管道化heredoc的多行语法； 这是便携式的吗？

Question

I'm familiar with this syntax:

cmd1 << EOF | cmd2
text
EOF

but just discovered that bash allows me to write:

cmd1 << EOF |
text
EOF
cmd2

(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?

Answer 1

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

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.

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

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

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

cat file |
cmd

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 */

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

Answer 2

Yes it's in the POSIX shell grammar. You can also have more than one here-doc for the same command (some other examples use two cat invocations, but this works as well):

cat <<EOF1 <<EOF2
first here-doc
EOF1
second here-doc
EOF2

This is contrived (using 2 here-docs for stdin), but if you think of providing input for different file descriptors it immediately makes sense.

There's also the possibility to drop the cat entirely . Why not make the here-document directly available to cmd :

cmd << EOF
input
here
EOF

Answer 3

Hmm, I suppose yes, according to the test in bash in POSIX mode:

$ bash --posix
$ cat <<EOF |
> ahoj
> nazdar
> EOF
> sed 's/a/b/'
bhoj
nbzdar

Answer 4

Hi, check this, for example

#!/bin/sh
( base32 -d | base64 -d )<<ENDOFTEXT
KNDWW42DNNSHS5ZXPJCG4MSVM5MVQVT2JFCTK3DELBFDCY2IIJYGE2JUJNHWS22LINVHQMCMNVFD
CWJQIIZVUV2JOVNEOVJLINTW6PIK
ENDOFTEXT

regards