xref/busybox-1.35.0/shell

http://www.opengroup.org/onlinepubs/9699919799/
Open Group Base Specifications Issue 7


http://www.opengroup.org/onlinepubs/9699919799/utilities/V3_chap01.html
Shell & Utilities

It says that any of the standard utilities may be implemented
as a regular shell built-in. It gives a list of utilities which
are usually implemented that way (and some of them can only
be implemented as built-ins, like "alias"):

alias
bg
cd
command
false
fc
fg
getopts
jobs
kill
newgrp
pwd
read
true
umask
unalias
wait


http://www.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html
Shell Command Language

It says that shell must implement special built-ins. Special built-ins
differ from regular ones by the fact that variable assignments
done on special builtin are *PRESERVED*. That is,

VAR=VAL special_builtin; echo $VAR

should print VAL.

(Another distinction is that an error in special built-in should
abort the shell, but this is not such a critical difference,
and moreover, at least bash's "set" does not follow this rule,
which is even codified in autoconf configure logic now...)

List of special builtins:

. file
: [argument...]
break [n]
continue [n]
eval [argument...]
exec [command [argument...]]
exit [n]
export name[=word]...
export -p
readonly name[=word]...
readonly -p
return [n]
set [-abCefhmnuvx] [-o option] [argument...]
set [+abCefhmnuvx] [+o option] [argument...]
set -- [argument...]
set -o
set +o
shift [n]
times
trap n [condition...]
trap [action condition...]
unset [-fv] name...

In practice, no one uses this obscure feature - none of these builtins
gives any special reasons to play such dirty tricks.

However. This section also says that *function invocation* should act
similar to special built-in. That is, variable assignments
done on function invocation should be preserved after function invocation.

This is significant: it is not unthinkable to want to run a function
with some variables set to special values. But because of the above,
it does not work: variable will "leak" out of the function.
Name		Date	Size	#Lines	LOC
..		-	-
ash_test/		26-Dec-2021	-	7,957	6,603
hush_test/		26-Dec-2021	-	8,477	7,127
Config.src	D	26-Dec-2021	5.5 KiB	190	154
Kbuild.src	D	26-Dec-2021	214	12	8
README	D	26-Dec-2021	2 KiB	83	67
README.job	D	26-Dec-2021	16.2 KiB	305	296
ash.c	D	26-Dec-2021	330.5 KiB	14,763	11,254
ash_doc.txt	D	26-Dec-2021	4.2 KiB	123	105
ash_ptr_hack.c	D	26-Dec-2021	974	30	12
ash_remove_unnecessary_code_in_backquote_expansion.patch	D	26-Dec-2021	4 KiB	136	114
brace.txt	D	26-Dec-2021	2.6 KiB	51	42
cttyhack.c	D	26-Dec-2021	5.5 KiB	189	91
hush.c	D	26-Dec-2021	336 KiB	12,201	8,319
hush_doc.txt	D	26-Dec-2021	5.1 KiB	144	120
hush_leaktool.sh	D	26-Dec-2021	566	26	19
match.c	D	26-Dec-2021	3.3 KiB	150	107
match.h	D	26-Dec-2021	806	35	24
math.c	D	26-Dec-2021	26 KiB	857	492
math.h	D	26-Dec-2021	3 KiB	91	21
random.c	D	26-Dec-2021	4 KiB	161	58
random.h	D	26-Dec-2021	871	42	18
shell_common.c	D	26-Dec-2021	16.5 KiB	690	497
shell_common.h	D	26-Dec-2021	1.5 KiB	60	27