CIS120 Book

CIS120 Linux Fundamentals by Scott Shaper

The tee Command

Think of the tee command like a photocopier for your command output. Just like a photocopier can make a copy while keeping the original, tee lets you save a copy of your command's output to a file while still showing it on your screen. It's perfect for when you want to both see what's happening and keep a record of it!

Quick Reference

Command	What It Does	Common Use
`tee`	Saves output to file while showing it on screen	Logging, debugging, saving command output

When to Use tee

Use tee when you want to:

Save command output while still seeing it on screen
Create a log of what you're doing
Debug a command by saving its output
Send output to multiple places at once
Keep a record of your work

Common Options

Option	What It Does	When to Use It
`-a`	Adds to end of file instead of overwriting	When you want to keep existing content
`-i`	Ignores Ctrl+C interruptions	When you don't want to accidentally stop
`-p`	Checks for writing errors	When debugging file writing problems

Practical Examples

Basic Usage

# Save directory listing while showing it
ls -l | tee filelist.txt

# Save a message to a file while showing it
echo "Hello, World!" | tee greeting.txt

# Save to multiple files at once
echo "Important note" | tee note1.txt note2.txt

Real-World Scenarios

# Save installation output while watching it
make install | tee install_log.txt

# Keep adding to a log file
date | tee -a daily_log.txt
who | tee -a daily_log.txt

# Debug a command while saving output
ls -l /nonexistent 2>&1 | tee -i error_log.txt

Advanced Usage

# Save full output and filtered results
ls -l | tee full_list.txt | grep ".txt" > text_files.txt

# Create a backup while processing
cat original.txt | tee backup.txt | sort > sorted.txt

# Monitor system while logging
top -b -n 1 | tee system_snapshot.txt

Tips for Success

Start simple: Try basic tee before adding options
Use -a carefully: Remember it adds to files instead of replacing them
Check permissions: Make sure you can write to the target files
Use descriptive names: Name your output files clearly

Common Mistakes to Avoid

Forgetting that tee overwrites files by default
Not using -a when you want to keep existing content
Using wrong file permissions
Forgetting the pipe (|) before tee

Best Practices

Use -a for logging to avoid losing data
Use descriptive filenames for your output
Check file permissions before using tee
Use -i for important operations you don't want interrupted
Combine with other commands for powerful data processing

Advanced Techniques

Multiple tee Usage

# Example 1: Save output at different stages
# This command:
# 1. Lists files (saved to stage1.txt)
# 2. Filters for .txt files (saved to stage2.txt)
# 3. Counts the lines (saved to stage3.txt)
ls -l | tee stage1.txt | grep ".txt" | tee stage2.txt | wc -l | tee stage3.txt

# Example 2: Save both original and filtered data
# This command:
# 1. Shows original file (saved to original.txt)
# 2. Finds error lines (saved to errors.txt)
# 3. Counts total error lines
cat data.txt | tee original.txt | grep "error" | tee errors.txt | wc -l

# Example 3: Create multiple backups
# This command:
# 1. Shows original file (saved to backup1.txt)
# 2. Sorts the data (saved to backup2.txt)
# 3. Removes duplicates (saved to final.txt)
cat file.txt | tee backup1.txt | sort | tee backup2.txt | uniq > final.txt

# Example 4: Back-to-back tee commands
# This command:
# 1. Shows file contents
# 2. Saves to backup.txt AND text.html at the same time
# Note: Both files get the exact same content
cat file.txt | tee backup.txt | tee text.html

# Example 5: Multiple back-to-back tee commands
# This command:
# 1. Shows file contents
# 2. Saves to three different files at once
cat file.txt | tee file1.txt | tee file2.txt | tee file3.txt

You can use tee as many times as you want in a pipeline. Each tee command:

Saves a copy of the data at that point in the pipeline
Passes the data along to the next command
Can save to one or more files

Think of it like taking snapshots at different stages of your work. Each tee command is like taking a picture of your data at that moment, while the data continues flowing to the next command.

You can even use tee commands back-to-back to save the same data to multiple files at once. Each tee command in the chain receives the exact same input and saves it to its file.

Complex Pipelines

# Save both normal and error output
command 2>&1 | tee output_and_errors.txt

# Process data while keeping original
cat data.txt | tee backup.txt | sort | uniq > unique.txt

# Monitor system while logging
top -b -n 1 | tee system_snapshot.txt

Debugging Scenarios

# Debug a script while running it
./myscript.sh 2>&1 | tee script_output.txt

# Monitor network while saving data
ping google.com | tee -a network_log.txt

# Track changes while processing
diff file1.txt file2.txt | tee changes.txt | wc -l