Bash if 语句中何时需要方括号？
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When are square brackets required in a Bash if statement?
提问by Misha Moroshko
Usually, I use square brackets in the if statement:
通常，我在 if 语句中使用方括号：
if [ "$name" = 'Bob' ]; then ...
But, when I check if grepsucceeded I don't use the square brackets:
但是，当我检查是否grep成功时，我不使用方括号：
if grep -q "$text" $file ; then ...
When are the square brackets necessary in the ifstatement?
if语句中什么时候需要方括号？
回答by chepner
The square brackets are a synonym for the testcommand. An ifstatement checks the exit status of a command in order to decide which branch to take. grep -q "$text"is a command, but "$name" = 'Bob'is not--it's just an expression. testis a command, which takes an expression and evaluates it:
方括号是test命令的同义词。一个if语句检查，以决定采取哪一个分支指令的退出状态。grep -q "$text"是一个命令，但"$name" = 'Bob'不是——它只是一个表达式。test是一个命令，它接受一个表达式并计算它：
if test "$name" = 'Bob'; then ...
Since square brackets are a synonym for the testcommand, you can then rewrite it as your original statement:
由于方括号是test命令的同义词，因此您可以将其重写为原始语句：
if [ "$name" = 'Bob' ]; then ...
回答by Keith Thompson
[is actually a command, equivalent (almost, see below) to the testcommand. It's not part of the shell syntax. (Both [and test, depending on the shell, are often built-in commands as well, but that doesn't affect their behavior, except perhaps for performance.)
[实际上是一个命令，相当于（几乎，见下文）test命令。它不是 shell 语法的一部分。（[和test，取决于shell，通常也是内置命令，但这不会影响它们的行为，除了性能。）
An ifstatement executes a command and executes the thenpart if the command succeeds, or the elsepart (if any) if it fails. (A command succeeds if it exits with a status ($?) of 0, fails if it exits with a non-zero status.)
一个if语句执行的命令和执行then如果命令成功的一部分，或者else一部分（如果有的话），如果它失败。（如果命令以状态 ( $?) 为 0退出，则命令成功，如果以非零状态退出，则命令失败。）
In
在
if [ "$name" = 'Bob' ]; then ...
the command is
命令是
[ "$name" = 'Bob' ]
(You could execute that same command directly, without the if.)
（您可以直接执行相同的命令，而无需if.）
In
在
if grep -q "$text" $file ; then ...
the command is
命令是
grep -q "$text" $file
man [or man testfor more information.
man [或man test了解更多信息。
FOOTNOTE: Well, the [command is almostequivalent to the testcommand. The difference is that [requires ]as its last argument, and testdoes not -- and in fact doesn't allow it (more precisely, testdoesn't treat a ]argument specially; for example it could be a valid file name). (It didn't haveto be implemented that way, but a [without a matching ]would have made a lot of people very very nervous.)
脚注：嗯，[命令几乎等同于test命令。不同之处在于，[需要]作为它的最后一个参数，而test不是——事实上不允许它（更准确地说，test不]特别对待一个参数；例如它可能是一个有效的文件名）。（它没有已经被实现这种方式，但一个[不匹配]会作出很多人非常非常紧张。）
回答by cha0site
The best way to think of the [ ... ]syntax, is to consider [to be a program - which it is!
[ ... ]考虑语法的最佳方式是将其视为[一个程序 - 确实如此！
Check this out:
看一下这个：
~ $ ls /usr/bin/\[
/usr/bin/[
on the other hand, you're probably not using that version of it since bashalso provides [as a shell built-in.
另一方面，您可能没有使用它的那个版本，因为它bash也[作为内置的 shell提供。
Anyway, to answer your question: What ifdoes is run the command you give it and see it the return value is 0or not. You use [to do other, more interesting comparisons such as string comparisons. See man [and man bash.
无论如何，要回答你的问题：什么if是运行你给它的命令，看看它的返回值是0不是。您习惯于[进行其他更有趣的比较，例如字符串比较。见man [和man bash。
