Linux Bash 有 do-while 循环吗？
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Does Linux Bash have a do-while loop?
提问by Willem Van Onsem
After some search on the Internet, it appears that Bash doesn't have a do-whileloop.
在 Internet 上进行了一些搜索后，Bash 似乎没有do-while循环。
Is this correct? Is there a reliable source to confirm this (the lack of evidence that there is a do-whileloop is not an argument that there isn't one, perhaps a statement is only unpopular)?
这样对吗？是否有可靠的来源来证实这一点（缺乏证据表明存在do-while循环并不是说不存在循环的论点，也许声明只是不受欢迎）？
Is it possible to define directives oneself and thus implement a do-whileloop? There is an algorithmic way to transform a do-while-loop in a while-loop, but this is notthe scope of this question.
是否可以自己定义指令从而实现do-while循环？有一种算法方法可以在 while循环中转换do- while循环，但这不是本问题的范围。
回答by rici
bash(or Posix shells in general) don't have an explicit syntax for a post-test loop (commonly known as a "do-while" loop) because the syntax would be redundant. The whilecompound statement allows you to write pre-test, post-test or mid-test loops, all with the same syntax.
bash（或一般的 Posix shell）没有用于测试后循环（通常称为“do-while”循环）的显式语法，因为该语法是多余的。该while复合语句可以让你写预试验，试验后或中试循环，都具有相同的语法。
Here's the semantics of a shell whileloop, from Posix:
这是while来自Posix的 shell循环的语义：
The format of the while loop is as follows:
while循环的格式如下：
while compound-list-1
do
compound-list-2
done
The compound-list-1 shall be executed, and if it has a non-zero exit status, the while command shall complete. Otherwise, the compound-list-2 shall be executed, and the process shall repeat.
将执行复合列表 1，如果它具有非零退出状态，则 while 命令将完成。否则，应执行复合列表 2，并重复该过程。
A "compound list" is a sequence of commands; the exit status of a compound list is the exit status of the last commandin the list.
“复合列表”是一个命令序列；复合列表的退出状态是列表中最后一个命令的退出状态。
That means that you could think of a whileloop as being written as follows:
这意味着您可以将while循环视为如下编写：
while
optional-pre-test-compound-list
condition
do
post-test-compound-list
done
That is, there is no requirement that the condition to be tested immediately follows the while keyword. So the equivalent to the C syntax:
也就是说，不要求待测试的条件紧跟在 while 关键字之后。所以等效于 C 语法：
do statements while (test);
is
是
while statements; test do :; done
The :between the do and the done is required, because the shell grammar does not allow empty statements. Since :is not a metacharacter, it must have whitespace or a metacharacter before and after it; otherwise, it would be parsed as part of the preceding or succeeding token. Since it's parsed as a command, it also needs a semicolon or newline after it; otherwise the doneis treated as an argument to :.
该:之间做和做是必需的，因为shell语法不允许空语句。由于:不是元字符，所以它前后必须有空格或元字符；否则，它将被解析为前一个或后继令牌的一部分。由于它被解析为命令，因此它后面还需要一个分号或换行符；否则 将done被视为 的参数:。
回答by Elliott Frisch
No, bash has no do-whileloop. The manualsection 3.2.4.1 Looping Constructslists until, whileand for. Notethere is no do-while.
不，bash 没有do-while循环。该手册节3.2.4.1循环结构列表until，while和for。注意没有do-while.
