Section 1.1: Utilizing Arrays in Shell Scripts

In Bash, a standard variable lacks a specific type, allowing it to be treated as an integer, decimal, or string based on the context. Typically, Bash variables store command outputs, algorithm parameters, and temporary values. Bash supports two types of arrays: one-dimensional (numerically indexed) and associative (key-value pairs). Manipulating Bash arrays is as intuitive as in other dynamically-typed languages like Python, PHP, or JavaScript. Here's how to create arrays in Bash:

#!/bin/bash

numbers=(2 4 5 2)

declare -a words

words[0]='Orange'

words[1]='Pineapple'

echo ${numbers[@]} ${words[@]}

This code outputs the contents of both arrays as follows:

To check the declaration of each array, use the declare built-in command as shown below:

You can also manipulate arrays, adding or removing items, processing elements, or sorting them with concise syntax. For instance, the following script eliminates invalid score values and prints the top three scores:

#!/bin/bash

declare -a marks

marks+=(75 65 80 102 26) # class A marks

marks+=(103 68) # class B marks

# Remove invalid marks

for i in "${!marks[@]}"; do

if ((marks[i] > 100)); then

unset "marks[$i]"

fi

done

# Sort all marks

marks_s=($(printf '%sn' "${marks[@]}" | sort -nr))

# Prints the top-3

echo ${marks_s[0]} ${marks_s[1]} ${marks_s[2]}

While the above script utilizes an external sort command, you can avoid this by implementing an efficient sorting algorithm directly in Bash.

Section 1.2: Creating Maps or Dictionaries

In certain coding situations, it's essential to store key-value pair data within your shell scripts. Key-value structures are commonly used to form dictionaries, maps, and caching containers via memoization. If you prefer Python for scripting, you might employ its built-in dictionary data structure. But how can you implement a similar concept in Bash?

Bash introduced associative arrays for key-value storage starting with version 4.0. Here’s a simple example of how to use associative arrays in Bash:

#!/bin/bash

declare -A marks=([john]=75 [doe]=82 [ann]=83 [ava]=72)

for key in "${!marks[@]}"; do

printf "$key t ${marks[$key]} n"

done

This code snippet extracts all dictionary keys using the !mapvar[@] syntax for iteration, producing output like this:

Bash allows you to manipulate associative array data with minimal syntax, resembling the ease of working with Python dictionaries. Consider the following example:

#!/bin/bash

read -p "Enter coords (i.e., [x]=10 [y]=12): " coords

declare -A "coords=($coords)"

if [ ! -v "coords[x]" ]; then

coords[x]=5

fi

if [ ! -v "coords[y]" ]; then

coords[y]=10

fi

for key in "${!coords[@]}"; do

printf "$key = ${coords[$key]} n"

done

In this script, user input for x and y coordinates is requested, and default values are assigned for any missing entries, which are then displayed in the terminal.