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  每天学习一个shell技能（6） - kaelinda

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          <h1>每天学习一个shell技能（6）</h1>

        <div class="read-more clearfix">
          <span class="date">2018/4/2</span>

          <span>posted in&nbsp;</span>

              <span class="posted-in"><a href='shell.html'>shell</a></span>


          <span class="comments">


          </span>

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      <div class="article-content">
      <h2 id="toc_0">test</h2>

<h3 id="toc_1">test文件</h3>

<pre><code class="language-text">if test -e ./test.sh
then
    echo &#39;文件已存在!&#39;
else
    echo &#39;文件不存在!&#39;
fi
</code></pre>

<ul>
<li>-e fileName ： 如果文件存在则为真</li>
<li>-r  fileName : 如果文件存在且可读则为真</li>
<li>-w fileName : 如果文件存在且可写则为真</li>
<li>-x fileName : 如果文件存在且可执行则为真</li>
<li>-s fileName: 文件存在且至少有一个字符</li>
<li>-d fileName : 文件存在且为目录</li>
<li>-f fileName: 文件存在且为文件</li>
<li>-c fileName:文件存在，且为字符型特殊文件</li>
<li>-b fileName: 如果文件存在且为块特殊文件则为真</li>
</ul>

<span id="more"></span><!-- more -->

<h3 id="toc_2">test字符串</h3>

<ul>
<li>= ：等于则为真</li>
<li>!= :不等于则为真</li>
<li>-z : 字符串长度为零则为真</li>
<li>-n : 字符串长度不为零则为真</li>
</ul>

<h3 id="toc_3">数值测试</h3>

<ul>
<li>-eq :等于</li>
<li>-ne : 不等于</li>
<li>-gt:大于</li>
<li>-lt:小于</li>
<li>-ge:大于等于</li>
<li>-le: 小于等于</li>
</ul>

<h2 id="toc_4">流程控制(条件)</h2>

<h3 id="toc_5">if</h3>

<pre><code class="language-text">if condition
then
    command1
    command2
    ...
    commandN
fi
</code></pre>

<h3 id="toc_6">if else</h3>

<pre><code class="language-text">if condition
then
    command1
    command2
    ...
    commandN
else
    command
fi
</code></pre>

<h3 id="toc_7">if else-if else</h3>

<pre><code class="language-text">if condition1
then
    command1
elif condition2
then
    command2
else
    commandN
fi
</code></pre>

<p>实例 用法：</p>

<pre><code class="language-text">a=10
b=20
if [ $a == $b ]
then
   echo &quot;a 等于 b&quot;
elif [ $a -gt $b ]
then
   echo &quot;a 大于 b&quot;
elif [ $a -lt $b ]
then
   echo &quot;a 小于 b&quot;
else
   echo &quot;没有符合的条件&quot;
fi
</code></pre>

<h2 id="toc_8">控制流（for 循环）</h2>

<pre><code class="language-text">for var in item1 item2 ... itemN
do
    command1
    command2
    ...
    commandN
done
</code></pre>

<p>改成一行：</p>

<pre><code class="language-text">for var in item1 item2 ... itemN; do command1; command2… done;
</code></pre>

<h2 id="toc_9">while</h2>

<pre><code class="language-text">while condition
do
    command
done
</code></pre>

<pre><code class="language-text">int=1
while(( $int&lt;=5 ))
do
    echo $int
    let &quot;int++&quot;
done
</code></pre>

<p>无限循环：</p>

<pre><code class="language-text">while :
do
    command
done

// 或者
while true
do
    command
done

// 或者
for (( ; ; ))

</code></pre>

<h2 id="toc_10">until 循环</h2>

<pre><code class="language-text">until condition
do
    command
done
</code></pre>

<h2 id="toc_11">case in</h2>

<pre><code class="language-text">case 值 in
模式1)
    command1
    command2
    ...
    commandN
    ;;
模式2）
    command1
    command2
    ...
    commandN
    ;;
esac
</code></pre>

<h2 id="toc_12">跳出循环</h2>

<h3 id="toc_13">break</h3>

<blockquote>
<p>break命令允许跳出所有循环（终止执行后面的所有循环）</p>
</blockquote>

<h3 id="toc_14">continue</h3>

<blockquote>
<p>continue命令与break命令类似，只有一点差别，它不会跳出所有循环，仅仅跳出当前循环</p>
</blockquote>

<pre><code class="language-text">while :
do
    echo -n &quot;输入 1 到 5 之间的数字: &quot;
    read aNum
    case $aNum in
        1|2|3|4|5) echo &quot;你输入的数字为 $aNum!&quot;
        ;;
        *) echo &quot;你输入的数字不是 1 到 5 之间的!&quot;
            continue
            echo &quot;游戏结束&quot;
        ;;
    esac
done
</code></pre>

<h3 id="toc_15">esac</h3>

<blockquote>
<p>case的语法和C family语言差别很大，它需要一个esac（就是case反过来）作为结束标记，每个case分支用右圆括号，用两个分号表示break。</p>
</blockquote>


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