问题描述
我看了看bash的手册页和 [表示，使用条件前pressions。然后我看着有条件防爆pressions部分，它列出了相同的运营商为测试（和 [）。
I looked at bash man page and the [[ says it uses Conditional Expressions. Then I looked at Conditional Expressions section and it lists the same operators as test (and [).
所以我在想，是什么样的区别 [和 [在bash？
So I wonder, what is the difference between [ and [[ in bash?
推荐答案
[是庆典的改进到 [命令。它有几个改进，使其成为一个更好的选择，如果你写针对的bash脚本。我最喜欢的是：
[[ is bash's improvement to the [ command. It has several enhancements that make it a better choice if you write scripts that target bash. My favorites are:
-
这是shell的语法特点，所以它有一些特殊的行为
[没有。你不再需要引用拼了命变量，因为[处理空字符串和字符串用空格更直观。例如，[你写
It is a syntactical feature of the shell, so it has some special behavior that
[doesn't have. You no longer have to quote variables like mad because[[handles empty strings and strings with whitespace more intuitively. For example, with[you have to write
if [ -f "$FILE" ]
要正确处理空字符串或文件名包含空格。随着 [引号是不必要的：
to correctly handle empty strings or file names with spaces in them. With [[ the quotes are unnecessary:
if [[ -f $FILE ]]
由于它是一个语法功能，它可以让您使用＆放大器;＆安培; 和 || 布尔测试和＆LT运营商; 和＆GT; 字符串比较。 [不能做到这一点，因为这是一个普通的命令和＆放大器;＆安培; ， || ，＆LT; 和＆GT; 不传递到常规命令作为命令行参数。
Because it is a syntactical feature, it lets you use && and || operators for boolean tests and < and > for string comparisons. [ cannot do this because it is a regular command and &&, ||, <, and > are not passed to regular commands as command-line arguments.
它有一个美好的 =〜运营商定期做前pression匹配。随着 [你可以写
It has a wonderful =~ operator for doing regular expression matches. With [ you might write
if [ "$ANSWER" = y -o "$ANSWER" = yes ]
使用 [你可以这样写为
if [[ $ANSWER =~ ^y(es)?$ ]]
它甚至可以让你访问捕获组，它在 BASH_REMATCH 存储。例如， $ {BASH_REMATCH [1]} 将ES，如果你输入一个完整的是上面。
It even lets you access the captured groups which it stores in BASH_REMATCH. For instance, ${BASH_REMATCH[1]} would be "es" if you typed a full "yes" above.
您获得模式匹配又名通配符是免费的。也许你如何键入yes那么严格。也许你没关系，如果用户键入的y什么。一应俱全：
You get pattern matching aka globbing for free. Maybe you're less strict about how to type yes. Maybe you're okay if the user types y-anything. Got you covered:
if [[ $ANSWER = y* ]]
请记住，这是一个bash的扩展，因此，如果你正在写sh兼容的脚本，那么你需要坚持使用 [。确保您已在＃！/斌/庆典家当您的脚本行，如果你使用双括号。
Keep in mind that it is a bash extension, so if you are writing sh-compatible scripts then you need to stick with [. Make sure you have the #!/bin/bash shebang line for your script if you use double brackets.
- Bash FAQ - "What is the difference between test, [ and [[ ?"
- Bash Practices - Bash Tests
- Server Fault - What is the difference between double and single brackets in bash?
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