号（backticks）.
    例子：
    A=`ls -l`
    把ls -l的结果赋给A    ls -l 作为命令来执行

**所有UNIX命令,要取结果或输出，都要用$( )或反引号\` \`**

**反引号\`  \`和$( )缺陷也相同，没回车换行，容易把多行合并**

**wc-l常与$( )合用取其输出，计算行数，进而确定循环次数**

**echo +$(  ) ――echo内也可执行命令，并通过$( )取该命令的输出** 再显示  
常见使用：echo内，用$( )取awk和sed对某些命令修饰后的效果

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   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> [macg@machome ~]$ vi test<br> #choose a iinterface to config<br> <span style="word-wrap:normal; word-break:normal; font-weight:bold">/sbin/ifconfig -a | awk '$2=="Link" {print($1,$3)}' &gt;iftmp</span><br> echo "choose one of interface to config"<br> i=1<br> <span style="word-wrap:normal; word-break:normal; font-weight:bold">count=$(more iftmp|wc -l)</span><br> while [ $i -le $count ]<br> do<br> echo "$i)--------&nbsp;<span style="word-wrap:normal; word-break:normal; font-weight:bold">$(</span>sed -n "${i}p" iftmp<span style="word-wrap:normal; word-break:normal; font-weight:bold">)</span>"<br> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span style="word-wrap:normal; word-break:normal; font-weight:bold">用sed 修饰输出</span><br> ii=$(($i+1))<br> done&nbsp;&nbsp;&nbsp;</td> 
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   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> [macg@machome ~]$ sh test<br> <br> choose one of interface to config<br> 1)-------- eth0 encap:Ethernet<br> 2)-------- lo encap:Local<br> 3)-------- sit0 encap:IPv6-in-IPv4<br> give the interface a host name[such as:xxx-hme0]:</td> 
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**$\{ \}和$( )的区别：$\{ \}  是做什么用的? --------避免在echo中变量被连读**  
 $\{i\}是区分其后紧接的字符串，避免“连读”

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   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> $ vi ttt.sh<br> <br> GONE="<span style="word-wrap:normal; word-break:normal; font-weight:bold">(ServerType|BindAddress|Port|AddModule|ClearModuleList|</span>"<br> GONE="<span style="word-wrap:normal; word-break:normal; font-weight:bold">${GONE}</span>AgentLog|RefererLog|RefererIgnore|FancyIndexing|"<br> echo $GONE</td> 
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   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> $ sh ttt.sh<br> <span style="word-wrap:normal; word-break:normal; font-weight:bold">(ServerType|BindAddress|Port|AddModule|ClearModuleList</span>|AgentLog|RefererLog|RefererIgnore|FancyIndexing<br> &nbsp;</td> 
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**$\{i\}可以在任何场合替代普通变量$i**

**expr 数学表达式**  
**shell语言里没有运算公式** ，必须借助expr语句实现：

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   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> expr 变量&nbsp;<span style="word-wrap:normal; word-break:normal; font-weight:bold">‘</span>+-x /<span style="word-wrap:normal; word-break:normal; font-weight:bold">’</span>&nbsp;变量</td> 
   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> expr $a&nbsp;<span style="word-wrap:normal; word-break:normal; font-weight:bold">'</span>+<span style="word-wrap:normal; word-break:normal; font-weight:bold">'</span>&nbsp;$b</td> 
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   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> expr 常数&nbsp;<span style="word-wrap:normal; word-break:normal; font-weight:bold">‘</span>+-x /<span style="word-wrap:normal; word-break:normal; font-weight:bold">’</span>&nbsp;常数</td> 
   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> [mac@machome ~]$ expr 3&nbsp;<span style="word-wrap:normal; word-break:normal; font-weight:bold">'</span>+<span style="word-wrap:normal; word-break:normal; font-weight:bold">'</span>&nbsp;2<br> 5</td> 
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**运算符号都要加引号** ,单引号双引号均可  
**输出是结果**  
  
  
 **为什么if expr 3 '+' 2 得5，但if仍走then ? 非0应该走else啊**  
因为 **expr表达式返回的是"返回值"，不是"结果"**

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   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> [mac@machome ~]$ vi test.sh<br> expr 3 '+' 2<br> echo $?</td> 
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   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> [mac@machome ~]$ sh test.sh<br> <span style="word-wrap:normal; word-break:normal; font-weight:bold">5&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;输出5</span><br style="font-weight:bold"> <span style="word-wrap:normal; word-break:normal; font-weight:bold">0&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;返回0</span>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span style="word-wrap:normal; word-break:normal; font-weight:bold">只要加法成功，返回值就是0，0为真</span></td> 
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**expr 表达式计算完后如何获得结果？——** **用 $(expr  … ) 或反引号\` expr  …\`**  
 其实就是取其输出，expr的输出就是结果

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   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> var = expr $a '+' $b<br> echo "final is $var"</td> 
   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> var=$(expr $a '+' $b)<br> echo "final is $var"</td> 
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   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> test.sh: line 7: var: command not found&nbsp;&nbsp;<br> 加法完成后结果赋值出错<br> final is &nbsp;</td> 
   <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> final is 7</td> 
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**expr在if语句里，不能用 \[ \]，因为这里把它当作command**

改成

不过如果要进行结果判定，加$(),就必须用\[ \]了, **因为把它当作表达式了**

**另一种运算格式i++如何实现？------ i=$(($i+1)** )

**(( ))与\[ \]作用完全相同**

**反面证明(( ))与\[ \]完全相同--------if (( ))**