bash treats numbers with leading 0 as octal

In a bash script, the number of the current week is used to select a different tape every week (rotation of 4 tapes) :

#!/bin/bash 

WEEK=`date +%W` # Week number (01..52)
TAPE=$((WEEK % 4))

This worked fine so far, but this weekend (it's the 8th weekend of the year) there was an error :

value too great for base (error token is "08")

After some digging, I found that bash treats numbers with a leading 0 as octal. But value 8 (and 9) don't exist in octal, so that's why it throws an error. With smaller numbers (01-07), there is no problem, because they are valid octal numbers, and for bigger numbers (10-...), the number is treated in decimal format, because there is no leading 0 anymore.

This problem can be fixed in two ways :

1) strip leading 0 from value

In bash you can do this with this command :

WEEK=${WEEK#0} # strip leading 0

This only removes a leading zero, a zero that is not leading will not be removed.

2) convert value to decimal

TAPE=$((10#$WEEK % 4)) # convert $WEEK to base-10 with '10#'

Note that you have to add the $ in front of the variable WEEK, to treat it is a variable, in order for the conversion (with 10#) to work.