bash ": ${foo=value}" 中冒号运算符的解释
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Explanation of colon operator in ": ${foo=value}"
提问by Peter Coulton
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:
我了解 bash 中的冒号运算符，它的作用类似于null，并且我知道它用于参数扩展以及其他方式，但有人可以解释一下：
: ${SOMETHING='value'}
From experimentation I know that this sets the environment variable $SOMETHINGto 'value'but why?
从实验中我知道这将环境变量设置为$SOMETHING，'value'但为什么呢？
"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.
“只是因为它确实”是一个有效的答案，但请指出它的文档（我似乎找不到），或者这种用法的正确名称会很有用。不过，我希望有一个更有启发性的解释。
回答by Kilian Foth
The expression ${SOMETHING='value'}sets SOMETHING to valueif it isn't already set. This is a useful operator to have in many situations. However, it also returns the assigned value, so if you simply executed
如果尚未设置${SOMETHING='value'}，value则表达式将 SOMETHING设置为。在许多情况下，这是一个有用的运算符。但是，它也会返回分配的值，因此如果您只是执行
${SOMETHING='value'}
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".
那么你的 shell 会尝试调用命令value。这可能会也可能不会做一些不需要的事情；至少它会抛出一条消息“值：未找到命令”。
To avoid this you can use the no-op :, which evaluates its argument and then throws it away, rather than executing it. This is documented here.
为了避免这种情况，您可以使用 no-op :，它会评估其参数，然后将其丢弃，而不是执行它。这在此处记录。
回答by Arnaud F.
Explained here : http://tldp.org/LDP/abs/html/parameter-substitution.html
在这里解释：http: //tldp.org/LDP/abs/html/parameter-substitution.html
If parameter not set, set it to default.
Both forms nearly equivalent. The : makes a difference only when $parameter has been declared and is null, [1] as above.
echo ${var=abc} # abc echo ${var=xyz} # abc # $var had already been set to abc, so it did not change.
如果未设置参数，则将其设置为默认值。
两种形式几乎相同。: 仅当 $parameter 已声明且为空时才会有所不同，[1] 如上所述。
echo ${var=abc} # abc echo ${var=xyz} # abc # $var had already been set to abc, so it did not change.
