Substring extraction (${var:offset:length}) in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to extract the substring 'world' from the variable.

Bash Scripting

text="hello world"
sub=${text:[1]:5}
echo "$sub"

Drag options to blanks, or click blank then click option'

A1

B0

C5

D6

Attempts:

3 left

💡 Hint

Common Mistakes

Using offset 0 or 1 extracts the wrong part of the string.

Forgetting that offset counts from zero.

The substring extraction syntax ${var:offset:length} starts at offset 6 to get 'world' from 'hello world'.

2fill in blank

medium

Complete the code to extract the first 4 characters from the variable.

Bash Scripting

filename="report2024.txt"
prefix=${filename:[1]:4}
echo "$prefix"

Drag options to blanks, or click blank then click option'

A4

B1

C0

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Starting at offset 1 misses the first character.

Using offset equal to length extracts nothing.

Offset 0 starts at the beginning, so ${filename:0:4} extracts 'repo'.

3fill in blank

hard

Fix the error in the code to correctly extract '2024' from the filename.

Bash Scripting

filename="report2024.txt"
year=${filename:[1]:4}
echo "$year"

Drag options to blanks, or click blank then click option'

A7

B6

C4

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using offset 7 or 4 extracts wrong parts.

Confusing offset with length.

The year '2024' starts at offset 6 in 'report2024.txt', so ${filename:6:4} extracts it correctly.

4fill in blank

hard

Fill both blanks to extract 'script' from the variable.

Bash Scripting

word="bashscript"
sub=${word:[1]:[2]
echo "$sub"

Drag options to blanks, or click blank then click option'

A4

B6

C7

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong offset or length extracts incorrect substring.

Mixing up offset and length values.

The substring 'script' starts at offset 4 and has length 6 in 'bashscript'.

5fill in blank

hard

Fill both blanks to extract 'log' from the filename variable.

Bash Scripting

filename="system.log.2024"
sub=${filename:[1]:[2]
echo "$sub"

Drag options to blanks, or click blank then click option'

A7

B3

C4

D6

Attempts:

3 left

💡 Hint

Common Mistakes

Using offset 6 or 4 extracts wrong substring.

Using length 4 extracts extra characters.

The substring 'log' starts at offset 7 and has length 3 in 'system.log.2024'.

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