🔄 Linux/Unix: I/O Streams – stdin, stdout, stderr Explained with Redirection Examples

🧲 Introduction – Why Learn About I/O Streams?

In Linux/Unix, every command interacts with input and output streams: standard input (stdin), standard output (stdout), and standard error (stderr). Mastering these streams is essential for scripting, debugging, and automating workflows efficiently.

🎯 In this guide, you’ll learn:

• What stdin, stdout, and stderr are and how they function
• How to redirect input/output using symbols like >, 2>, and <
• Real-life usage for log management, silent execution, and piping

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📥 What is stdin (Standard Input)?

• File Descriptor: 0
• Function: Accepts user input from keyboard or piped data
• Default Source: Keyboard or input file

🧪 Example:

cat > notes.txt

Then type:

This is input from stdin.

(Press Ctrl+D to end input)

✅ Input is saved into notes.txt

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📤 What is stdout (Standard Output)?

• File Descriptor: 1
• Function: Sends normal output from commands to the screen
• Default Destination: Terminal

🧪 Example:

echo "Hello, world!"

Output:

Hello, world!

✅ You can redirect this output to a file:

echo "Hello" > greeting.txt

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❌ What is stderr (Standard Error)?

• File Descriptor: 2
• Function: Outputs error messages separately from stdout
• Default Destination: Terminal

🧪 Example:

ls non_existing_file

Output (to stderr):

ls: cannot access 'non_existing_file': No such file or directory

✅ Redirect error output:

ls non_existing_file 2> error.log

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🔁 Redirection Operators – Explained

Operator	Meaning	Example
>	Redirect stdout (overwrite)	ls > list.txt
>>	Append stdout	echo "new line" >> file.txt
2>	Redirect stderr	command 2> errors.txt
2>>	Append stderr	command 2>> errors.txt
&>	Redirect both stdout and stderr	command &> output.log
<	Redirect stdin from a file	wc -l < file.txt
<<	Here-document (multi-line stdin)	cat << EOF ... EOF

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📦 Combine Output & Error Redirection

🧪 Example:

ls existing.txt missing.txt > out.txt 2> err.txt

• out.txt gets the listing of existing.txt
• err.txt captures the error for missing.txt

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🧪 Silence Output or Errors

Discard Output:

command > /dev/null          # Discard stdout
command 2> /dev/null         # Discard stderr
command &> /dev/null         # Discard both

✅ Useful for cron jobs or background tasks

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🔗 Pipe (|) vs Redirection

• Redirection (>): Sends output to file
• Pipe (|): Sends stdout of one command into stdin of another

🧪 Example:

ls -l | grep ".sh"

✅ Lists .sh files using ls output as input for grep

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📌 Summary – Recap & Next Steps

Linux I/O streams provide powerful tools for managing inputs and outputs in scripts and commands. Understanding stdin, stdout, and stderr helps you filter outputs, handle errors, and build reliable automation.

🔍 Key Takeaways:

• stdin (0) = input, stdout (1) = normal output, stderr (2) = error output
• Use >, >>, 2>, &> to redirect output
• Combine redirections for advanced logging and debugging
• Use pipes (|) to chain commands using output as input

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❓ FAQs

❓ How can I save both output and errors into one file?
✅ Use:

command &> combined.log

❓ What is the purpose of /dev/null?
✅ It’s a black hole for output. Any data sent there is discarded.

❓ How do I redirect stderr to stdout?
✅ Use:

command 2>&1

This merges error output into standard output.

❓ Can I redirect input to a command from a file?
✅ Yes:

sort < names.txt

❓ What happens if I use > on an existing file?
✅ It overwrites the file. Use >> to append instead.

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