How to iterate over associative arrays in Bash

Bash Problem Overview

Based on an associative array in a Bash script, I need to iterate over it to get the key and value.

#!/bin/bash

declare -A array
array[foo]=bar
array[bar]=foo

I actually don't understand how to get the key while using a for-in loop.

Bash Solutions

Solution 1 - Bash

The keys are accessed using an exclamation point: ${!array[@]}, the values are accessed using ${array[@]}.

You can iterate over the key/value pairs like this:

for i in "${!array[@]}"
do
  echo "key  : $i"
  echo "value: ${array[$i]}"
done

Note the use of quotes around the variable in the for statement (plus the use of @ instead of *). This is necessary in case any keys include spaces.

The confusion in the other answer comes from the fact that your question includes "foo" and "bar" for both the keys and the values.

Solution 2 - Bash

You can access the keys with ${!array[@]}:

bash-4.0$ echo "${!array[@]}"
foo bar

Then, iterating over the key/value pairs is easy:

for i in "${!array[@]}"
do
  echo "key :" $i
  echo "value:" ${array[$i]}
done

Solution 3 - Bash

Use this higher order function to prevent the pyramid of doom

foreach(){ 
  arr="$(declare -p $1)" ; eval "declare -A f="${arr#*=}; 
  for i in ${!f[@]}; do $2 "$i" "${f[$i]}"; done
}

example:

$ bar(){ echo "$1 -> $2"; }
$ declare -A foo["flap"]="three four" foo["flop"]="one two"
$ foreach foo bar
flap -> three four
flop -> one two

Solution 4 - Bash

Welcome to input associative array 2.0!

    clear
    echo "Welcome to input associative array 2.0! (Spaces in keys and values now supported)"
    unset array
    declare -A array
    read -p 'Enter number for array size: ' num
    for (( i=0; i < num; i++ ))
        do
            echo -n "(pair $(( $i+1 )))"
            read -p ' Enter key: ' k
            read -p '         Enter value: ' v
            echo " "
            array[$k]=$v
        done
    echo " "
    echo "The keys are: " ${!array[@]}
    echo "The values are: " ${array[@]}
    echo " "
    echo "Key <-> Value"
    echo "-------------"
    for i in "${!array[@]}"; do echo $i "<->" ${array[$i]}; done
    echo " "
    echo "Thanks for using input associative array 2.0!"

Output:

Welcome to input associative array 2.0! (Spaces in keys and values now supported)
Enter number for array size: 4
(pair 1) Enter key: Key Number 1
         Enter value: Value#1

(pair 2) Enter key: Key Two
         Enter value: Value2

(pair 3) Enter key: Key3
         Enter value: Val3

(pair 4) Enter key: Key4
         Enter value: Value4


The keys are:  Key4 Key3 Key Number 1 Key Two
The values are:  Value4 Val3 Value#1 Value2

Key <-> Value
-------------
Key4 <-> Value4
Key3 <-> Val3
Key Number 1 <-> Value#1
Key Two <-> Value2

Thanks for using input associative array 2.0!

Input associative array 1.0

(keys and values that contain spaces are not supported)

    clear
    echo "Welcome to input associative array! (written by mO extraordinaire!)"
    unset array
    declare -A array
    read -p 'Enter number for array size: ' num
    for (( i=0; i < num; i++ ))
        do
            read -p 'Enter key and value separated by a space: ' k v
            array[$k]=$v
        done
    echo " "
    echo "The keys are: " ${!array[@]}
    echo "The values are: " ${array[@]}
    echo " "
    echo "Key <-> Value"
    echo "-------------"
    for i in ${!array[@]}; do echo $i "<->" ${array[$i]}; done
    echo " "
    echo "Thanks for using input associative array!"