tee command: print to the screen and save to a file

Syntax

command | tee [options] file...

Whatever comes in goes both to the screen and to the file.

Try it first

Keep the output of a grep while still reading it.

$ grep ERROR logs/app.log | tee errors.txt
ERROR connection refused
ERROR timeout
ERROR connection refused

You saw the result, and the same lines went into the file.

$ ls
errors.txt  logs/
$ cat errors.txt
ERROR connection refused
ERROR timeout
ERROR connection refused

With > the screen stays empty. tee is for when you want to watch and keep.

Saving from the middle of a pipe

tee passes its input straight through, so it can sit anywhere in a pipeline.

$ grep ERROR logs/app.log | tee errors.txt | wc -l
3

The answer you wanted was the count, 3, and the list that produced it is still in errors.txt. It is also a handy way to inspect the middle of a long pipeline.

Appending

-a adds to the end instead of replacing.

$ echo 'ERROR manual entry' | tee -a errors.txt
ERROR manual entry
$ cat errors.txt
ERROR connection refused
ERROR timeout
ERROR connection refused
ERROR manual entry

Writing to a file you do not own

This is where tee really earns its place. Files under /etc are not yours to write.

$ echo '10.0.0.1 db' >> /etc/hosts
zsh: permission denied: /etc/hosts

Adding sudo changes nothing, because the >> is carried out by your shell. tee moves the writing into a program that can run as root.

$ echo '10.0.0.1 db' | sudo tee -a /etc/hosts
10.0.0.1 db
$ cat /etc/hosts
127.0.0.1 localhost
10.0.0.1 db

sudo tee -a is the standard way to add a line to a system config file.

Several files at once

List the names and they all get the same content.

$ grep INFO logs/app.log | tee info.txt summary.txt
INFO  server started
INFO  retrying
$ ls
errors.txt  info.txt  logs/  summary.txt

When you actually reach for it

tee is what you type to keep a record without interrupting the flow.

Things that trip people up

Forgetting -a overwrites. tee run.log starts the file again every time. To accumulate, always add -a.

sudo does not apply to >. Redirection is the shell's job, so sudo only elevates the command. That is exactly why sudo tee exists.

It always prints. tee cannot be quiet. When you only want the file, use > or >>.