blocks|key|2768957|text|它们在将Perl脚本包装在另一个文件中时非常有用，就像pl2bat一样。Perl在文件的开头看不到批处理命令，这就抛弃了它的行号概念。Perl源代码开头的#line指令弥补了这一点。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2768958|entityMap^0|25|5|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

They're useful when wrapping a Perl script in another file, like pl2bat does. Perl doesn't see the batch commands at the beginning of the file which throws off its idea of line numbers. A <code>#line</code> directive at the beginning of the Perl source compensates for this.

blocks|key|199127|text|通常，这样的标记被放入经过预处理或机械生成的代码中，以便引用人类生成的源代码。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|199128|例如，如果有一个程序将Python转换为Perl，它可能会插入一个|199129|#+line+812+"foo.py"|code-block|syntax|javascript|199130|因此，错误消息将指向原始Python代码，这对程序员来说更有意义。|199131|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|O|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Usually such markers are put into code that has been pre-processed or mechanically generated in order to refer back to the human-generated source.

For example, if there was a program that converted Python to Perl, it might insert a 

<pre><code># line 812 "foo.py"
</code></pre>

so that error messages would refer to the original Python code which would make more sense to the programmer.

blocks|key|199388|text|除了前面提到的原因之外，perl还有一个(强烈建议您不要这么做的)+-P选项，它可以在Perl文件执行之前通过C预处理器运行它。由于大多数C预处理器在包含或删除文件的一部分时将使用行指令，因此任何错误都将从它们在原始源中的位置报告，而不是从处理后的源中报告。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|199389|如果您正在生成字符串形式的代码，然后将其传递给eval，则行指令也非常有用。通常，如果在这类代码中出现警告或错误，您会得到类似于"died+at+(eval+1)+line+1“的错误报告。使用line指令，您可以提供有用的文件名和行号。|199390|entityMap^0|C|4|Y|2|0|N|4|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|R|8|@]|D|@]|E|$]]]|I|$]]

In addition to the already mentioned reasons <code>perl</code> has a (strongly discouraged) <code>-P</code> option that runs the Perl file through a C preprocessor before it is executed. Since most C preprocessor's will use line directives when they include or remove part of a file so any errors will be reported from where they were located in the original source instead of the processed source.

Line directives can also be very useful if you are generating code in strings that is then passed to <code>eval</code>. Normally if there is a warning or error in such code you get an error reported like "died at (eval 1) line 1." Using line directives you can supply a useful file name and line number.

blocks|key|2769051|text|我见过几次人们错误地将当前行号写到#line指令中。它应该包含下一行的数字。linetest.pl的示例代码(使用ksh包装器为perl脚本设置环境变量)：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2769052|1++#!/usr/bin/ksh
2++MY_ENV_VAR='something'
3++export+MY_ENV_VAR
4++/usr/bin/perl+-x+$0+$@+2>&1
5++exit+$?
6
7++#!/usr/bin/perl
8++#line+9
9++print+"MY_ENV_VAR+is+$ENV{MY_ENV_VAR}\n";
10+die+"This+is+line+10.";|code-block|syntax|javascript|2769053|运行脚本并检查结果：|2769054|$+./linetest.pl
MY_ENV_VAR+is+something
This+is+line+10.+at+./linetest.pl+line+10.|2769055|在第8行写入#line+9后，您可以看到行号匹配。|offset|length|style|CODE|2769056|entityMap^0|0|0|0|0|6|7|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@$M|X|N|Y|O|P]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

I've seen several times that people incorrectly write the current line's number into the #line directive. It should contain the next line's number. Example code of linetest.pl (using a ksh wrapper to set an environment variable for the perl script):

<pre><code>1 #!/usr/bin/ksh
2 MY_ENV_VAR='something'
3 export MY_ENV_VAR
4 /usr/bin/perl -x $0 $@ 2&gt;&amp;1
5 exit $?
6
7 #!/usr/bin/perl
8 #line 9
9 print "MY_ENV_VAR is $ENV{MY_ENV_VAR}\n";
10 die "This is line 10.";
</code></pre>

Run the script and check the result:

<pre><code>$ ./linetest.pl
MY_ENV_VAR is something
This is line 10. at ./linetest.pl line 10.
</code></pre>

You can see that line numbers are matching after writing <code>#line 9</code> on line 8.

blocks|key|199205|text|在shell脚本中执行perl+-e时，#line指令也非常有用。我写|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|199206|perl+-e+'#line+X
more+perl+code+here...
'|code-block|syntax|javascript|199207|其中X是当前shell脚本行%2B1，因此任何Perl错误都会告诉我失败的语句所在的Perl+shell行。|BOLD|199208|entityMap^0|B|7|0|0|Z|F|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|S|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|T|8|@$9|U|A|V|B|M]]|D|@]|E|$]]|$1|N|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|O|$]]

The #line directive is also very helpful when doing <code>perl -e</code> in a shell script. I write
<pre><code>perl -e '#line X
more perl code here...
'
</code></pre>
where X is the current shell script line +1 so that any Perl errors tell me the shell line where the failed Perl statement is.

<a href="http://perldoc.perl.org/perlsyn.html#Plain-Old-Comments-%28Not!%29" rel="noreferrer">Line directives</a> (#line) are used to reconfigure perl's idea of the current filename and line number. When is this required to get right filename and line number (in an error message)?

What's the purpose of perl's #line directives?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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 (# line )用于重新配置perl的当前文件名和行号。什么时候需要这样才能获得正确的文件名和行号(在错误消息中)？

问perl的#line指令的用途是什么？
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问perl的#line指令的用途是什么？EN