The `/var/` Directory

Since the FHS requires Linux to mount /usr/ as read-only, any programs that write log files or need spool/ or lock/ directories should write them to the /var/ directory. The FHS states /var/ is for:

...variable data files. This includes spool directories and files, administrative and logging data, and transient and temporary files.

Below are some of the directories found within the /var/ directory:

/var
   |- account/
   |- arpwatch/
   |- cache/
   |- crash/
   |- db/
   |- empty/
   |- ftp/
   |- gdm/
   |- kerberos/
   |- lib/
   |- local/
   |- lock/
   |- log/
   |- mail -> spool/mail/
   |- mailman/
   |- named/
   |- nis/
   |- opt/
   |- preserve/
   |- run/
   +- spool/
       |- at/
       |- clientmqueue/
       |- cron/
       |- cups/
       |- exim/
       |- lpd/
       |- mail/
       |- mailman/
       |- mqueue/
       |- news/
       |- postfix/
       |- repackage/
       |- rwho/
       |- samba/
       |- squid/
       |- squirrelmail/
       |- up2date/
       |- uucp
       |- uucppublic/
       |- vbox/
|- tmp/
|- tux/
|- www/
|- yp/

System log files, such as messages and lastlog, go in the /var/log/ directory. The /var/lib/rpm/ directory contains RPM system databases. Lock files go in the /var/lock/ directory, usually in directories for the program using the file. The /var/spool/ directory has subdirectories for programs in which data files are stored.