|The Linux System Administrators' Guide: Version 0.6.2|
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The root filesystem should generally be small, since it contains very critical files and a small, infrequently modified filesystem has a better chance of not getting corrupted. A corrupted root filesystem will generally mean that the system becomes unbootable except with special measures (e.g., from a floppy), so you don't want to risk it.
The root directory generally doesn't contain any files, except perhaps the standard boot image for the system, usually called /vmlinuz. All other files are in subdirectories in the root filesystems:
Commands needed during bootup that might be used by normal users (probably after bootup).
Like /bin, but the commands are not intended for normal users, although they may use them if necessary and allowed.
Configuration files specific to the machine.
The home directory for user root.
Shared libraries needed by the programs on the root filesystem.
Loadable kernel modules, especially those that are needed to boot the system when recovering from disasters (e.g., network and filesystem drivers).
Temporary files. Programs running after bootup should use /var/tmp, not /tmp, since the former is probably on a disk with more space.
Files used by the bootstrap loader, e.g., LILO. Kernel images are often kept here instead of in the root directory. If there are many kernel images, the directory can easily grow rather big, and it might be better to keep it in a separate filesystem. Another reason would be to make sure the kernel images are within the first 1024 cylinders of an IDE disk.
Mount point for temporary mounts by the system administrator. Programs aren't supposed to mount on /mnt automatically. /mnt might be divided into subdirectories (e.g., /mnt/dosa might be the floppy drive using an MS-DOS filesystem, and /mnt/exta might be the same with an ext2 filesystem).
Mount points for the other filesystems.