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

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.