The reason is that "012" is not a number. "12" is a number and you can increment that to "13". But "012" and "013" are strings and you cannot do calculations with strings.

You haven't told us yet what your shell is and in fact it didn't matter that much because what we did up to now was common ground to all of them. We are leaving this area and maybe i will tell you something you can't use because you have a different shell - which is why you should always state what your environment (OS and shell and their versions most prominently) is.

In Korn shell (ksh) you can use typeset to make a variable have leading zeroes and having a certain length like this (try it on the command line):

Code:

$ typeset -RZ3 variable=12
$ echo $variable
012
$ (( variable += 1 ))
$ echo $variable
013

Variables in shell do not really have a certain "type". Of course it will lead to an error if you try to multiply "abc" by 3 but if you create a string "ab12cd", then somehow (you now know how) get rid of the characters you can multiply the remaining "12" and it will give you "60", which you still can use as a string. What we did above was to create a string, right-aligned (-R) in the length of 3 where every "free"space is filled with zeroes. When we calculate with it like a number, the shell will silently drop the leading zeroes to make the number "12" from "012", then do the calculation and, when writing the result back, realign and refill it (because of the typeset directive, so that "013" is the final content.

I hope this helps.

bakunin