Linux in a Nutshell, Fifth Edition

Variables | 641

Bash and Korn

This is the Title of the Book, eMatter Edition

Copyright © 2006 O’Reilly & Associates, Inc. All rights reserved.

Individual elements may also be assigned to:

message[0]=hi This is the hard way

message[1]=there

message[2]=how

message[3]=are

message[4]=you

message[5]=today

Declaring arrays is not required. Any valid reference to a subscripted variable can

create an array.

When referencing arrays, use the ${ … } syntax. This isn’t needed when refer-

encing arrays inside (( )) (the form of let that does automatic quoting). Note that [

and ] are typed literally (i.e., they don’t stand for optional syntax).

ksh93 provides associative arrays, where the indices are strings instead of

numbers (as in awk). In this case, [ and ] act like double quotes. Associative

arrays are created with typeset -A. A special syntax allows assigning to multiple

elements at once:

data=([joe]=30 [mary]=25)

The values would be retrieved as ${data[joe]} and ${data[mary]}.

Discipline Functions (ksh93 only)

Along with structured variables, ksh93 introduces discipline functions. These are

special functions that are called whenever a variable’s value is accessed or

changed. For a shell variable named x, you can define the following functions:

x.get

Called when x’s value is retrieved ($x).

x.set

Called when x’s value is changed (x=2).

x.unset

Called when x is unset (unset x).

Within the discipline functions, special ...