Consider the following:

#!/bin/bash
month='08'

for m in $( seq 1 $(( ${month} - 1 )) ); do
    rm "backupfile-${m}.tar.bz2"
done

When run, it produces the following error:

$ /bin/bash ./test.sh
test.sh: line 6: 08: value too great for base (error token is "08")

According to man 1 bash:

Constants with a leading 0 are interpreted as octal numbers.

Solution 1: force the number to be interpreted as decimal:

#!/bin/bash
month='08'

for m in $( seq 1 $(( 10#${month} - 1 )) ); do
    rm "backupfile-${m}.tar.bz2"
done

Solution 2: strip the leading zero:

#!/bin/bash
month='08'

for m in $( seq 1 $(( ${month#0} - 1 )) ); do
    rm "backupfile-${m}.tar.bz2"
done

(or if there are multiple leading zeros:)

#!/bin/bash
month='08'

for m in $( seq 1 $(( $(echo "${month}" | sed 's/^0*//') - 1 )) ); do
    rm "backupfile-${m}.tar.bz2"
done

Source

Bennett Perkins