Ubuntu adheres to the Filesystem Hierarchy Standard for directory and file naming. This standard allows users and software programs to predict the location of files and directories. The root level directory is represented simply by the slash /. At the root level, all Ubuntu systems include these directories:
|bin||Essential command binaries|
|boot||Static files of the boot loader|
|etc||Host-specific system configuration|
|home||User home directories|
|lib||Essential shared libraries and kernel modules|
|media||Mount point for removable media|
|mnt||Mount point for mounting a file system temporarily|
|proc||Virtual directory for system information|
|root||Home directory for the root user|
|sbin||Essential system binaries|
|srv||Data for services provided by this system|
|sys||Virtual directory for kernel data structures|
|opt||Add-on application software packages|
The following is a list of important considerations regarding directories and partitions.
The root partition / must always physically contain /etc, /bin, /sbin, /lib and /dev, otherwise you won't be able to boot. Typically 100 MB is needed for the root partition, but this may vary.
/usr: all user programs (/usr/bin), libraries (/usr/lib), documentation (/usr/share/doc), etc., are in this directory. This part of the file system needs most of the space. You should provide at least 500 MB of disk space for a minimal installation, and at least 1.5 GB if you want to install a standard Ubuntu desktop. If you want to install more packages you should increase the amount of space you give this directory.
/home: every user will put his data into a subdirectory of this directory. The size of this depends on how many users will be using the system and what files are to be stored in their directories. Depending on your planned usage you should reserve about 100 MB for each user, but adapt this value to your needs.
/var: all variable data like news articles, e-mails, web sites, the packaging system cache, etc. will be placed under this directory. The size of this directory depends greatly on the usage of your computer, but for most people will be dictated by the package management tool's overhead. If you are going to do a full installation of just about everything Ubuntu has to offer, all in one session, setting aside 2 or 3 gigabytes of space for /var should be sufficient. If you are going to install in pieces (that is to say, install services and utilities, followed by text stuff, then X, ...), you can get away with 300 - 500 megabytes in /var. If hard drive space is at a premium and you don't plan on doing major system updates, you can get by with as little as 30 or 40 megabytes in /var.
/tmp: if a program creates temporary data it will most likely go in /tmp. 20-50 MB should usually be enough.