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Indexed and associative arrays in bash

Linux  #linux  #bash 

Indexed arrays

In bash, indexed arrays can be created in two ways

  1. Declaring explicitly: declare -a my_arr
  2. Creating with initial values: other_arr=(a b c)
declare -a my_arr # declare a new array
other_arry=() # create new array

Associative arrays or maps

Associative arrays are key value pairs and can be declared using declare keyword with -A switch

declare -A associative_arr

Adding elements to arrays

  • Indexed arrays
declare -a my_arr
my_arr+=(a)
my_arr+=(b)
  • Associative arrays
declare -A map
map=([foo]="bar" ["baz"]="other")
map["key"]="value"

Size of the array

#my_arr[@] - returns the size of array (see for loop example below)

Getting elements from arrays

There are two ways to get elements from arrays

  1. my_arr[@] - returns all elements of arrays as separate words
  2. my_arr[*] - returns all elements of arrays as a single string
  • Iterating over associative array
#!/bin/bash
declare -A map
map=([foo]="bar" ["baz"]="other")
map["key"]="value"

for value in "${map[@]}"; do 
    echo "$value"
done

Output

bar
other
value
  • Iterating over indexed array
#/bin/bash
declare -a arr # indexed array
arr+=(a)
arr+=(b c)

for value in "${arr[@]}"; do 
    echo "${value}"
done

Output

a
b
c

Another way to iterate over indexed array is using for loop

#!/bin/bash

declare -a arr
arr+=(a)
arr+=(b c)

size=${#arr[@]}
for ((i=0;i<$size;i++)); do 
    echo "index $i, value: ${arr[$i]}"
done

Output

index 0, value: a
index 1, value: b
index 2, value: c

Iterating over keys of arrays

!arr[@] returns the keys of arrays, in case of indexed arrays these are indices while for associative arrays this will return actual keys.

  • Example
#!/bin/bash

# indexed array
declare -a arr
arr+=(a)
arr+=(b c)

for k in "${!arr[@]}"; do 
    echo "keys $k"
done 

# associative array
echo -e "\n"

declare -A map
map=([foo]="bar" ["baz"]="other")
map["key"]="value"

for value in "${map[@]}"; do 
    echo "$value"
done

Output

keys 0
keys 1
keys 2


bar
other
value

Deleting element from array

  • Associative array: unset map[key]
  • Indexed array: unset arr[index]
#!/bin/bash

declare -a arr
arr+=(a)
arr+=(b c)

unset arr[0] # delete element at index 0

for k in "${!arr[@]}"; do 
    echo "keys $k"
done

Output

keys 1
keys 2

Note that after deletion indices will remain same for other elements i.e. there is no element at position 0 now, only index 1 & 2 has values

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