stdin redirection (<) in Linux CLI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read input from a file named 'input.txt' using stdin redirection.

Linux CLI

cat [1]

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

A< input.txt

B> input.txt

Cinput.txt

D>> input.txt

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' or '>>' which redirect output instead of input.

Just writing the filename without '<' does not redirect stdin.

Using '< input.txt' redirects the content of 'input.txt' as standard input to the 'cat' command.

2fill in blank

medium

Complete the command to count the number of lines in 'data.txt' using stdin redirection.

Linux CLI

wc -l [1]

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

A>> data.txt

Bdata.txt

C> data.txt

D< data.txt

Attempts:

3 left

💡 Hint

Common Mistakes

Using the filename without '<' which makes 'wc' treat it as an argument.

Using '>' or '>>' which redirect output, not input.

Using '< data.txt' sends the file content as input to 'wc -l' which counts lines.

3fill in blank

hard

Fix the error in the command to sort the contents of 'list.txt' using stdin redirection.

Linux CLI

sort [1]

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

Alist.txt

B> list.txt

C< list.txt

D>> list.txt

Attempts:

3 left

💡 Hint

Common Mistakes

Using the filename without '<' which makes 'sort' treat it as an argument.

Using '>' or '>>' which redirect output and overwrite the file.

To sort contents from a file using stdin redirection, use '< list.txt' so 'sort' reads from the file.

4fill in blank

hard

Fill both blanks to filter lines containing 'error' from 'log.txt' using stdin redirection and grep.

Linux CLI

grep [1] [2]

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

A"error"

B< log.txt

Clog.txt

D-i

Attempts:

3 left

💡 Hint

Common Mistakes

Using the filename as an argument instead of redirecting input.

Forgetting to quote the search pattern.

Use 'grep "error" < log.txt' to search for 'error' in the file by redirecting input.

5fill in blank

hard

Fill all three blanks to print words longer than 5 letters from 'words.txt' using stdin redirection and awk (assuming one word per line).

Linux CLI

awk '[1] [2]' [3]

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

Alength($0) > 5

B< words.txt

C{print $0}

D> words.txt

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' for input redirection.

Omitting the print action in awk.

Not quoting the awk script properly.

Use 'awk 'length($0) > 5 {print $0}' < words.txt' to print words (lines) longer than 5 characters from the file.

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