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 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/tmp may be mountpoints. Thus it is a little bit comparable to the difference between
/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
/tmp may be, and sometimes is, cleaned on reboot.
/var/tmp is preserved between reboots.
The following directory shall exist on conforming systems and shall be used as described:
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 /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.
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.
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
/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
g+s,+t permissions, but not on
/var/tmp is rarely mounted from RAM or swap.