在cat命令语法奇怪 [英] Strange grammar in cat command
问题描述
首先,我很抱歉,如果这个问题可以通过网络搜索来回答,但我无法找到任何东西。
First of all, I apologize if this question can be answered with a web search, but I couldn't find anything.
有是我所见过的重复文件cat命令语法部分。
There is some grammar in the cat command which I've seen to "repeat" files.
cat file{,}
等同于调用
cat file file
此外,
cat file{,}{,}{,}{,}
重复文件
不四次,但16倍。
此外,
repeats file
not four times, but 16 times.
In addition,
cat file{,,}
重复文件
3倍。
我想更多地了解这个语法。这叫什么?它是内置到猫或者是抽壳特征?是否有这种语法更多的功能?
I would like to know more about this grammar. What is it called? Is it built into cat or is it a shell feature? Are there more features of this grammar?
推荐答案
此功能被称为的括号扩展。
一般来说,你可以写文件{1,2,3}
和之前的bash它扩展到文件1文件2文件3
运行该命令。
Generally you can write file{1,2,3}
and bash expands it to file1 file2 file3
before running the command.
如果你写
mkdir foo{1,2,3}{a,b}
这将是等同于
mkdir foo1a foo1b foo2a foo2b foo3a foo3b
和将创建的6目录。
在您的情况下( {,}
)要添加什么,没有什么,因此获得两倍的同一个词。
In your case ({,}
) you are adding nothing and nothing and therefore get the same word twice.
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