Output Redirection

Output redirection lets you send command output to files instead of the screen. It's a fundamental concept for automating tasks and saving results.

Standard Streams

Linux has three standard streams:

Stream	Number	Description
stdin	0	Standard input (keyboard)
stdout	1	Standard output (screen)
stderr	2	Standard error (screen)

Basic Redirection: >

Send output to a file (overwrites):

echo "Hello" > greeting.txt
ls > files.txt

Append: >>

Add to a file (doesn't overwrite):

echo "Line 1" > file.txt
echo "Line 2" >> file.txt

Exercise: Create a File with Redirection

Create a file using echo and redirection:

Redirecting stderr

Errors go to stderr (stream 2):

ls nonexistent 2> errors.txt     # Redirect errors only
ls * 2> /dev/null                # Discard errors

Redirect Both stdout and stderr

command > output.txt 2>&1        # Both to same file
command &> output.txt            # Shorthand (bash)
command > out.txt 2> err.txt     # Separate files

The Null Device

/dev/null discards output:

command > /dev/null              # Discard output
command 2> /dev/null             # Discard errors
command &> /dev/null             # Discard everything

Input Redirection

Read input from a file:

sort < unsorted.txt
wc -l < file.txt

Here Documents

Provide multi-line input:

cat << EOF > config.txt
server=localhost
port=3000
debug=true
EOF

Common Patterns

Save Command Output

date > timestamp.txt
ps aux > processes.txt

Create Quick Notes

echo "Remember to commit" >> TODO.txt

Log Output

./script.sh > script.log 2>&1

Discard Unwanted Output

find / -name "file" 2> /dev/null

Overwrite vs Append

Operator	Effect
>	Create/overwrite file
>>	Create/append to file

echo "First" > file.txt   # File contains: First
echo "Second" > file.txt  # File contains: Second (overwritten)
echo "Third" >> file.txt  # File contains: Second\nThird

Exercise: Append to File

Append a new line to an existing file:

Key Takeaways

• > redirects output to a file (overwrites)
• >> appends to a file
• 2> redirects errors
• &> redirects both output and errors
• /dev/null discards unwanted output