stdin redirection (<) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this command using stdin redirection?

Given a file named numbers.txt containing:
1
2
3
What is the output of the command cat < numbers.txt?

A 1 2 3

B numbers.txt

C cat: numbers.txt: No such file or directory

D 1\n2\n3

💡 Hint

Think about what cat does when reading from standard input.

The command cat < numbers.txt reads the contents of numbers.txt through stdin and outputs the lines as they are, each on its own line.

💻 Command Output

intermediate

What error occurs when redirecting stdin from a non-existent file?

What happens when you run sort < missingfile.txt if missingfile.txt does not exist?

A bash: missingfile.txt: No such file or directory

B No output, command waits for input

C sort: standard input: No such file or directory

D sort: cannot open missingfile.txt for reading: Permission denied

💡 Hint

Consider how the shell handles input redirection from a file that does not exist.

The shell tries to open the file before running the command. If the file does not exist, the shell reports an error like 'No such file or directory'.

📝 Syntax

advanced

Which command correctly uses stdin redirection to count lines in a file?

You want to count the number of lines in data.txt using wc -l and stdin redirection. Which command is correct?

A wc -l < data.txt

B wc < -l data.txt

C wc -l data.txt <

D < wc -l data.txt

💡 Hint

Remember the syntax for input redirection: command < filename

The correct syntax places the redirection operator after the command and before the filename: wc -l < data.txt reads data.txt as stdin for wc -l.

🚀 Application

advanced

How to use stdin redirection with a script that reads input?

You have a script process.sh that reads lines from standard input. Which command correctly feeds input.txt to the script using stdin redirection?

A ./process.sh input.txt

B cat input.txt | ./process.sh

C ./process.sh < input.txt

D ./process.sh > input.txt

💡 Hint

Think about how stdin redirection works versus passing a filename as an argument.

Using < input.txt sends the file content as standard input to the script. Option C works but uses a pipe, not stdin redirection. Option C passes filename as argument, not stdin. Option C redirects output, not input.

🧠 Conceptual

expert

What happens if you combine stdin redirection with a command that also reads a file argument?

Consider the command: grep 'error' < log.txt log2.txt
What will happen?

A grep searches 'error' in log2.txt and also reads log.txt from stdin

B grep searches 'error' only in log2.txt, ignoring stdin

C grep searches 'error' in log2.txt and ignores stdin redirection

D grep searches 'error' in log.txt twice, once from stdin and once from log2.txt

💡 Hint

Think about how stdin redirection and file arguments work together in commands.

The shell redirects stdin from log.txt, but since log2.txt is provided as a file argument, grep reads only from log2.txt and ignores standard input.