Linux in a Nutshell, 6th Edition

Arrays

Bash supports one-dimensional arrays. Elements are referenced by an index; the first element is numbered 0 and there is no upper limit on the number of elements. Arrays are initialized with a special form of assignment:

message=(hi there how are you today)

where each value (in this example, each word) becomes an element of the array.

Elements may also be assigned individually:

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.

Bash also provides associative arrays, where the indices are strings instead of numbers. In this case, [ and ] act like double quotes. Associative arrays are created with declare -A arrayname. Unlike indexed arrays, when assigning a value to an associative array, a subscript is always required.

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

Syntax	Effect
${ name[ i]}	Use element i of array name. i can be any arithmetic expression as described under let.
${ name}	Use element 0 of indexed array name.
${ name[*]}, ${ name[@]}	Use all elements of array name.
${# name[*]}, ${# name[@]}	Use the number of elements in array name.

The built-in commands declare, local, and readonly accept the -a option for an indexed array and the ...