What is the difference between /tmp and /var/tmp?

Question

On most FHS systems, there is a /tmp folder as well as a /var/tmp folder. What is the functional difference between the two?

Answer 1

/tmp is meant as fast (possibly small) storage with a short TTL. Many systems clean /tmp very fast - on some systems it is even mounted as RAM-disk. /var/tmp is normally located on a physical disk, is larger and can hold temporary files for a longer time. Some systems also clean /var/tmp - but with a longer TTL.

Also note that /var/tmp might not be avaiable in the early boot-process, as /var and/or /var/tmp may be mountpoints. Thus it is a little bit comparable to the difference between /bin and /usr/bin. The first is available during early boot - the later after the system has mounted everything. So most boot-scripts will use /tmp and not /var/tmp for temporary files.

Another (upcoming) location on Linux for temporary files is /dev/shm.

Answer 2

/tmp may be, and sometimes is, cleaned on reboot. /var/tmp is preserved between reboots.

See the Wikipedia article on the FHS.

Answer 3

They have the same purpose and functionality. Every version of UNIX/Linux will handle these directories differently. Historically, before the advent of RAM/swap based filesystems, you had disk-less systems where the / and /usr filesystems would be read-only and /var (variable) would be read-write. The /tmp name would be a symbolic link to /var/tmp. Later, disk-less systems fell out of style, disk space became cheaper (to have larger root filesytems) and technology allowed for filesystems mounted from memory instead of disk. The /var/tmp directory fell out of style, but is still used by some programs.

These days, more security are set up by default on /tmp, like g+s,+t permissions, but not on /var/tmp. Additionally, /var/tmp is rarely mounted from RAM or swap.