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Translated by cposture (#4187)
* Translated by cposture * Translating by cposture * translating partly * translating partly 75 * Translated by cposture * Translated by cposture * Translating by cposture * Translating partly 50 * Translting partly 70 * Translating partly 75:wq * Translated by cposture * Translated by cposture
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陈家启
Ezio
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Translating by cposture 2016.07.09
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Create Your Own Shell in Python - Part II
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===========================================
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In [part 1][1], we already created a main shell loop, tokenized command input, and executed a command by fork and exec. In this part, we will solve the remaining problmes. First, `cd test_dir2` does not change our current directory. Second, we still have no way to exit from our shell gracefully.
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### Step 4: Built-in Commands
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The statement “cd test_dir2 does not change our current directory” is true and false in some senses. It’s true in the sense that after executing the command, we are still at the same directory. However, the directory is actullay changed, but, it’s changed in the child process.
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Remember that we fork a child process, then, exec the command which does not happen on a parent process. The result is we just change the current directory of a child process, not the directory of a parent process.
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Then, the child process exits, and the parent process continues with the same intact directory.
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Therefore, this kind of commands must be built-in with the shell itself. It must be executed in the shell process without forking.
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#### cd
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Let’s start with cd command.
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We first create a builtins directory. Each built-in command will be put inside this directory.
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```
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yosh_project
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|-- yosh
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|-- builtins
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| |-- __init__.py
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| |-- cd.py
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|-- __init__.py
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|-- shell.py
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```
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In cd.py, we implement our own cd command by using a system call os.chdir.
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```
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import os
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from yosh.constants import *
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def cd(args):
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os.chdir(args[0])
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return SHELL_STATUS_RUN
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```
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Notice that we return shell running status from a built-in function. Therefore, we move constants into yosh/constants.py to be used across the project.
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```
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yosh_project
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|-- yosh
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|-- builtins
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| |-- __init__.py
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| |-- cd.py
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|-- __init__.py
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|-- constants.py
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|-- shell.py
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```
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In constants.py, we put shell status constants here.
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```
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SHELL_STATUS_STOP = 0
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SHELL_STATUS_RUN = 1
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```
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Now, our built-in cd is ready. Let’s modify our shell.py to handle built-in functions.
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```
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...
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# Import constants
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from yosh.constants import *
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# Hash map to store built-in function name and reference as key and value
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built_in_cmds = {}
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def tokenize(string):
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return shlex.split(string)
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def execute(cmd_tokens):
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# Extract command name and arguments from tokens
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cmd_name = cmd_tokens[0]
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cmd_args = cmd_tokens[1:]
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# If the command is a built-in command, invoke its function with arguments
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if cmd_name in built_in_cmds:
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return built_in_cmds[cmd_name](cmd_args)
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...
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```
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We use a Python dictionary built_in_cmds as a hash map to store our built-in functions. In execute function, we extract command name and arguments. If the command name is in our hash map, we call that built-in function.
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(Note: built_in_cmds[cmd_name] returns the function reference that can be invoked with arguments immediately.)
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We are almost ready to use the built-in cd function. The last thing is to add cd function into the built_in_cmds map.
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```
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...
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# Import all built-in function references
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from yosh.builtins import *
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...
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# Register a built-in function to built-in command hash map
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def register_command(name, func):
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built_in_cmds[name] = func
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# Register all built-in commands here
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def init():
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register_command("cd", cd)
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def main():
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# Init shell before starting the main loop
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init()
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shell_loop()
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```
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We define register_command function for adding a built-in function to our built-in commmand hash map. Then, we define init function and register the built-in cd function there.
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Notice the line register_command("cd", cd). The first argument is a command name. The second argument is a reference to a function. In order to let cd, in the second argument, refer to the cd function reference in yosh/builtins/cd.py, we have to put the following line in yosh/builtins/__init__.py.
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```
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from yosh.builtins.cd import *
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```
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Therefore, in yosh/shell.py, when we import * from yosh.builtins, we get cd function reference that is already imported by yosh.builtins.
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We’ve done preparing our code. Let’s try by running our shell as a module python -m yosh.shell at the same level as the yosh directory.
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Now, our cd command should change our shell directory correctly while non-built-in commands still work too. Cool.
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#### exit
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Here comes the last piece: to exit gracefully.
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We need a function that changes the shell status to be SHELL_STATUS_STOP. So, the shell loop will naturally break and the shell program will end and exit.
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As same as cd, if we fork and exec exit in a child process, the parent process will still remain inact. Therefore, the exit function is needed to be a shell built-in function.
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Let’s start by creating a new file called exit.py in the builtins folder.
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```
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yosh_project
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|-- yosh
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|-- builtins
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| |-- __init__.py
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| |-- cd.py
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| |-- exit.py
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|-- __init__.py
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|-- constants.py
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|-- shell.py
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```
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The exit.py defines the exit function that just returns the status to break the main loop.
