$ uname -srm
NetBSD 1.2B i386
$ #1
$ echo ${10}
Syntax error: Bad substitution
$ #2
$ a=abcd; echo XX${a#$a}YY XX${a#abcd}YY
XXabcd XXabcd
$ #3
$ set -- ""; a=$1; echo "$# '$1' '${1:-wwww}' '${a:-wwww}'"
1 '' '' 'wwww'
$ #4
$ hd () hexdump -e '"%06_ax " 8/1 " %02x"' -e '"  " "%_p"' -e '"\n"' "$@"
$ a=xy; echo -n "$((${#a}+1))" | hd
000000  33 20 2b 31 90 a7 04     3 +1...
$ 

Hiroyuki Ito