Linux CLI 11🐧 Input/Output and redirection

Every command in Linux works with three standard streams — and you can redirect them to files, other commands, or the void. This is one of the most powerful concepts in the shell.

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The Three Standard Streams

Every process on Linux has three standard I/O streams:

Stream	Name	Default Source/Destination	File Descriptor
stdin	Standard Input	Keyboard	0
stdout	Standard Output	Terminal	1
stderr	Standard Error	Terminal	2

Visual:

┌──────────────────────────────────────┐
│              Program                  │
│                                       │
│  stdin (0)  ──→  ┌─────────┐          │
│  Keyboard        │  Process │          │
│                  └────┬────┘          │
│                       │               │
│         stdout (1) ←──┤               │
│         stderr (2) ←──┘               │
│              │                        │
│              ▼                        │
│           Terminal                    │
└──────────────────────────────────────┘

Key points:

• stdin — where the program reads input (usually the keyboard)
• stdout — where the program writes regular output (usually the terminal)
• stderr — where the program writes errors (usually the terminal)
• Each stream has a file descriptor (a number the kernel uses)
• stdin = 0, stdout = 1, stderr = 2

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Basic Redirection: stdout

echo "I am Kronos" > output.txt

This redirects the stdout of echo to the file output.txt — overwriting anything that was there.

What happens:

• Without >: output goes to the terminal
• With >: output goes to a file (terminal shows nothing)
• The file is created if it doesn’t exist
• The file is overwritten if it does exist

Examples:

$ echo "Hello" > file.txt
$ cat file.txt
Hello

$ echo "World" > file.txt    # Overwrites!
$ cat file.txt
World

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Redirection Operators

Linux has several operators to redirect the standard streams:

Operator	Description	Example
>	Redirect stdout to a file (overwrite)	echo "Hi" > file.txt
>>	Redirect stdout to a file (append)	echo "Hi" >> file.txt
<	Redirect stdin from a file	sort < input.txt
2>	Redirect stderr to a file	ls bad 2> error.txt
2>>	Redirect stderr (append)	ls bad 2>> error.txt
&>	Redirect both stdout and stderr to a file	cmd &> all.txt
&>>	Redirect both (append)	cmd &>> all.txt
2>&1	Redirect stderr to where stdout is going	cmd > out.txt 2>&1
1>	Explicitly redirect stdout (same as >)	cmd 1> out.txt

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The >> Operator — Append

echo "My son is Zeus" >> output.txt
cat output.txt

Redirects stdout to a file but appends instead of overwriting.

Comparison:

# > overwrites
$ echo "Line 1" > file.txt
$ echo "Line 2" > file.txt
$ cat file.txt
Line 2

# >> appends
$ echo "Line 1" > file.txt
$ echo "Line 2" >> file.txt
$ cat file.txt
Line 1
Line 2

Visual:

> :  [old content] ──→ [new content]      (old erased)
>> : [old content] ──→ [old content + new] (old kept)

When to use >>:

• Adding to log files
• Building up a file incrementally
• Preserving previous data

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The < Operator — Redirect stdin

sort < output.txt

Redirects stdin from a file — the command reads from the file instead of the keyboard.

Example:

# Without redirect — sort waits for keyboard input
$ sort
banana
apple
cherry
# Press Ctrl+D to finish
apple
banana
cherry

# With redirect — reads from file
$ sort < fruits.txt
apple
banana
cherry

Common use cases:

sort < data.txt          # Sort a file
wc -l < data.txt         # Count lines
grep "error" < log.txt   # Search a file

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Redirecting stderr (2>)

ls out.txt
ls out.txt 2> error.txt
cat error.txt

Why separate stdout and stderr?

• Regular output and error messages are different things
• Sometimes you want to save only errors
• Or hide errors while showing output

Example:

# ls out.txt — file doesn't exist
$ ls out.txt
ls: cannot access 'out.txt': No such file or directory

# Redirect the error to a file
$ ls out.txt 2> error.txt
# (no output on terminal)

$ cat error.txt
ls: cannot access 'out.txt': No such file or directory

File descriptors in the command:

Syntax	Meaning
2>	Redirect stderr (fd 2) to a file
1>	Redirect stdout (fd 1) to a file
0<	Redirect stdin (fd 0) from a file

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Redirecting stdout and stderr

To Different Files

ls out.txt output.txt > lslog.txt 2> error.txt

• stdout → lslog.txt
• stderr → error.txt

Example:

$ ls existing.txt missing.txt > success.txt 2> errors.txt

$ cat success.txt
existing.txt

$ cat errors.txt
ls: cannot access 'missing.txt': No such file or directory

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To the Same File

ls out.txt &> log.txt

• Both stdout and stderr → log.txt

Equivalent forms:

# Modern (bash 4+)
cmd &> file.txt

# Traditional (POSIX-compatible)
cmd > file.txt 2>&1

Note: The order matters! 2>&1 must come after >.

cmd > file.txt 2>&1    # ✅ Both go to file.txt
cmd 2>&1 > file.txt    # ❌ stderr goes to terminal, stdout to file

Why? 2>&1 means “make fd 2 point where fd 1 currently points.” If > hasn’t redirected fd 1 yet, fd 2 goes to the terminal.