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```
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from yosh.constants import *
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def exit(args):
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return SHELL_STATUS_STOP
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```
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Then, we import the exit function reference in `yosh/builtins/__init__.py`.
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```
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from yosh.builtins.cd import *
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from yosh.builtins.exit import *
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```
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Finally, in shell.py, we register the exit command in `init()` function.
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```
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...
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# Register all built-in commands here
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def init():
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register_command("cd", cd)
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register_command("exit", exit)
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...
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```
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That’s all!
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Try running python -m yosh.shell. Now you can enter exit to quit the program gracefully.
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### Final Thought
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I hope you enjoy creating yosh (your own shell) like I do. However, my version of yosh is still in an early stage. I don’t handle several corner cases that can corrupt the shell. There are a lot of built-in commands that I don’t cover. Some non-built-in commands can also be implemented as built-in commands to improve performance (avoid new process creation time). And, a ton of features are not yet implemented (see Common features and Differing features).
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I’ve provided the source code at github.com/supasate/yosh. Feel free to fork and play around.
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Now, it’s your turn to make it real Your Own SHell.
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Happy Coding!
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--------------------------------------------------------------------------------
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via: https://hackercollider.com/articles/2016/07/06/create-your-own-shell-in-python-part-2/
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作者：[Supasate Choochaisri][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://disqus.com/by/supasate_choochaisri/
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[1]: https://hackercollider.com/articles/2016/07/05/create-your-own-shell-in-python-part-1/
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[2]: http://tldp.org/LDP/Bash-Beginners-Guide/html/x7243.html
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[3]: http://www.tldp.org/LDP/intro-linux/html/x12249.html
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[4]: https://github.com/supasate/yosh
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@ -0,0 +1,216 @@
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使用 Python 创建你自己的 Shell:Part II
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===========================================
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在[part 1][1] 中，我们已经创建了一个主要的 shell 循环、切分了的命令输入，以及通过 `fork` 和 `exec` 执行命令。在这部分，我们将会解决剩下的问题。首先，`cd test_dir2` 命令无法修改我们的当前目录。其次，我们仍无法优雅地从 shell 中退出。
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### 步骤 4：内置命令
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“cd test_dir2 无法修改我们的当前目录” 这句话是对的，但在某种意义上也是错的。在执行完该命令之后，我们仍然处在同一目录，从这个意义上讲，它是对的。然而，目录实际上已经被修改，只不过它是在子进程中被修改。
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还记得我们 fork 了一个子进程，然后执行命令，执行命令的过程没有发生在父进程上。结果是我们只是改变了子进程的当前目录，而不是父进程的目录。
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然后子进程退出，而父进程在原封不动的目录下继续运行。
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因此，这类与 shell 自己相关的命令必须是内置命令。它必须在 shell 进程中执行而没有分叉（forking）。
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#### cd
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让我们从 `cd` 命令开始。
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我们首先创建一个 `builtins` 目录。每一个内置命令都会被放进这个目录中。
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```shell
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yosh_project
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|-- yosh
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|-- builtins
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| |-- __init__.py
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| |-- cd.py
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|-- __init__.py
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|-- shell.py
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```
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在 `cd.py` 中，我们通过使用系统调用 `os.chdir` 实现自己的 `cd` 命令。
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```python
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import os
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from yosh.constants import *
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def cd(args):
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os.chdir(args[0])
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return SHELL_STATUS_RUN
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```
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注意，我们会从内置函数返回 shell 的运行状态。所以，为了能够在项目中继续使用常量，我们将它们移至 `yosh/constants.py`。
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```shell
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yosh_project
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|-- yosh
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|-- builtins
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| |-- __init__.py
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| |-- cd.py
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|-- __init__.py
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|-- constants.py
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|-- shell.py
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```
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在 `constants.py` 中，我们将状态常量都放在这里。
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```python
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SHELL_STATUS_STOP = 0
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SHELL_STATUS_RUN = 1
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```
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现在，我们的内置 `cd` 已经准备好了。让我们修改 `shell.py` 来处理这些内置函数。
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```python
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...
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# Import constants
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from yosh.constants import *
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# Hash map to store built-in function name and reference as key and value
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built_in_cmds = {}
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def tokenize(string):
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return shlex.split(string)
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def execute(cmd_tokens):
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# Extract command name and arguments from tokens
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cmd_name = cmd_tokens[0]
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cmd_args = cmd_tokens[1:]
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# If the command is a built-in command, invoke its function with arguments
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if cmd_name in built_in_cmds:
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return built_in_cmds[cmd_name](cmd_args)
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...
