如何使用双括号或单括号、圆括号、花括号 [英] How to use double or single brackets, parentheses, curly braces

问题描述

我对 Bash 中括号、圆括号、花括号的使用以及它们的双重或单一形式之间的区别感到困惑.有明确的解释吗?

I am confused by the usage of brackets, parentheses, curly braces in Bash, as well as the difference between their double or single forms. Is there a clear explanation?

推荐答案

在 Bash 中，test 和 [ 是 shell 内置的.

In Bash, test and [ are shell builtins.

双括号，它是一个 shell 关键字，可启用附加功能.例如，您可以使用 && 和 || 而不是 -a 和 -o ，还有正则表达式匹配运算符 =~.

The double bracket, which is a shell keyword, enables additional functionality. For example, you can use && and || instead of -a and -o and there's a regular expression matching operator =~.

另外，在一个简单的测试中，双方括号的评估似乎比单方括号快很多.

Also, in a simple test, double square brackets seem to evaluate quite a lot quicker than single ones.

$ time for ((i=0; i<10000000; i++)); do [[ "$i" = 1000 ]]; done

real    0m24.548s
user    0m24.337s
sys 0m0.036s
$ time for ((i=0; i<10000000; i++)); do [ "$i" = 1000 ]; done

real    0m33.478s
user    0m33.478s
sys 0m0.000s

大括号除了用于分隔变量名外还用于 参数扩展，因此您可以执行以下操作:

The braces, in addition to delimiting a variable name are used for parameter expansion so you can do things like:

• 截断变量的内容

• Truncate the contents of a variable

$ var="abcde"; echo ${var%d*}
abc

进行类似于 sed

$ var="abcde"; echo ${var/de/12}
abc12

使用默认值

Use a default value

$ default="hello"; unset var; echo ${var:-$default}
hello

还有几个

and several more

此外，大括号扩展创建通常在循环中迭代的字符串列表:

Also, brace expansions create lists of strings which are typically iterated over in loops:

$ echo f{oo,ee,a}d
food feed fad

$ mv error.log{,.OLD}
(error.log is renamed to error.log.OLD because the brace expression
expands to "mv error.log error.log.OLD")

$ for num in {000..2}; do echo "$num"; done
000
001
002

$ echo {00..8..2}
00 02 04 06 08

$ echo {D..T..4}
D H L P T

请注意，在 Bash 4 之前，前导零和增量功能不可用.

Note that the leading zero and increment features weren't available before Bash 4.

感谢 gboffi 提醒我大括号扩展.

Thanks to gboffi for reminding me about brace expansions.

双括号用于算术运算:

((a++))

((meaning = 42))

for ((i=0; i<10; i++))

echo $((a + b + (14 * c)))

并且它们使您能够省略整数和数组变量上的美元符号，并在运算符周围包含空格以提高可读性.

and they enable you to omit the dollar signs on integer and array variables and include spaces around operators for readability.

单括号也用于数组索引:

array[4]="hello"

element=${array[index]}

右侧的(大多数/所有?)数组引用需要花括号.

Curly brace are required for (most/all?) array references on the right hand side.

ephemient 的 评论提醒我括号也用于子shell.并且它们用于创建数组.

ephemient's comment reminded me that parentheses are also used for subshells. And that they are used to create arrays.

array=(1 2 3)
echo ${array[1]}
2

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