tee Command — Writing Output to Both Screen and File¶

The tee command reads data from standard input and writes it simultaneously to the terminal and one or more files. It is an essential Linux utility for logging command output, debugging shell scripts, monitoring automation, and building CI/CD pipelines.

Learning Path¶

Linux Mastery → Module 3: Text Processing → Lesson 14

Difficulty: Beginner → Intermediate

Reading Time: 30 Minutes

Course Progress

Course: Linux Mastery

Module: Text Processing

Lesson: 14 of 18

What You'll Learn¶

After completing this lesson, you'll be able to:

Understand the tee command
Save command output while displaying it
Append output to files
Write to multiple files
Use tee in shell pipelines
Log automation output
Apply tee in DevOps and CI/CD workflows

Prerequisites¶

Before starting this lesson, complete:

Module 1 – Linux Fundamentals
Module 2 – Linux Command Line Essentials
Module 3 Lessons 1–13

Why Learn tee?¶

Normally:

ls > files.txt

Output goes to the file.

Nothing appears on the screen.

Or:

ls

Output appears on the screen.

Nothing is saved.

What if you need both?

ls | tee files.txt

Now you can:

View the output
Save it to a file

At the same time.

What is tee?¶

The tee command copies its input to:

Standard Output (Terminal)
One or more files

Syntax:

tee [OPTIONS] FILE...

Think of it as a T-junction in a pipe.

Command
   │
   ▼
  tee
 ┌──┴──┐
 ▼     ▼
Screen File

Basic Usage¶

echo "Welcome to REBASH Academy" | tee output.txt

Output on screen:

Welcome to REBASH Academy

File:

cat output.txt

Output:

Welcome to REBASH Academy

Save Command Output¶

ls -l | tee files.txt

You will see the directory listing while also saving it.

Append Instead of Overwrite¶

Normally:

tee output.txt

overwrites the file.

Append instead:

echo "Docker" | tee -a output.txt

View:

cat output.txt

Output:

Linux

Docker

Write to Multiple Files¶

echo "Cloud Computing" | tee file1.txt file2.txt file3.txt

All files receive the same output.

Use with grep¶

Save matching lines.

grep ERROR application.log | tee errors.log

Display:

ERROR Database Connection Failed

The same output is saved to errors.log.

Use with sort¶

sort employees.txt | tee sorted.txt

Use with wc¶

cat access.log | tee backup.log | wc -l

Workflow:

access.log
      │
      ▼
     tee
   ↙     ↘
backup.log  wc -l

Use with find¶

find . -name "*.sh" | tee scripts.txt

Logging Script Output¶

./backup.sh | tee backup.log

You can monitor progress in real time while keeping a log for later review.

Capture Both Standard Output and Errors¶

Normally, tee captures only standard output.

To capture both:

command 2>&1 | tee output.log

Example:

ls /existing /missing 2>&1 | tee result.log

Output on screen:

/existing

ls: cannot access '/missing': No such file or directory

The same content is saved to result.log.

Ignore Interrupt Signals¶

Useful for long-running processes.

tee -i output.log

The -i option ignores interrupt signals (Ctrl+C) sent to tee.

Common tee Options¶

Option	Description
`-a`	Append instead of overwrite
`-i`	Ignore interrupt signals

Combining with Other Commands¶

Monitor disk usage.

df -h | tee disk-report.txt

Save running processes.

ps -ef | tee processes.txt

Save Kubernetes Pods.

kubectl get pods | tee pods.txt

Save Docker containers.

docker ps | tee containers.txt

Save environment variables.

printenv | tee env.txt

Real Production Examples¶

Save deployment logs.

kubectl apply -f deployment.yaml | tee deployment.log

Save Terraform output.

terraform apply | tee terraform.log

Save Ansible execution.

ansible-playbook site.yml | tee ansible.log

Save Helm installation.

helm install myapp chart/ | tee helm.log

Save Docker build logs.

docker build -t app . | tee build.log

Production Perspective¶

The tee command is one of the most frequently used utilities in:

CI/CD pipelines
DevOps automation
System administration
Deployment scripts
Infrastructure provisioning
Production troubleshooting

It allows engineers to monitor commands while simultaneously creating logs for auditing and debugging.

