Bash if 语句中何时需要方括号？
通常，我在 if 语句中使用方括号：
if [ "$name" = 'Bob' ]; then ...
但是，当我检查 grep 是否成功时，我不使用方括号：
if grep -q "$text" $file ; then ...
if 中何时需要方括号> 声明？
Usually, I use square brackets in the if statement:
if [ "$name" = 'Bob' ]; then ...
But, when I check if grep succeeded I don't use the square brackets:
if grep -q "$text" $file ; then ...
When are the square brackets necessary in the if statement?
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方括号是
test命令的同义词。if语句检查命令的退出状态，以便决定采用哪个分支。grep -q "$text"是一个命令，但"$name" = 'Bob'不是 - 它只是一个表达式。test是一个命令，它接受一个表达式并对其求值：由于方括号是
test命令的同义词，因此您可以将其重写为原始语句：The square brackets are a synonym for the
testcommand. Anifstatement checks the exit status of a command in order to decide which branch to take.grep -q "$text"is a command, but"$name" = 'Bob'is not--it's just an expression.testis a command, which takes an expression and evaluates it:Since square brackets are a synonym for the
testcommand, you can then rewrite it as your original statement:[实际上是一个命令，相当于（几乎，见下文）test命令。它不是 shell 语法的一部分。 （[和test，取决于 shell，通常也是内置命令，但这不会影响它们的行为，除了性能之外。）
if语句执行命令，如果命令成功则执行then部分，如果失败则执行else部分（如果有）。 （如果命令以状态 ($?) 0 退出，则命令成功；如果以非零状态退出，命令失败。）在
命令中是
（您可以直接执行相同的命令，无需
if。）在
命令中使用
man [或man test了解更多信息。脚注：嗯，
[命令几乎等同于test命令。不同之处在于[需要]作为其最后一个参数，而test不需要 - 事实上不允许它（更准确地说） ，test不会特殊对待]参数；例如，它可能是有效的文件名）。 （它没有必须以这种方式实现，但是没有匹配]的[会让很多人非常紧张.)[is actually a command, equivalent (almost, see below) to thetestcommand. It's not part of the shell syntax. (Both[andtest, depending on the shell, are often built-in commands as well, but that doesn't affect their behavior, except perhaps for performance.)An
ifstatement executes a command and executes thethenpart if the command succeeds, or theelsepart (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.)In
the command is
(You could execute that same command directly, without the
if.)In
the command is
man [orman testfor more information.FOOTNOTE: Well, the
[command is almost equivalent to thetestcommand. The difference is that[requires]as its last argument, andtestdoes 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 have to be implemented that way, but a[without a matching]would have made a lot of people very very nervous.)考虑
[ ... ]语法的最佳方式是将[视为一个程序 - 确实如此！检查一下：
另一方面，您可能没有使用它的该版本，因为
bash还提供[作为内置 shell。无论如何，回答你的问题：
if所做的是运行你给它的命令，看看它的返回值是0或不是。您可以使用[进行其他更有趣的比较，例如字符串比较。请参阅man [和man bash。The best way to think of the
[ ... ]syntax, is to consider[to be a program - which it is!Check this out:
on the other hand, you're probably not using that version of it since
bashalso provides[as a shell built-in.Anyway, to answer your question: What
ifdoes is run the command you give it and see it the return value is0or not. You use[to do other, more interesting comparisons such as string comparisons. Seeman [andman bash.