LeetCode 195: Tenth Line Solution in Shell Explained

Problem Statement

In LeetCode 195, you’re given a text file file.txt with one or more lines of text. Your task is to write a one-line bash command to:

• Read the file.
• Output the tenth line if it exists.
• Output nothing (empty result) if the file has fewer than 10 lines.

This differs from file transposition like LeetCode 194: Transpose File, focusing on line extraction rather than matrix manipulation.

Constraints

• File exists and is readable (file.txt).
• Each line is a string of printable ASCII characters.
• No guarantee of at least 10 lines.

Example

Let’s see some cases:

file.txt:
Line1
Line2
Line3
Line4
Line5
Line6
Line7
Line8
Line9
Line10
Line11

Output:
Line10
Explanation: Tenth line is "Line10".

file.txt:
Line1
Line2
Line3

Output:
(empty)
Explanation: Fewer than 10 lines, no output.

file.txt: (10 lines exactly)
a
b
c
d
e
f
g
h
i
j

Output:
j
Explanation: Tenth line is "j".

These examples show we’re extracting the tenth line or nothing.

Understanding the Problem

How do you solve LeetCode 195: Tenth Line in a bash command? We need to:

• Read file.txt and count lines.
• Output only the tenth line if it exists.
• Return nothing if there are fewer than 10 lines.

For a file with 11 lines, we want "Line10"; for 3 lines, we output nothing. We need a concise command using tools like sed, awk, or head/tail, not a frequency task like LeetCode 192: Word Frequency. We’ll use: 1. sed Command: Precise, efficient—our best solution. 2. awk with NR: Alternative approach.

Let’s dive into the best solution.

Best Solution: sed Command Approach

Explanation

sed Command extracts the tenth line by:

• Using sed with the -n flag to suppress default output.
• Specifying 10p to print only line 10.
• Reading file.txt directly.

This ensures O(n) time (n = lines, stops at line 10 in practice with optimization), O(1) space (sed’s buffer), and simplicity with a single tool, making it the most efficient and clear one-line solution. If fewer than 10 lines, sed outputs nothing by default.

For "Line1\nLine2\n...\nLine10\nLine11":

• sed -n '10p': Prints "Line10".

Step-by-Step Example

Example 1: file.txt = "Line1\nLine2\n...\nLine10\nLine11" (11 lines)

Goal: Return "Line10".

• Step 1: Read file with sed.
• sed -n '10p' file.txt.
• Step 2: Process with sed.
• -n: Suppress all output except specified.
• 10p: Print line 10 (e.g., "Line10").
• Lines 1-9, 11+: Ignored.
• Step 3: Output.
• "Line10".
• Finish: Return "Line10".

Example 2: file.txt = "Line1\nLine2\nLine3" (3 lines)

Goal: Return nothing.

• Step 1: sed -n '10p' file.txt.
• Step 2: Process.
• Only 3 lines, no line 10 exists.
• -n: No output by default.
• Step 3: Output.
• (empty).
• Finish: Return nothing.

How the Code Works (sed Command) – Detailed Line-by-Line Explanation

Here’s the bash command with a thorough breakdown (as a one-liner):

# Command: sed -n '10p' file.txt

# Line 1: sed with no default output
sed -n
    # -n: Suppress automatic printing (e.g., only print what’s specified)

# Line 2: Print tenth line
'10p'
    # 10: Line number 10
    # p: Print command (e.g., outputs "Line10" if exists)
    # No match (e.g., <10 lines) → no output

# Line 3: Input file
file.txt
    # Source file (e.g., "Line1\nLine2\n...\nLine10")

This detailed breakdown clarifies how the sed command efficiently extracts the tenth line.

Alternative: awk with NR Approach

Explanation

awk with NR extracts the tenth line by:

• Using awk to process lines.
• Checking NR (line number) against 10.
• Printing the line if NR == 10.

It’s a practical alternative, O(n) time (n = lines), O(1) space, and straightforward, though slightly less concise than sed due to explicit condition syntax.

For "Line1\nLine2\n...\nLine10":

• NR==10: Prints "Line10".

Step-by-Step Example (Alternative)

For "Line1\nLine2\n...\nLine10\nLine11":

• Step 1: awk 'NR==10' file.txt.
• Step 2: Process:
• NR=1 to 9: No match, no output.
• NR=10: "Line10" → print.
• NR=11: No match.
• Step 3: Output "Line10".
• Finish: Same as sed.

For 3 lines:

• Step 1: awk 'NR==10' file.txt.
• Step 2: NR never reaches 10 → no output.
• Finish: Empty.

How the Code Works (awk)

# Command: awk 'NR==10' file.txt

awk 'NR==10' file.txt
    # NR==10: Condition for line 10 (e.g., prints "Line10")
    # file.txt: Input file

Complexity

• sed Command:
• Time: O(n) – reads until line 10 or end.
• Space: O(1) – minimal buffer.
• awk with NR:
• Time: O(n) – reads all lines.
• Space: O(1) – minimal buffer.

Efficiency Notes

sed Command is the best solution with O(n) time and O(1) space, offering brevity and efficiency by stopping at line 10 (optimized internally)—awk with NR matches complexity but processes all lines, making it equally efficient but slightly less concise.

Key Insights

• sed -n: Targeted output.
• awk NR: Line number check.
• Tenth: Specific extraction.

Additional Example

file.txt:
a
b
c
d
e
f
g
h
i
j
k
Output:
j
Explanation: Tenth line is "j".

Edge Cases

• Fewer than 10: Empty output.
• Exactly 10: Tenth line output.
• Empty File: Empty output.

Both solutions handle these well.

Final Thoughts

LeetCode 195: Tenth Line in shell is a straightforward line-extraction challenge. The sed Command solution excels with its simplicity and efficiency, while awk with NR offers a clear alternative. Want more shell? Try LeetCode 194: Transpose File for text manipulation or explore Python Basics for coding skills. Ready to practice? Solve LeetCode 195 on LeetCode with an 11-line file, aiming for the tenth line—test your skills now!