Use tee to Append to Files in Privileged Permission, not Output Redirection

Sometimes, we want to append text to a file which we don't have permission to write to. Let's say /etc/fstab. It's tempted to use the command below

$ sudo echo "# test line" >> /etc/fstab
-bash: /etc/fstab: Permission denied

However, we get permission denied error, which is unexpected because we have already elevated the permission to root with sudo.

The actual reason for this to happen is that sudo only applies to echo, an external command. So, echo runs in privileged mode. It is bash shell, which still runs in normal permission, who is responsible to do output redirection. Therefore, we finally get permission issue.

How would we do this properly? One of the possible is solution is to use tee -a to append text.

$ echo -e "# test line 1\n# test line 2" | sudo tee -a /etc/fstab
# test line 1
# test line 2
$ tail -2 /etc/fstab
# test line 1
# test line 2