# Bash shell scripting and octal values


While rechecking my [rdup](/projects/rdup) test-suite
one of the tests failed. On closer inspection it was due to the
following line:

    DAY_BEFORE=$(( $(date +%d) - 1 )) 

When `$(date +%d)` is 10 this yields:

    $ DAY_BEFORE=$(( 10 - 1 ))
    $ echo $DAY_BEFORE
    9

Also with octal values (those starting with a leading zero), it also
works:

    $ DAY_BEFORE=$(( 06 - 1 )) 
    $ echo $DAY_BEFORE
    5

So when does this go wrong? When the day is `08` or `09` (as it is
today):

    $ DAY_BEFORE=$(( $(date +%d) - 1 ))
    bash: 09: value too great for base (error token is "09")

My fix was not to let `date` generate the leading zero in the first
place:

    $ DAY_BEFORE=$(( $(date +%_d) - 1 ))
    $ echo $DAY_BEFORE
    8

Moral of the story: 

> Be careful with leading zeros in Bash, as they might get interpreted
> as octal values.