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A Bash array is a one-dimensional data structure capable of storing multiple values under a single variable identifier. Bash supports two types of arrays: indexed arrays, which use zero-based integers as keys, and associative arrays (introduced in Bash 4.0), which use arbitrary strings as keys. Bash arrays are dynamically sized, mutable, and do not require contiguous memory allocation.

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Declaration and Initialization

Indexed Arrays Indexed arrays can be declared explicitly using the declare built-in or implicitly through assignment. 

# Explicit declaration
declare -a my_indexed_array


# Implicit declaration via compound assignment
my_indexed_array=(value0 value1 value2)


# Sparse assignment (indices do not need to be sequential)
my_indexed_array[0]="first"
my_indexed_array[5]="sixth"
Associative Arrays Associative arrays must be explicitly declared using the -A flag before they can be initialized or used. 

# Explicit declaration (mandatory)
declare -A my_assoc_array


# Compound assignment
my_assoc_array=([key1]="value1" [key2]="value2")


# Individual assignment
my_assoc_array[key3]="value3"

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Accessing Elements

Accessing array elements requires parameter expansion using curly braces {} to prevent the shell from interpreting the index brackets as literal characters. 

# Access a specific element
echo "${my_indexed_array[0]}"
echo "${my_assoc_array[key1]}"


# Access all elements
echo "${my_indexed_array[@]}"
echo "${my_indexed_array[*]}"
Note on Expansion: When enclosed in double quotes, "${array[@]}" expands each element as a separate word, preserving whitespace within elements. "${array[*]}" expands all elements into a single word, separated by the first character of the IFS (Internal Field Separator) variable.

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Modifying Arrays

Appending Elements The += operator appends elements to an indexed array without requiring knowledge of the current highest index. 

# Append multiple elements
my_indexed_array+=(value3 value4)


# Append to an associative array
my_assoc_array+=([key4]="value4")
Deleting Elements The unset built-in removes specific elements or the entire array from memory. 

# Delete a specific element (creates a sparse array)
unset 'my_indexed_array[1]'
unset 'my_assoc_array[key1]'


# Delete the entire array
unset my_indexed_array

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Array Metadata

Bash provides specific parameter expansion syntax to retrieve metadata about the array, such as its size or its keys. 

# Get the total number of elements in the array
length="${#my_indexed_array[@]}"


# Get the string length of a specific element
elem_length="${#my_indexed_array[0]}"


# Get all keys/indices currently in use
indices=("${!my_indexed_array[@]}")
keys=("${!my_assoc_array[@]}")

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Array Slicing

Indexed arrays support slicing via parameter expansion, allowing extraction of a subset of elements based on an offset and length. 

# Syntax: ${array[@]:offset:length}

# Extract 2 elements starting from index 1
slice=("${my_indexed_array[@]:1:2}")
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