busybox/shell
Shawn Landden 58598eb709 ash: optional sleep builtin
function                                             old     new   delta
sleepcmd                                               -      10     +10
builtintab                                           352     360      +8
.rodata                                           105264  105271      +7
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(add/remove: 1/0 grow/shrink: 2/0 up/down: 25/0)               Total: 25 bytes

Signed-off-by: Shawn Landden <shawnlandden@tutanota.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
2022-08-27 19:56:21 +02:00
..
ash_test ash: fix ifs cleanup on error paths 2022-08-02 11:18:11 +02:00
hush_test ash: fix ifs cleanup on error paths 2022-08-02 11:18:11 +02:00
ash_doc.txt
ash_ptr_hack.c
ash_remove_unnecessary_code_in_backquote_expansion.patch ash: save Ron's patch from oblivion 2021-06-06 13:01:25 +02:00
ash.c ash: optional sleep builtin 2022-08-27 19:56:21 +02:00
brace.txt
Config.src nologin: make it possible to build it as single applet 2020-06-24 15:05:22 +02:00
cttyhack.c config: update size information 2018-12-28 03:20:17 +01:00
hush_doc.txt
hush_leaktool.sh
hush.c ash,hush: use HOME for tab completion and prompts 2022-06-26 18:05:50 +02:00
Kbuild.src Make it possible to select "sh" and "bash" aliases without selecting ash or hush 2016-12-23 16:56:43 +01:00
match.c style fix 2022-05-01 17:06:00 +02:00
match.h
math.c shell: fix parsing of $(( (v)++ + NUM )) 2021-09-26 13:29:25 +02:00
math.h shell: move all definitions of strto_arith_t() together 2019-05-26 14:02:10 +02:00
random.c whitespace fixes 2018-07-17 15:04:17 +02:00
random.h
README
README.job
shell_common.c shell: add comments about SIGINT-related problems 2022-01-16 23:54:46 +01:00
shell_common.h hush: fix "export PS1=xyz" and "local PS1=xyz" messing up prompt 2019-05-14 18:56:04 +02:00

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.