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. - Note that I set
F'up-to CUS. And post the response there as a f'up to this post.

Janis

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

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

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

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

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

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