perl - 本地在标准流/裸字文件句柄 : STDIN? 上做什么

Question

我正在考虑做

open(*STDIN, "<", "/dev/null" );
open(my $fh, "-|", "/bin/bash", "/tmp/foo");
print for <$fh>;'

但是，我想*STDIN恢复，后来，所以我尝试了。

{
  open(local *STDIN, "<", "/dev/null" );
  open(my $fh, "-|", "/bin/bash", "/tmp/foo");
  print for <$fh>;
}

你可以用 cat 试试这个带和不带 local像这样的关键字，

{
  # remove local and it works,
  open(local *STDIN, "<", "/dev/null" );
  open(my $fh, "-|", "/bin/cat");
  print for <$fh>;
}

只有没有 local将 cat阅读自 /dev/null .那么local有什么用实际上是在一个裸字文件句柄上做的吗？

Answer 1

open ...,'-|'和 system从标准输入读取的调用将尝试从文件描述符 0 读取。如果你不搞砸，外部程序将从与 perl 的标准输入相同的输入流中读取。

# reads from standard input
open my $fh, '-|', "/bin/cat";
while (<fh>) { print }

# reads from /tmp/foo
open STDIN, "<", "/tmp/foo";       # replaces fd0 with handle to /tmp/foo
open my $fh, '-|', "/bin/cat";
while (<$fh>) { print }

# reads from standard input
open local *STDIN, "<", "/tmp/foo";  # doesn't close fd0, creates new fd
open my $fh, '-|',  "/bin/cat";
while (<$fh>) { print }

# reads from /tmp/foo
close STDIN;
open FOO, "<", "/tmp/foo";       # fileno(FOO) should be 0 now
open my $fh, '-|', "/bin/cat";
while (<$fh>) { print }