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[#]: subject: (Read and write files with Bash)
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[#]: via: (https://opensource.com/article/21/3/input-output-bash)
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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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Read and write files with Bash
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======
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Learn the different ways Bash reads and writes data and when to use each
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method.
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![bash logo on green background][1]
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When you're scripting with Bash, sometimes you need to read data from or write data to a file. Sometimes a file may contain configuration options, and other times the file is the data 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 Bash and other [POSIX][2] shells.
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### Install Bash
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If you're on Linux, you probably already have Bash. If not, you can find it in your software repository.
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On macOS, you can use the default terminal, either Bash or [Zsh][3], depending on the macOS version you're running.
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On Windows, there are several ways to experience Bash, including Microsoft's officially supported [Windows Subsystem for Linux][4] (WSL).
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Once you have Bash installed, open your favorite text editor and get ready to code.
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### Reading a file with Bash
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In addition to being [a shell][5], Bash is a scripting language. There are several ways to read data from Bash: You can create a sort of data stream and parse the output, or you can load data into memory. Both are valid methods of ingesting information, but each has pretty specific use cases.
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#### Source a file in Bash
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When you "source" a file in Bash, you cause Bash to read the contents of a file with the expectation that it contains valid data that Bash can fit into its established data model. You won't source data from any old file, but you can use this method to read configuration files and functions.
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For instance, create a file called `example.sh` and enter this into it:
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```
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#!/bin/sh
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greet opensource.com
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echo "The meaning of life is $var"
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```
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Run the code to see it fail:
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```
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$ bash ./example.sh
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./example.sh: line 3: greet: command not found
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The meaning of life is
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```
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Bash doesn't have a command called `greet`, so it could not execute that line, and it has no record of a variable called `var`, so there is no known meaning of life. To fix this problem, create a file called `include.sh`:
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```
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greet() {
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echo "Hello ${1}"
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}
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var=42
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```
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Revise your `example.sh` script to include a `source` command:
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```
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#!/bin/sh
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source include.sh
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greet opensource.com
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echo "The meaning of life is $var"
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```
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Run the script to see it work:
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```
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$ bash ./example.sh
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Hello opensource.com
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The meaning of life is 42
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```
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The `greet` command is brought into your shell environment because it is defined in the `include.sh` file, and it even recognizes the argument (`opensource.com` in this example). The variable `var` is set and imported, too.
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#### Parse a file in Bash
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The other way to get data "into" Bash is to parse it as a data stream. There are many ways to do this. You can use `grep` or `cat` or any command that takes data and pipes it to stdout. Alternately, you can use what is built into Bash: the redirect. Redirection on its own isn't very useful, so in this example, I also use the built-in `echo` command to print the results of the redirect:
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```
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#!/bin/sh
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echo $( < include.sh )
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```
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Save this as `stream.sh` and run it to see the results:
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```
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$ bash ./stream.sh
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greet() { echo "Hello ${1}" } var=42
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$
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```
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For each line in the `include.sh` file, Bash prints (or echoes) the line to your terminal. Piping it first to an appropriate parser is a common way to read data with Bash. For instance, assume for a moment that `include.sh` is a configuration file with key and value pairs separated by an equal (`=`) sign. You could obtain values with `awk` or even `cut`:
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```
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#!/bin/sh
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myVar=`grep var include.sh | cut -d'=' -f2`
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echo $myVar
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```
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Try running the script:
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```
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$ bash ./stream.sh
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42
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```
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### Writing data to a file with Bash
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Whether you're storing data your user created with your application or just metadata about what the user did in an application (for instance, game saves or recent songs played), there are many good reasons to store data for later use. In Bash, you can save data to files using common shell redirection.
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For instance, to create a new file containing output, use a single redirect token:
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```
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#!/bin/sh
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TZ=UTC
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date > date.txt
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```
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Run the script a few times:
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```
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$ bash ./date.sh
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$ cat date.txt
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Tue Feb 23 22:25:06 UTC 2021
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$ bash ./date.sh
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$ cat date.txt
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Tue Feb 23 22:25:12 UTC 2021
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```
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To append data, use the double redirect tokens:
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```
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#!/bin/sh
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TZ=UTC
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date >> date.txt
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```
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Run the script a few times:
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```
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$ bash ./date.sh
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$ bash ./date.sh
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$ bash ./date.sh
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$ cat date.txt
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Tue Feb 23 22:25:12 UTC 2021
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Tue Feb 23 22:25:17 UTC 2021
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Tue Feb 23 22:25:19 UTC 2021
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Tue Feb 23 22:25:22 UTC 2021
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```
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### Bash for easy programming
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Bash excels at being easy to learn because, with just a few basic concepts, you can build complex programs. For the full documentation, refer to the [excellent Bash documentation][6] on GNU.org.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/3/input-output-bash
