What is the difference between /tmp and /var/tmp?
On most FHS systems, there is a /tmp folder as well as a /var/tmp folder. What is the functional difference between the two?
/tmp may be, and sometimes is, cleaned on reboot. /var/tmp is preserved between reboots.
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.
/tmp is meant as fast (possibly small) storage with a short lifetime. 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 less often.
Also note that /var/tmp might not be available 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 latter 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.
POSIX Base Specifications, Issue 7 on /tmp:
The following directory shall exist on conforming systems and shall be
used as described:/tmp
A directory made available for applications that need a place to create temporary files. Applications shall be allowed to create files
in this directory, but shall not assume that such files are preserved
between invocations of the application.
The File Hierarchy Standard 2.3 on /tmp:
The /tmp directory must be made available for programs that require
temporary files.Programs must not assume that any files or directories in /tmp are
preserved between invocations of the program.Rationale
IEEE standard P1003.2 (POSIX, part 2) makes requirements that are
similar to the above section.Although data stored in /tmp may be deleted in a site-specific manner,
it is recommended that files and directories located in /tmp be
deleted whenever the system is booted.FHS added this recommendation on the basis of historical precedent and
common practice, but did not make it a requirement because system
administration is not within the scope of this standard.
POSIX does not specify /var/tmp.
The FHS does though:
The /var/tmp directory is made available for programs that require
temporary files or directories that are preserved between system
reboots. Therefore, data stored in /var/tmp is more persistent than
data in /tmp.Files and directories located in /var/tmp must not
be deleted when the system is booted. Although data stored in
/var/tmp is typically deleted in a site-specific manner, it is recommended that deletions occur at a less frequent interval than
/tmp.