什么是空操作，用例[：]在bash？ [英] What is the use case of noop [:] in bash?

问题描述

我搜索了空操作在bash（:)，但无法找到任何有用信息。这是什么经营者的具体目的或用途呢？

I searched for noop in bash (:), but was not able to find any good information. What is the exact purpose or use case of this operator?

我试过以下，它的工作是这样对我来说：

I tried following and it's working like this for me:

[mandy@root]$ a=11
[mandy@root]$ b=20
[mandy@root]$ c=30
[mandy@root]$ echo $a; : echo $b ; echo $c
10
30

请让我知道，该运营商实时或者它是强制性使用它的任何地方的用例。

Please let me know, any use case of this operator in real time or any place where it is mandatory to use it.

推荐答案

它的存在，更由于历史的原因。结肠内置：完全等同于真正。这是传统的方法是使用真正当返回值是很重要的，例如在一个无限循环：

It's there more for historical reasons. The colon builtin : is exactly equivalent to true. It's traditional to use true when the return value is important, for example in an infinite loop:

while true; do
  echo 'Going on forever'
done

这是传统方法是使用：当shell语法需要一个命令，但是你什么都没有做。

It's traditional to use : when the shell syntax requires a command but you have nothing to do.

while keep_waiting; do
  : # busy-wait
done

的：内置的历史可以追溯到汤普森壳，这是 present 中的的Unix V6 。 ：是汤普森shell的转到语句的标签指示。标签可以是任何文字，所以：弯了腰为注释指示器（如果没有转到评论，然后：注释是一个有效的注释）。该 Bourne shell的没有转到但保留：

The : builtin dates all the way back to the Thompson shell, it was present in Unix v6. : was a label indicator for the Thompson shell's goto statement. The label could be any text, so : doubled up as a comment indicator (if there is no goto comment, then : comment is effectively a comment). The Bourne shell didn't have goto but kept :.

一个使用常见的成语：是的 ：$ {VAR = VALUE} ，这台 VAR 到值如果是未设置并执行咱这 VAR 已被设置。这种结构只存在于一个变量替换的形式，而这个变量替换需要以某种方式命令的一部分：无操作指令提供很好

A common idiom that uses : is : ${var=VALUE}, which sets var to VALUE if it was unset and does nothing if var was already set. This construct only exists in the form of a variable substitution, and this variable substitution needs to be part of a command somehow: a no-op command serves nicely.

又见什么用处呢结肠服务内嵌的？。

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