From 0fccdf0d57fb8ed4b88df6681d15822f3c4e784f Mon Sep 17 00:00:00 2001
From: Bart Schaefer <schaefer@zsh.org>
Date: Wed, 24 Jan 2024 17:42:36 -0800
Subject: 52476 + cf. 52479: more FAQs about nofork substitution

---
 ChangeLog  |  2 ++
 Etc/FAQ.yo | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index 66397f10a..d51d527b8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,7 @@
 2024-01-24  Bart Schaefer  <schaefer@zsh.org>
 
+	* 52476 + cf. 52479: Etc/FAQ.yo: more about nofork substitution
+
 	* 52477: Src/Modules/curses.c: fix "zcurses mouse delay ..."
 
 	* 52473: Doc/Zsh/mod_zutil.yo, Src/Modules/zutil.c: zstyle -q
diff --git a/Etc/FAQ.yo b/Etc/FAQ.yo
index 270a04608..7d46e9192 100644
--- a/Etc/FAQ.yo
+++ b/Etc/FAQ.yo
@@ -1067,6 +1067,44 @@ 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.
+  The latter is consistent with mytt(${|...}) from mksh but differs from
+  bash and ksh, so in emulation modes, newlines are stripped from command
+  output (not from mytt(REPLY) assignments).
+
+  When mytt(command) is myem(not) a builtin, mytt(${ 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?)
-- 
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