[PATCH] More stuff for Etc/FAQ

[PATCH] More stuff for Etc/FAQ

[Bart Schaefer]

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The file-slurping thread on zsh-users brought me here.

Anything else worth mentioning?

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diff --git a/Etc/FAQ.yo b/Etc/FAQ.yo
index 270a04608..98bc5dce4 100644
--- a/Etc/FAQ.yo
+++ b/Etc/FAQ.yo
@@ -1067,6 +1067,41 @@ label(211)
   quoted, that is, included in a quoted string or prefixed by backslash.
   These substitutions first become usable after zsh 5.9.
 
+sect(Comparisons of forking and non-forking command substitution)
+
+  mytt(${ command }) and variants may change the caller's options by using
+  mytt(setopt) and may modify the caller's local parameters, including the
+  positional parameters mytt($1), mytt($2), etc., via both assignments and
+  mytt(set -- pos1 pos2 etc).  Nothing that happens within mytt($(command))
+  affects the caller.
+
+  mytt($(command)) removes trailing newlines from the output of mytt(command)
+  when substituting, whereas mytt(${ command }) and its variants do not.
+
+  When mytt(command) is myem(not) a builtin, mytt(${ command }) does fork, and
+  typically forks the same number of times as mytt($(command)), because in
+  the latter case zsh usually optimizes the final fork into an exec.
+
+  Redirecting input from files has subtle differences:
+
+  mytt($(<file)) is optimized to read from mytt(file) without forking, but
+  per above it removes trailing newlines.
+
+  mytt(${<file}) is a substitution error.
+
+  mytt(${ <file }) copies mytt(file) using the mytt(NULLCMD) programs, then
+  reads and substitutes the contents of the copy.  Also, this fails if the
+  mytt(CSH_NULLCMD) or mytt(SH_NULLCMD) options are in effect, so it does
+  not work in emulation modes.
+
+  mytt(${|<file}) copies mytt(file) to the standard output using mytt(NULLCMD)
+  but substitutes nothing because there is no assignment to mytt(REPLY).  It
+  fails in emulation modes.
+
+  mytt(${|IFS= read -rd '' <file}) is therefore the best solution for files
+  that do not contain nul bytes, because it copies the file directly into
+  the local mytt(REPLY) and then substitutes that.
+
 chapter(How to get various things to work)
 
 sect(Why does mytt($var) where mytt(var="foo bar") not do what I expect?)

Re: [PATCH] More stuff for Etc/FAQ

[Lawrence Velázquez]

On Sat, Jan 13, 2024, at 3:48 PM, Bart Schaefer wrote:
> The file-slurping thread on zsh-users brought me here.
>
> Anything else worth mentioning?
>
> [...]
>
> +  mytt($(command)) removes trailing newlines from the output of mytt(command)
> +  when substituting, whereas mytt(${ command }) and its variants do not.

Hm, this diverges from the behavior of (m)ksh and (soon) bash:

	% cat ./nofork_test; echo
	echo "$KSH_VERSION$BASH_VERSION$ZSH_PATCHLEVEL"
	foo=${ echo abc; }
	typeset -p foo

	% ksh ./nofork_test; echo
	Version AJM 93u+ 2012-08-01
	foo=abc

	% mksh ./nofork_test; echo
	@(#)MIRBSD KSH R59 2020/10/31
	typeset foo=abc

	% Reference/bash/bash ./nofork_test; echo
	5.3.0(2)-devel
	declare -- foo="abc"

	% zsh/zsh/Src/zsh ./nofork_test
	zsh-5.9-342-gdde1259
	typeset foo=$'abc\n'

If this is intentional, then it might be worth mentioning in the
FAQ, to warn users who are used to the other behavior.

-- 
vq

Re: [PATCH] More stuff for Etc/FAQ

[Bart Schaefer]

On Sun, Jan 14, 2024 at 12:47 PM Lawrence Velázquez <larryv@zsh.org> wrote:
>
> > +  mytt($(command)) removes trailing newlines from the output of mytt(command)
> > +  when substituting, whereas mytt(${ command }) and its variants do not.
>
> Hm, this diverges from the behavior of (m)ksh and (soon) bash:

This was based on the discussion (and Sebastian's sample code) of how
mksh ${|REPLY=$'abc\n'} works.  I thought it better to be consistent
for all of these forms.

I'm disinclined to change it for native zsh,  but I will look at
whether it can easily be changed either based on emulation mode or a
setopt (SH_WORD_SPLIT maybe?  Hrm).

Defintely worth mentioning in the FAQ tho.

Re: [PATCH] More stuff for Etc/FAQ

[Lawrence Velázquez]

On Sun, Jan 14, 2024, at 5:37 PM, Bart Schaefer wrote:
> On Sun, Jan 14, 2024 at 12:47 PM Lawrence Velázquez <larryv@zsh.org> wrote:
>>
>> > +  mytt($(command)) removes trailing newlines from the output of mytt(command)
>> > +  when substituting, whereas mytt(${ command }) and its variants do not.
>>
>> Hm, this diverges from the behavior of (m)ksh and (soon) bash:
>
> This was based on the discussion (and Sebastian's sample code) of how
> mksh ${|REPLY=$'abc\n'} works.  I thought it better to be consistent
> for all of these forms.

I'm inclined to agree, despite the inconsistency with $(...) and
the other shells.  In zsh it will be easy to use something like

    foo=${${ cmd }%$'\n'}

if desired, while in the other shells preserving trailing LFs is
still annoying.


> Defintely worth mentioning in the FAQ tho.

A quick aside in the manual might also be good.


-- 
vq