Indexed array declaration in Bash Scripting

Introduction

Arrays let you store many values in one place. Indexed arrays use numbers to find each value easily.

You want to keep a list of file names to process one by one.

You need to store multiple user inputs and use them later.

You want to save a list of commands to run in order.

You want to group related data like days of the week or colors.

Syntax

Bash Scripting

array_name=(value1 value2 value3 ...)
# or
declare -a array_name
array_name=(value1 value2 value3 ...)

Use parentheses () to list values separated by spaces.

Array indexes start at 0 by default.

Examples

Creates an array named fruits with three items. Prints the first item: apple.

Bash Scripting

fruits=(apple banana cherry)
echo ${fruits[0]}

Declares an indexed array colors, assigns three colors, and prints the third item: blue.

Bash Scripting

declare -a colors
colors=(red green blue)
echo ${colors[2]}

Creates an empty array and prints its length: 0.

Bash Scripting

empty_array=()
echo ${#empty_array[@]}

Array with one number. Prints 10.

Bash Scripting

numbers=(10)
echo ${numbers[0]}

Sample Program

This script shows how to declare an indexed array, access items by index, add a new item, and get the array length.

Bash Scripting

#!/bin/bash

# Declare an indexed array of fruits
fruits=(apple banana cherry)

# Print all fruits
echo "All fruits: ${fruits[@]}"

# Print the first fruit
echo "First fruit: ${fruits[0]}"

# Add a new fruit
fruits+=(date)

# Print all fruits after adding
echo "Fruits after adding one: ${fruits[@]}"

# Print number of fruits
echo "Number of fruits: ${#fruits[@]}"

Important Notes

Access array elements using ${array_name[index]} where index starts at 0.

Use ${#array_name[@]} to get the number of elements in the array.

Adding elements can be done with array+=(new_value).

Arrays in bash are zero-based and can hold strings or numbers.

Common mistake: forgetting parentheses when declaring arrays causes errors.

Use indexed arrays when order matters and you want to access by position.

Summary

Indexed arrays store multiple values accessed by number starting at zero.

Declare arrays with parentheses and separate values by spaces.

Use ${array[index]} to get an item and ${#array[@]} to get the count.

Practice

1. What is the correct way to declare an indexed array named fruits with values apple, banana, and cherry in Bash?

easy

A. fruits=[apple, banana, cherry]

B. fruits=("apple, banana, cherry")

C. fruits={apple banana cherry}

D. fruits=(apple banana cherry)

Solution

1. Step 1: Understand Bash array declaration syntax

   Bash arrays use parentheses with space-separated values, no commas or braces.

2. Step 2: Check each option's syntax

   fruits=(apple banana cherry) uses parentheses and spaces correctly. Square brackets and braces are incorrect. fruits=("apple, banana, cherry") treats all values as one string.

3. Final Answer:

   fruits=(apple banana cherry) -> Option D

4. Quick Check:

   Parentheses + spaces = correct array declaration [OK]

Hint: Use parentheses and spaces, no commas or braces [OK]

Common Mistakes:

• Using square brackets instead of parentheses
• Including commas between values
• Putting all values inside quotes as one string

2. Which of the following is the correct syntax to declare an empty indexed array named numbers in Bash?

easy

A. numbers={}

B. numbers=()

C. numbers=[]

D. numbers=""

Solution

1. Step 1: Recall empty array declaration in Bash

   An empty indexed array is declared with empty parentheses.

2. Step 2: Evaluate each option

   numbers=() uses empty parentheses correctly. Braces and square brackets are invalid. numbers="" assigns an empty string, not an array.

3. Final Answer:

   numbers=() -> Option B

4. Quick Check:

   Empty parentheses declare empty array [OK]

Hint: Empty parentheses mean empty array [OK]

Common Mistakes:

• Using square brackets or braces instead of parentheses
• Assigning empty string instead of array
• Confusing array declaration with variable assignment

3. Given the array declaration colors=(red green blue), what will be the output of echo ${colors[1]}?

medium

A. green

B. colors[1]

C. blue

D. red

Solution

1. Step 1: Understand array indexing in Bash

   Indexed arrays start at 0, so index 1 is the second element.

2. Step 2: Identify the element at index 1

   colors[0] = red, colors[1] = green, colors[2] = blue.

3. Final Answer:

   green -> Option A

4. Quick Check:

   Index 1 = second element = green [OK]

Hint: Array indexes start at zero [OK]

Common Mistakes:

• Thinking array starts at 1 instead of 0
• Printing the variable name instead of value
• Confusing index with value

4. What is wrong with this array declaration?

myarray=(one two three four five six seven eight nine ten

medium

A. Array name is invalid

B. Values should be quoted

C. Missing closing parenthesis

D. Values must be comma separated

Solution

1. Step 1: Check array syntax completeness

   The declaration starts with '(' but does not have a closing ')'.

2. Step 2: Verify other syntax rules

   Values do not need quotes if no spaces, array name is valid, commas are not used in Bash arrays.

3. Final Answer:

   Missing closing parenthesis -> Option C

4. Quick Check:

   Parentheses must open and close [OK]

Hint: Always close parentheses in array declaration [OK]

Common Mistakes:

• Forgetting the closing parenthesis
• Adding commas between values
• Quoting values unnecessarily

5. How can you declare an indexed array days with the values Mon, Tue, and Wed, then print the total number of elements in the array?

hard

A. days=(Mon Tue Wed); echo ${#days[@]}

B. days=[Mon Tue Wed]; echo ${#days}

C. days=(Mon, Tue, Wed); echo ${#days[@]}

D. days=(Mon Tue Wed); echo ${days.length}

Solution

1. Step 1: Declare the array correctly

   Use parentheses with space-separated values: days=(Mon Tue Wed).

2. Step 2: Use correct syntax to get array length

   Use ${#days[@]} to get the number of elements in the array.

3. Final Answer:

   days=(Mon Tue Wed); echo ${#days[@]} -> Option A

4. Quick Check:

   Parentheses + ${#array[@]} = count [OK]

Hint: Use ${#array[@]} to get array length [OK]

Common Mistakes:

• Using square brackets for array declaration
• Using commas between values
• Using incorrect syntax for length like ${#days} or .length