How to use the Tee command in Linux

If you've ever used pipe and redirection in Linux shell, sometimes you'll need to use the tee utility.

What does Tee hold?

A command like ls will display the contents of the current directory. In other words, it displays these content into a stdout (standard output), usually your screen, or more accurately a virtual terminal screen.

A command like ls > file123 will not display anything on the screen. That's because the > sign redirects all output to a file instead of displaying it as stdout. Now file123 will be filled with the contents shown earlier on the screen.

To display the contents of the folder on the screen and write it to a file, you must use two commands. But with a tee , you can do both at the same time.

 ls | tee file123 

Why should Tee be used when you can run a similar command twice?

In the above example, it is clear that you don't need a tee if you can execute ls normally, then redo and redirect the output to a file. However, you will encounter situations in which the output is unique. Imagine the situation you are trying to diagnose a problem. You run:

 diagnose | tee error.log 

The errors you get may be unique. You want them to be displayed on the screen so you can see what's going on when checking everything. But you also want those errors to be saved in a file, to review later or paste the output into a discussion forum and consult people about it.

Another common situation that you may need to tee is when you want to take the output of the command to a location that only root users can read or write. The following command does not work:

 /sbin/blkid > /root/somefile 

Then you might think, just use sudo ! But you will be surprised when this command is not working either:

 sudo blkid > /root/somefile 

That's because after sudo blkid executes, you are still logged in as not a root user. And your shell (usually bash) tries to write to / root / somefile with your regular user information. To solve this, you can use the tee :

 /sbin/blkid | sudo tee /root/somefile 

Append text and redirect errors

Tee is a simple but useful command. A command | tee somefile basic command | tee somefile usually sufficient for most situations. However, there are 2 cases where you will need these tips.

The first thing to know is a tee , by default, always overwrites a file. If you run:

 ls | tee somefile 

Then run:

 ls /tmp | tee somefile 

The second command will overwrite the content of somefile and you will only see the contents of the last command executed. To change this behavior, you can create text that links tee instead of overwriting. To do so, simply use the -a switch command.

 ls | tee -a somefile 

The second thing to know is that not all outputs are the same. Error messages are handled differently and although they appear on the screen, they are not considered stdout, but are considered stderr and not handled by tee . The following is an example of grep .

 grep -r L2TP /etc | tee somefile 

The results shown will look like the following image:

Notice Permission denied is written into stderr. The only thing written into stdout is the highlighted text. That's why you notice that the content of 'somefile' is what is shown in the image below:

In this case, grep is used to search for text and is useful when error messages are not redirected to the file. Error messages only fill the file with unnecessary information. You just want to see the results found. But when you need an error message, use 2>&1 , to redirect stderr to stdout.

 grep -r L2TP /etc 2>&1 | tee somefile 

With this command, you will notice that somefile now also contains error messages.

Hopefully, this guide includes everything you need to make the most of the tee . But if you encounter some difficult situation with a tee , leave comments in the comment section below for everyone to share.

Wish you use the successful tee command!