Where is this =(:) construct documented?

Where is this =(:) construct documented?

[Zach Riggle]

It's insane, cool, and I can't find this documented anywhere.  It
creates a temporary file that is automatically deleted... but only
after the line / expression terminates.

( ... ) creates a subshell (terminology?)
( : ) creates a subshell that does nothing
=bash evaluates to the path to bash, e.g. /usr/bin/bash
$(...) captures the output of a subshell, e.g. x=$(echo foo)

If follows, then, that =(:) shows the path to... what? Apparently, a
scope-tracked, auto-deleting temporary file.  *Where is this
documented?*

# wat.zsh ####
wat() {
    >&2 echo IN WAT
    >&2 ls -la "$1"
    echo "$1"
}ls -la $(wat =(:))
#############

# example #####
$ zsh wat.sh
IN WAT
-rw-------  1 zachriggle  wheel  0 Jun 11 14:13 /tmp/zshfMERth
ls: /tmp/zshfMERth: No such file or directory
#############

What's more insane is that the temporary file will be auto-populated
with the output of the =() construct.  Where is THAT documented?

#############
$ (){ echo $1; cat $1 } =( echo allo )
/tmp/zshpfFAOp
allo
#############


Zach Riggle

Re: Where is this =(:) construct documented?

[Zach Riggle]

Formatting fix, I accidentally a newline.

# wat.zsh ####
wat() {
    >&2 echo IN WAT
    >&2 ls -la "$1"
    echo "$1"
}
ls -la $(wat =(:))
#############

Zach Riggle

On Fri, Jun 11, 2021 at 2:17 PM Zach Riggle <zachriggle@gmail.com> wrote:
>
> It's insane, cool, and I can't find this documented anywhere.  It
> creates a temporary file that is automatically deleted... but only
> after the line / expression terminates.
>
> ( ... ) creates a subshell (terminology?)
> ( : ) creates a subshell that does nothing
> =bash evaluates to the path to bash, e.g. /usr/bin/bash
> $(...) captures the output of a subshell, e.g. x=$(echo foo)
>
> If follows, then, that =(:) shows the path to... what? Apparently, a
> scope-tracked, auto-deleting temporary file.  *Where is this
> documented?*
>
> # wat.zsh ####
> wat() {
>     >&2 echo IN WAT
>     >&2 ls -la "$1"
>     echo "$1"
> }ls -la $(wat =(:))
> #############
>
> # example #####
> $ zsh wat.sh
> IN WAT
> -rw-------  1 zachriggle  wheel  0 Jun 11 14:13 /tmp/zshfMERth
> ls: /tmp/zshfMERth: No such file or directory
> #############
>
> What's more insane is that the temporary file will be auto-populated
> with the output of the =() construct.  Where is THAT documented?
>
> #############
> $ (){ echo $1; cat $1 } =( echo allo )
> /tmp/zshpfFAOp
> allo
> #############
>
>
> Zach Riggle

Re: Where is this =(:) construct documented?

[Stephane Chazelas]

2021-06-11 14:17:08 -0500, Zach Riggle:
> It's insane, cool, and I can't find this documented anywhere.  It
> creates a temporary file that is automatically deleted... but only
> after the line / expression terminates.
[...]
> What's more insane is that the temporary file will be auto-populated
> with the output of the =() construct.  Where is THAT documented?
[...]

See

info zsh "Process Substitution"

Or:

https://zsh.sourceforge.io/Doc/Release/Expansion.html#Process-Substitution

That's the third form of process substitution.

ksh introduced the first two <(...) and >(...) in the 80s. zsh
added that third form in the 90s.

While A <(B) and A >(B) are IPC mechanisms in that A and B are
started concurrently and connected via a pipe (<(B) and >(B)
expand respectively to the path of the reading of a pipe and
writing end of a pipe, in A =(B), B is started first, its output
collected into a temp file, and the path of that temp file
passed to A when B has finished (and later removed when A
finishes).

-- 
Stephane

Re: Where is this =(:) construct documented?

