Convert string into integer in bash script - "Leading Zero" number error

Question

In a text file, test.txt, I have the next information:

sl-gs5 desconnected Wed Oct 10 08:00:01 EDT 2012 1001 

I want to extract the hour of the event by the next command line:

hour=$(grep -n sl-gs5 test.txt | tail -1 | cut -d' ' -f6 | awk -F ":" '{print $1}') 

and I got "08". When I try to add 1,

 14 echo $((hour+1)) 

I receive the next error message:

./test2.sh: line 14: 08: value too great for base (error token is "08") 

If variables in Bash are untyped, why?

Answer 1

See ARITHMETIC EVALUATION in man bash:

Constants with a leading 0 are interpreted as octal numbers.

You can remove the leading zero by parameter expansion:

hour=${hour#0} 

or force base-10 interpretation:

$((10#$hour + 1)) 

Answer 2

what I'd call a hack, but given that you're only processing hour values, you can do

 hour=08  echo $(( ${hour#0} +1 ))  9  hour=10   echo $(( ${hour#0} +1))  11 

with little risk.

IHTH.