This article introduces a useful Linux command tee.

The function of tee command, as described in its man page, is

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

The first time I saw this command, I was wondering why I need that. If I want to check what was written into the file, just use cat to show the content of the file. Well, one obvious situation where you may want to use tee is to display the output of a command and save the output to file for later reference.

In addition to this common usage, there are some special cases where tee can come in handy.

Pipe needs root privilege

Sometimes you need to send the output to a file which requires root privilege. Say you are downloading some domain resolution entries for your organization’s internal servers. You want to add them to the /etc/hosts file. Normally you issue a command like this

wget -O- http://example.com/hostsfile >> /etc/hosts

Unfortunately this does not work and you are told that

bash: /etc/hosts: Permission denied

because you don’t have write permission to the hosts file. Now you will probably try this

sudo wget -O- http://example.com/hostsfile  >> /etc/hosts

Again it does not work because sudo only applies to wget and it cannot reach the redirection target. With tee you can achieve your goal with

wget -O- http://example.com/hostsfile | sudo tee -a /etc/hosts

The -a option sets the output to be appended.

Vim needs root privilege

Another situation is that people like me often forget to run vim with sudo to modify system config files. For example, you want to modify some configuration entries for PHP, with the following command

vim /etc/php5/apache2/php.ini

Once you’ve done the changes, you start type :wq to save and exit the editor. Oops! Vim tells you that you don’t have permission:

E45: 'readonly' option is set (add ! to override)

Is it unfortunate that you have to close the editor without saving the changes and reopen it with sudo and redo all the changes? Absolutely not. What I used to do was to use :save command to save the current content to somewhere I have write permission and then use sudo mv to overwrite the actual config file I would like to change. But that seems a little bit too much typing. Now you can solve this problem in place with tee. Type the following command in Vim:

:w !sudo tee %

This basically tells vim to run a shell command sudo tee with the current file name %. Note that Vim will warn you that the file content has changed. This is because the overwrite is conducted outside Vim. But that’s fine.