sh：如何避免破坏编号的文件描述符？

发布于 2024-11-18 01:46:10 字数 499 浏览 2 评论 0原文

当我有时，

  exec 3>>file               # file descriptor 3 now points to file
  [ $dryrun ] && exec 3>&1   # or possibly to stdout
  echo "running">&3
  exec 3>&-                  # and is now closed

我担心文件描述符 3 可能指向相关函数之外的内容。我该如何处理这个问题？

是否有内置的 next_available_fd ？
有没有办法将 fd3 复制到变量，然后在函数完成后将其复制回来？
- 在这种情况下我应该担心线程和并发写入 fd3 吗？
我在 sh，但也许 bash/ksh/zsh 有这个问题的答案？

原文

When I have

  exec 3>>file               # file descriptor 3 now points to file
  [ $dryrun ] && exec 3>&1   # or possibly to stdout
  echo "running">&3
  exec 3>&-                  # and is now closed

I'm worried about what file descriptor 3 may have pointed to outside of the function in question. How can I handle this?

Is there a builtin next_available_fd?
Is there a way to duplicate fd3 to a variable, then dup it back once the function is done?
- and should I worry about threading and concurrent writes to fd3 in this case?
I'm in sh, but maybe bash/ksh/zsh has an answer to this?

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酒解孤独 2024-11-25 01:46:10

您可以（使用 bash，我没有尝试使用其他 shell）执行以下操作，而不是使用 exec 在函数内重定向文件描述符：

foo() {
  test $dryrun && exec 3>&1
  echo running >&3
} 3>>file

foo
more_commands

在此设置中，“运行”将转到文件或原始标准输出，具体取决于$dryrun 和 more_commands 将具有 fd 3，就像调用 foo 之前一样。

Instead of using exec to redirect the file descriptor within the function, you can (with bash, I haven't tried with other shells) do:

foo() {
  test $dryrun && exec 3>&1
  echo running >&3
} 3>>file

foo
more_commands

In this setup, "running" will go to either the file or to the original stdout depending on $dryrun, and more_commands will have fd 3 as it was before foo was called.

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吃素的狼 2024-11-25 01:46:10

我不知道像 next_available_fd 这样简单的事情，但是要获得您想要的功能（临时重定向文件描述符而不影响函数外部）可以在 bash 中按以下方式完成（我不知道不知道 sh):

exec 3>file3
exec 1>file1

echo "something">&3
echo "something else"

f31 () {
        echo "something">&3
}
f31 3>&1

f13 () {
        echo "something else"
}
f13 >&3

echo "something">&3
echo "something else"

生成的 file1:

something else
something
something else

和 file3:

something
something else
something

这表明重定向仅限于每种情况下的函数调用。

I don't know about anything as simple as next_available_fd, but to get the functionality that you want (temporarily redirecting a file descriptor without affecting it outside the function) can be accomplished as follows in bash (I don't know about sh):

exec 3>file3
exec 1>file1

echo "something">&3
echo "something else"

f31 () {
        echo "something">&3
}
f31 3>&1

f13 () {
        echo "something else"
}
f13 >&3

echo "something">&3
echo "something else"

The resulting file1: