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translating---geekpi
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Caffeinated 6.828: Exercise: Shell
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======
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This assignment will make you more familiar with the Unix system call interface and the shell by implementing several features in a small shell. You can do this assignment on any operating system that supports the Unix API (a Linux Athena machine, your laptop with Linux or Mac OS, etc.). Please submit your shell to the the [submission web site][1] at any time before the first lecture.
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While you shouldn't be shy about emailing the [staff mailing list][2] if you get stuck or don't understand something in this exercise, we do expect you to be able to handle this level of C programming on your own for the rest of the class. If you're not very familiar with C, consider this a quick check to see how familiar you are. Again, do feel encouraged to ask us for help if you have any questions.
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Download the [skeleton][3] of the xv6 shell, and look it over. The skeleton shell contains two main parts: parsing shell commands and implementing them. The parser recognizes only simple shell commands such as the following:
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```
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ls > y
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cat < y | sort | uniq | wc > y1
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cat y1
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rm y1
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ls | sort | uniq | wc
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rm y
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```
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Cut and paste these commands into a file `t.sh`
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You can compile the skeleton shell as follows:
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```
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$ gcc sh.c
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```
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which produces a file named `a.out`, which you can run:
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```
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$ ./a.out < t.sh
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```
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This execution will panic because you have not implemented several features. In the rest of this assignment you will implement those features.
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### Executing simple commands
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Implement simple commands, such as:
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```
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$ ls
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```
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The parser already builds an `execcmd` for you, so the only code you have to write is for the ' ' case in `runcmd`. To test that you can run "ls". You might find it useful to look at the manual page for `exec`; type `man 3 exec`.
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You do not have to implement quoting (i.e., treating the text between double-quotes as a single argument).
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### I/O redirection
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Implement I/O redirection commands so that you can run:
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```
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echo "6.828 is cool" > x.txt
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cat < x.txt
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```
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The parser already recognizes '>' and '<', and builds a `redircmd` for you, so your job is just filling out the missing code in `runcmd` for those symbols. Make sure your implementation runs correctly with the above test input. You might find the man pages for `open` (`man 2 open`) and `close` useful.
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Note that this shell will not process quotes in the same way that `bash`, `tcsh`, `zsh` or other UNIX shells will, and your sample file `x.txt` is expected to contain the quotes.
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### Implement pipes
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Implement pipes so that you can run command pipelines such as:
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```
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$ ls | sort | uniq | wc
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```
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The parser already recognizes "|", and builds a `pipecmd` for you, so the only code you must write is for the '|' case in `runcmd`. Test that you can run the above pipeline. You might find the man pages for `pipe`, `fork`, `close`, and `dup` useful.
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Now you should be able the following command correctly:
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```
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$ ./a.out < t.sh
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```
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Don't forget to submit your solution to the [submission web site][1], with or without challenge solutions.
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### Challenge exercises
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If you'd like to experiment more, you can add any feature of your choice to your shell. You might try one of the following suggestions:
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* Implement lists of commands, separated by `;`
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* Implement subshells by implementing `(` and `)`
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* Implement running commands in the background by supporting `&` and `wait`
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* Implement quoting of arguments
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All of these require making changing to the parser and the `runcmd` function.
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--------------------------------------------------------------------------------
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via: https://sipb.mit.edu/iap/6.828/lab/shell/
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作者：[mit][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://sipb.mit.edu
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[1]:https://exokernel.scripts.mit.edu/submit/
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[2]:mailto:sipb-iap-6.828@mit.edu
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[3]:https://sipb.mit.edu/iap/6.828/files/sh.c
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Caffeinated 6.828：练习 shell
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======
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通过在 shell 中实现多项功能，该作业将使你更加熟悉 Unix 系统调用接口和 shell。你可以在支持 Unix API 的任何操作系统（一台 Linux Athena 机器、装有 Linux 或 Mac OS 的笔记本电脑等）上完成此作业。请在第一次上课前将你的 shell 提交到[网站][1]。
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如果你在练习中遇到困难或不理解某些内容时，你不要害羞给[员工邮件列表][2]发送邮件，但我们确实希望全班的人能够自行处理这级别的 C 编程。如果你对 C 不是很熟悉，可以认为这个是你对 C 熟悉程度的检查。再说一次，如果你有任何问题，鼓励你向我们寻求帮助。
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下载 xv6 shell 的[框架][3]，然后查看它。框架 shell 包含两个主要部分：解析 shell 命令并实现它们。解析器只能识别简单的 shell 命令，如下所示：
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```
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ls > y
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cat < y | sort | uniq | wc > y1
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cat y1
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rm y1
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ls | sort | uniq | wc
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rm y
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```
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将这些命令剪切并粘贴到 `t.sh `中。
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你可以按如下方式编译框架 shell：
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```
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$ gcc sh.c
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```
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它会生成一个名为 `a.out` 的文件，你可以运行它：
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```
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$ ./a.out < t.sh
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```
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执行会崩溃，因为你还没有实现几个功能。在本作业的其余部分中，你将实现这些功能。
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### 执行简单的命令
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实现简单的命令，例如：
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```
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$ ls
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```
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解析器已经为你构建了一个 `execcmd`，所以你唯一需要编写的代码是 `runcmd` 中的 case ' '。要测试你可以运行 “ls”。你可能会发现查看 `exec` 的手册页是很有用的。输入 `man 3 exec`。
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你不必实现引用（即将双引号之间的文本视为单个参数）。
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### I/O 重定向
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实现 I/O 重定向命令，这样你可以运行：
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```
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echo "6.828 is cool" > x.txt
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cat < x.txt
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```
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解析器已经识别出 '>' 和 '<'，并且为你构建了一个 `redircmd`，所以你的工作就是在 `runcmd` 中为这些符号填写缺少的代码。确保你的实现在上面的测试输入中正确运行。你可能会发现 `open`（`man 2 open`） 和 `close` 的 man 手册页很有用。
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请注意，此 shell 不会像 `bash`、`tcsh`、`zsh` 或其他 UNIX shell 那样处理引号，并且你的示例文件 `x.txt` 预计包含引号。
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### 实现管道
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实现管道，这样你可以运行命令管道，例如：
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```
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$ ls | sort | uniq | wc
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```
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解析器已经识别出 “|”，并且为你构建了一个 `pipecmd`，所以你必须编写的唯一代码是 `runcmd` 中的 case '|'。测试你可以运行上面的管道。你可能会发现 `pipe`、`fork`、`close` 和 `dup` 的 man 手册页很有用。
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现在你应该可以正确地使用以下命令：
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```
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$ ./a.out < t.sh
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```
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无论是否完成挑战任务，不要忘记将你的答案提交给[网站][1]。
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### 挑战练习
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如果你想进一步尝试，可以将所选的任何功能添加到你的 shell。你可以尝试以下建议之一：
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* 实现由 `;` 分隔的命令列表
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* 通过实现 `(` 和 `)` 来实现子 shell
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* 通过支持 `＆` 和 `wait` 在后台执行命令
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* 实现参数引用
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所有这些都需要改变解析器和 `runcmd` 函数。
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--------------------------------------------------------------------------------
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via: https://sipb.mit.edu/iap/6.828/lab/shell/
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作者：[mit][a]
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译者：[geekpi](https://github.com/geekpi)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://sipb.mit.edu
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[1]:https://exokernel.scripts.mit.edu/submit/
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[2]:mailto:sipb-iap-6.828@mit.edu
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[3]:https://sipb.mit.edu/iap/6.828/files/sh.c
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