Tail: Command Line Debug

Tail: Command Line Debug with Will Hetherington In this video, we are going to discuss the tail command. It's a Unix tool, and by default prints the last 10 lines of input you provide to standard output, which makes it really useful for debugging. This might have you asking, what is standard output? Well, by default this is your terminal window. In documentation, you'll often see this shortened to just "stdout," standard out. Today we're going to use tail to examine some log files and then to watch those logs in real time and then a practical application using tail to debug a white screen of death with a Drupal install. So let's take a look at the most basic example. By default my web server's log files are located in /var/log/apache2. And if we take a look at the access log, by default tail will print the last 10 lines from the file you provide. Let's take a quick moment to look at the manual page, which you can open by typing "man tail" at your terminal. I'm not going to discuss all of the options that you could pass to tail, just the two that I find most useful. That would be -n or --lines. And you can pass an integer with the number of lines you'd like to return. The other is -f or --follow, and this will keep showing output as the file grows. So let's take a look at examples of both of those. If we take the -n flag and pass, say, the number 50, then we're going to get 50 lines of output back from tail, or rather tail's going to provide 50 lines of output from the file that we passed to it if indeed 50 lines exist in the file. Otherwise it will show as many as it can. My other favorite is -f or --follow. So the -f flag or --follow, it's really useful for watching your log files in real time. So you'll notice at the bottom of my terminal, I don't have a prompt currently. I do have a flashing cursor but no prompt. And the reason for that is that using the -f flag means that tail will keep the file that we passed open to it, and it will keep printing the output to the terminal window as soon as it's written to the file. So to show that in action, I am just going to refresh the browser on the left-hand side a couple of times, and you'll see the log rows on the right-hand side happily being printed by tail. Let's say, for a second, that you're developing a new module for your Drupal install. Everything's been going swimmingly until you press F5 or click refresh on your browser, and the dreaded white screen of death appears. At this point you probably don't have much idea about the problem that you've arrived at. This is where tail can really come to the rescue. So first things first. We're going to want to take a look at our web server's error log. This is most often, when you arrive at a white screen of death, where you'll find more information. Running the tail command and the path to our error.log, you will be able to see the last 10 lines of output which will hopefully give us some clues as to what the problem might be. So just by reading the last couple of lines in this error.log, you'll see that it says the allowed memory size has been exhausted. So this is most likely an issue with the memory limit in your PHP configuration. Now to be clear, tail doesn't try and help us fix the problem. It really helps us to pinpoint or narrow down what or where it might be. In this case, my issue was a problem with PHP's max memory, and after increasing the allowed max memory, I've restarted my web server, and my site is back in action. So in this video, we've covered examining your access and your error logs with tail, also how to watch those log files in real time with tail, and then a practical application where we use tail to analyze a white screen of death from a Drupal install from a log perspective. Thanks for watching.