为什么Perl的全球电话返回undef？ [英] Why does Perl's glob return undef for every other call?

问题描述

我不一定要寻找更好的方式来做到这一点，而是对输出的解释将非常感激。最近，一位高级程序员问我为什么他的代码工作，但只有一个实例。我来了解的是，它发挥其他作用。这是我的例子：

I'm not necessarily looking for a better way to do this, rather an explanations of the output would greatly be appreciated. Recently, a senior programmer asked me why his code worked but only for one instance. What I came to find out was that it worked every other occurrence. Here is my example:

#!/usr/bin/perl -w
use strict;

my @list_env_vars = (
    '$SERVER',
    '$SERVER',
    '$SERVER',
    '$SERVER',
    '$SERVER',
    '$SERVER',
);

foreach (@list_env_vars){
    print "$_ = ".glob()."\n";
}

perl的输出5.004：

which output for perl 5.004:

$SERVER = UNIX_SERVER
$SERVER =
$SERVER = UNIX_SERVER
$SERVER =
$SERVER = UNIX_SERVER
$SERVER =

或输出perl 5.10：

or output for perl 5.10:

$SITE = $SITE
Use of uninitialized value in concatenation (.) or string at glob_test.pl line 14.
$SITE =
$SITE = $SITE
Use of uninitialized value in concatenation (.) or string at glob_test.pl line 14.
$SITE =
$SITE = $SITE
Use of uninitialized value in concatenation (.) or string at glob_test.pl line 14.
$SITE =

我个人从来没有使用过glob（），所以我没有能力回答他。我阅读了 perldoc glob 文档，并遵循 File :: Glob 链接，仍然找不到任何可以解释输出的内容。任何帮助将不胜感激。

I personally have never used glob() in this fashion so I was ill equipped to answer him. I read through perldoc glob documentation and followed the File::Glob link on that page and still couldn’t find anything that would explain the output. Any help would be much appreciated.

推荐答案

glob 在标量上下文中：

glob in scalar context:

在标量上下文中，glob遍历这样的文件名扩展，当列表耗尽时返回undef。

In scalar context, glob iterates through such filename expansions, returning undef when the list is exhausted.

在

foreach (@list_env_vars){
    print "$_ = ".glob()."\n";
}

glob（）真的有 glob（$ _）。每次迭代， $ _ 包含字符串 $ SERVER 。鉴于环境变量不变， $ SERVER 扩展为相同的字符串。第一次返回此字符串。接下来，列表用尽，因此返回 undef 。第三次，我们重新开始。 ...

The glob() there really is glob($_). Every iteration, $_ contains the string $SERVER. Given that the environment variable does not change, $SERVER is expanded to the same string. First time, this string is returned. Next, the list is exhausted, so undef is returned. Third time, we start over. ...

澄清：第二个呼叫的参数与第一个呼叫的参数相同，因为那里无法重置 glob 的迭代器。

Clarification: It does not matter that the argument to the second call is the same as the one for the first call since there is no way to reset glob's iterator.

您可以使用以下示例（当前目录包含文件'1.a'，1.b'，'2.a'和'2.b'）：

You can see this more clearly using the following example (current directory contains files '1.a', 1.b', '2.a' and '2.b'):

#!/usr/bin/perl -w
use strict;

my @patterns = (
    '*.a',
    '*.b',
);

for my $v ( @patterns ) {
    print "$v = ", scalar glob($v), "\n";
}

输出：

C:\Temp> d
*.a = 1.a
*.b = 2.a

我建议您通过％ENV 哈希来访问环境变量：

I would recommend accessing environment variables via the %ENV hash:

my @list_env_vars = ($ENV{SERVER}) x 6;

或

my @list_env_vars = @ENV{qw(HOME TEMP SERVER)};

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