Ruby，exec、system 和 %x() 或反引号之间的区别 [英] Ruby, Difference between exec, system and %x() or Backticks

问题描述

以下 Ruby 方法有什么区别?

What is the difference between the following Ruby methods?

exec、system 和 %x() 或 反引号

我知道它们用于通过 Ruby 以编程方式执行终端命令，但我想知道为什么有三种不同的方法来执行此操作.

I know they are used to execute terminal commands programmatically via Ruby, but I'd like to know why there are three different ways to do this.

推荐答案

系统

系统 方法调用系统程序.您必须将此命令作为字符串参数提供给此方法.例如:

The system method calls a system program. You have to provide the command as a string argument to this method. For example:

>> system("date")
Wed Sep 4 22:03:44 CEST 2013
=> true

被调用的程序将使用您的 Ruby 程序当前的 STDIN、STDOUT 和 STDERR 对象.实际上，实际的返回值是true、false 或nil.在示例中，日期是通过 STDIN 的 IO 对象打印的.如果进程以零状态退出，该方法将返回 true，如果进程以非零状态退出，则返回 false，如果进程以非零状态退出，则返回 nil执行失败.

The invoked program will use the current STDIN, STDOUT and STDERR objects of your Ruby program. In fact, the actual return value is either true, false or nil. In the example the date was printed through the IO object of STDIN. The method will return true if the process exited with a zero status, false if the process exited with a non-zero status and nil if the execution failed.

从 Ruby 2.6 开始，传递 exception: true 将引发异常而不是返回 false 或 nil:

As of Ruby 2.6, passing exception: true will raise an exception instead of returning false or nil:

>> system('invalid')
=> nil

>> system('invalid', exception: true)
Traceback (most recent call last):
...
Errno::ENOENT (No such file or directory - invalid)

另一个副作用是全局变量 $? 设置为 Process::Status 对象.该对象将包含有关调用本身的信息，包括被调用进程的进程标识符 (PID) 和退出状态.

Another side effect is that the global variable $? is set to a Process::Status object. This object will contain information about the call itself, including the process identifier (PID) of the invoked process and the exit status.

>> system("date")
Wed Sep 4 22:11:02 CEST 2013
=> true
>> $?
=> #<Process::Status: pid 15470 exit 0>

反引号

反引号 (``) 调用系统程序并返回其输出.与第一种方法相反，该命令不是通过字符串提供的，而是通过将其放在反引号对中来提供的.

Backticks (``) call a system program and return its output. As opposed to the first approach, the command is not provided through a string, but by putting it inside a backticks pair.

>> `date`
=> Wed Sep 4 22:22:51 CEST 2013   

全局变量 $? 也是通过反引号设置的.使用反引号，您还可以使用字符串插值.

The global variable $? is set through the backticks, too. With backticks you can also make use string interpolation.

%x()

使用 %x 是反引号样式的替代方法.它也会返回输出.就像它的亲戚 %w 和 %q (以及其他)一样，只要括号样式的分隔符匹配，任何分隔符就足够了.这意味着 %x(date)、%x{date} 和 %x-date- 都是同义词.像反引号 %x 可以利用字符串插值.

Using %x is an alternative to the backticks style. It will return the output, too. Like its relatives %w and %q (among others), any delimiter will suffice as long as bracket-style delimiters match. This means %x(date), %x{date} and %x-date- are all synonyms. Like backticks %x can make use of string interpolation.

执行

通过使用 Kernel#exec 当前进程(您的 Ruby 脚本)被替换为通过 exec 调用的进程.该方法可以将字符串作为参数.在这种情况下，字符串将受到外壳扩展.当使用多个参数时，则使用第一个参数执行程序，并提供以下参数作为要调用的程序的参数.

By using Kernel#exec the current process (your Ruby script) is replaced with the process invoked through exec. The method can take a string as argument. In this case the string will be subject to shell expansion. When using more than one argument, then the first one is used to execute a program and the following are provided as arguments to the program to be invoked.

Open3.popen3

有时所需的信息会写入标准输入或标准错误，您也需要控制这些信息.这里 Open3.popen3 派上用场:

Sometimes the required information is written to standard input or standard error and you need to get control over those as well. Here Open3.popen3 comes in handy:

require 'open3'

Open3.popen3("curl http://example.com") do |stdin, stdout, stderr, thread|
   pid = thread.pid
   puts stdout.read.chomp
end

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