Spark修煉之道（基礎篇）——Linux大資料開發基礎：第十節：Shell程式設計入門（二)

參考書目：linux shell scripting cookbook

本節主要內容

1. shell命令列中的基本運算
2. 檔案描述符與檔案重定向

1. shell命令列中的基本運算

通過上一節我們知道，Linux Bash shell 命令列的變數都被解析成字串，如

<code class="hljs perl has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">//變數被解析為字串
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span>:~<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">/ShellLearning/chapter</span>09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># first=1</span>
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span>:~<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">/ShellLearning/chapter</span>09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># second=2</span>

//並不會進行加法操作，而是兩個字串拼接
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span>:~<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">/ShellLearning/chapter</span>09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># $first+$second</span>
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>+<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>: command <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">not</span> found
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

那如何要進行基本的加減乘除等運算，怎麼辦呢？有三種命令可以做到，它們是let, (( )), and []。先來看一下let命令

<code class="hljs coffeescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span>使用<span class="hljs-reserved" style="box-sizing: border-box;">let</span>命令，會進行加法運算
root<span class="hljs-property" style="box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># let result=first+second</span>

root<span class="hljs-property" style="box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># echo $result</span>
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

let命令還提供c、c++風格的自增、自減等操作，如：

<code class="hljs coffeescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">root<span class="hljs-property" style="box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># first=2</span>
root<span class="hljs-property" style="box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># $first</span>
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>: command <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">not</span> found
<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span>自增操作
root<span class="hljs-property" style="box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># let first++</span>
root<span class="hljs-property" style="box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># echo ${first}</span>
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>
//自減操作
root<span class="hljs-property" style="box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># let first--</span>
root<span class="hljs-property" style="box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># echo ${first}</span>
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>
//相當於<span class="hljs-reserved" style="box-sizing: border-box;">let</span> first=first+<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>
root<span class="hljs-property" style="box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># let first+=10</span>
root<span class="hljs-property" style="box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># echo ${first}</span>
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12</span>
//相當於<span class="hljs-reserved" style="box-sizing: border-box;">let</span> first=first-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>，其它操作如乘、除類似
root<span class="hljs-property" style="box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># let first-=10</span>
root<span class="hljs-property" style="box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># echo ${first}</span>
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul>

[]命令的功能與let命令類似，如

<code class="hljs perl has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span>:~<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">/ShellLearning/chapter</span>09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># first=5</span>
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span>:~<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">/ShellLearning/chapter</span>09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># second=6</span>
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span>:~<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">/ShellLearning/chapter</span>09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># result=$[first+second]</span>
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span>:~<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">/ShellLearning/chapter</span>09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># echo $result </span>
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">11</span>
//result=<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$[</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$first</span>+<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$second</span>]與result=<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$[</span>first+second]等同
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span>:~<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">/ShellLearning/chapter</span>09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># result=$[$first+$second]</span>
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span>:~<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">/ShellLearning/chapter</span>09<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># echo $result </span>
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">11</span>

</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>

也可以使用(( )) 命令進行，如：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:~/ShellLearning/chapter09</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># reslut=$((first+second))</span>
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:~/ShellLearning/chapter09</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># echo $result </span>
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">11</span>
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

需要注意的是上述命令只對整型數值有效，不適用於浮點數

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:~/ShellLearning/chapter09</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># result=$[first+second]</span>
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">bash:</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5.5</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span> syntax <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">error:</span> invalid arithmetic operator (error token is <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">".5"</span>)

root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:~/ShellLearning/chapter09</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># let resul=first+second</span>
<span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">bash:</span> <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">let:</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5.5</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:</span> syntax <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">error:</span> invalid arithmetic operator (error token is <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">".5"</span>)

</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

如果有浮點數參與運算，可以將echo與bc命令結合起來使用，程式碼如下：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:~/ShellLearning/chapter09</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># echo "$first+$second" | bc</span>
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12.0</span>
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:~/ShellLearning/chapter09</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># echo "$first*$second" | bc</span>
<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">35.7</span>
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

