Substring extraction (${var:offset:length}) in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this substring extraction?

Given the variable assignment text="automation", what is the output of echo ${text:4:3}?

Bash Scripting

text="automation"
echo ${text:4:3}

A mat

B uto

C tom

D oma

💡 Hint

Remember that offset starts at 0 and length is how many characters to extract.

The string 'automation' indexed from 0 is: a(0) u(1) t(2) o(3) m(4) a(5) t(6) i(7) o(8) n(9). Starting at offset 4, length 3 extracts 'm', 'a', 't'.

💻 Command Output

intermediate

What happens if length exceeds string length?

Given word="script", what is the output of echo ${word:3:10}?

Bash Scripting

word="script"
echo ${word:3:10}

A pt

B ipt

C ip

D Error: length too long

💡 Hint

If length goes beyond the string end, bash extracts until the end.

Starting at offset 3 ('r'(0), 'i'(1), 'p'(2), 't'(3)), length 10 tries to extract beyond string end, but bash returns till end: 'ipt'.

📝 Syntax

advanced

Which option correctly extracts substring with negative offset?

Given data="automation", which command outputs the last 4 characters using substring extraction?

Bash Scripting

data="automation"

A echo ${data:-4:4}

B echo ${data: -4:4}

C echo ${data: -4}

D echo ${data: -4:-1}

💡 Hint

Negative offset counts from the end; length can be omitted to go to string end.

Option C uses negative offset -4 and no length, extracting last 4 chars 'tion'. Option C has a space causing syntax error. Option C uses wrong syntax. Option C has negative length which is invalid.

🔧 Debug

advanced

Why does this substring extraction fail?

Given str="automation", why does echo ${str:3,4} produce an error?

Bash Scripting

str="automation"
echo ${str:3,4}

A Comma is invalid syntax; should use colon between offset and length

B Offset 3 is out of range for this string

C Length 4 is too large and causes error

D Variable name must be uppercase

💡 Hint

Check the syntax for substring extraction carefully.

Substring extraction syntax uses colons, not commas. Using a comma causes a syntax error.

🚀 Application

expert

Extract middle word from a string using substring extraction

Given sentence="learn bash scripting easily", which command extracts the word 'bash' using substring extraction only (no external commands)?

Bash Scripting

sentence="learn bash scripting easily"

A echo ${sentence:6:4}

B echo ${sentence:6:5}

C echo ${sentence:6:3}

D echo ${sentence:7:4}

💡 Hint

Count characters including spaces to find offset and length of 'bash'.

The word 'bash' starts at offset 6 (counting from 0) and has length 4. So ${sentence:6:4} extracts 'bash'.

Practice

1. What does the expression ${var:2:3} do in bash scripting?

easy

A. Extracts 3 characters from var starting at index 2

B. Extracts 2 characters from var starting at index 3

C. Extracts 3 characters from var starting at index 0

D. Extracts the entire string var ignoring offset and length

Solution

1. Step 1: Understand substring syntax

   The syntax ${var:offset:length} extracts a substring from var starting at offset with length characters.

2. Step 2: Apply to given expression

   Here, offset is 2 and length is 3, so it extracts 3 characters starting from index 2 (0-based).

3. Final Answer:

   Extracts 3 characters from var starting at index 2 -> Option A

4. Quick Check:

   Substring = characters from index 2, length 3 [OK]

Hint: Offset is start index, length is number of chars [OK]

Common Mistakes:

• Confusing offset with length
• Forgetting offset starts at 0
• Assuming length is mandatory

2. Which of the following is the correct syntax to extract 4 characters starting at position 1 from variable text?

easy

A. ${text:1:4}

B. ${text:4:1}

C. ${text:1-4}

D. ${text:1,4}

Solution

1. Step 1: Recall correct substring syntax

   The correct syntax uses colon separators: ${var:offset:length}.

2. Step 2: Match syntax to options

   ${text:1:4} matches this exactly with offset 1 and length 4. Others use invalid separators or wrong order.

3. Final Answer:

   ${text:1:4} -> Option A

4. Quick Check:

   Correct syntax uses colons [OK]

Hint: Use colons, not commas or dashes [OK]

Common Mistakes:

• Using commas or dashes instead of colons
• Swapping offset and length
• Missing braces around variable

3. Given filename="report2024.txt", what is the output of echo ${filename:6:4}?

medium

A. port

B. 2024

C. 202

D. txt

Solution

1. Step 1: Identify substring start and length

   Offset is 6, length is 4. The string is "report2024.txt".

2. Step 2: Extract substring from position 6

   Counting from 0: r(0) e(1) p(2) o(3) r(4) t(5) 2(6) 0(7) 2(8) 4(9). Extract 4 chars starting at index 6: "2024".

3. Final Answer:

   2024 -> Option B

4. Quick Check:

   Substring at 6 length 4 = "2024" [OK]

Hint: Count from zero, then take length chars [OK]

Common Mistakes:

• Starting count at 1 instead of 0
• Extracting wrong substring length
• Confusing characters with digits

4. What is wrong with this code snippet?

var="hello world"
echo ${var:5,3}

medium

A. Offset 5 is out of range

B. Length 3 is too large

C. Comma is invalid separator; should use colon

D. Variable name is incorrect

Solution

1. Step 1: Check substring syntax

   The substring syntax requires colons between offset and length, not commas.

2. Step 2: Identify error in code

   The code uses a comma ${var:5,3} which is invalid and causes a syntax error.

3. Final Answer:

   Comma is invalid separator; should use colon -> Option C

4. Quick Check:

   Use colons, not commas [OK]

Hint: Use colons between offset and length [OK]

Common Mistakes:

• Using commas instead of colons
• Assuming offset out of range causes error
• Misnaming variable

5. You have a date string date="2024-06-15". Which command extracts the month part ("06") using substring extraction?

hard

A. ${date:3:2}

B. ${date:6:2}

C. ${date:4:2}

D. ${date:5:2}

Solution

1. Step 1: Locate month position in date string

   The string is "2024-06-15". Counting from 0: 2(0) 0(1) 2(2) 4(3) -(4) 0(5) 6(6) -(7) 1(8) 5(9). Month starts at index 5.

2. Step 2: Extract 2 characters starting at index 5

   Using ${date:5:2} extracts "06" which is the month.

3. Final Answer:

   ${date:5:2} -> Option D

4. Quick Check:

   Month = substring at 5 length 2 [OK]

Hint: Count chars from zero, dash counts as char [OK]

Common Mistakes:

• Starting count at 1 instead of 0
• Extracting wrong substring length
• Confusing day and month positions