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52476 + cf. 52479: more FAQs about nofork substitution

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Bart Schaefer 2024-01-24 17:42:36 -08:00
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@ -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

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Etc/FAQ.yo
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@ -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?)