2. 檔案描述符與檔案重定向

在Linux作業系統當中，檔案描述符（File descriptors ）與檔案的輸入輸出相關，用整數表示，最常用的三種檔案描述符號為stdin、stdout及stderr。stdin表示標準輸入（standard input），檔案描述符為0；stdout表示標準輸出（standard output），檔案描述符為1；stderr表示標準錯誤（standard error），檔案描述為2。 
標準輸出指的是命令執行正常時顯示到終端的資訊，如

<code class="hljs ruleslanguage has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//stdout</span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-array" style="box-sizing: border-box;"># echo </span><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Shell Scripting 1"</span> 
Shell Scripting <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

標準錯誤指的是命令執行異常時顯示到終端的資訊，如

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">//<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">stderr</span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># cat ls</span>
cat: ls: No such <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">file</span> <span class="hljs-operator" style="box-sizing: border-box;">or</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">directory</span>
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

如果不想將這些資訊輸出到終端，而是輸出到檔案中儲存起來，這時就需要用到重定向，具體程式碼如下：

<code class="hljs ruleslanguage has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//利用>命令將標準輸出重定向輸出到檔案,>命令首先清空shell.txt檔案</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//然後將資訊寫到檔案當中，相當於覆蓋了以前檔案的內容</span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-array" style="box-sizing: border-box;"># echo </span><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Shell Scripting 1"</span> > shell.txt
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//以追加的方式將標準輸出重定向到檔案</span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-array" style="box-sizing: border-box;"># echo </span><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Shell Scripting 2"</span> >> shell.txt
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-array" style="box-sizing: border-box;"># cat shell.txt </span>
Shell Scripting <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>
Shell Scripting <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

剛提到echo “Shell Scripting 1” > shell.txt命令是將標準輸出（檔案描述符為1）重定向到檔案shell.txt當中，它其實相當於echo “Shell Scripting 1” 1> shell.txt，只不過預設可以省略：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:~/ShellLearning/chapter10</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># echo "Shell Scripting 1" 1> shell2.txt </span>
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:~/ShellLearning/chapter10</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># cat shell2.txt </span>
<span class="hljs-constant" style="box-sizing: border-box;">Shell</span> <span class="hljs-constant" style="box-sizing: border-box;">Scripting</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

同樣標準錯誤輸出也可以重定向到檔案當中，與標準輸出重定向不同的是，其檔案描述符不能省略

<code class="hljs mel has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//2>將標準錯誤資訊重定向到檔案中，這裡的檔案描述符2不能省略</span>
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter10# cat <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">ls</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>> shellError.txt
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter10# cat shellError.txt 
cat: <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">ls</span>: No such <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">file</span> or directory

<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//標準錯誤的檔案描述符2不能省略，因為省略的話預設是標準輸出</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//會將錯誤資訊輸出到終端</span>
root<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@sparkmaster</span>:~/ShellLearning/chapter10# cat <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">ls</span> > shellError.txt
cat: <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">ls</span>: No such <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">file</span> or directory
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

