Bash Arrays

Internal

• bash

Indexed Arrays

External

• http://www.gnu.org/software/bash/manual/html_node/Arrays.html#Arrays
• http://tldp.org/LDP/abs/html/arrays.html

Overview

bash indexed arrays are 0-based and unidimensional. No explicit declaration is necessary if at least one element is initialized as described below:

Declaration

Arrays do not need explicit declarations, they will be automatically declared upon the first assignment of any of their elements.

my_array_var[0]="something"

They can be explicitly declared, though:

declare -a my_array_var

Previously declared arrays can be listed with:

declare -a

An array variable can be "undeclared" with "unset"

unset my_array_var

Assignment

Individual Elements

Individual elements are initialized by using ${var-name[index]}=value notation:

a[0]="something"

Entire Array

The (...) syntax can be used to initialize an entire array. The values must be separated by space:

a=("X" "Y" "Z")

If we want initialize only specific elements, we can use this syntax:

a=([0]="X" [4]="Y" [8]="Z")

Reference

Element arrays can be referenced with ${var-name[index]} notation:

echo ${a[0]}

If the array variable was not previously declared, or if the specific element was not initialized, a reference attempt will return the blank string.

Use Cases

Read the Content of a File into An Array

Each line will be assigned to an element of the array:

readarray < ./myfile.txt -t my_array_var;

DO NOT

readarray

I found cases when this does not work.

Alternative to research: The read builtin accepts a -a option to assign a list of words read from the standard input to an array, and can read values from the standard input into individual array elements.