52476 + cf. 52479: more FAQs about nofork substitution

1 files changed, 38 insertions, 0 deletions

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?)