重定向時可以根據將重定向命令結合起來使用，如

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">//cmd命令無效，會產生標準錯誤，此時錯誤資訊會重定向到檔案<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">stderr</span>.txt檔案當中
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># cmd 2>stderr.txt 1>stdout.txt</span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># cat stderr.txt </span>
No <span class="hljs-command" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">command</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cmd'</span> <span class="hljs-title" style="box-sizing: border-box;">found</span>, <span class="hljs-title" style="box-sizing: border-box;">did</span> <span class="hljs-title" style="box-sizing: border-box;">you</span> <span class="hljs-title" style="box-sizing: border-box;">mean</span>:</span>
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'dcmd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'devscripts'</span> (main)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'wmd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'wml'</span> (universe)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'tcmd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'tcm'</span> (universe)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cmp'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'diffutils'</span> (main)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'qcmd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'renameutils'</span> (universe)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'mmd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'mtools'</span> (main)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cm'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'config-manager'</span> (universe)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'mcd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'mtools'</span> (main)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'icmd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'renameutils'</span> (universe)
cmd: <span class="hljs-command" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">command</span> <span class="hljs-title" style="box-sizing: border-box;">not</span> <span class="hljs-title" style="box-sizing: border-box;">found</span></span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
//stdout.txt中無內容</span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># cat stdout.txt </span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
//ls命令合法，會產生標準輸出，此時會被重定向到stdout.txt檔案當中</span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># ls 2>stderr.txt 1>stdout.txt </span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># cat stdout.txt </span>
shell2.txt
shellError.txt
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">shell</span>.txt
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">stderr</span>.txt
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">stdout</span>.txt
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li></ul>

在實際使用時，有些時候可能會將標準輸出與標準錯誤輸出都重定向到一個檔案，此時可以使用下列命令

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">//&>將標準錯誤輸出轉換為標準輸出，相當於<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>>&<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># cmd &> output.txt</span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># cat output.txt </span>
No <span class="hljs-command" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">command</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cmd'</span> <span class="hljs-title" style="box-sizing: border-box;">found</span>, <span class="hljs-title" style="box-sizing: border-box;">did</span> <span class="hljs-title" style="box-sizing: border-box;">you</span> <span class="hljs-title" style="box-sizing: border-box;">mean</span>:</span>
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'dcmd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'devscripts'</span> (main)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'wmd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'wml'</span> (universe)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'tcmd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'tcm'</span> (universe)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cmp'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'diffutils'</span> (main)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'qcmd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'renameutils'</span> (universe)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'mmd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'mtools'</span> (main)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'cm'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'config-manager'</span> (universe)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'mcd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'mtools'</span> (main)
 Command <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'icmd'</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">from</span> package <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'renameutils'</span> (universe)
cmd: <span class="hljs-command" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">command</span> <span class="hljs-title" style="box-sizing: border-box;">not</span> <span class="hljs-title" style="box-sizing: border-box;">found</span></span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># ls &>output.txt </span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># cat output.txt </span>
output.txt
shell2.txt
shellError.txt
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">shell</span>.txt
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">stderr</span>.txt
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">stdout</span>.txt
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li></ul>

有時命令執行時，對於出錯資訊我們並不關心，又不想浪費儲存空間儲存這些錯誤資訊，此時可以將其丟棄，具體做法是將標準錯誤輸出重定向到/dev/null檔案當中，/dev/null就像一個垃圾黑洞

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//將錯誤資訊丟棄</span>
root@sparkmaster:~<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">/ShellLearning/</span>chapter10# cmd <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>> <span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">/dev/</span><span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">null</span>
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

標準錯誤輸出或標準輸出還可以作為管道命令的標準輸入，例如：

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">//標準輸出作為另外一個命令的標準輸入
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># cat stdout.txt | more</span>
shell2.txt
shellError.txt
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">shell</span>.txt
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">stderr</span>.txt
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">stdout</span>.txt<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
//標準錯誤輸出作為另一個命令的標準輸入</span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;"># ls + | more</span>
ls: cannot access +: No such <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">file</span> <span class="hljs-operator" style="box-sizing: border-box;">or</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">directory</span>
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>

有時我們既想將標準錯誤輸出或標準輸出重定向到一個檔案當中，又想它作為另外一個命令的標準輸入，這時可以使用tee命令

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">//標準輸出重定向到檔案teeStdout<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>當中，同時又作為more的標準輸入
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"># ls  | tee teeStdout.txt | more</span>
output<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
shell2<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
shellError<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
shell<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
stderr<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
stdout<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
teeStdout<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
root@sparkmaster:~/ShellLearning/chapter10<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"># cat teeStdout.txt </span>
output<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
shell2<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
shellError<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
shell<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
stderr<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
stdout<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
teeStdout<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.txt</span>
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li></ul>