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[#]: subject: (Manipulate data in files with Lua)
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[#]: via: (https://opensource.com/article/21/3/lua-files)
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[#]: author: (Seth Kenlon https://opensource.com/users/seth)
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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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Manipulate data in files with Lua
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======
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Understand how Lua handles reading and writing data.
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![Person standing in front of a giant computer screen with numbers, data][1]
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Some data is ephemeral, stored in RAM, and only significant while an application is running. But some data is meant to be persistent, stored on a hard drive for later use. When you program, whether you're working on a simple script or a complex suite of tools, it's common to need to read and write files. Sometimes a file may contain configuration options, and other times the file is the data that your user is creating with your application. Every language handles this task a little differently, and this article demonstrates how to handle data files with Lua.
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### Installing Lua
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If you're on Linux, you can install Lua from your distribution's software repository. On macOS, you can install Lua from [MacPorts][2] or [Homebrew][3]. On Windows, you can install Lua from [Chocolatey][4].
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Once you have Lua installed, open your favorite text editor and get ready to code.
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### Reading a file with Lua
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Lua uses the `io` library for data input and output. The following example creates a function called `ingest` to read data from a file and then parses it with the `:read` function. When opening a file in Lua, there are several modes you can enable. Because I just need to read data from this file, I use the `r` (for "read") mode:
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```
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function ingest(file)
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local f = io.open(file, "r")
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local lines = f:read("*all")
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f:close()
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return(lines)
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end
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myfile=ingest("example.txt")
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print(myfile)
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```
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In the code, notice that the variable `myfile` is created to trigger the `ingest` function, and therefore, it receives whatever that function returns. The `ingest` function returns the lines (from a variable intuitively called `lines`) of the file. When the contents of the `myfile` variable are printed in the final step, the lines of the file appear in the terminal.
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If the file `example.txt` contains configuration options, then I would write some additional code to parse that data, probably using another Lua library depending on whether the configuration was stored as an INI file or YAML file or some other format. If the data were an SVG graphic, I'd write extra code to parse XML, probably using an SVG library for Lua. In other words, the data your code reads can be manipulated once it's loaded into memory, but all that's required to load it is the `io` library.
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### Writing data to a file with Lua
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Whether you're storing data your user is creating with your application or just metadata about what the user is doing in an application (for instance, game saves or recent songs played), there are many good reasons to store data for later use. In Lua, this is achieved through the `io` library by opening a file, writing data into it, and closing the file:
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```
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function exgest(file)
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local f = io.open(file, "a")
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io.output(f)
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io.write("hello world\n")
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io.close(f)
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end
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exgest("example.txt")
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```
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To read data from the file, I open the file in `r` mode, but this time I use `a` (for "append") to write data to the end of the file. Because I'm writing plain text into a file, I added my own newline character (`\n`). Often, you're not writing raw text into a file, and you'll probably use an additional library to write a specific format instead. For instance, you might use an INI or YAML library to help write configuration files, an XML library to write XML, and so on.
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### File modes
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When opening files in Lua, there are some safeguards and parameters to define how a file should be handled. The default is `r`, which permits you to read data only:
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* **r** for read only
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* **w** to overwrite or create a new file if it doesn't already exist
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* **r+** to read and overwrite
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* **a** to append data to a file or make a new file if it doesn't already exist
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* **a+** to read data, append data to a file, or make a new file if it doesn't already exist
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There are a few others (`b` for binary formats, for instance), but those are the most common. For the full documentation, refer to the excellent Lua documentation on [Lua.org/manual][5].
