Re: Negative offsets in variable substitution

[Chris F.A. Johnson]

On Tue, 25 Jan 2005, Bob Dowling wrote:

> Version of bash:        2.05b.0(1)-release
>
> Problem duplicated on AMD64/SuSE9.1 GNU Linux and Apple G5/MacOS 10.3.6. 
> Both ship wit the same version of bash as given above.
>
> "man bash" says in the Parameter Expansion section under Substring Expansion 
> (${parameter:offset:length}) that "[i]f  offset  evaluates  to  a number less 
> than zero, the value is used as an offset from the end of the value  of 
> parameter."
>
> However, experience suggests otherwise:
>
> $ xyz='abcdefghijkl'
> $ echo ${xyz}
> abcdefghijkl
> $ echo ${xyz:3}         # Positive offset - OK
> defghijkl
> $ echo ${xyz:-3}        # Negative offset - Not OK
> abcdefghijkl
>
> I don't know whether this is a bug in the manual page or the bash program. 
> Or in my understanding of the manual page.

   Take a look at the other parameter expansions (and what :- means).

$ xyz=
$ echo ${xyz:-3}
3
$ echo ${xyz:-3:1}
3:1
$ xyz=abcdefghijkl
$ echo ${xyz: -3}
jkl
$ echo ${xyz: -3:1}
j

--
        Chris F.A. Johnson                      http://cfaj.freeshell.org
        =================================================================
                Everything in moderation -- including moderation