tee Command Cheat Sheet

tee reads from standard input and writes to both standard output and one or a more files. It's named after the "T" pipe junction used in plumbing.

Synopsis

command | tee [OPTION]... [FILE]...

Run a command, see the output, AND save it to a file.

ls -la | tee filelist.txt

Don't overwrite the file; append to it.

echo "Server restart: $(date)" | tee -a system.log

You cannot do sudo command > /etc/file because the redirection > runs as your user, not root. Use tee instead:

echo "127.0.0.1 my.site" | sudo tee -a /etc/hosts

Ignore interrupt signals (Ctrl+C).

long_running_command | tee -i log.txt

Process the data in different ways by using process substitution.

echo "Hello" | tee >(md5sum) >(sha256sum) | wc -c

Insert tee in the middle of a pipe to inspect data at that stage.

cat raw.txt | tee stage1.log | grep "Error" | tee stage2.log | sort

Stdout: tee always passes the input to stdout, so the next command in the pipe receives it unmodified.
Stderr: tee only captures stdout. To capture stderr too, redirect it first: command 2>&1 | tee log.txt.