Arrays in Bash

An array is a variable that holds multiple values under one name. Instead of creating ten separate variables for ten values, store all of them in a single array and access each one by its position number.

What Is an Array?

┌──────────────────────────────────────────────────────┐
│  Without Array:                                      │
│  city1="Delhi"                                       │
│  city2="Mumbai"                                      │
│  city3="Pune"                                        │
│                                                      │
│  With Array:                                         │
│  cities=("Delhi" "Mumbai" "Pune")                    │
│                                                      │
│  Index:    0        1        2                       │
└──────────────────────────────────────────────────────┘

Declaring an Array

#!/bin/bash
fruits=("Apple" "Banana" "Mango" "Orange")

An array can also be declared explicitly:

declare -a colors
colors[0]="Red"
colors[1]="Green"
colors[2]="Blue"

Accessing Array Elements

Access an individual element using its index number inside curly braces. Array indices start at 0.

#!/bin/bash
fruits=("Apple" "Banana" "Mango" "Orange")
echo ${fruits[0]}   # First element
echo ${fruits[2]}   # Third element

Output:

Apple
Mango

Accessing All Elements

#!/bin/bash
fruits=("Apple" "Banana" "Mango" "Orange")
echo ${fruits[@]}   # All elements
echo ${fruits[*]}   # All elements (alternate syntax)

Output:

Apple Banana Mango Orange
Apple Banana Mango Orange

Array Length

#!/bin/bash
fruits=("Apple" "Banana" "Mango" "Orange")
echo "Total fruits: ${#fruits[@]}"

Output:

Total fruits: 4

Modifying Array Elements

#!/bin/bash
fruits=("Apple" "Banana" "Mango")
fruits[1]="Grapes"   # Replace Banana with Grapes
echo ${fruits[@]}

Output:

Apple Grapes Mango

Adding Elements to an Array

#!/bin/bash
fruits=("Apple" "Banana")
fruits+=("Mango" "Orange")
echo ${fruits[@]}

Output:

Apple Banana Mango Orange

Removing an Element

#!/bin/bash
fruits=("Apple" "Banana" "Mango" "Orange")
unset fruits[2]   # Remove Mango (index 2)
echo ${fruits[@]}

Output:

Apple Banana Orange

Looping Over an Array

Using for Loop

#!/bin/bash
cities=("Delhi" "Mumbai" "Chennai" "Kolkata")
for city in "${cities[@]}"
do
  echo "City: $city"
done

Output:

City: Delhi
City: Mumbai
City: Chennai
City: Kolkata

Loop with Index

#!/bin/bash
fruits=("Apple" "Banana" "Mango")
for i in "${!fruits[@]}"
do
  echo "Index $i: ${fruits[$i]}"
done

Output:

Index 0: Apple
Index 1: Banana
Index 2: Mango

Array Slicing

Extract a portion of an array using ${array[@]:start:count}.

#!/bin/bash
nums=(10 20 30 40 50 60)
echo ${nums[@]:2:3}   # Start at index 2, take 3 items

Output:

30 40 50

Associative Arrays (Key-Value Pairs)

Associative arrays use text keys instead of numbers. This feature requires Bash version 4 or higher.

#!/bin/bash
declare -A student
student["name"]="Ravi"
student["age"]="22"
student["city"]="Hyderabad"

echo "Name: ${student["name"]}"
echo "Age:  ${student["age"]}"
echo "City: ${student["city"]}"

Output:

Name: Ravi
Age:  22
City: Hyderabad

Loop Over Associative Array

#!/bin/bash
declare -A capital
capital["India"]="New Delhi"
capital["Japan"]="Tokyo"
capital["France"]="Paris"

for country in "${!capital[@]}"
do
  echo "$country → ${capital[$country]}"
done

Output:

India → New Delhi
Japan → Tokyo
France → Paris

Indexed vs Associative Array Comparison

Practical Example – Student Score Report

#!/bin/bash
declare -A scores
scores["Alice"]=88
scores["Bob"]=72
scores["Carol"]=95

for student in "${!scores[@]}"
do
  if [ ${scores[$student]} -ge 90 ]; then
    grade="A"
  elif [ ${scores[$student]} -ge 75 ]; then
    grade="B"
  else
    grade="C"
  fi
  printf "%-10s Score: %d  Grade: %s\n" "$student" "${scores[$student]}" "$grade"
done

Output:

Alice      Score: 88  Grade: B
Bob        Score: 72  Grade: C
Carol      Score: 95  Grade: A

Key Takeaways

• Arrays hold multiple values under one name using numeric indices starting at 0.
• Use ${array[@]} to access all elements and ${#array[@]} for the count.
• Use += to append new elements and unset to remove one.
• Loop over arrays using for item in "${array[@]}".
• Associative arrays use string keys and require declare -A.