Substring extraction (${var:offset:length}) in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the syntax ${var:offset:length} do in bash scripting?

It extracts a substring from the variable var, starting at position offset and taking length characters.

beginner

If var="hello world", what is the result of ${var:6:5}?

The substring starting at position 6 with length 5 is world.

intermediate

What happens if the length in ${var:offset:length} is omitted?

The substring will include all characters from offset to the end of the string.

intermediate

Can the offset be negative in ${var:offset:length}? What does it mean?

Yes, a negative offset counts from the end of the string backwards. For example, ${var: -3} starts 3 characters from the end.

beginner

How would you extract the first 4 characters of a variable var in bash?

Use ${var:0:4} to get the first 4 characters starting at position 0.

What does ${var:3:2} extract from var="abcdefg"?

A "ef"

B "cd"

C "de"

D "bc"

Starting at position 3 (0-based), the substring is "de".

If var="hello", what does ${var:1} return?

A "h"

B "he"

C "o"

D "ello"

Without length, it returns substring from offset 1 to end: "ello".

What happens if offset is negative in ${var:offset:length}?

A Counts from start of string

B Counts from end of string backwards

C Causes an error

D Returns empty string

Negative offset counts backwards from the end of the string.

Which command extracts the first 3 characters of var?

A ${var:0:3}

B ${var:3:0}

C ${var:3:3}

D ${var:1:3}

Start at 0, take 3 characters extracts the first 3 characters.

If var="bash scripting", what does ${var:5} return?

A "scripting"

B "bash scripting"

C "script"

D "bash"

Offset 5 means start at 6th character to end, which is "scripting".

Explain how to extract a substring from a bash variable using the syntax ${var:offset:length}. Include what happens if length is omitted or offset is negative.

Think about how you cut a piece from a string, starting point and size.

Describe a real-life example where substring extraction in bash scripting can be useful.

Imagine you have a filename and want just the extension.

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