用于传递Heredoc的多行语法；这是便携式的吗? [英] Multiline syntax for piping a heredoc; is this portable?

问题描述

我熟悉以下语法:

cmd1 << EOF | cmd2
text
EOF

但是刚刚发现bash允许我写:

but just discovered that bash allows me to write:

cmd1 << EOF |
text
EOF
cmd2

(heredoc用作cmd1的输入，而cmd1的输出通过管道传递到cmd2).这似乎是一个非常奇怪的语法.便携吗?

(the heredoc is used as input to cmd1, and the output of cmd1 is piped to cmd2). This seems like a very odd syntax. Is it portable?

推荐答案

是的，POSIX标准允许这样做. 根据2008年版本:

Yes, the POSIX standard allows this. According to the 2008 version:

此处文档应被视为一个单词，该单词以 下一个<newline>并继续直到有一行仅包含 分隔符和<newline>，中间没有<blank>字符. 然后，如果有的话，下一个此处文档开始.

The here-document shall be treated as a single word that begins after the next <newline> and continues until there is a line containing only the delimiter and a <newline>, with no <blank> characters in between. Then the next here-document starts, if there is one.

并在同一行中包含以下多个"here-documents"示例:

And includes this example of multiple "here-documents" in the same line:

cat <<eof1; cat <<eof2
Hi,
eof1
Helene.
eof2

因此，进行重定向或管道操作没有问题.您的示例类似于以下内容:

So there is no problem doing redirections or pipes. Your example is similar to something like this:

cat file |
cmd

shell语法(在链接页面的更下方)包括以下定义:

And the shell grammar (further down on the linked page) includes these definitions:

pipe_sequence    :                             command
                 | pipe_sequence '|' linebreak command

newline_list     :              NEWLINE
                 | newline_list NEWLINE
                 ;
linebreak        : newline_list
                 | /* empty */

因此，管道符号可以在行尾跟随，并且仍被视为管道的一部分.

So a pipe symbol can be followed by an end-of-line and still be considered part of a pipeline.

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