stdin redirection (<) in Linux CLI - Mini Project: Build & Apply

Using stdin Redirection (<) in Linux CLI

📖 Scenario: You have a text file with a list of names. You want to use a command that reads from this file as if you typed the names directly into the command.

🎯 Goal: Learn how to use stdin redirection (

📋 What You'll Learn

Create a text file named names.txt with specific names.

Use stdin redirection (<) to pass the file content to the cat command.

Display the content of the file using the command with stdin redirection.

💡 Why This Matters

🌍 Real World

Many Linux commands can read input from files using stdin redirection. This helps automate tasks without typing input manually.

💼 Career

Understanding stdin redirection is essential for system administrators, developers, and anyone working with command-line tools to automate workflows.

1

Create the names.txt file

Create a file called names.txt with these exact three lines: Alice, Bob, and Charlie.

Linux CLI

# Use a command to create names.txt with Alice, Bob, and Charlie
# Your code here

Hint

Use the echo command with -e to add new lines and redirect output to names.txt.

2

Prepare to use stdin redirection

Use the cat command with stdin redirection (<) to read from the file names.txt. Write the command without running it.

Linux CLI

echo -e "Alice\nBob\nCharlie" > names.txt
# Write the command using cat and < to read names.txt
# Your code here

Hint

Use cat < names.txt to read the file content through stdin redirection.

3

Run the command with stdin redirection

Run the command cat < names.txt to display the content of names.txt using stdin redirection.

Linux CLI

echo -e "Alice\nBob\nCharlie" > names.txt
cat < names.txt
# Run the command above to see the output
# Your code here

Hint

Just run the command cat < names.txt and observe the output.

4

Confirm understanding by printing a message

Print the message File content displayed using stdin redirection after showing the file content.

Linux CLI

echo -e "Alice\nBob\nCharlie" > names.txt
cat < names.txt
# Print the confirmation message below
# Your code here

Hint

Use printf or echo to print the message after the file content.

Practice

1. What does the command sort < names.txt do in Linux?

easy

A. It sorts the lines inside the file names.txt and shows the result.

B. It saves the sorted output into names.txt.

C. It renames the file names.txt to sort.

D. It deletes the file names.txt after sorting.

Solution

1. Step 1: Understand stdin redirection with <

   The < symbol sends the content of names.txt as input to the sort command.

2. Step 2: Effect of the command

   The sort command reads the file content and outputs the sorted lines to the screen (standard output).

3. Final Answer:

   It sorts the lines inside the file names.txt and shows the result. -> Option A

4. Quick Check:

   stdin redirection < sends file content as input [OK]

Hint: Remember: < sends file content as input to command [OK]

Common Mistakes:

• Thinking < saves output to the file
• Confusing < with > (output redirection)
• Assuming the file is modified

2. Which of the following is the correct syntax to use stdin redirection to provide input from a file named input.txt to the command wc -l?

easy

A. < wc -l input.txt

B. wc -l > input.txt

C. wc -l input.txt <

D. wc -l < input.txt

Solution

1. Step 1: Identify correct stdin redirection syntax

   The correct way to redirect input is command < filename. So wc -l < input.txt is correct.

2. Step 2: Check other options

   wc -l > input.txt uses > which is output redirection. Options C and D have incorrect order or syntax.

3. Final Answer:

   wc -l < input.txt -> Option D

4. Quick Check:

   stdin redirection syntax = command < file [OK]

Hint: Use < between command and filename for input redirection [OK]

Common Mistakes:

• Using > instead of < for input
• Placing < after filename
• Starting command line with <

3. Given a file numbers.txt containing the lines:

3
1
2

What is the output of the command sort < numbers.txt?

medium

A. 1
2
3

B. 3 1 2

C. 2
3
1

D. Syntax error

Solution

1. Step 1: Understand the input file content

   The file numbers.txt has three lines: 3, 1, and 2.

2. Step 2: Effect of sort < numbers.txt

   The sort command reads the lines and outputs them sorted ascending: 1, 2, 3.

3. Final Answer:

   1 2 3 -> Option A

4. Quick Check:

   sort command sorts lines from stdin [OK]

Hint: Sort reads from stdin when using < redirection [OK]

Common Mistakes:

• Assuming output is original order
• Confusing input and output redirection
• Expecting numbers sorted numerically without options

4. You run the command grep 'apple' < fruits.txt but get no output, even though fruits.txt contains lines with 'apple'. What is the most likely error?

medium

A. The file fruits.txt is empty.

B. You used output redirection instead of input redirection.

C. The pattern 'apple' is case-sensitive and does not match the file content.

D. The command syntax is incorrect; grep does not support input redirection.

Solution

1. Step 1: Check if input redirection is valid for grep

   grep supports reading from stdin, so grep 'apple' < fruits.txt is valid syntax.

2. Step 2: Consider case sensitivity of grep

   By default, grep is case-sensitive. If the file has 'Apple' or 'APPLE', it won't match 'apple'.

3. Final Answer:

   The pattern 'apple' is case-sensitive and does not match the file content. -> Option C

4. Quick Check:

   grep is case-sensitive by default [OK]

Hint: Check case sensitivity when grep finds no matches [OK]

Common Mistakes:

• Assuming input redirection is wrong for grep
• Ignoring case differences in pattern
• Thinking file is empty without checking

5. You want to count how many lines in data.txt contain the word 'error' ignoring case, using stdin redirection. Which command achieves this?

hard

A. wc -l < grep -i 'error' data.txt

B. grep -i 'error' < data.txt | wc -l

C. grep 'error' > data.txt | wc -l

D. grep -i 'error' > data.txt | wc -l

Solution

1. Step 1: Use grep with case-insensitive option and stdin redirection

   grep -i 'error' < data.txt reads data.txt ignoring case.

2. Step 2: Pipe output to wc -l to count matching lines

   Adding | wc -l counts the number of lines output by grep.

3. Final Answer:

   grep -i 'error' < data.txt | wc -l -> Option B

4. Quick Check:

   Use grep -i with < and pipe to wc -l [OK]

Hint: Pipe grep output to wc -l for counting lines [OK]

Common Mistakes:

• Using > instead of < for input
• Placing wc -l before grep
• Redirecting output to input file mistakenly