On 28.02.2024 00:50, Janis Papanagnou wrote:

On 28.02.2024 00:10, Lawrence D'Oliveiro wrote:

On Tue, 27 Feb 2024 13:18:20 +0100, Janis Papanagnou wrote:

On 26.02.2024 21:31, Lawrence D'Oliveiro wrote:

Question: How would you do two separate <<-strings in the same shell
command?

Can you give an example what you intend here? (With what semantics?)

Since '<<' is redirecting the here-document text to stdin of the command >>> you can have only one channel.

Perl lets you do something like

func(<<EOD1, <<EOD2);
... contents of first string ...
EOD1
... contents of second string ...
EOD2

But this doesn’t work in Bash. However, in a Posix shell, remember you can >> specify the number of the file descriptor you want to redirect, e.g.

diff -u /dev/fd/8 /dev/fd/9 8<<'EOD1' 9<<'EOD2'
... contents of first string ...
EOD1
... contents of second string ...
EOD2

Note I add the single quotes to prevent expansion of “$”-sequences within
the strings. (I think this might be needed in Perl, too.)

I see. - Yes, you can do that in POSIX shells as well. [...]

You found a syntactic solution already, but that requires some
non-standard /dev/fd OS support. Newer shells allow also process
substitution diff <(...) <(...) to support pipes where files
are expected, but that also relies on the existence of /dev/fd,
AFAICT.

With standard mechanisms when I want to read from two individual
stdin sources I do (for example) something like

exec 3<&0
while read <&3 ; read <&3 ; read <&3 # every 3rd line data #1
do echo $REPLY
read ; read ; echo $REPLY # every 2nd line data #2
done 3<<EOT1 <<EOT2
line 1
line 2
line 3
line 4
line 5
line 6
line 7
EOT1
line A
line B
line C
line D
line E
EOT2

With actually the same syntax at least for the here documents.

Otherwise, in your diff sample, use named pipes?

Janis

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On 2024-02-28, Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:

You found a syntactic solution already, but that requires some
non-standard /dev/fd OS support. Newer shells allow also process
substitution diff <(...) <(...) to support pipes where files
are expected, but that also relies on the existence of /dev/fd,
AFAICT.

Bash, at least, can alternatively use named pipes.
This is on FreeBSD:

bash$ cat s
#!/bin/sh
echo "$@"
ls -l "$@"

bash$ ./s <(echo hello)
/tmp//sh-np.Uawtgm
prw------- 1 naddy wheel 0 Feb 28 16:19 /tmp//sh-np.Uawtgm

--
Christian "naddy" Weisgerber naddy@mips.inka.de

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On 28.02.2024 16:22, Christian Weisgerber wrote:

On 2024-02-28, Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:

You found a syntactic solution already, but that requires some
non-standard /dev/fd OS support. Newer shells allow also process
substitution diff <(...) <(...) to support pipes where files
are expected, but that also relies on the existence of /dev/fd,
AFAICT.

Bash, at least, can alternatively use named pipes.

Yes, below you're using newer shells' process substitution (as I
also did above). It implicitly requires an OS supporting /dev/fd .

The original poster (from comp.lang.c) used also a /dev/fd based
version like this
diff -u /dev/fd/8 /dev/fd/9 8<<'EOD1' 9<<'EOD2'
which appears overly complicated and it's explicitly also relying
on /dev/fd .

This is on FreeBSD:

bash$ cat s
#!/bin/sh
echo "$@"
ls -l "$@"

bash$ ./s <(echo hello)
/tmp//sh-np.Uawtgm
prw------- 1 naddy wheel 0 Feb 28 16:19 /tmp//sh-np.Uawtgm

With the OP's original question:

Question: How would you do two separate <<-strings in the same
shell command?

it would be interesting to see some neat solution that does not
rely implicitly (by process substitution) or explicitly on /dev/fd .

The OP gave an example how it can be done in Perl:
func(<<EOD1, <<EOD2);
... contents of first string ...
EOD1
... contents of second string ...
EOD2
and is probably seeking a terse alternative in shell.

Janis

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On 2024-02-28, Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:

Bash, at least, can alternatively use named pipes.

Yes, below you're using newer shells' process substitution (as I
also did above). It implicitly requires an OS supporting /dev/fd .

It does not! That was my point. Bash implements process substitution
with temporary named pipes on systems where /dev/fd/* may not be
available:

bash$ ./s <(echo hello)
/tmp//sh-np.Uawtgm
prw------- 1 naddy wheel 0 Feb 28 16:19 /tmp//sh-np.Uawtgm

--
Christian "naddy" Weisgerber naddy@mips.inka.de

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On 28.02.2024 19:12, Christian Weisgerber wrote:

On 2024-02-28, Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:

Yes, below you're using newer shells' process substitution (as I
also did above). It implicitly requires an OS supporting /dev/fd .

It does not! That was my point. Bash implements process substitution
with temporary named pipes on systems where /dev/fd/* may not be
available:

That's interesting! And you are right; the bash man page indeed says:
"Process substitution is supported on systems that support
named pipes (FIFOs) or the /dev/fd method of naming open
files."

The Bolsky/Korn Kornshell book says that Kornshell supports process substitution on Unix systems that support /dev/fd/ and I'd have
expected that Bash adopted that from Kornshell (as it did with so
many Kornshell features). But probably not...

Or does Kornshell also support it independently of /dev/fd/ also
with named pipes? (I cannot check that on my system.)

Janis

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On 2024-02-29, Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:

The Bolsky/Korn Kornshell book says that Kornshell supports process substitution on Unix systems that support /dev/fd/ and I'd have
expected that Bash adopted that from Kornshell (as it did with so
many Kornshell features). But probably not...

Or does Kornshell also support it independently of /dev/fd/ also
with named pipes? (I cannot check that on my system.)

Checking the nearest available ksh93 source, I see:

#### COMPILE-TIME OPTIONS ####

...
DEVFD Use the more secure /dev/fd mechanism instead of FIFOs for
process substitutions. On by default on OSs with /dev/fd.
...

--
Christian "naddy" Weisgerber naddy@mips.inka.de

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