How does the tee command works?

After coming across about 3 tee explanations I regard as undidactic, I would ask for a simple, general, and stepped (if possible) explanation about this command, aimed for Linux newcomers.

I do understand we use it either through a pipe, on a command’s stdout, or alternatively, directly a certain file, but I think I miss what the command actually do with this content and when it is useful.

So here’s what I ask, hoping to find a didactic explanation, served to newcomers in a clear, stepped way:

  1. How does the command work with either with a command’s stdout, or alternatively, the files themselves?

  2. Why is it common to say tee reads standard input? I mean, if I do ls -l, tee doesn’t read the syntax ls -l itself but rather the stdout it printed into the session.

Also, if you want, please share a practical example from your daily work as to when tee is very useful to you?

Asked By: user149572


From the tee manual on my system:

The tee utility copies standard input to standard output, making a copy
in zero or more files. The output is unbuffered.

So, it reads from standard input and copies it to standard output, and while doing so also duplicates the stream into one or several files.

In the following pipeline, tee would take the output of the first command in the pipeline and copy it to standard output (the terminal) while also making copies of it in the files one, two and three:

$ somecommand | tee one two three

tee has many uses, one is in conjunction with sudo to redirect output to a file owned by root:

$ somecommand | sudo tee /root/somefile >/dev/null

The following would not have worked since the redirection happens as the unprivileged user (it would also run somecommand as root which may be unwanted):

$ sudo somecommand >/root/somefile

An artificial example of appending a fixed set of lines to many files at once (to all users ~/.profile files, assuming that the * expands to the usernames and that the expanded command line does not become too long for the shell to handle):

$ tee -a /home/*/.profile <<'END_NEWPATH'

A real example of using tee:

time doas 2>&1 | tee build.out |  grep '^=*>'

This is me building the OpenBSD base system. doas is the OpenBSD “equivalent” of sudo and is a small shell script that does the building (essentially cd /usr/src && make obj && make build). I’d like to store the output of the whole build process, including any errors or warnings, but I don’t want to have everything spewing out into my terminal. To this end I use tee to save everything into build.out and then grep to only get a hint of where in the process we are in the terminal.

See also the other questions here that are tagged with the tag.

Answered By: Kusalananda
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