[LUG] Bash substrings

["Darke, Clive" <Clive.darke@xxxxxx>]

Any BASH experts out there?

I am looking at substrings, and do not seem to be able to get negative
offsets to work the way the man pages say/imply - 
"If  offset  evaluates  to  a number less than zero, the value is
used as an offset from the end of the value  of  parameter."   

${var:offset:length}  works fine if all are positive
${var:-3:3}  I thought would give me that last 3 chars of $var, but gives me
the whole string.
${var:3,-3} Note the comma.  This DOES work (found by accident), but I can't
find this syntax documented. 

Incidently (in this last example) it does not matter what number I have as
the offset - even beyond the length of $var - but I must have one.

Any ideas? What is going on here?

Clive

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