Indexed array declaration in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an indexed array in Bash scripting?

An indexed array in Bash is a list of values stored in a single variable, where each value has a numeric index starting from 0.

beginner

How do you declare an indexed array with values in Bash?

Use syntax: array_name=(value1 value2 value3). For example: fruits=(apple banana cherry).

beginner

How do you access the second element of an indexed array named colors?

Use ${colors[1]} because indexing starts at 0.

intermediate

How to add a new element to an existing indexed array in Bash?

Use syntax: array_name+=(new_value). For example: fruits+=('orange') adds 'orange' to the fruits array.

beginner

What command shows all elements of an indexed array named my_array?

Use echo ${my_array[@]} to print all elements separated by spaces.

How do you declare an indexed array named pets with elements 'dog', 'cat', and 'bird'?

A pets<dog cat bird>

B pets=[dog, cat, bird]

C pets=(dog cat bird)

D pets={dog cat bird}

In Bash, indexed arrays are declared with parentheses and space-separated values.

What is the index of the first element in a Bash indexed array?

A 1

B -1

C Depends on the array

D 0

Bash arrays start indexing at 0, like many programming languages.

How do you print all elements of an indexed array colors?

A echo ${colors[@]}

B echo ${colors[*]}

C echo colors

D echo colors[]

Both ${colors[@]} and ${colors[*]} print all elements, but ${colors[@]} is preferred for separate elements.

How to add 'pear' to an existing array fruits?

A fruits.append('pear')

B fruits+=('pear')

C fruits.push('pear')

D fruits = fruits + 'pear'

In Bash, use += with parentheses to add elements to an array.

Which syntax is correct to access the third element of array numbers?

A ${numbers[2]}

B ${numbers[3]}

C ${numbers(2)}

D ${numbers{2}}

Indexing starts at 0, so the third element is at index 2.

Explain how to declare and access elements in an indexed array in Bash.

Think about how you list items in a shopping list and then pick one by its position.

Describe how to add a new element to an existing indexed array in Bash.

Imagine adding a new item to your list without rewriting the whole list.

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