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```
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我们使用一个 python 字典变量 `built_in_cmds` 作为哈希映射（hash map），以存储我们的内置函数。我们在 `execute` 函数中提取命令的名字和参数。如果该命令在我们的哈希映射中，则调用对应的内置函数。
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（提示：`built_in_cmds[cmd_name]` 返回能直接使用参数调用的函数引用的。)
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我们差不多准备好使用内置的 `cd` 函数了。最后一步是将 `cd` 函数添加到 `built_in_cmds` 映射中。
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```
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...
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# Import all built-in function references
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from yosh.builtins import *
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...
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# Register a built-in function to built-in command hash map
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def register_command(name, func):
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built_in_cmds[name] = func
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# Register all built-in commands here
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def init():
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register_command("cd", cd)
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def main():
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# Init shell before starting the main loop
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init()
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shell_loop()
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```
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我们定义了 `register_command` 函数，以添加一个内置函数到我们内置的命令哈希映射。接着，我们定义 `init` 函数并且在这里注册内置的 `cd` 函数。
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注意这行 `register_command("cd", cd)` 。第一个参数为命令的名字。第二个参数为一个函数引用。为了能够让第二个参数 `cd` 引用到 `yosh/builtins/cd.py` 中的 `cd` 函数引用，我们必须将以下这行代码放在 `yosh/builtins/__init__.py` 文件中。
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```
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from yosh.builtins.cd import *
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```
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因此，在 `yosh/shell.py` 中，当我们从 `yosh.builtins` 导入 `*` 时，我们可以得到已经通过 `yosh.builtins` 导入的 `cd` 函数引用。
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我们已经准备好了代码。让我们尝试在 `yosh` 同级目录下以模块形式运行我们的 shell，`python -m yosh.shell`。
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现在，`cd` 命令可以正确修改我们的 shell 目录了，同时非内置命令仍然可以工作。非常好！
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#### exit
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最后一块终于来了：优雅地退出。
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我们需要一个可以修改 shell 状态为 `SHELL_STATUS_STOP` 的函数。这样，shell 循环可以自然地结束，shell 将到达终点而退出。
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和 `cd` 一样，如果我们在子进程中 fork 和执行 `exit` 函数，其对父进程是不起作用的。因此，`exit` 函数需要成为一个 shell 内置函数。
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让我们从这开始：在 `builtins` 目录下创建一个名为 `exit.py` 的新文件。
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```
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yosh_project
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|-- yosh
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|-- builtins
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| |-- __init__.py
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| |-- cd.py
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| |-- exit.py
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|-- __init__.py
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|-- constants.py
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|-- shell.py
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```
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`exit.py` 定义了一个 `exit` 函数，该函数仅仅返回一个可以退出主循环的状态。
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```
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from yosh.constants import *
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def exit(args):
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return SHELL_STATUS_STOP
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```
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然后，我们导入位于 `yosh/builtins/__init__.py` 文件的 `exit` 函数引用。
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```
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from yosh.builtins.cd import *
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from yosh.builtins.exit import *
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```
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最后，我们在 `shell.py` 中的 `init()` 函数注册 `exit` 命令。
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```
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...
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# Register all built-in commands here
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def init():
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register_command("cd", cd)
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register_command("exit", exit)
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...
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```
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到此为止！
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尝试执行 `python -m yosh.shell`。现在你可以输入 `exit` 优雅地退出程序了。
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### 最后的想法
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我希望你能像我一样享受创建 `yosh` （**y**our **o**wn **sh**ell）的过程。但我的 `yosh` 版本仍处于早期阶段。我没有处理一些会使 shell 崩溃的极端状况。还有很多我没有覆盖的内置命令。为了提高性能，一些非内置命令也可以实现为内置命令（避免新进程创建时间）。同时，大量的功能还没有实现（请看 [公共特性](http://tldp.org/LDP/Bash-Beginners-Guide/html/x7243.html) 和 [不同特性](http://www.tldp.org/LDP/intro-linux/html/x12249.html)）
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我已经在 github.com/supasate/yosh 中提供了源代码。请随意 fork 和尝试。
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现在该是创建你真正自己拥有的 Shell 的时候了。
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Happy Coding!
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--------------------------------------------------------------------------------
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||||
via: https://hackercollider.com/articles/2016/07/06/create-your-own-shell-in-python-part-2/
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作者：[Supasate Choochaisri][a]
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译者：[cposture](https://github.com/cposture)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: https://disqus.com/by/supasate_choochaisri/
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[1]: https://hackercollider.com/articles/2016/07/05/create-your-own-shell-in-python-part-1/
|
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[2]: http://tldp.org/LDP/Bash-Beginners-Guide/html/x7243.html
|
||||
[3]: http://www.tldp.org/LDP/intro-linux/html/x12249.html
|
||||
[4]: https://github.com/supasate/yosh
|
||||
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Reference in New Issue
Block a user