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The /dev/null — The Void

ls d1 1> /dev/null
ls d1 2> /dev/null

/dev/null is a special file that accepts input and does nothing with it — the black hole of Linux.

Command	Effect
cmd > /dev/null	Discard stdout
cmd 2> /dev/null	Discard stderr
cmd &> /dev/null	Discard everything

Examples:

# Hide stdout, show stderr
$ ls existing.txt missing.txt > /dev/null
ls: cannot access 'missing.txt': No such file or directory

# Hide stderr, show stdout
$ ls existing.txt missing.txt 2> /dev/null
existing.txt

# Hide everything
$ ls existing.txt missing.txt &> /dev/null
# (no output)

# Check if a command succeeds silently
$ if ls /nonexistent &> /dev/null; then
    echo "exists"
else
    echo "not found"
fi

Why it’s useful:

• Testing — run commands without noise
• Cron jobs — prevent email spam from output
• Scripts — hide messages you don’t need
• Silencing errors — hide expected errors

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Complete Example Session

# ============================================
# PART 1: REDIRECT STDOUT
# ============================================

$ echo "I am Kronos" > output.txt
$ cat output.txt
I am Kronos

# Overwrite
$ echo "I am Zeus" > output.txt
$ cat output.txt
I am Zeus

# ============================================
# PART 2: APPEND WITH >>
# ============================================

$ echo "My son is Zeus" >> output.txt
$ cat output.txt
I am Zeus
My son is Zeus

$ echo "My wife is Hera" >> output.txt
$ cat output.txt
I am Zeus
My son is Zeus
My wife is Hera

# ============================================
# PART 3: REDIRECT STDIN WITH <
# ============================================

$ cat > fruits.txt
banana
apple
cherry
# Press Ctrl+D

$ sort < fruits.txt
apple
banana
cherry

# ============================================
# PART 4: REDIRECT STDERR WITH 2>
# ============================================

$ ls out.txt
ls: cannot access 'out.txt': No such file or directory

$ ls out.txt 2> error.txt
$ cat error.txt
ls: cannot access 'out.txt': No such file or directory

# ============================================
# PART 5: REDIRECT BOTH STREAMS
# ============================================

# To different files
$ ls output.txt missing.txt > success.txt 2> errors.txt
$ cat success.txt
output.txt
$ cat errors.txt
ls: cannot access 'missing.txt': No such file or directory

# To the same file
$ ls output.txt missing.txt &> combined.txt
$ cat combined.txt
ls: cannot access 'missing.txt': No such file or directory
output.txt

# Traditional syntax
$ ls output.txt missing.txt > combined.txt 2>&1

# ============================================
# PART 6: /dev/null
# ============================================

# Hide stdout
$ ls output.txt missing.txt > /dev/null
ls: cannot access 'missing.txt': No such file or directory

# Hide stderr
$ ls output.txt missing.txt 2> /dev/null
output.txt

# Hide everything
$ ls output.txt missing.txt &> /dev/null
# (no output)

# ============================================
# PART 7: PRACTICAL EXAMPLES
# ============================================

# Log output of a script
$ ./backup.sh > backup.log 2>&1

# Save errors separately
$ ./backup.sh > backup.log 2> backup-errors.log

# Discard output but keep errors
$ ./quiet.sh > /dev/null

# Silent check
$ if ping -c 1 google.com &> /dev/null; then
    echo "Online"
else
    echo "Offline"
fi

# Redirect output of multiple commands
$ { echo "Line 1"; echo "Line 2"; } > multi.txt
$ cat multi.txt
Line 1
Line 2

# Redirect with tee (view AND save)
$ ls | tee ls-output.txt
# Shows output AND saves to file

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Quick Reference

Redirection Operators

Operator	Description
>	Redirect stdout (overwrite)
>>	Redirect stdout (append)
<	Redirect stdin from file
2>	Redirect stderr (overwrite)
2>>	Redirect stderr (append)
&>	Redirect both stdout + stderr (overwrite)
&>>	Redirect both (append)
2>&1	Point stderr to where stdout goes
> /dev/null	Discard stdout
2> /dev/null	Discard stderr
&> /dev/null	Discard everything

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File Descriptors

FD	Name	Default
0	stdin	Keyboard
1	stdout	Terminal
2	stderr	Terminal

---

/dev/null

Aspect	Description
Purpose	Discards everything written to it
Use case	Silencing output/errors
Type	Special character device
Also	Reading from it returns EOF immediately

