How to calculate available disk space on Linux?

Checking disk space on a Windows PC is easy -- just right-click on a drive like the C: drive and pick "Properties". But if you have a dedicated server or a Virtual Private Server (VPS) hosted for you at a hosting company, the chances are that it's running some version of Linux, and that you have to connect to it via a command line interface, looking something like the little black box in the image below. This means that most simple administrative commands, like checking free disk space, can no longer be accomplished with something intuitive like right-clicking; most administrative tasks correspond to a typed command that has to be memorized.

Most dedicated servers and Virtual Private Servers come with plenty of disk space, but if you are running any software that you wrote yourself or that was custom-written for you, a small error in the program could cause disk space to fill up quickly. Another possibility is that a remote user might be connecting to a service running on your machine (for example, browsing your website) and generating a lot of traffic, and the webserver writes a record of each transaction to a log file, and if you're getting an enormous amount of traffic, the log file could grow so large that you could start to run out of disk space.

If you're connected to your server via a command like interface like the following:

then to check disk space, type the "df -h" command:

Unfortunately the output is not particularly clear about which of these numbers you need to pay attention to. Look in the "Mounted on" column, and look for a row that has "/" listed in that column (it will usually be the first row of the output). In that row, look for the percentage listed in the "Use%" column. That is the percentage of your disk that is full. (Things get a little more complicated if you have more than one hard disk installed, but in cases like that, the hosting company who installed the extra hard disk should be able to tell you how to check how full it is.)

Many programs running on Linux will fail with mysterious error messages if you have run out of disk space. (Unfortunately it's rare for a program to simply check how much disk space is left, and if not enough is free, output an error saying "Not enough available disk space!" It's far more common for a program to simply fail with a cryptic error.) If everything seemed to be working fine yesterday but a program is giving a strange error today, and nothing else has changed, do "df -h" to check if a lack of disk space is the problem.

Even if disk space is not the problem, if you're posting a message asking for help in a Linux forum because you're running into an error that you've never seen before, run "df -h" just so that you can tell people, "I checked and I have plenty of disk space left" -- just to rule out one more possible cause of the problem :)

Bennett Haselton is a technology and political blogger who also maintains a page about how to bypass the FortiNet/FortiGuard Internet filter.