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### Lua and files
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Like other programming languages, Lua has plenty of library support to access a filesystem to read and write data. Because Lua has a consistent and simple syntax, it's easy to perform complex processing on data in files of any format. Try using Lua for your next software project, or as an API for your C or C++ project.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/3/lua-files
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作者：[Seth Kenlon][a]
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选题：[lujun9972][b]
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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://opensource.com/users/seth
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/data_metrics_analytics_desktop_laptop.png?itok=9QXd7AUr (Person standing in front of a giant computer screen with numbers, data)
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[2]: https://opensource.com/article/20/11/macports
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[3]: https://opensource.com/article/20/6/homebrew-mac
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[4]: https://opensource.com/article/20/3/chocolatey
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[5]: http://lua.org/manual
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@ -0,0 +1,97 @@
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[#]: subject: (Manipulate data in files with Lua)
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[#]: via: (https://opensource.com/article/21/3/lua-files)
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[#]: author: (Seth Kenlon https://opensource.com/users/seth)
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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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用 Lua 操作文件中的数据
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======
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了解 Lua 如何处理数据的读写。
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![Person standing in front of a giant computer screen with numbers, data][1]
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有些数据是临时的，存储在 RAM 中，只有在应用运行时才有意义。但有些数据是要持久的，存储在硬盘上供以后使用。当你编程时，无论是简单的脚本还是复杂的工具套件，通常都需要读取和写入文件。有时文件可能包含配置选项，而另一些时候这个文件是你的用户用你的应用创建的数据。每种语言都会以不同的方式处理这项任务，本文将演示如何使用 Lua 处理文件数据。
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### 安装 Lua
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如果你使用的是 Linux，你可以从你的发行版软件库中安装 Lua。在 macOS 上，你可以从 [MacPorts][2] 或 [Homebrew][3] 安装 Lua。在 Windows 上，你可以从 [Chocolatey][4] 安装 Lua。
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安装 Lua 后，打开你最喜欢的文本编辑器并准备开始。
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### 用 Lua 读取文件
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Lua 使用 `io` 库进行数据输入和输出。下面的例子创建了一个名为 `ingest` 的函数来从文件中读取数据，然后用 `:read` 函数进行解析。在 Lua 中打开一个文件时，有几种模式可以启用。因为我只需要从这个文件中读取数据，所以我使用 `r`（代表”读“）模式：
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```
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function ingest(file)
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local f = io.open(file, "r")
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local lines = f:read("*all")
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f:close()
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return(lines)
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end
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myfile=ingest("example.txt")
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print(myfile)
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```
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在这段代码中，注意到变量 `myfile` 是为了触发 `ingest` 函数而创建的，因此，它接收该函数返回的任何内容。`ingest` 函数返回文件的行数(从一个称为 `lines` 的变量中)。当最后一步打印 `myfile` 变量的内容时，文件的行数就会出现在终端中。
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如果文件 `example.txt` 中包含了配置选项，那么我会写一些额外的代码来解析这些数据，可能会使用另一个 Lua 库，这取决于配置是以 INI 文件还是 YAML 文件或其他格式存储。如果数据是 SVG 图形，我会写额外的代码来解析 XML，可能会使用 Lua 的 SVG 库。换句话说，你的代码读取的数据一旦加载到内存中，就可以进行操作，但是它们都需要加载 `io` 库。
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### 用 Lua 将数据写入文件
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无论你是要存储用户用你的应用创建的数据，还是仅仅是关于用户在应用中做了什么的元数据（例如，游戏保存或最近播放的歌曲），都有很多很好的理由来存储数据供以后使用。在 Lua 中，这是通过 `io` 库实现的，打开一个文件，将数据写入其中，然后关闭文件：
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```
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function exgest(file)
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local f = io.open(file, "a")
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io.output(f)
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io.write("hello world\n")
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io.close(f)
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end
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exgest("example.txt")
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```
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为了从文件中读取数据，我以 `r` 模式打开文件，但这次我使用 `a` （用于”追加“）将数据写到文件的末尾。因为我是将纯文本写入文件，所以我添加了自己的换行符（`/n`）。通常情况下，你并不是将原始文本写入文件，你可能会使用一个额外的库来代替写入一个特定的格式。例如，你可能会使用 INI 或 YAML 库来帮助编写配置文件，使用 XML 库来编写 XML，等等。
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### 文件模式
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在 Lua 中打开文件时，有一些保护措施和参数来定义如何处理文件。默认值是 `r`，允许你只读数据：
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* **r** 只读
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* **w** 如果文件不存在，覆盖或创建一个新文件。
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* **r+** 读取和覆盖。
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* **a** 追加数据到文件中，或在文件不存在的情况下创建一个新文件。
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* **a+** 读取数据，将数据追加到文件中，或文件不存在的话，创建一个新文件。
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还有一些其他的(例如，`b` 代表二进制格式)，但这些是最常见的。关于完整的文档，请参考 [Lua.org/manual][5] 上的优秀 Lua 文档。
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### Lua 和文件
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和其他编程语言一样，Lua 有大量的库支持来访问文件系统来读写数据。因为 Lua 有一个一致且简单语法，所以很容易对任何格式的文件数据进行复杂的处理。试着在你的下一个软件项目中使用 Lua，或者作为 C 或 C++ 项目的 API。
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/3/lua-files
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作者：[Seth Kenlon][a]
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选题：[lujun9972][b]
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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://opensource.com/users/seth
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/data_metrics_analytics_desktop_laptop.png?itok=9QXd7AUr (Person standing in front of a giant computer screen with numbers, data)
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[2]: https://opensource.com/article/20/11/macports
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[3]: https://opensource.com/article/20/6/homebrew-mac
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[4]: https://opensource.com/article/20/3/chocolatey
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[5]: http://lua.org/manual
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