Hash Tables in Bash

1. Declaring and Adding <key,value> Pairs in Dictionary Variable in Bash
2. Retrieving <key,value> Pairs From the Dictionary in Bash
3. Updating <key,value> in a Dictionary in Bash
4. Removing a <key,value> Pair From the Dictionary
5. Iterating a Dictionary in Bash in Bash

A dictionary, also called a hashmap, or associative array, is one of the most fundamental data structures in any programming language to store a collection of things.

A dictionary is a collection of keys, each of which has a value associated with it. You can insert, retrieve, or change a value in a dictionary using the matching key.

Although Bash is not a general-purpose programming language, it natively supports hashmaps in versions four and higher.

Declaring and Adding <key,value> Pairs in Dictionary Variable in Bash

Variables in Bash are not strongly typed, so you must impose type-like behavior by stating a variable’s attribute.

To utilize a dictionary in Bash, use the declare statement with the -A option which means associative array, to declare a dictionary variable.

$ declare -A dict

We’ve now declared a variable called dict, which can be used like a dictionary. To add <key,value> pairs to the dictionary, we’ll follow the following syntax:

$ name_of_dictionary[key]=value

Let’s add <key, value> pairs using the syntax above.

$ dict[1]=Nil
$ dict[2]=Esh

We have added two <key, value> pairs where keys are 1 and 2 and values are Nil and Esh.

Retrieving <key,value> Pairs From the Dictionary in Bash

To check corresponding values in the dictionary with a key, we have to add $ with braces {} to the dictionary variable we defined.

$ echo ${dict[1]}

Output:

Nil

Updating <key,value> in a Dictionary in Bash

To update an existing <key, value> in the dictionary, we must insert a new <key, value> and overwrite the existing value.

$ dict[1]=Nilesh

If we retrieve the value of key 1, then:

$ echo ${dict[1]}

Output:

Nilesh

As we can see, Nil of key 1 is overwritten by Nilesh.

Removing a <key,value> Pair From the Dictionary

We use the unset command to remove a <key, value> pair.

$ unset dict[1]

The command above will remove the <key, value> pair with key 1.

Iterating a Dictionary in Bash in Bash

In Bash, we can use a for loop to iterate a dictionary. Let’s look at an example:

#!/bin/bash
declare -A dict
dict[1]=Nil
dict[2]=Esh
for key in "${!dict[@]}"; do 
    echo "$key ${dict[$key]}" 
done

We rewrote the preceding command in a script and ran it in a Bash terminal. "" is used to handle the keys with spaces.

Output: