bash value too great for base - even when string?

[many_miles]

I've read the FAQs about posting and I've a fairly good handle on bash programming.

I am trying to get the numerical value of "last month". Since this is August, the last month value I'm looking for is 7.

Obviously, I can just subtract one from "this month" as long as this month is not 1. If this month is one, last month is 12.

As you can see, I am using the date +%m command for obtaining the two digit month number.

To do that, I have this code


#!/bin/bash

# First find out what this month is...
THIS_MONTH_NUM=`date +%m`

if [ $THIS_MONTH_NUM -eq 1 ]; then
SRCH_MONTH_NUM=12
else
SRCH_MONTH_NUM=$((THIS_MONTH_NUM-1))
fi


But, on my Hardy Heron Ubuntu, because $THIS_MONTH_NUM contains 08, that produces:


./show_last_month: line 9: 08: value too great for base (error token is "08")


So, I figured I could force a string comparison by replacing line 6 with


if [ "$THIS_MONTH_NUM" == "01" ]; then

but, I still get the same error.

I've seen this other posting which I can use as a workaround
http://ubuntuforums.org/showthread.php?p=4206419

But, I am more curious as to why it's not a stringwise comparison...

[ghostdog74]

use GNU date


# date +%m -d "1 month ago"
07

[lswb]

The reason for the error is that bash will try to interpret numbers with a leading 0 as octal (base 8 instead of base 10). Since "8" is not a valid numerical digit in base 8, it causes the error. You can force it to recognize "08" as a base 10 number by prefixing it with "10#", i.e. "10#08" (substitute X for "10" to change to base X IIRC you can use up to base36 arithmetic by using upper and lower case letters for digits)

So you could change line 4 to THIS_MONTH_NUM="10#`date +%m`"

[many_miles]

use GNU date


Wow, that's much simpler than my solution. Thanks.

[many_miles]

The reason for the error is that bash will try to interpret numbers with a leading 0 as octal (base 8 instead of base 10). Since "8" is not a valid numerical digit in base 8, it causes the error. You can force it to recognize "08" as a base 10 number by prefixing it with "10#", i.e. "10#08" (substitute X for "10" to change to base X IIRC you can use up to base36 arithmetic by using upper and lower case letters for digits)

So you could change line 4 to THIS_MONTH_NUM="10#`date +%m`"

But, even when the number is within quotes? I thought that quotes and the use of the == operator would force a string comparison.

[lswb]

But, even when the number is within quotes? I thought that quotes and the use of the == operator would force a string comparison.

It should and when I tested that code on my system, it does just that. Have you checked for any typos or misplaced "fi" and such?