Managing Filesystems and Files

This chapter gives you an introduction how Linux systems handle devices, filesystems as well as users, groups and permissions.

It is a very simple topic and it is good to fully understand this topic when handling your data stored in the filesystems.


The first thing to understand while dealing with the Linux filesystem is that everything can be accessed as a file. Even system resources and hardware has a file representation that is used to access it. Those special files usually live in /dev.

The second thing to know is that all files are addressed in a tree which starting location for all files is '/' - which is pronounced 'root'.

Any filesystem, be it stored on a removeable floppy disk, a CD-ROM, fixed hard disc, ZIP disk, USB or IEEE1294 disk or stick or a network resource can be attached to any point inside this tree hierarchy. The attach process is called 'mounting' and the directory the filesystem is mounted on is called 'mount-point'.

It is common practice to store '/usr' or '/home' on different filesystems for performance or maintenance reasons. Also removable media is usually mounted into '/mnt' - for example '/mnt/floppy' or '/mnt/cdrom'.

The command to mount a filesystem to a directory is:

mount [-t fstype] something somewhere'

The filesystem type is normally detected automatically and specifying it is optional.

Filesystems that are always available can be automatically mounted when the systems boots up. Which filesystems are mounted at boot-up is controlled by /etc/fstab. All of the mounted filesystems get unmounted at shutdown and must be remounted at startup.

The layout of /etc/fstab is very simple. It lists the device name - the source, followed by where in the file hierarchy it gets mounted - the mount-point, as well as the filesystem type alogn with some more options:

# Device                Mountpoint   FStype  Options     Dump  Pass#
/dev/discs/disc0/part1  swap         swap    defaults    0     0
none                    /dev         devfs   defaults    0     0
none                    /proc        proc    defaults    0     0
/dev/discs/disc0/part2  /            auto    defaults    0     0
/dev/discs/disc0/part3  /home        auto    defaults    0     0
/dev/cdroms/cdrom0      /mnt/cdrom   iso9660 ro,noauto   0     0

The process (/proc) and device (/dev) filesystem (see \cite{devfs}) do not have a source specified since they are virtually created inside the kernel.

The swap partition does not have a mount-point since it is not part of the filesystem - used exclusively used by the kernel as temporary storage for virtual memory.

The FStype tells the system how the partition is formatted. Usual filesystems used as system root include: exT2, ext3, reiserfs or xfs. But normally the type can be automatically detected and thus auto can specified.

The options 'default' specifies that the partition should be mounted using system defaults: that is read/write-able. The 'ro' option on the CD drive declares it read only. The additional option of 'noauto' indicates that this partition should not be automatically mounted at boot up.

Your T2 Linux system should list some more filesystems like '/dev/pts ' and '/dev/shm' which are used for pseudo-terminals and shared-memory. They are automatically created by stone and can normally be left untouched.

Permissions, Users and Groups

Unix system are based on a very strong security model, the user must have the correct permission for any kind of operation. Each file has three permission sets. One for the owner, one for the group and one for everyone else. One set of permissions describe the possibility to read, write or execute the file. Often one set is displayed as a row of letter in the form 'rwx': where 'r' stands for read access, 'w' for write access and 'x' for the possiblity to execute the file or enter the directory. A missing access right is replace by a -. Another form of notation is a octal number you get by 'r-bit*4 + w-bit*2 + x-bit'. like "6" for 'rw-'. To form the three sets they are also put into a row like: "660" meaning the same as 'rw-rw----'.

Since the device-nodes in /dev are some kind of file too, these permissions are also used for hardware access via these device-nodes. So for accessing a IDE/SCSI device (e.g. a cd-writer or ZIP), sound-card or other hardware devices the user must have the permissions (the right) to do so.

The persmissions are manipulated with the following tools: chmod(8), chown(8), chgrp(8).

Where chown can also modify the group in one set. They are specified spererated by a colon (for historic reasons a dot '.' is also possible, but it should be avoided and of course does not work with owner names containing a '.'):

chown rene:users t2-article.pdf

The system users and groups are edited via: useradd(8), userdel(8), usermod(8) groupadd(8), groupdel(8), groupmod(8) - or by using the appropriated STONE module.

Why Should a User Bother?

Because even at normal workstation - even at home where only one person might use the computer - security is a must in the todays networked world.

Imaging everyone could just read or even write data on your computer! Electronic mail, financial data, pictures or audio files are personal property that needs to be protected.

Additionally a normal user should not be able to accidentally damage the operating system by replacing or removing an important system file - the operation should stay intact regardless whatever the user might apply to the system.

Of course the same especially applies to real networked environment in companies.

To access any kind of hardware or data you must have the permissions to do so. For example by default in T2 Linux the sound-card devices are usable for members of the group sound, the video4linux video devices can be utilized by users in the group video, and so on. Make sure you are a member of that group before complaining something does not work!