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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/bash_command_line.png?itok=k4z94W2U (bash logo on green background)
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[2]: https://opensource.com/article/19/7/what-posix-richard-stallman-explains
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[3]: https://opensource.com/article/19/9/getting-started-zsh
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[4]: https://opensource.com/article/19/7/ways-get-started-linux#wsl
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[5]: https://www.redhat.com/sysadmin/terminals-shells-consoles
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[6]: http://gnu.org/software/bash
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200
translated/tech/20210324 Read and write files with Bash.md
Normal file
200
translated/tech/20210324 Read and write files with Bash.md
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@ -0,0 +1,200 @@
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[#]: subject: (Read and write files with Bash)
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[#]: via: (https://opensource.com/article/21/3/input-output-bash)
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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 Bash 读写文件
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======
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学习 Bash 读取和写入数据的不同方式，以及何时使用每种方法。
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![bash logo on green background][1]
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当你使用 Bash 编写脚本时，有时你需要从一个文件中读取数据或向一个文件写入数据。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。每种语言处理这个任务的方式都有些不同，本文将演示如何使用 Bash 和其他 [POSIX][2] shell 处理数据文件。
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### 安装 Bash
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如果你在使用 Linux，你可能已经有了 Bash。如果没有，你可以在你的软件仓库里找到它。
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在 macOS 上，你可以使用默认终端，Bash 或 [Zsh][3]，这取决于你运行的 macOS 版本。
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在 Windows 上，有几种方法可以体验 Bash，包括微软官方支持的 [Windows Subsystem for Linux][4]（WSL）。
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安装 Bash 后，打开你最喜欢的文本编辑器并准备开始。
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### 使用 Bash 读取文件
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除了是 [shell][5] 之外，Bash 还是一种脚本语言。有几种方法可以从 Bash 中读取数据。你可以创建一种数据流并解析输出, 或者你可以将数据加载到内存中. 这两种方法都是有效的获取信息的方法，但每种方法都有相当具体的用例。
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#### 在 Bash 中 source 文件
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当你在 Bash 中 “source” 一个文件时，你会让 Bash 读取文件的内容，期望它包含有效的数据，Bash 可以将这些数据放入它建立的数据模型中。你不会从任何旧文件中获取数据，但你可以使用这种方法来读取配置文件和函数。
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例如，创建一个名为 `example.sh` 的文件，并输入以下内容：
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```
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#!/bin/sh
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greet opensource.com
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echo "The meaning of life is $var"
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```
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运行这段代码，看见失败了：
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```
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$ bash ./example.sh
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./example.sh: line 3: greet: command not found
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The meaning of life is
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```
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Bash 没有一个叫 `greet` 的命令，所以无法执行那一行，也没有一个叫 `var` 的变量记录，所以文件没有意义。为了解决这个问题，建立一个名为 `include.sh` 的文件：
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```
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greet() {
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echo "Hello ${1}"
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}
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var=42
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```
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修改你的 `example.sh` 脚本，加入 `source` 命令：
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```
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#!/bin/sh
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source include.sh
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greet opensource.com
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echo "The meaning of life is $var"
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```
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运行脚本，看见已经可以了：
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```
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$ bash ./example.sh
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Hello opensource.com
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The meaning of life is 42
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```
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`greet` 命令被带入你的 shell 环境，因为它被定义在 `include.sh` 文件中，它甚至可以识别参数（本例中的 `opensource.com`）。变量 `var` 也被设置和导入。
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#### 在 Bash 中解析文件
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另一种让数据“进入” Bash 的方法是将其解析为数据流。有很多方法可以做到这一点. 你可以使用 `grep` 或 `cat` 或任何可以获取数据并管道输出到标准输出的命令。另外，你可以使用 Bash 内置的东西：重定向。重定向本身并不是很有用, 所以在这个例子中, 我也使用内置的 `echo` 命令来打印重定向的结果：
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```
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#!/bin/sh
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echo $( < include.sh )
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```
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将其保存为 `stream.sh` 并运行它来查看结果：
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```
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$ bash ./stream.sh
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greet() { echo "Hello ${1}" } var=42
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$
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```
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对于 `include.sh` 文件中的每一行，Bash 都会将该行打印（或 echo）到你的终端。先用管道把它传送到一个合适的解析器是用 Bash 读取数据的常用方法。例如, 假设 `include.sh` 是一个配置文件, 它的键和值对用一个等号（`=`）分开. 你可以用 `awk` 甚至 `cut` 来获取值：
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```
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#!/bin/sh
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myVar=`grep var include.sh | cut -d'=' -f2`
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echo $myVar
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```
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试着运行这个脚本：
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```
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$ bash ./stream.sh
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42
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```
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### 用 Bash 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 Bash 中，你可以使用常见的 shell 重定向将数据保存到文件中。
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例如, 要创建一个包含输出的新文件, 使用一个重定向符号：
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```
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#!/bin/sh
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TZ=UTC
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date > date.txt
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```
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运行脚本几次：
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```
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$ bash ./date.sh
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$ cat date.txt
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Tue Feb 23 22:25:06 UTC 2021
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$ bash ./date.sh
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$ cat date.txt
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Tue Feb 23 22:25:12 UTC 2021
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```
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要追加数据，使用两个重定向符号：
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```
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#!/bin/sh
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TZ=UTC
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date >> date.txt
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```
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运行脚本几次：
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```
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$ bash ./date.sh
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$ bash ./date.sh
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$ bash ./date.sh
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$ cat date.txt
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Tue Feb 23 22:25:12 UTC 2021
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Tue Feb 23 22:25:17 UTC 2021
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Tue Feb 23 22:25:19 UTC 2021
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Tue Feb 23 22:25:22 UTC 2021
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```
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### Bash 轻松编程
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Bash 的优势在于简单易学，因为只需要一些基本的概念，你就可以构建复杂的程序。完整的文档请参考 GNU.org 上的[优秀的 Bash 文档][6]。
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/3/input-output-bash
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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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||||
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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/bash_command_line.png?itok=k4z94W2U (bash logo on green background)
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[2]: https://opensource.com/article/19/7/what-posix-richard-stallman-explains
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[3]: https://opensource.com/article/19/9/getting-started-zsh
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[4]: https://opensource.com/article/19/7/ways-get-started-linux#wsl
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[5]: https://www.redhat.com/sysadmin/terminals-shells-consoles
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[6]: http://gnu.org/software/bash
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