Hands-on Lab¶

Task 1¶

Display and save text.

echo "Linux" | tee linux.txt

Task 2¶

Append another line.

echo "Docker" | tee -a linux.txt

Task 3¶

Display the file.

cat linux.txt

Task 4¶

Save directory listing.

ls -l | tee files.txt

Task 5¶

Save process list.

ps -ef | tee processes.txt

Task 6¶

Write to multiple files.

echo "REBASH Academy" | tee file1.txt file2.txt

Task 7¶

Capture both output and errors.

ls /tmp /invalid 2>&1 | tee errors.log

Task 8¶

Count files while saving the list.

ls | tee listing.txt | wc -l

Command Deep Dive¶

Command	Purpose	Production Example
`tee file`	Save output	Reports
`tee -a`	Append logs	Automation
`tee file1 file2`	Multiple outputs	Backups
`2>&1 \\| tee`	Capture stdout + stderr	Troubleshooting

Production Troubleshooting Scenario¶

Scenario

A DevOps engineer is deploying an application to Kubernetes.

Tasks:

Watch deployment progress live.
Save deployment logs.
Capture errors.
Share the log with the support team.

Solution:

kubectl apply -f deployment.yaml 2>&1 | tee deployment.log

Benefits:

Live monitoring
Permanent log file
Easier troubleshooting
Audit trail

Mini Challenge¶

Perform the following:

Save the output of df -h while displaying it.
Save the output of ps -ef.
Append a custom message to an existing log.
Capture errors from an invalid command.
Save the list of shell scripts in the current directory.
Write the same output to two different files.

Best Practices¶

Use tee in deployment and automation scripts.
Use -a for log files that should be preserved.
Capture both stdout and stderr using 2>&1.
Use descriptive log file names.
Rotate or archive large log files regularly.

Performance Tip¶

For long-running commands that produce continuous output:

tail -f application.log | tee live-monitor.log

This allows real-time monitoring while creating a persistent copy for later analysis.

Common Mistakes¶

❌ Forgetting -a.

✅ Without -a, the file is overwritten.

Incorrect:

echo "New Entry" | tee log.txt

Correct:

echo "New Entry" | tee -a log.txt

❌ Assuming tee captures error output automatically.

✅ Incorrect:

command | tee output.log

Correct:

command 2>&1 | tee output.log

❌ Using tee without considering log file growth.

✅ Regularly rotate or clean up logs in long-running systems.

Interview Questions¶

Beginner¶

What is the purpose of the tee command?
How do you append output to a file?
How do you write to multiple files?
Why is tee useful in pipelines?

Intermediate¶

Explain the difference between > and tee.
How do you capture both standard output and standard error?
What does the -i option do?
Why is tee commonly used in shell scripts?

Architect Level¶

How would you log a Kubernetes deployment while monitoring it live?
Why is tee valuable in CI/CD pipelines?
How would you design a deployment script that logs every step without hiding console output?

Summary¶

In this lesson, you learned:

Displaying and saving output simultaneously
Appending to log files
Writing to multiple files
Capturing errors
Using tee in shell pipelines
Real-world DevOps and automation use cases

The tee command is a simple yet powerful utility that bridges the gap between visibility and logging. It is widely used in Linux administration, automation, CI/CD, and production troubleshooting.

Key Takeaways¶

tee writes output to both the terminal and files.
Use -a to append instead of overwrite.
Use 2>&1 | tee to capture both standard output and errors.
tee integrates naturally with pipelines.
It is a standard tool for logging deployments, builds, and automation tasks.

What's Next?¶

xargs Command — Building Powerful Command Pipelines in Linux

In the next lesson, you'll learn:

Passing input as command arguments
Running commands on multiple files
Combining find with xargs
Parallel execution
Production automation examples