Although it is getting harder and harder to run out of disk space, some of us still manage to do it. Whether it's yesterday you had all the space in the world and today you have nothing or the space just dwindled away over time doesn't really matter: you need to either add a bigger disk or get rid of some of the junk.

If this was sudden, you should check a couple of obvious suspects: /tmp files, log files, and device files that got accidentally removed and thus became ordinary files when something writes to them. If the device were a tape drive or another disk, you could be writing a lot without realizing it.

Don't blindly delete large log files without at least doing a "tail": the contents may alert you to a ongoing system problem.

You might want to look for core files too:

find / -type f -name core -exec ls -l {} \;

or

find / -type f -name core -exec rm {} \;

If you are running a mail server, check its spool directory: undeliverable messages can be backing up.

Although unusual, it is also possible that your filesystem is confused: running fsck (single user mode or unmounted of course) can fix that. Because fsck varies greatly between OSes, check your man page and look for options - for example SCO has "-ofull" and "-s" (which reconstructs the freelist even if nothing seems to be wrong). For Mac, you can use fsck or boot from the install CD and use Disk Utility from there.

Remember that if a process has a file open, the space you remove will not appear in the free list until the process closes the file, either of its own choice or through being killed. Use "lsof" to show what process(es) are using a file.

By the way, in the examples below I've tried to use generic examples that should work on any Unix/Linux. However, syntax can be slightly different (for an example see find large files). If you are not familiar with find, sort etc., experiment and read the man pages before doing anything drastic.

Although time consuming, the following procedure can be used to track down where your space has been used.

cd /

du -s *
 

(Some folks like to use "du -cks *", which is easy to remember as "ducks". For "human readable", add an "h" to the end of that.)

This command will print the number of blocks used by each directory and file in the root. Most likely, the largest number printed is where you should be looking. You can then cd to that directory, and examine it. If it has sub-directories, you might use:

find . -type d -exec du -s {} \;
 

You can search for "large" files by cd'ing to a suspect directory ( or even starting at /, if you must), and typing:`

find . -size +5000 -print
 

which will print the names of all files over 5,000 blocks (2,560,000) bytes. This may find many, many files, so you might want to refine it with larger numbers. You might also want to sort it:

find / -size +2000 -exec ls -s {} \; | sort -nr | more
 
To limit find to the filesystem you are on, add "-mount":
find . -mount -size +5000 -print

If you are using Mac OS X:

 mdfind 'kMDItemFSSize > 20000000'

will find files over 20,000,000 bytes.

You may not be looking for a large file per se. A directory that contains a very large number of file can be just as bad. Find those with:

find / -type d -size +5 -print

Again, you'll want to use "lsof" to see what (if anything) might be using that directory.

What I often do when on a quest to find big files is:

du -sk * | sort -nk 1 | pg
which will place the fattest files at the end of the list, making them a bit easier to spot

SOURCE: A.P. Lawrence - http://aplawrence.com/Unixart/find_large_files.html