This document summarize some code I use for Bash arrays.

For more information see Bash Reference Manual - Arrays

To access all the elements of an array, @, * are used. # will get you the number of elements. As a processed short excerpt from that manual:

• "${arr[@]}" expands each element of arr to a separate word;
• "${arr[*]}" expands to a single word with the value of each array member separated by the first character of the IFS variable;
• "${#arr[*]}" and "${#arr[@]}" expands to the number of elements in the array.

To read more about @, * and # in Bash see Bash Reference Manual - Special Parameters.

Indexed arrays

#!/usr/bin/bash

# explicit declaration of an indexed array
declare -a arr

display_array_info() {
    printf "%d elements, keys: %s, values: %s\n" "${#arr[@]}" "${!arr[*]}" "${arr[*]}"
}

# assignment with a sequence of values
arr=(value0 value1 value2)

# display the array's values
echo "${arr[@]}"
# display the array as a string, values delimited by IFS
echo "${arr[*]}"
# ... array keys ...
echo "${!arr[@]}"
# ... and number of elements
echo "${#arr[@]}"

# add an element to the end of the array
arr+=(value3)
display_array_info

# add / change an element
arr[7]=v7
display_array_info

# change the last element (negative indices count back from the end of the array)
arr[-1]=v77
display_array_info

# this actually adds an element because arr[6] was not defined before
arr[-2]=v66
display_array_info

# delete the second element
unset 'arr[1]'
display_array_info

# rebuild the array to use continuous indices
arr=("${arr[@]}")
display_array_info

The output of the previous script is:

value0 value1 value2
value0 value1 value2
0 1 2
3
4 elements, keys: 0 1 2 3, values: value0 value1 value2 value3
5 elements, keys: 0 1 2 3 7, values: value0 value1 value2 value3 v7
5 elements, keys: 0 1 2 3 7, values: value0 value1 value2 value3 v77
6 elements, keys: 0 1 2 3 6 7, values: value0 value1 value2 value3 v66 v77
5 elements, keys: 0 2 3 6 7, values: value0 value2 value3 v66 v77
5 elements, keys: 0 1 2 3 4, values: value0 value2 value3 v66 v77

Associative arrays

#!/usr/bin/bash

# explicit declaration of an associative array
declare -A arr

display_array_info() {
    for key in "${!arr[@]}"; do
        printf "arr[%s] = %s\n" "$key" "${arr[$key]}"
    done
    printf "%d elements\n\n" "${#arr[@]}"
}

# assignment with statements
arr=([key0]=value0 [key1]=value1 [key2]=value2)

# display the array's values
echo "${arr[@]}"
# display the array as a string, values delimited by IFS
echo "${arr[*]}"
# ... array keys ...
echo "${!arr[@]}"
# ... and number of elements
echo "${#arr[@]}"
echo

# add / change an element
arr[7]=aaa
arr['key7']=bbb
arr[-1]=ccc
arr[key0]=ddd
display_array_info

# delete an element
unset 'arr[key1]'
display_array_info

The output of the previous script is:

value2 value0 value1
value2 value0 value1
key2 key0 key1
3

arr[-1] = ccc
arr[key7] = bbb
arr[key2] = value2
arr[key0] = ddd
arr[key1] = value1
arr[7] = aaa
6 elements

arr[-1] = ccc
arr[key7] = bbb
arr[key2] = value2
arr[key0] = ddd
arr[7] = aaa
5 elements

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