Array slicing in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is array slicing in bash scripting?

Array slicing is a way to extract a part of an array by specifying a start index and length, similar to cutting a piece from a list.

beginner

How do you slice an array named arr from index 2 to get 3 elements?

Use ${arr[@]:2:3}. This extracts 3 elements starting from index 2.

intermediate

What happens if the length in array slicing is omitted in bash?

If length is omitted, slicing extracts all elements from the start index to the end of the array.

beginner

Given arr=(a b c d e), what is the output of echo ${arr[@]:1:2}?

The output is b c. It slices 2 elements starting from index 1.

intermediate

Can array slicing in bash use negative indices?

Yes, negative start indices count from the end of the array backwards. For example, ${arr[@]: -2} gets the last two elements.

In bash, what does ${arr[@]:3} do?

A Extracts all elements starting from index 3 to the end

B Extracts the first 3 elements

C Extracts 3 elements starting from index 0

D Extracts 3 elements starting from index 3

Without length, slicing extracts from the start index (3) to the end.

How do you get the last 2 elements of an array arr in bash?

A ${arr[@]: -2}

B ${arr[@]:2:2}

C ${arr[@]:-1}

D ${arr[@]:0:2}

Negative start index counts from the end; -2 means last two elements.

What is the index of the first element in a bash array?

A 1

B -1

C 0

D Depends on array size

Bash arrays start indexing at 0.

If arr=(x y z), what does ${arr[@]:1:5} return?

A z

B y z

C x y z

D Error

Length longer than available elements returns all from start index without error.

Which syntax correctly slices an array arr from index 0 for 2 elements?

A ${arr:0:2}

B ${arr[0]:2}

C ${arr[2]:0}

D ${arr[@]:0:2}

Correct syntax uses ${arr[@]:start:length}.

Explain how to extract a slice from a bash array and what happens if the length is omitted.

Think about cutting a piece from a list starting at a position.

Describe how negative indices work in bash array slicing and give an example.

Negative numbers count backwards from the end.

Practice

1. In Bash scripting, what does array slicing do when you use ${array[@]:start:length}?

easy

A. Sorts the array elements alphabetically

B. Deletes elements from the array starting at start index

C. Extracts a part of the array starting at start index with length elements

D. Reverses the array elements from start to length

Solution

1. Step 1: Understand array slicing syntax

   In Bash, ${array[@]:start:length} extracts elements from the array starting at index start and takes length elements.

2. Step 2: Compare options with this behavior

   Only Extracts a part of the array starting at start index with length elements correctly describes this extraction. Other options describe unrelated operations.

3. Final Answer:

   Extracts a part of the array starting at start index with length elements -> Option C

4. Quick Check:

   Array slicing = Extract part [OK]

Hint: Remember slicing picks elements, not deletes or sorts [OK]

Common Mistakes:

• Confusing slicing with deleting elements
• Thinking slicing sorts or reverses the array
• Mixing up start index and length meaning

2. Which of the following is the correct syntax to slice 3 elements from index 2 in a Bash array named arr?

easy

A. ${arr[2:3]}

B. ${arr[@]:2:3}

C. ${arr:2:3}

D. ${arr[2..4]}

Solution

1. Step 1: Recall correct array slicing syntax

   In Bash, to slice an array, use ${array[@]:start:length}. The @ is needed to refer to all elements.

2. Step 2: Check each option

   ${arr[@]:2:3} matches the correct syntax: ${arr[@]:2:3}. ${arr:2:3} misses @, which is required for arrays. Options B and D use invalid syntax.

3. Final Answer:

   ${arr[@]:2:3} -> Option B

4. Quick Check:

   Use ${array[@]:start:length} for slicing [OK]

Hint: Use @ inside braces for array slicing syntax [OK]

Common Mistakes:

• Omitting @ when slicing arrays
• Using square brackets with colon syntax
• Trying to slice with invalid range notation

3. Given the Bash array arr=(a b c d e), what is the output of echo ${arr[@]:1:3}?

medium

A. a b c

B. c d e

C. b c d e

D. b c d

Solution

1. Step 1: Identify the array and slice parameters

   The array is (a b c d e). The slice starts at index 1 and takes 3 elements.

2. Step 2: Extract elements from index 1 to 3

   Index 0 = a, 1 = b, 2 = c, 3 = d, 4 = e. Taking 3 elements from index 1 gives: b, c, d.

3. Final Answer:

   b c d -> Option D

4. Quick Check:

   Slice from 1 length 3 = b c d [OK]

Hint: Count from zero, pick length elements starting at start [OK]

Common Mistakes:

• Starting count at 1 instead of 0
• Taking wrong number of elements
• Confusing output with full array

4. What is wrong with this Bash code snippet for slicing an array?

arr=(x y z)
echo ${arr:1:2}

medium

A. It will output an empty string

B. It will output only one element instead of two

C. It will cause a syntax error because slicing is not allowed

D. It will output 'y z' correctly

Solution

1. Step 1: Check slicing syntax correctness

   The code uses ${arr:1:2} without @. For arrays, @ is needed to slice multiple elements.

2. Step 2: Understand the effect of missing @

   Without @, Bash uses the first element x as a scalar string and slices it from index 1 length 2. Since x has length 1, it outputs an empty string.

3. Final Answer:

   It will output an empty string -> Option A

4. Quick Check:

   Missing @ slices first elem as string -> empty [OK]

Hint: Always use @ for array slicing, else string slicing happens [OK]

Common Mistakes:

• Omitting @ and expecting array slice
• Assuming no error but wrong output
• Confusing string and array slicing syntax

5. You have a Bash array arr=(1 2 3 4 5). You want to extract the last 3 elements using array slicing. Which command will do this correctly?

hard

A. echo ${arr[@]:-3}

B. echo ${arr[@]: -3:3}

C. echo ${arr[@]:3:3}

D. echo ${arr[@]:3}

Solution

1. Step 1: Understand negative index slicing in Bash

   Using a negative start index slices from the end. ${arr[@]:-3} extracts from 3rd from end to the end (last 3 elements).

2. Step 2: Check each option

   ${arr[@]: -3:3} has space after : causing bad substitution. ${arr[@]:3:3} slices from index 3 length 3 but only 2 elements remain (4 5). ${arr[@]:3} slices from 3 to end (4 5). Only ${arr[@]:-3} gets exactly the last 3: 3 4 5.

3. Final Answer:

   echo ${arr[@]:-3} -> Option A

4. Quick Check:

   Negative start index slices from end [OK]

Hint: Use negative start index without length to get last elements [OK]

Common Mistakes:

• Adding space after ':' before negative offset
• Using slice length larger than remaining elements
• Not knowing negative indices slice from array end