Hack51.Free Up Disk Space Now

Hack 51. Free Up Disk Space Now

Moving large files to another partition isn't always an option, especially if running services are holding them open. Here are a few tips for truncating large files in emergency situations.

Server consolidation takes planning, and it usually means adjusting the way you set up your OS installations. Running multiple services on a single OS image means not only increased network traffic to the same hardware, but increased disk usage for logfiles.

What's more is that administrators' thirst for more data about the services they run has resulted in a tendency for logging to be more verbose these days than it was in the past, partially because the tools for analyzing the data are getting better.

However, someday you'll inevitably be faced with a situation where you're receiving pages from some form of service monitoring agent telling you that your web server has stopped responding to requests. When you log in, you immediately type df h to see if what you suspect is true, and it isyour verbose logging has just bitten you by filling up the partition, leaving your web server unable to write to its logfiles, and it has subsequently stopped serving pages and become useless. What to do?

There are several commands you can use to deal with this. If the service is completely dead, you could actually move the file to another partition, or simply run rm -f logfile if you know that the data is not particularly useful. If the service is still running, however, and needs its logfile to be available in order to do anything useful, truncation may be the way to go. Some admins have a watchdog script that polls for large files created by noncritical services and truncates them before they get out of control, without having to restart the service. A command that might appear in a script to do this (which can also be issued at a command line) is:

 $ cat /dev/null >  filename  

Obviously, you should run this command as root if the file you are truncating requires elevated privileges. Why use /dev/null? You could also use the following command:

 $ cat >  filename  

This is certainly a little shorter, but the downfall here is that it doesn't exit by itselfyou need to terminate it manually. On the command line, that means typing Ctrl-C to exit.

While these commands definitely work, I'd like to show you what I believe to be the shortest file truncation command known to bash. It goes a little something like this:

 $ > filename 

The above command has no dependency on anything except for the redirection operator >. Essentially, you are redirecting what's on the left of the operator (which is to say, nothing) into the file in question. What makes this perfectly elegant is that it exits all by itself and leaves behind a file of zero bytes in length. What more could an admin ask for?

Technically, understanding what has happened above involves knowing how redirection in the shell works. In the bash shell, if the redirection operator is pointing to the right (i.e., >), what is being directed is the standard output of whatever is on the left. Since we've specified no command on the lefthand side, the standard output is nothing, and our redirection operator happily overwrites our large file, replacing the contents with…nothing.