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Best Practices

✅ Do This:

# Use >> for logs
echo "$(date): backup started" >> backup.log

# Separate errors from output
./script.sh > output.log 2> errors.log

# Combine when debugging
./script.sh > all.log 2>&1

# Order matters with 2>&1
cmd > file 2>&1         # ✅ Both to file
cmd 2>&1 > file         # ❌ Wrong order

# Use /dev/null to silence
cron-job &> /dev/null

# Test with tee to see AND save
long-command | tee output.txt

# Check exit codes even with redirection
if cmd > /dev/null 2>&1; then
    echo "success"
fi

❌ Don’t Do This:

# Don't overwrite important files with >
cat important.log > important.log   # ❌ Erases file!
cat important.log >> important.log  # ✅ Appends

# Don't redirect stderr without redirecting stdout
cmd 2> errors.txt                   # ✅ stderr saved, stdout shown
cmd > output.txt                    # ✅ stdout saved, stderr shown
cmd 2> errors.txt > output.txt      # ✅ Both saved separately

# Don't use > when you want to keep old content
echo "new log entry" > log.txt      # ❌ Overwrites whole log
echo "new log entry" >> log.txt     # ✅ Appends to log

# Don't forget the order with 2>&1
cmd 2>&1 > file.txt                 # ❌ stderr goes to terminal!
cmd > file.txt 2>&1                 # ✅ Correct

# Don't redirect to /dev/null when debugging
./script.sh &> /dev/null            # ❌ You see nothing!
./script.sh                         # ✅ Debug normally

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Common Pitfalls

Pitfall	Problem	Solution
> overwrites	Loses old data	Use >> for appending
2>&1 order	stderr not redirected	Put 2>&1 after >
Redirection on nonexistent dir	Error	Create dir first
/dev/null on important data	Data lost	Use tee to save
Confusing stderr and stdout	Wrong file gets error	Use 2> for stderr
Bash-specific &> in scripts	Not portable	Use > file 2>&1

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Real-World Examples

1. Logging Script Output

#!/bin/bash
# Run backup, save both stdout and stderr to log
./backup.sh >> /var/log/backup.log 2>&1

# Save output and errors separately
./backup.sh > backup-success.log 2> backup-error.log

2. Silent Cron Jobs

# In crontab — run every hour, no email
0 * * * * /usr/local/bin/cleanup.sh &> /dev/null

3. Test If Command Exists

if command -v git &> /dev/null; then
    echo "Git is installed"
else
    echo "Please install Git"
fi

4. Suppress Expected Errors

# Find files, ignore permission errors
find / -name "*.conf" 2> /dev/null

# List directories, ignore "not found" errors
ls -d */ 2> /dev/null

5. Build Incremental Log

#!/bin/bash
LOG="build.log"

echo "===== Build started $(date) =====" >> $LOG
make >> $LOG 2>&1
echo "===== Build finished $(date) =====" >> $LOG

6. View and Save with tee

# See output AND save to file
ls -la | tee directory-listing.txt

# Append instead of overwrite
ls -la | tee -a directory-listing.txt

# Save and see, but suppress errors
command 2> /dev/null | tee output.txt

7. Redirect Only Errors

# Run script, show output, save errors
./script.sh 2> errors.log

# Run script, discard output, see errors
./script.sh > /dev/null

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Visual: Redirection Flow

BEFORE (no redirection):
   Command  ──stdout──→  Terminal
            ──stderr──→  Terminal

AFTER (echo "Hi" > file.txt):
   Command  ──stdout──→  file.txt
            ──stderr──→  Terminal

AFTER (cmd &> file.txt):
   Command  ──stdout──→  file.txt
            ──stderr──→  file.txt

AFTER (cmd 2> /dev/null):
   Command  ──stdout──→  Terminal
            ──stderr──→  /dev/null (discarded)

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Summary

Concept	Description
stdin (0)	Input, usually keyboard
stdout (1)	Output, usually terminal
stderr (2)	Errors, usually terminal
>	Redirect stdout, overwrite
>>	Redirect stdout, append
<	Redirect stdin from file
2>	Redirect stderr
&>	Redirect both
2>&1	Point stderr to stdout
/dev/null	Discards everything

Key takeaways:

• Every command has three streams: stdin (0), stdout (1), stderr (2)
• > overwrites, >> appends
• 2> redirects errors separately from output
• &> or > file 2>&1 redirects both streams
• /dev/null is the void — use it to silence output or errors
• Order matters — 2>&1 must come after > to work correctly
• Use tee when you want to see AND save output

Remember: Redirection is what makes the shell composable. It lets you chain commands, save output, discard noise, and control exactly where every byte goes. Master >, >>, 2>, and /dev/null, and you’ll write far more powerful scripts. The three streams are the plumbing of the command line — once you understand them, everything clicks!

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