Various bits of what is known about busybox shells, in no particular order.

2008-02-14
ash: does not restore tty pgrp if killed by HUP. Symptom: Midnight Commander
is backgrounded if you started ash under it, and then killed it with HUP.

2007-11-23
hush: fixed bogus glob handling; fixed exec <"$1"; added test and echo builtins

2007-06-13
hush: exec <"$1" doesn't do parameter subst

2007-05-24
hush: environment-related memory leak plugged, with net code size
decrease.

2007-05-24
hush: '( echo ${name )' will show syntax error message, but prompt
doesn't return (need to press <enter>). Pressing Ctrl-C, <enter>,
'( echo ${name )' again, Ctrl-C segfaults.

2007-05-21
hush: environment cannot be handled by libc routines as they are leaky
(by API design and thus unfixable): hush will leak memory in this script,
bash does not:
pid=$$
while true; do
    unset t;
    t=111111111111111111111111111111111111111111111111111111111111111111111111
    export t
    ps -o vsz,pid,comm | grep " $pid "
done
The fix is to not use setenv/putenv/unsetenv but manipulate env ourself. TODO.
hush: meanwhile, first three command subst bugs mentioned below are fixed. :)

2007-05-06
hush: more bugs spotted. Comparison with bash:
bash-3.2# echo "TEST`date;echo;echo`BEST"
TESTSun May  6 09:21:05 CEST 2007BEST         [we dont strip eols]
bash-3.2# echo "TEST`echo '$(echo ZZ)'`BEST"
TEST$(echo ZZ)BEST                            [we execute inner echo]
bash-3.2# echo "TEST`echo "'"`BEST"
TEST'BEST                                     [we totally mess up this one]
bash-3.2# echo `sleep 5`
[Ctrl-C should work, Ctrl-Z should do nothing][we totally mess up this one]
bash-3.2# if true; then
> [Ctrl-C]
bash-3.2#                                     [we re-issue "> "]
bash-3.2# if echo `sleep 5`; then
> true; fi                                    [we execute sleep before "> "]

2007-05-04
hush: made ctrl-Z/C work correctly for "while true; do true; done"
(namely, it backgrounds/interrupts entire "while")

2007-05-03
hush: new bug spotted: Ctrl-C on "while true; do true; done" doesn't
work right:
# while true; do true; done
[1] 0 true <-- pressing Ctrl-C several times...
[2] 0 true
[3] 0 true
Segmentation fault

2007-05-03
hush: update on "sleep 1 | exit 3; echo $?" bug.
parse_stream_outer() repeatedly calls parse_stream().
parse_stream() is now fixed to stop on ';' in this example,
fixing it (parse_stream_outer() will call parse_stream() 1st time,
execute the parse tree, call parse_stream() 2nd time and execute the tree).
But it's not the end of story.
In more complex situations we _must_ parse way farther before executing.
Example #2: "{ sleep 1 | exit 3; echo $?; ...few_lines... } >file".
Because of redirection, we cannot execute 1st pipe before we parse it all.
We probably need to learn to store $var expressions in parse tree.
Debug printing of parse tree would be nice too.

2007-04-28
hush: Ctrl-C and Ctrl-Z for single NOFORK commands are working.
Memory and other resource leaks (opendir) are not addressed
(testcase is "rm -i" interrupted by ctrl-c).

2007-04-21
hush: "sleep 5 | sleep 6" + Ctrl-Z + fg seems to work.
"rm -i" + Ctrl-C, "sleep 5" + Ctrl-Z still doesn't work
for SH_STANDALONE case :(

2007-04-21
hush: fixed non-backgrounding of "sleep 1 &" and totally broken
"sleep 1 | sleep 2 &". Noticed a bug where successive jobs
get numbers 1,2,3 even when job #1 has exited before job# 2 is started.
(bash reuses #1 in this case)

2007-04-21
hush: "sleep 1 | exit 3; echo $?" prints 0 because $? is substituted
_before_ pipe gets executed!! run_list_real() already has "pipe;echo"
parsed and handed to it for execution, so it sees "pipe"; "echo 0".

2007-04-21
hush: removed setsid() and made job control sort-of-sometimes-work.
Ctrl-C in "rm -i" works now except for SH_STANDALONE case.
"sleep 1 | exit 3" + "echo $?" works, "sleep 1 | exit 3; echo $?"
shows exitcode 0 (should be 3). "sleep 1 | sleep 2 &" fails horribly.

2007-04-14
lash, hush: both do setsid() and as a result don't have ctty!
Ctrl-C doesn't work for any child (try rm -i), etc...
lash: bare ">file" doesn't create a file (hush works)