I’ve read about the Unix 'tee' command before but never found a reason to use it until the last few weeks.

One of the things I repeatedly do by mistake is open /etc/hosts without sudo and then try to make changes to it:

I always used to close the file and then re-open it with sudo but I recently came across an approach which allows us to use 'tee' to get around the problem.

Once we’ve made our change we can run the following command:

The ':w' is usually used to write the buffer to the current file but here we’re substituting a shell command to receive the buffer instead.

In this case we’re sending the buffer to the 'sudo tee %' command where '%' refers to the current file name.

tee is defined like so:

So we’re sending the buffer for the current file to tee which then saves it to the current file. Since we’ve run tee with sudo it will give us the appropriate permissions to write to the file that we didn’t initially have.

This Stack Overflow post explains what’s going on in more detail.

Another thing we’ve been working on is hooking up the logs of our various services to logstash and to test it out we needed to write to files in the /var/log directory.

We started out trying to echo a message to the end of the apache log file like this:

As you can see that doesn’t work since we don’t have permissions to write to files in /var/log so we need to find another way to do that and Nick pointed out that tee would do the job here as well.

Passing the '-a' flag to tee means that it appends standard input to the file instead of replacing the whole file like before. If we don’t want standard in to be directed to standard out we can redirect that to /dev/null like so: