busybox/shell
Denys Vlasenko eaf9436b08 ash: [REDIR] Move null redirect checks into caller
Upstream commit:

    Date: Thu, 27 May 2010 14:21:17 +0800
    [REDIR] Move null redirect checks into caller

    The null redirect checks were added as an optimisation to avoid
    unnecessary memory allocations.  However, we could avoid this
    completely by simply making the caller avoid making a redirection
    unless it is not null.

    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

function                                             old     new   delta
evaltree                                             784     809     +25
evalcommand                                         1251    1261     +10
hashvar                                               59      62      +3
dotcmd                                               321     319      -2
clearredir                                            37      30      -7
popredir                                             183     162     -21
redirect                                            1264    1233     -31
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/4 up/down: 63/-61)            Total: -23 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
2016-10-25 21:46:03 +02:00
..
ash_test ash testsuite: fix false positives 2016-10-07 04:07:05 +02:00
hush_test sh testsuite: add tests for exitcode on failure to exec 2016-10-03 00:55:34 +02:00
ash_doc.txt
ash_ptr_hack.c *: make GNU licensing statement forms more regular 2010-08-16 20:14:46 +02:00
ash.c ash: [REDIR] Move null redirect checks into caller 2016-10-25 21:46:03 +02:00
brace.txt
Config.src config: disentangle PREFER_APPLETS from SH_STANDALONE and SH_NOFORK 2016-07-22 18:48:38 +02:00
cttyhack.c cttyhack: handle multiple consoles found in sysfs 2012-02-04 21:55:01 +01:00
hush_doc.txt
hush_leaktool.sh
hush.c ash,hush: set exit code 127 in "sh /does/not/exist" case 2016-10-25 17:00:13 +02:00
Kbuild.src *: make GNU licensing statement forms more regular 2010-08-16 20:14:46 +02:00
match.c shell/match.c: shrink by dropping double bool inversion 2010-09-12 15:06:42 +02:00
match.h hush: optimize #[#] and %[%] for speed. size -2 bytes. 2010-09-04 21:21:07 +02:00
math.c typo fix in comment 2014-11-20 01:43:30 +01:00
math.h move endofname() to libbb 2013-02-26 00:36:53 +01:00
random.c ash,hush: fix a thinko about 2^64-1 factorization 2014-03-15 09:25:46 +01:00
random.h ash,hush: improve randomness of $RANDOM, add easy-ish way to test it 2014-03-13 12:52:43 +01:00
README
README.job
shell_common.c ash: [VAR] Initialise OPTIND after importing environment 2016-09-30 14:46:41 +02:00
shell_common.h ash: [VAR] Initialise OPTIND after importing environment 2016-09-30 14:46:41 +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.