[Roman Perepelitsa]

On Fri, Jun 11, 2021 at 9:53 PM Stephane Chazelas <stephane@chazelas.org> wrote:
>
> That's the third form of process substitution.

And the colon in =(:) is a builtin that does nothing (the same as
true). It's mandated by POSIX.

Roman.

Re: Where is this =(:) construct documented?

[Stephane Chazelas]

2021-06-11 20:53:22 +0100, Stephane Chazelas:
[...]
> info zsh "Process Substitution"
[...]

For information, searching in info (with / or s or Ctrl+S) these
days uses extended regexps. So, to search for =(, you need to
search for =\( or toggle regexp matching off first with R. See
info info / for details.

[...]
> That's the third form of process substitution.
> 
> ksh introduced the first two <(...) and >(...) in the 80s. zsh
> added that third form in the 90s.

Already in ksh86. In ksh86 you could actually also use A (B) in
place of A <(B) though it looks like it was removed in later
versions. rc also had process substitution with a different
syntax IIRC.

See also:

https://unix.stackexchange.com/questions/49918/when-was-process-substitution-first-introduced/49952#49952

and

https://unix.stackexchange.com/questions/181937/how-create-a-temporary-file-in-shell-script/181996#181996

-- 
Stephane

Re: Where is this =(:) construct documented?

[Zach Riggle]

Thanks for the help, all!  This was impossible to Google for!

Most obfuscated I could make it (to mess with coworkers) is...

(){<<<$1} =(:)

Zach Riggle

On Fri, Jun 11, 2021 at 3:04 PM Stephane Chazelas <stephane@chazelas.org> wrote:
>
> 2021-06-11 20:53:22 +0100, Stephane Chazelas:
> [...]
> > info zsh "Process Substitution"
> [...]
>
> For information, searching in info (with / or s or Ctrl+S) these
> days uses extended regexps. So, to search for =(, you need to
> search for =\( or toggle regexp matching off first with R. See
> info info / for details.
>
> [...]
> > That's the third form of process substitution.
> >
> > ksh introduced the first two <(...) and >(...) in the 80s. zsh
> > added that third form in the 90s.
>
> Already in ksh86. In ksh86 you could actually also use A (B) in
> place of A <(B) though it looks like it was removed in later
> versions. rc also had process substitution with a different
> syntax IIRC.
>
> See also:
>
> https://unix.stackexchange.com/questions/49918/when-was-process-substitution-first-introduced/49952#49952
>
> and
>
> https://unix.stackexchange.com/questions/181937/how-create-a-temporary-file-in-shell-script/181996#181996
>
> --
> Stephane

Re: Where is this =(:) construct documented?

[Vincent Lefevre]

On 2021-06-11 15:39:29 -0500, Zach Riggle wrote:
> Thanks for the help, all!  This was impossible to Google for!
> 
> Most obfuscated I could make it (to mess with coworkers) is...
> 
> (){<<<$1} =(:)

You need to search in the man pages (I recommend to do that with
"man zshall" so that all zsh man pages are searched).

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

Re: Where is this =(:) construct documented?

[Stephane Chazelas]

2021-06-14 14:44:17 +0200, Vincent Lefevre:
> On 2021-06-11 15:39:29 -0500, Zach Riggle wrote:
> > Thanks for the help, all!  This was impossible to Google for!
> > 
> > Most obfuscated I could make it (to mess with coworkers) is...
> > 
> > (){<<<$1} =(:)
> 
> You need to search in the man pages (I recommend to do that with
> "man zshall" so that all zsh man pages are searched).
[...]

I wouldn't recommend man for a manual this big. I don't think
I've read zsh's man page in the last decade.

IMO, info is much more appropriate especially considering the
zsh manual there has a very good index and table of contents (i
and I commands with completion are your friends in info, zsh
also has completion for index entries so "info zsh topic<Tab>"
often works).

On some systems, you need to install a zsh-doc package or
equivalent though for the info pages to be available.

-- 
Stephane