bash 使用大括号参数列表多次执行命令
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Execute command multiple times with curly brackets arguments list
提问by Haralan Dobrev
When I needed to run a command multiple times with a different argument I used this approach (without understanding it fully):
当我需要使用不同的参数多次运行命令时，我使用了这种方法（没有完全理解它）：
touch {a,b,c}
Which is equivalent of:
这相当于：
touch a
touch b
touch c
I think the same could be accomplished with the following loop:
我认为同样可以通过以下循环来完成：
for file in {a,b,c}; do touch $file; done
However I've stumbled across a case where this does not work:
但是，我偶然发现了一个不起作用的情况：
pear channel-discover {"pear.phpunit.de","pear.symfony-project.com"}
I have a few questions:
我有几个问题：
- What is the name of what is happening in the first example and what exactly is happening?
- Is it better to use this approach for simple things rather than for-in loops?
- Why the pear command is not working like that? Should the command script implement some techniques to handle such arguments or is it the shell responsible for that?
- 第一个示例中发生的事情的名称是什么，究竟发生了什么？
- 将这种方法用于简单的事情而不是 for-in 循环更好吗？
- 为什么 pear 命令不能这样工作？命令脚本是否应该实现一些技术来处理这些参数，还是由 shell 负责？
回答by Shawn Chin
That's called Brace Expansion, which expands to a space-separated list of the given strings.
这称为Brace Expansion，它扩展为给定字符串的空格分隔列表。
So touch {a,b,c}would be equivalent to
所以touch {a,b,c}将相当于
touch a b c
While touch {a,b,c}xwould be equivalent to:
虽然touch {a,b,c}x相当于：
touch ax bx cx
You pearcommand would essentially be run as:
您的pear命令基本上将运行为：
pear channel-discover pear.phpunit.de pear.symfony-project.com
which may not be what you expected. If you want the command to be run once for each string, use a for loop (which answers your 2nd question), or use a combination of brace expansion and xargs.
这可能不是您所期望的。如果您希望命令为每个字符串运行一次，请使用 for 循环（它回答您的第二个问题），或使用大括号扩展和 xargs 的组合。
回答by Jens
The problem is that contrary to your expectation, the brace expansionof
问题是，违背你的期望，在括号扩展的
touch {a,b,c}
is equivalent to
相当于
touch a b c # NOT 3 separate invocations.
(Use echo {a,b,c}to verify). It appears that pear channel-discoverdoes not accept twoargs. You will probably see the same error with
（echo {a,b,c}用于验证）。似乎pear channel-discover不接受两个参数。您可能会看到相同的错误
pear channel-discover pear.phpunit.de pear.symfony-project.com
回答by yo'
Well, you have two options:
那么，你有两个选择：
for i in "pear.phpunit.de" "pear.symfony-project.com"
do
pear channel-discover "$i"
done
Or a one-liner (but calling xargsinstead of using bash internals):
或单行（但调用xargs而不是使用 bash 内部结构）：
echo "pear.phpunit.de" "pear.symfony-project.com" | xargs -n1 pear channel-discover
The former is certainly easier to read by the human, the time efficiency will be basically the same.
前者当然更容易被人类阅读，时间效率会基本相同。
