冒号运算符的说明及QUOT;：$ {富=值}＆QUOT; [英] Explanation of colon operator in ": ${foo=value}"

问题描述

据我所知在bash，它如同空，我知道这是在参数扩展使用，以及使用其他方式，但有人可以解释这个

I understand the colon operator in bash that acts like a null, and I know it's used in parameter expansion, as well as being used other ways, but can someone explain this:

: ${SOMETHING='value'}

从实验我知道，这个设置环境变量 $ SOMETHING 到'值'但是为什么呢？

就因为它是一个有效的答案，但再请点我为此使用它的文档（这我似乎无法找到）或正确的名称将是有益的。我希望有一个虽然更富有启发性的解释。

"Just because it does" is a valid answer but then please point me to the documentation for it (which I can't seem to find) or a proper name for this usage would be useful. I'm hoping there's a more enlightening explanation though.

推荐答案

这位前pression设置的东西 如果尚未设置值。这是一个有用的操作者在许多情况下。然而，它也返回指定的值，所以如果你只是执行

The expression sets SOMETHING to value if it isn't already set. This is a useful operator to have in many situations. However, it also returns the assigned value, so if you simply executed

${SOMETHING='value'}

那么你的shell会尝试调用命令值。这可能会或可能不会做不必要的东西;在至少它会抛出一个消息值：找不到命令。

then your shell would try to invoke the command value. This might or might not do something unwanted; at the least it would throw a message "value: command not found".

要避免这一点，你可以使用无操作：，它计算它的参数，然后它扔了出去，而不是执行它

To avoid this you can use the no-op :, which evaluates its argument and then throws it away, rather than executing it.

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