在 bash 脚本中重绕 stdin [英] rewinding stdin in a bash script

问题描述

是否有一种简单的方法可以在我的 bash 脚本中倒带" /dev/stdin ，该脚本已经从输入管道中读取了全部或部分内容?

Is there a simple way to "rewind" /dev/stdin inside my bash script which already read all or some portion from the input pipe?

应用程序:我在第 1 部分中编写了一个简单的 MDA，它逐行读取来自 fetchmail 的一封电子邮件，如下所示:

Application: I wrote a simple MDA that in part 1, reads a single email from fetchmail line by line, like so:

while read -a linA; do
    echo -e "$[++linenum]:	${#linA[@]},${linA[*]}" > /dev/null  # verbose
    [ "${linA[0]}" = "Date:" ] && unset linA[0] && mailDate="${linA[*]}"
    [ "${linA[0]}" = "Subject:" ] && unset linA[0] && mailSubject="${linA[*]}"
    [ "$mailSubject" = "Courtesy Fill Notification" ] || break  # if wrong subject then thank you, we're done with this mail
done

并且在处理结束时，我希望将整个消息保存到一个文件中，既用于调试，又使管道的编写端看到其整个输出已被读取，并且不会返回失败(因此将邮件保持在邮箱中的未读状态).

and at the end of processing, I wish to save the entire message into a file, both for debugging, and so that the writer-side of the pipe sees that its entire output had been read, and not return failure (therefore keeping the message as unread in the mailbox).

推荐答案

读取管道是破坏性的；没有办法在管道上寻找:

Reading a pipe is destructive; there is no way to seek on a pipe:

ESPIPE (29): Illegal seek

错误编号来自 MacOS X，但名称是传统的(并且是 POSIX 强制要求的)并指示限制的来源.

The error number is from MacOS X, but the name is traditional (and POSIX-mandated) and gives an indication of where the restriction comes from.

因此，如果您想重新处理 shell 脚本中的输入，您需要将标准输入存储在一个文件中，以便您可以重新处理它:

So, if you want to reprocess input in a shell script, you will need to stash the standard input away in a file so you can reprocess it:

tmp=${TMPDIR:-/tmp}/xx.$$    # Consider mktemp or something
trap "rm -f $tmp.1; exit 1" 0 1 2 3 13 15  # Exit, HUP, INT, QUIT, PIPE, TERM

tee $tmp.1 |
while read -a linA
do
    ...
done

...reprocess $tmp.1 here...

rm -f $tmp.1
trap 0
exit $exit_status

唯一需要注意的陷阱是由于管道的原因，在父 shell 中无法访问 while 循环中设置的变量.如果这是一个问题，您可以使用:

The only trap to watch is that because of the pipeline, variables set in the while loop are not accessible in the parent shell. If that's a problem, you can use:

tee $tmp.1 |
{
while read -a linA
do
    ...
done

...reprocess $tmp.1 here...
}

大括号将用于 I/O 重定向目的的语句分组.tee 进程会因为 EOF 而完成，所以当 while read 检测到 EOF 时文件就会完成，所以访问 $tmp.1 在循环之后.

The braces group the statements for I/O redirection purposes. The tee process will have completed because of EOF so the file will be complete when the while read detects EOF, so it is safe to access $tmp.1 after the loop.

需要注意的一件事是 tee 会使它对终端输入无响应.文件不会有问题；管道输入不太可能成为问题.但是，tee 可能会完全缓冲其输出，而不是行缓冲其输出，因此在您输入多行输入之前，读取循环可能看不到任何内容.

The one thing to watch is that tee will make this unresponsive to terminal input. Files won't be a problem; piped input is unlikely to be a problem. However, tee will probably fully buffer its output, rather than line buffering its output, so the read loop may not see anything until you've typed many lines of input.

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