Associative arrays in Bash

One of my favorite pastimes these days is to set BingChat to creative mode, ask it to teach me a trick about topic X, and then write a short blog post about it to reinforce my understanding. Some of the things it comes up with are absolutely delightful. In the spirit of that, I asked it to teach me a Shell trick that I can use to mimic maps or dictionaries in a shell environment. I didn’t even know what I was expecting.

It didn’t disappoint and introduced me to the idea of associative arrays in Bash. This data structure is basically the Bash equivalent of a map.

First, we have our usual arrays which are containers that can store multiple values, indexed by numbers. Associative arrays are similar, but they use strings as keys instead of numbers. For example, if you want to store the names of some fruits in a regular array, you can use:

fruits=(apple banana cherry)

This will create an array called fruits with three elements. You can access the elements by using the index number inside brackets, such as:

echo ${fruits[0]}

This prints:

apple

You can also use a range of indices to get a slice of the array, such as:

echo ${fruits[@]:1:2}

This will print:

banana cherry

Moreover, you can use * or @ to get all the elements of the array, such as:

echo ${fruits[*]}

This returns:

apple banana cherry

Associative arrays are declared with the declare -A command, and then assigned values using the = operator and brackets. For example, if you want to store the prices of some fruits in an associative array, you can use:

declare -A prices
prices[apple]=1.00
prices[banana]=0.50
prices[cherry]=2.00

This will create an associative array called prices with three key-value pairs. You can access the values by using the keys inside brackets, such as:

echo ${prices[apple]}

This will print:

1.00

Similar to regular arrays, you can use * or @ to get all the keys or values of the associative array. Run the following command to get all the keys of the prices associative array:

echo ${!prices[*]}

This will print:

apple banana cherry

To get the values, run:

echo ${prices[@]}

This returns:

1.00 0.50 2.00

Arrays and associative arrays can be useful when you want to store and manipulate complex data structures in bash. You can use them to perform arithmetic operations, string operations, or loop over them with for or while commands. For example, you can use:

for fruit in ${!prices[*]}; do
    echo "$fruit costs ${prices[$fruit]}";
done

In the above snippet, we iterate through the keys of prices in a for loop. The ${!prices[*]} notation expands to a list of all the keys in the prices array. Inside the loop, we print the key-value pairs, where $fruit represents the current key and ${prices[$fruit]} represents the corresponding value. So in each iteration, the snippet will output the name of each fruit along with its corresponding price.

Running the snippet will print:

apple costs 1.00
banana costs 0.50
cherry costs 2.00

A more practical example

Here’s a script that downloads three famous RFCs using cURL. We’re using an associative array for bookkeeping purposes.

#!/usr/bin/env bash

set -euo pipefail

declare -A rfc_urls

base_url="https://www.rfc-editor.org/rfc"

rfc_urls["http-error"]="${base_url}/rfc7808.txt"
rfc_urls["http-one"]="${base_url}/rfc7231.txt"
rfc_urls["datetime-format"]="${base_url}/rfc3339.txt"

echo "======================"
echo "start downloading rfcs"
echo "======================"
echo ""

for key in ${!rfc_urls[*]}; do
    value=${rfc_urls[$key]}
    echo "Downloading rfcs ${key}: ${value}"
    curl -OJLs "${value}"
done

echo ""
echo "======================"
echo "done downloading rfcs"
echo "======================"

Running this will download the RFCs in the current directory:

======================
start downloading rfcs
======================

Downloading rfcs http-error: https://www.rfc-editor.org/rfc/rfc7808.txt
Downloading rfcs datetime-format: https://www.rfc-editor.org/rfc/rfc3339.txt
Downloading rfcs http-one: https://www.rfc-editor.org/rfc/rfc7231.txt

======================
done downloading rfcs
======================

The script begins by declaring an associative array called rfc_urls. This array serves as a convenient way to keep track of the RFCs we want to download. Each key in the array represents a unique identifier for an RFC, while the corresponding value holds the complete URL to download that specific RFC.

Next, we set the base_url variable to https://www.rfc-editor.org/rfc, which will be used as the base URL for all RFC downloads.

Inside a loop that iterates over the keys of the rfc_urls array, we retrieve the URL value associated with each key. To provide a progress update, we echo a message indicating the RFC being downloaded.

Using the curl command with the options -OJLs, we initiate the download process. The -O flag ensures that the remote file is saved with its original filename, while the -J flag takes advantage of the Content-Disposition header in the HTTP response to determine the filename. We include the -L flag to follow redirects, and the -s flag to silence curl’s progress output.

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