Can't increment a 0-padded number past 8 in busybox sh - linux

Question

this is the code I am using to save files from a camera and name them from 0001 onward. The camera is running Busybox, and it has an ash shell inside.
The code is based on a previous answer by Charles Duffy here.
#!/bin/sh
# Snapshot script
cd /mnt/0/foto
sleep 1
set -- *.jpg # put the sorted list of picture namefiles on argv ( the number of files on the list can be requested by echo $# )
while [ $# -gt 1 ]; do # as long as there's more than one...
shift # ...some rows are shifted until only one remains
done
if [ "$1" = "*.jpg" ]; then # If cycle to determine if argv is empty because there is no jpg file present in the dir. #argv is set so that following cmds can start the sequence from 0 on.
set -- snapfull0000.jpg
else
echo "Piu' di un file jpg trovato."
fi
num=${1#*snapfull} # $1 is the first row of $#. The alphabetical part of the filename is removed.
num=${num%.*} # removes the suffix after the name.
num=$(printf "%04d" "$(($num + 1))") # the variable is updated to the next digit and the number is padded (zeroes are added)
# echoes for debug
echo "variabile num="$num # shows the number recognized in the latest filename
echo "\$#="$# # displays num of argv variables
echo "\$1="$1 # displays the first arg variable
wget http://127.0.0.1/snapfull.php -O "snapfull${num}.jpg" # the snapshot is requested to the camera, with the sequential naming of the jpeg file.
This is what I get on the cmd line during the script operation. I manually ran the script nine times, but after the saving of file snapfull0008.jpg, as you can see in the last lines, files are named only snapfull0000.jpg.
# ./snap4.sh
variable num=0001
$#=1
$1=snapfull0000.jpg
Connecting to 127.0.0.1 (127.0.0.1:80)
127.0.0.1 127.0.0.1 - [05/Dec/2014:20:22:22 +0000] "GET /snapfull.php HTTP/1.1" 302 0 "-" "Wget"
snapfull0001.jpg 100% |*******************************| 246k --:--:-- ETA
# ./snap4.sh
More than a jpg file found.
variable num=0002
$#=1
$1=snapfull0001.jpg
Connecting to 127.0.0.1 (127.0.0.1:80)
127.0.0.1 127.0.0.1 - [05/Dec/2014:20:22:32 +0000] "GET /snapfull.php HTTP/1.1" 302 0 "-" "Wget"
snapfull0002.jpg 100% |*******************************| 249k --:--:-- ETA
# ./snap4.sh
More than a jpg file found.
variable num=0003
$#=1
$1=snapfull0002.jpg
Connecting to 127.0.0.1 (127.0.0.1:80)
127.0.0.1 127.0.0.1 - [05/Dec/2014:20:22:38 +0000] "GET /snapfull.php HTTP/1.1" 302 0 "-" "Wget"
snapfull0003.jpg 100% |*******************************| 248k --:--:-- ETA
# ./snap4.sh
More than a jpg file found.
variable num=0004
$#=1
$1=snapfull0003.jpg
Connecting to 127.0.0.1 (127.0.0.1:80)
127.0.0.1 127.0.0.1 - [05/Dec/2014:20:22:43 +0000] "GET /snapfull.php HTTP/1.1" 302 0 "-" "Wget"
snapfull0004.jpg 100% |*******************************| 330k --:--:-- ETA
# ./snap4.sh
More than a jpg file found.
variable num=0005
$#=1
$1=snapfull0004.jpg
Connecting to 127.0.0.1 (127.0.0.1:80)
127.0.0.1 127.0.0.1 - [05/Dec/2014:20:22:51 +0000] "GET /snapfull.php HTTP/1.1" 302 0 "-" "Wget"
snapfull0005.jpg 100% |*******************************| 308k --:--:-- ETA
# ./snap4.sh
More than a jpg file found.
variable num=0006
$#=1
$1=snapfull0005.jpg
Connecting to 127.0.0.1 (127.0.0.1:80)
127.0.0.1 127.0.0.1 - [05/Dec/2014:20:22:55 +0000] "GET /snapfull.php HTTP/1.1" 302 0 "-" "Wget"
snapfull0006.jpg 100% |*******************************| 315k --:--:-- ETA
# ./snap4.sh
More than a jpg file found.
variable num=0007
$#=1
$1=snapfull0006.jpg
Connecting to 127.0.0.1 (127.0.0.1:80)
127.0.0.1 127.0.0.1 - [05/Dec/2014:20:22:59 +0000] "GET /snapfull.php HTTP/1.1" 302 0 "-" "Wget"
snapfull0007.jpg 100% |*******************************| 316k --:--:-- ETA
# ./snap4.sh
More than a jpg file found.
variable num=0008
$#=1
$1=snapfull0007.jpg
Connecting to 127.0.0.1 (127.0.0.1:80)
127.0.0.1 127.0.0.1 - [05/Dec/2014:20:23:04 +0000] "GET /snapfull.php HTTP/1.1" 302 0 "-" "Wget"
snapfull0008.jpg 100% |*******************************| 317k --:--:-- ETA
# ./snap4.sh
More than a jpg file found.
variable num=0000
$#=1
$1=snapfull0008.jpg
Connecting to 127.0.0.1 (127.0.0.1:80)
127.0.0.1 127.0.0.1 - [05/Dec/2014:20:23:10 +0000] "GET /snapfull.php HTTP/1.1" 302 0 "-" "Wget"
snapfull0000.jpg 100% |*******************************| 318k --:--:-- ETA
What could be the cause of the sequence stopping after file number 8?

Answer 1

The problem is that leading 0s cause a number to be read as octal.
In bash, using $((10#$num)) will force decimal. Thus:
num=$(printf "%04d" "$((10#$num + 1))")
To work with busybox ash, you'll need to strip the 0s. One way to do this which will work even in busybox ash:
while [ "${num:0:1}" = 0 ]; do
num=${num:1}
done
num=$(printf '%04d' "$((num + 1))")
See the below transcript showing use (tested with ash from busybox v1.22.1):
$ num=0008
$ while [ "${num:0:1}" = 0 ]; do
> num=${num:1}
> done
$ num=$(printf '%04d' "$((num + 1))")
$ echo "$num"
0009
If your shell doesn't support even the baseline set of parameter expansions required by POSIX, you could instead end up using:
num=$(echo "$num" | sed -e 's/^0*//')
num=$(printf '%04d' "$(($num + 1))")
...though this would imply that your busybox was built with a shell other than ash, a decision I would strongly suggest reconsidering.