Linux tee Command: Syntax, Options, Examples

Introduction

When a user executes a command in a Linux interactive shell, the command prints the output to the terminal's standard output (stdout) stream. However, shell redirection and piping operators can redirect or pipe this output according to the user's needs.

In this tutorial, you will learn how to use the tee command in Linux to manage the output of a command.

Prerequisites

• A system running Linux.
• Command-line access.
• A user account with sudo privileges.

Linux tee Command Syntax

The basic syntax for the tee command is:

[command] | tee [options] [filename]

The example below uses tee to create an example.txt file that stores information about an enp0s3 network interface.

ifconfig enp0s3 | tee example.txt

The same information is also shown in the terminal output.

The following cat command confirms that tee successfully wrote the output of ifconfig to example.txt:

cat example.txt

tee Command Options

Below is the list of the options available to use with the tee command:

Option	Description
-a, --append	Do not overwrite a file if it exists already. Append the new content instead.
-i, --ignore-interrupts	Ignore interrupt signals.
-p, <b>--output-error</b>[=MODE]	Set write error behaviors. The following are the available modes: warn - diagnose errors. warn-nopipe - diagnose errors when writing to non-pipe outputs. The default mode when another mode is not specified. exit - exit on errors. exit-nopipe - exit on errors when writing to non-pipe outputs.
--help	Print the help manual.
--version	Show the command version.

tee Command and Piping

The tee command reads standard input (stdin) and writes it to both standard output (stdout) and one or more files.

Since its purpose is to forward a command output, tee is always used as a part of a pipe. The common usage scenarios for the tee command include capturing logs, debugging, and analyzing data streams.

Linux tee Command Examples

The following sections provide examples of the most common uses of the tee command.

Append to File

Overwriting the file's content is the default behavior of the tee command. Use argument -a (or --append) to add the command output to the end of the file.

[command] | tee -a [filename]

For example, use the echo command to append a line of text to a file:

echo "This text will be added" | tee -a example.txt

Confirm the successful addition with the cat command:

Write to Multiple Files

Use tee followed by any number of files to write the same output to each of them:

[command] | tee [options] [filename1] [filename2]...

The example below shows writing the output of the echo command to two files:

echo "Adding to multiple files" | tee example1.txt example2.txt

The ls command shows that tee successfully created files example1.txt and example2.txt.

Hide Output

Use the following syntax to instruct tee to store the command output in a file and skip the terminal output:

[command] | tee [options] [filename] >/dev/null

In the example below, tee creates a file containing network interface data, skipping the standard output:

ifconfig enp0s3 | tee example.txt >/dev/null

Redirect Output of One Command to Another

tee does not have to be the last command in the pipeline. Use it to forward the output to another command:

[command] | tee [options] [filename] | [command]

In the following example, tee stores the output of the ls command to example.txt and passes the content of that file to the grep command, which finds and displays all instances of the word "example":

ls | tee example.txt | grep "example"

Ignore Interrupts

To allow tee to exit correctly even after the previous command has been interrupted, add the argument -i (or --ignore-interrupts):

[command] | tee -i [filename]

The following example shows tee writing output from the ping command and completing the action successfully even after ping is interrupted with ctrl+ c:

ping google.com | tee -i example.txt

Using tee with sudo

To enable tee to write to a root-owned file or file belonging to another user, place the sudo command right before tee.

[command] | sudo tee [options] [filename]

The example below shows an unsuccessful attempt to write to the root-owned sudoex.txt. When the sudo command is added, the operation completes:

echo "Added text" | tee -a sudoex.txt

Using tee in Vim Editor

If you open and edit a root-owned file in Vim without using the sudo command, trying to save changes produces an error:

To override this error, type the following into Vim:

:w !sudo tee %

After you enter the sudo password, Vim displays a warning but writes the changes to the file.

Use tee Command with Bash Script

The tee command is often found in bash scripts. Consider the following example of a script named testbash.sh:

#!bin/bash
LOGFILE=/tmp/log-files-$(date +%d%m%Y)
echo "Hello World" | tee -a $LOGFILE

The script above prints the "Hello World" message and stores the output in a log file. Executing the script creates a log file in the tmp directory. The log contains the output of the script:

Watch Log Files

Writing script output to a log file is usually performed with the (>) operator:

./testbash.sh > testbash.log

The command above creates a log file but does not write anything to standard output.

Use tee to create a log file and see the output in the terminal:

./testbash.sh | tee testbash.log

See Help and Version Information

See the current version of the tee command by typing:

tee --version

For the instructions regarding the tee command syntax and the available arguments, use the command's help argument:

tee --help

Conclusion

By reading this tutorial, you learned how to use the tee command in a pipeline to manage the command output. The article also talked about the use of tee in bash scripts.

Read more about shell commands in this Linux commands cheat sheet.