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在Ruby/Chef中，如果“如果”和唯一的_if;

发布于 2025-02-11 18:51:53 字数 341 浏览 2 评论 0 原文

在厨师（因此，Ruby）中，我看到了两种宣布有条件的方式

resource 'foo' do
  echo "Ubuntu"
end if node['platform'] == 'ubuntu'

，而

resource 'foo' do
  echo "Ubuntu"
  only_if node['platform'] == 'ubuntu'
end

这些有效地做同样的事情吗？在官方文档中，似乎“只有_if”是首选的方式，我找不到许多“结束”的示例，但只是好奇他们是否（看起来像他们看起来一样）条件是正确的）。

谢谢！

原文

In chef (therefor, ruby), I've seen two ways of declaring conditionals

resource 'foo' do
  echo "Ubuntu"
end if node['platform'] == 'ubuntu'

and

resource 'foo' do
  echo "Ubuntu"
  only_if node['platform'] == 'ubuntu'
end

Don't these effectively do the same thing? In the official docs, it seems "only_if" is the preferred way, and I can't find many examples of the "end if", but just curious if they, as they seem, do the same thing (execute the block only if the conditional is true).

Thanks!

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∝单色的世界 2025-02-18 18:51:53

Chef Rourcut Guard Recus vs.表达后的基本

guard scrause 这是厨师DSL的一部分。但是，做...如果是ruby 修饰符控制表达式（有时称为后条件）应用于块，该块以相同的功能，以正常的 ruby if/then then语句 works works。请注意，即使在要评估的表达式之后放置了，但仍会对后条件进行评估 first 。

将唯一的视为特定于厨师的资源语句。另一种只是句法糖，由Ruby的解释器支持，您引用的示例（假设它在Ruby Resource Block外部在厨师中起作用；我不费心地测试它）与在更为标准的IF-IF-IF-这样的陈述：

if node['platform'] == 'ubuntu'
  resource 'foo' do
    echo "Ubuntu"
  end 
end

大多数厨师资源都应遵循当前的样式指南和DSL功能，但是后条件在惯用性红宝石中非常普遍，因为它们强调了表达方式而不是条件，并且因为它们允许更简短的代码。

Chef Resource Guard Clauses vs. Expression Post-Conditions

only_if is a guard clause that's part of the Chef DSL. However, do...end if is a Ruby modifier control expression (sometimes called a post-condition) applied to a block that functions the same way a normal Ruby if/then statement works. Note that even though the if is placed after the expression to be evaluated, the post-condition is still evaluated first.

Think of only_if as a Chef-specific resource statement. The other is just syntactic sugar supported by Ruby's interpreter, and the example you cited (assuming it works in Chef outside a Ruby resource block; I didn't bother to test it) is the same as writing the block inside a more standard if-statement like this one:

if node['platform'] == 'ubuntu'
  resource 'foo' do
    echo "Ubuntu"
  end 
end

Most Chef resources should follow the current style guides and DSL features, but post-conditions are very common in idiomatic Ruby because they emphasize the expression rather than the conditional and because they allow for more brevity of code.

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倒数 2025-02-18 18:51:53

如果在收敛时表现出结束时， bealts_if 有微妙的区别（在厨师speak中）。简而言之，当厨师 - client 启动时，它会编译食谱并创建一个将在节点上收敛的资源集合。

为了举例来说，假设我们有一个食谱> Code> cookbook1 在食谱中只有1个资源。当我们在下面的方案中运行此类食谱时：

方案1 ：

使用 do ..结束如果：

当条件不匹配时，将资源从汇编中删除。因此，将没有资源可以运行。当 node ['platform'] 不是 ubuntu 时，从Chef-Client运行的示例输出。

Compiling Cookbooks...
Converging 0 resources

方案2 ：

使用唯一保护

资源保留在集合中，但是当 node ['platform'] 不是 ubuntu 。

Compiling Cookbooks...
Converging 1 resources
Recipe: cookbook1::default
  * resource[foo] action run (skipped due to only_if)

简而言之，如果条件将在“编译”阶段本身过程中运行，例如，例如。厨师资源在“融合”阶段运行。请参阅 Infra Client Client Documentation 有关详细信息。

There is a subtle difference in how only_if and end if behave when a node is converged (in Chef speak). In simple terms, when chef-client starts, it compiles the cookbooks and creates a collection of resources that will converge on the node.

For the sake of example, let's say we have a cookbook cookbook1 with only 1 resource in the recipe. When we run such cookbook in below scenarios:

Scenario 1:

Using do .. end if:

The resource is removed from the compilation when the condition is not matched. So there will be no resources to run. Example output from chef-client run when node['platform'] is not ubuntu.

Compiling Cookbooks...
Converging 0 resources

Scenario 2:

Using only_if guard

The resource remains in the collection, but it is skipped when node['platform'] is not ubuntu.

Compiling Cookbooks...
Converging 1 resources
Recipe: cookbook1::default
  * resource[foo] action run (skipped due to only_if)

In short, pure Ruby code, such as if conditions will run during "compile" phase itself. Chef resources run during the "converge" phase. See the Chef Infra Client documentation for details.