From: Bart Schaefer <schaefer@brasslantern.com>
To: Zsh hackers list <zsh-workers@zsh.org>
Subject: [PATCH] More stuff for Etc/FAQ
Date: Sat, 13 Jan 2024 12:48:44 -0800 [thread overview]
Message-ID: <CAH+w=7YUUkKQ=P-+C8TNoK5CgH0aYssE_EFsXR8tJG8zyqizuQ@mail.gmail.com> (raw)
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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?)
next reply other threads:[~2024-01-13 20:49 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-13 20:48 Bart Schaefer [this message]
2024-01-14 20:46 ` Lawrence Velázquez
2024-01-14 22:37 ` Bart Schaefer
2024-01-15 0:39 ` Lawrence Velázquez
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