Learn Linux Shell Scripting - Fundamentals of Bash 4.4

tee

A command that was seemingly created to work in tandem with a pipe is tee. The description on the man page should tell most of the story:

tee - read from standard input and write to standard output and files

So, in essence, sending something to the stdin of tee (via a pipe!) allows us to save that output to both your Terminal and a file at the same time.

This is often most useful when using interactive commands; it allows you to follow the output live, but also write it to a (log) file for later review. Updating a system provides a good example for the use case of tee:

sudo apt upgrade -y | tee /tmp/upgrade.log

We can make it even better by sending all output to tee, including stderr:

sudo apt upgrade -y |& tee /tmp/upgrade.log

The output ...