tee

What it does

How to write it

tee [OPTION]... [FILE]...

Most used

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$ run as a normal user# run as root (or with sudo)

1. 1See the output and save it

   $dnf check-update | tee updates.txt

   You watch the list and keep a copy for your change ticket.

2. 2Write a root-owned file from a normal user

   $echo "net.ipv4.ip_forward = 1" | sudo tee /etc/sysctl.d/90-forward.conf

   sudo echo ... > file fails because the redirect is not done by sudo.

3. 3Add a line to a root-owned file

   $echo "192.168.122.20 server2" | sudo tee -a /etc/hosts

   -a appends. Without it the file would be replaced.

4. 4Hide tee's copy on the screen

   $echo "export EDITOR=vim" | sudo tee /etc/profile.d/editor.sh > /dev/null

   Only the file gets the text.

5. 5Write several files at once

   $ip -br a | tee /tmp/net-$(hostname).txt /srv/reports/net-latest.txt

   One command, two copies.

6. 6Log a long script and still watch it

   $./upgrade.sh 2>&1 | tee -a /var/tmp/upgrade-$(date +%F).log

   2>&1 sends errors into the pipe too, so the log is complete.

7. 7Save the middle of a pipe

   $journalctl -u sshd --since today | tee ssh-today.log | grep -c "Failed password"

   The full log is saved; the screen shows only the count of failed logins.

8. 8Write a heredoc to a protected file

   $sudo tee /etc/yum.repos.d/local.repo > /dev/null <<'EOF'
   [local-baseos]
   name=Local BaseOS
   baseurl=file:///repo/BaseOS
   gpgcheck=0
   EOF

   A clean way to create config files in scripts that run as a normal user with sudo.

9. 9Keep running when you press Ctrl+C

   $ping server2 | tee -i ping.log

   With -i, tee is not stopped by Ctrl+C, so the log gets the last lines.

10. 10Record a kubectl apply for the change log

    $kubectl apply -f deploy.yaml 2>&1 | tee -a changes.log

    Every change is visible now and stored for later.

11. 11Count and save matches at the same time

    $grep "ERROR" app.log | tee errors.txt | wc -l

    The number on the screen, the lines in the file.

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