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From: Lem Novantotto <Lem@none.invalid>
Newsgroups: comp.unix.shell
Subject: Re: Different variable assignments
Date: Sat, 12 Oct 2024 23:07:41 -0000 (UTC)
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Il Sat, 12 Oct 2024 17:57:16 +0200, Frank Winkler ha scritto:

> I'm still thinking about the difference between "< <(...)" and "<<<`...`"

As already pointed out by Lawrence:

1) "command2 <<<`command1`": `command2`, equal to the preferable[*]
$(command2), can be seen as the output of command2: a string. So with
<<< you are telling the shell to take this string as input on the 
standard input of command2. So: execute command1, then take its output
as the input of command2. This is called *command* substitution:
substitution of a command with its output.

[*] No characters are special between parenthesis: easier.

2) "command2 < <(process_of_command1)": here process, which is the
"active running instance" of command1 - enough: remove the difference,
think of command1 and stop - is run with its output connected to a
named FIFO pipe file (or to some file in /dev/fd).
So <(process_of_command1) is expanded as the name of this file.
The first < is simple input redirection.
So: execute command1, connect the output of its process to a special
file, redirect this special file to input of command2.
This is called *process* output substitution: substitute a
process with the name of its output file.
Process substitution does work only on systems supporting named pipes
or /dev/fd/... so it's less universal than command substitution.

Some practical differences. For example, try:

$ cat <<<$(while true; do echo yes; done)

Nothing: cat is waiting for the other stuff to end. But it won't end,
in this case! Gawsh!

$ cat < <( while true; do echo yes; done)

Rock'n'roll!

> And I would be happy with this one if there was a notation "the other
> way round"  ... something like "... >>> $var"

You're more the straightforward type of guy, eh! That's fine! ;)
But, sorry, '>>>' is still to come. In bash, at least. :-)
-- 
Bye, Lem