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linux基础/shell中的特殊替换结构.html
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linux基础/shell中的特殊替换结构.html
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<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<meta name="tool" content="leanote-desktop-app">
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<title>shell中的特殊替换结构</title>
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<body>
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<div class="note-container">
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<h1 class="title" id="leanote-title">shell中的特殊替换结构</h1>
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<div class="content-html" id="leanote-content"><p>A,${var:-string}和${var:=string}<br>若变量var为空，则用在命令行中用string来替换${var:-string}，否则变量var不为空时，则用变量var的值来替换${var:-string}<br>如： <br>$ echo $newvar</p><p>$ echo ${newvar:-a} <br>a <br>$ echo $newvar ###变量newvar的值仍然是空，但上一命令行中${newvar:-a}被替换成了a</p><p>$ newvar=b <br>$ echo ${newvar:-a} ###变量newvar的值不为空时，此命令行中的${newvar:-b}被替换为$newvar，即b <br>b <br>$</p><p>对于${var:=string}的替换规则和${var:-string}是一样的，所不同之处是${var:=string}若var为空时，用string替换${var:=string}的同时，把string赋给变量var：</p><p><br>$ echo $newvar</p><p>$ echo ${newvar:=a} <br>a <br>$ echo $newvar ###变量newvar被赋值为a，同时${newvar:=a}被替换成a <br>a <br>$ echo ${newvar:=b} ###变量newvar不为空(其值已被赋为a)，则${newvar:=b}被替换为newvar的值(即b)<br>a <br>$ echo $newvar <br>a</p><p>${var:=string}很常用的一种用法是，判断某个变量是否赋值，没有的话则给它赋上一个默认值。<br>如设置默认的编辑器： <br>PHP 代码:<br>echo You use editor: ${EDITOR:=/bin/vi}</p><p>B,${var:+string}<br>${var:+string}的替换规则和上面的相反，即只有当var不是空的时候才替换成string，若var为空时则不替换或者说是替换成变量 var的值，即空值。(因为变量var此时为空，所以这两种说法是等价的) <br>$ echo $newvar <br>a <br>$ echo ${newvar:+b} <br>b <br>$ echo $newvar <br>a <br>$ newvar= <br>$ echo ${newvar:+b}</p><p>$</p><p>C,${var:?string}<br>替换规则为：若变量var不为空，则用变量var的值来替换${var:?string}；若变量var为空，则把string输出到标准错误中，并从脚本中退出。我们可利用此特性来检查是否设置了变量的值。 <br>$ newvar= <br>$ echo ${newvar:?没有设置newvar的值} <br>bash: newvar: 没有设置newvar的值 <br>$ newvar=a <br>$ echo ${newvar:?没有设置newvar的值} <br>a <br>$</p><p>补充扩展：在上面这五种替换结构中string不一定是常值的，可用另外一个变量的值或是一种命令的输出。 <br>$ echo ${var:-`date`} <br>日 3月 6 02:10:39 CST 2005 <br>$ echo ${var:-$(date)} <br>日 3月 6 02:11:46 CST 2005 <br>$ a=test <br>$ echo ${var:-$a} <br>test <br>$</p><p><br></p></div>
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