substr EXPR,OFFSET
$[ to (but don't do that). If
OFFSET is negative, starts that far from the end of the string. If
LEN is omitted, returns everything to the end of the string. If
LEN is negative, leaves that many characters off the end of the string.
You can use the substr function as an lvalue, in which case
EXPR must be an lvalue. If you assign something shorter than
LEN, the string will shrink, and if you assign something longer than
LEN, the string will grow to accommodate it. To keep the string the same length you may need to pad or chop your value using sprintf.