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是否禁止在"for NAME"中使用NAME后的分号...?

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问题描述

bash手册将for复合语句的语法列出为

The bash manual lists the syntax for the for compound statement as

for name [ [ in [ word ... ] ] ; ] do list ; done

for name [ [ in [ word ... ] ] ; ] do list ; done

表示如果省略 in 子句，则 do 前的分号是可选的. [笔记2].

which implies that the semicolon before do is optional if the in clause is omitted. [Note 2].

但是，Posix规范仅列出了for_clause的以下三种产品:

However, the Posix specification lists only the following three productions for for_clause:

for_clause       : For name linebreak                            do_group
                 | For name linebreak in          sequential_sep do_group
                 | For name linebreak in wordlist sequential_sep do_group
                 ;

作为参考，linebreak是NEWLINE的可能为空的序列，而sequential_sep是分号或NEWLINE的可能，随后可能是NEWLINE的序列:

For reference, linebreak is a possibly-empty sequence of NEWLINE while sequential_sep is either a semicolon or a NEWLINE, possibly followed by a sequence of NEWLINE:

newline_list     :              NEWLINE
                 | newline_list NEWLINE
                 ;
linebreak        : newline_list
                 | /* empty */
                 ;
separator        : separator_op linebreak
                 | newline_list
                 ;
sequential_sep   : ';' linebreak
                 | newline_list
                 ;

据我所知，它禁止使用语法for foo; do :; done.

As far as I can see, that prohibits the syntax for foo; do :; done.

实际上，我尝试过的所有shell(bash，dash，ksh和zsh)都可以接受for foo; do :; done和for foo do :; done，而无需抱怨，无论Posix或自己的文档如何[注3].

In practice, all the shells I tried (bash, dash, ksh and zsh) accept both for foo; do :; done and for foo do :; done without complaint, regardless of Posix or their own documentation [Note 3].

这是Posix标准在语法上的偶然遗漏吗?还是应该在该语法中使用分号被视为对标准的(通常实现的)扩展?

Is this an accidental omission in the grammar in the Posix standard, or should the use of the semicolon in that syntax be considered a (commonly-implemented) extension to the standard?

在for loop的XCU描述中，Posix似乎坚持换行:

In the XCU description of the for loop, Posix seems to insist on newlines:

for name [ in [word ... ]]
do
compound-list
done

for name [ in [word ... ]]
do
compound-list
done

但是，在基本原理"卷中清楚地表明，语法旨在成为硬道理:

However, in the Rationale volume, it is made clear that the grammar is intended to be the last word:

注释

  1. 显然，这是第一个将的问题配对的SO问题. shell 和 language-lawyer .没有 idle-curiosity 的问题，可能更合适.

  1. Apparently this is the first SO question which pairs shell and language-lawyer. There is no idle-curiosity, which might have been more appropriate.

bash手册对换行符并不十分明确；它说的是:

The bash manual is not entirely explicit about newlines; what it says is:

很明显，可以用换行符替换 done 之前的分号，但是似乎没有提到可以对 .

That makes it clear that the semicolon preceding done can be replaced by a newline, but does not seem to mention that the same transformation can be performed on the semicolon preceding do.

ksh和zsh似乎都坚持在name之后使用分号或换行符，尽管实现方式并不坚持使用分号或换行符.

Both ksh and zsh seem to insist that there be either a semicolon or a newline after the name, although the implementations don't insist on it.

ksh联机帮助页列出了以下语法:

The ksh manpage lists the syntax as:

(我相信;do和;done中的分号代表分号或换行符".我无法找到任何明确的声明来表示这种效果，但这是理解语法描述的唯一方法)

(I believe that the semicolon in ;do and ;done represents "a semicolon or a newline". I can't find any definite statement to that effect but it is the only way to make sense of the syntax description.)

zsh手册显示:

推荐答案

很漂亮！我没有确切的答案，但这是源代码对此的解释:

Nicely spotted! I don't have a definite answer, but here is what the source code says about it:

在来自AT& T UNIX v7的原始Bourne shell :

(shell has just read `for name`):
       IF skipnl()==INSYM
       THEN chkword();
        t->forlst=item(0);
        IF wdval!=NL ANDF wdval!=';'
        THEN    synbad();
        FI
        chkpr(wdval); skipnl();
       FI
       chksym(DOSYM|BRSYM);

鉴于此摘要，这似乎不是一个有意识的设计决定.这只是将分号作为in组的一部分来处理的副作用，当没有"in"时，会完全跳过分号.

Given this snippet, it does not appear to be a conscious design decision. It's just a side effect of the semicolon being handled as part of the in group, which is skipped entirely when there is no "in".

Dash同意无效在伯恩(Bourne)中，但将其添加为扩展名:

Dash agrees that it's not valid in Bourne, but adds it as an extension:

        /*
         * Newline or semicolon here is optional (but note
         * that the original Bourne shell only allowed NL).
         */

Ksh93 声称它是有效的，但未提及上下文:

Ksh93 claims that it's valid, but says nothing of the context:

/* 'for i;do cmd' is valid syntax */
else if(tok==';')
    while((tok=sh_lex(lexp))==NL);

Bash没有评论，但明确添加了对这种情况:

Bash has no comment, but explicitly adds support for this case:

for_command:    FOR WORD newline_list DO compound_list DONE
            {
              $$ = make_for_command ($2, add_string_to_list ("\"$@\"", (WORD_LIST *)NULL), $5, word_lineno[word_top]);
              if (word_top > 0) word_top--;
            }
...
    |   FOR WORD ';' newline_list DO compound_list DONE
            {
              $$ = make_for_command ($2, add_string_to_list ("\"$@\"", (WORD_LIST *)NULL), $6, word_lineno[word_top]);
              if (word_top > 0) word_top--;
            }

在zsh中，它只是的副作用解析器:

while (tok == SEPER)
    zshlex();

其中(SEPER是 ;或换行).因此，zsh很高兴接受此循环:

where (SEPER is ; or linefeed). Due to this, zsh happily accepts this loop:

for foo; ;
;
; ; ; ; ;
; do echo cow; done

对我来说，所有这些都表明POSIX中的故意遗漏，并且作为扩展被广泛而有意地支持.

To me, this all points to an intentional omission in POSIX, and widely and intentionally supported as an extension.

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08-13 10:36
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