Chapter 9
Arrays
An array is a special type of variable that contains a set of values, accessed by a key (also known as an index). Unless otherwise specified, arrays in bash are indexed from zero, so the first element in the array is ${array[0]}, not ${array[1]}. This is not intuitive to all, but it comes from the fact that bash is written in the C programming language, which also works in this way.
It is possible to have sparse arrays, so for non-contiguous data such as mapping a few PIDs to their process names, you can store pid[35420]=httpd -k ssl without having to have all of the other 35,419 items stored in the array. This can be useful, although it is difficult to know which index(es) might actually have values stored with them.
Arrays in the shell are only one-dimensional. This means that if you want to model a chess board, it is not possible to access square c6 as ${chessboard[2][5]}. Instead, you would have to find some way to flatten out the board into a linear 64-item line, so ${chessboard[0]} through ${chessboard[7]} are the first row, ${chessboard[8]} through ${chessboard[15]} are the second row, and so on. The alternative is to have 8 arrays of 8, which is how Recipe 17-1 deals with multiple rows of characters.
New to bash version 4 are associative arrays. These arrays have text instead of a number as their index, so you can keep track of race results using ${points[Ferrari]} and ${points[McLaren]} rather than ${points[0]} and ${points[1]} and then having a lookup ...
