tee — split command output to stdout and a file — garron.me

What is tee

tee reads from stdin and writes to stdout and to one or more files simultaneously. It acts as a T-junction in a pipeline — the data flows through to the terminal while also being saved.

Basic usage

command | tee file.txt

The output appears on screen as usual, and is also written to file.txt.

Example — capture the output of free:

free | tee free.txt

You see the output on screen and free.txt contains the same content.

Append instead of overwrite

By default tee overwrites the file. Use -a to append:

command | tee -a log.txt

Useful for accumulating log entries across multiple runs.

Write to multiple files

command | tee file1.txt file2.txt

Both files receive the same output.

Pass output to another command

tee still writes to stdout, so you can keep piping:

command | tee file.txt | grep error

This saves the full output to file.txt while passing only matching lines to grep.

The sudo tee pattern

A very common situation: you want to write to a file owned by root, but your shell runs as a regular user. This fails:

sudo echo "text" > /etc/file.conf   # WRONG — redirection runs as your user

The correct pattern is:

echo "text" | sudo tee /etc/file.conf

Or to append:

echo "text" | sudo tee -a /etc/file.conf

tee runs as root (via sudo) and handles the write, while echo runs as your user.

Suppress screen output

If you only want the file and not the terminal output, redirect stdout to /dev/null:

command | tee file.txt > /dev/null

Practical examples

Save a build log while watching it:

make 2>&1 | tee build.log

The 2>&1 merges stderr into stdout so both are captured.

Capture package installation output:

apt install nginx | sudo tee /var/log/nginx-install.log

Write a config block to a root-owned file:

echo "net.ipv4.ip_forward=1" | sudo tee -a /etc/sysctl.conf
sudo sysctl -p

tee — split command output to stdout and a file