stdin redirection (<) in Linux CLI - Step-by-Step Execution

Concept Flow - stdin redirection (<)

Start command expecting input

↓

Check for '<' operator

Yes↓

Open file specified after '<'

↓

Redirect file content to command's stdin

↓

Command reads input from file

↓

Command executes using file input

↓

Output

This flow shows how the shell redirects input from a file to a command using '<', letting the command read from the file instead of keyboard.

Execution Sample

Linux CLI

cat < input.txt

This command uses stdin redirection to make 'cat' read from 'input.txt' instead of keyboard.

Execution Table

Step	Action	File Opened	Input Source for Command	Command Output
1	Shell sees 'cat < input.txt'	input.txt	stdin redirected from input.txt
2	Shell opens 'input.txt' for reading	input.txt	stdin redirected from input.txt
3	Command 'cat' starts, reads from stdin	input.txt	reads content of input.txt
4	Command outputs content read from input.txt	input.txt	reads content of input.txt	Content of input.txt displayed
5	Command finishes execution	input.txt	stdin closed	Output complete

💡 Command finishes after reading all input from 'input.txt' redirected via '<'

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5
stdin	keyboard	file input.txt opened	reading from input.txt	closed
command_output	none	none	buffering content	content displayed

Key Moments - 2 Insights

Why does the command read from the file instead of waiting for keyboard input?

Because the shell redirects stdin from the file using '<', as shown in execution_table step 2 and 3, so the command reads from the file, not keyboard.

What happens if the file after '<' does not exist?

The shell cannot open the file (step 2 fails), so the command does not run and an error is shown.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the command start reading input from the file?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Check the 'Input Source for Command' column in execution_table row for Step 3

According to variable_tracker, what is the state of 'stdin' after Step 5?

A keyboard

B closed

C file input.txt opened

D reading from input.txt

💡 Hint

Look at the 'stdin' row in variable_tracker after Step 5

If the file 'input.txt' is empty, what will the command output be?

A Content of input.txt displayed

B Error message

C No output (empty)

D Command waits for keyboard input

💡 Hint

Refer to execution_table Step 4 output when reading file content

Concept Snapshot

stdin redirection (<):
Use '<' to send file content as input to a command.
Syntax: command < filename
The command reads from the file instead of keyboard.
If file missing, command errors.
Useful to automate input from files.

Full Transcript

This visual trace shows how stdin redirection works in Linux shell. When you run a command like 'cat < input.txt', the shell sees the '<' operator and opens the file 'input.txt'. It then redirects the command's standard input to read from this file instead of the keyboard. The command reads the file content as if typed by the user and outputs it. After reading all content, the command finishes. If the file does not exist, the shell will show an error and the command won't run. This lets you automate commands that normally wait for keyboard input by feeding them file data instead.

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