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.