Each directory in UNIX is just another file in UNIX like any other. Directories contain information about the names of files and their locations on disk. As more and more files are added to a directory, the directory itself grows larger and larger. However, the directory does not shrink when files are removed.
# ls -ld /tmp
drwxrwxrwt 14 root root 4096 2007-10-29 11:14 /tmp
This directory is 4096 bytes (4k) in size. There is no way to actually shrink this directory. To reduce its size, the directory must be moved (renamed) and its contents copied into a new directory.
This could be done this way:
# mv /tmp /tmp.old
# mkdir /tmp
# chmod 1777 /tmp
The chmod will change the /tmp directory permissions to the appropriate ones. Note that the “sticky” bit is set, which means that files may be created, accessed, and deleted by anyone, but that files may be deleted only by their owner.
In the case of the /tmp directory, it may be best to reboot before deleting the old directory to avoid problems with programs using /tmp.