Linux: What’s filling up my almost full filesystem?

Let’s say you realize (maybe because you got an alarm for it) that a particular filesystem — let’s say /qwerty — is full, or almost full, and you want to find out what’s taking up the most space. Simply enter something like:

du -k -x /qwerty | sort -n | tail -20

to get a list of the directories taking up the most space in that filesystem, sorted by size, with the largest ones at the bottom.

Note that the “/qwerty“, in this case, should be the filesystem’s top directory, not some subdirectory of it. In other words, it should be something that shows up on a “df” command.

Explanation:

du -k -x” shows the subdirectories (and their total sizes) of the specified directory (or of the current one, if you don’t specify one). “-k” means that sizes are reported in kilobytes (KB) — this is not mandatory, but different versions of “du” may use other units, and this one is easy to read. “-x” means “don’t go out of the specified filesystem”, and that’s quite important here, as you could have a “/qwerty/asdfgh” filesystem, mounted in a subdirectory of “/qwerty“, but since it’s a different filesystem (appearing separately in the results of a “df” command) you won’t want to include it in this list.

sort -n” is a simple numeric sort, and “tail -20” just means “show only the last 20 lines. 20 is a reasonable number that fits in most terminals at their default sizes (typically 80×25), so that you don’t have to scroll up.

(Yes, this is very basic, but, hey, one of the goals of this blog is to document stuff I help co-workers with, since, at least in theory, if someone has a question about something they need to do, then many other people may have the same doubt/need as well, and we have a couple of new team members whose backgrounds are not Linux/Unix-related, so…)

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