UNIX file system basics

File system basics

A file system is a logical collection of files on a partition or disk. A partition is a container of information that can span the entire hard disk if needed.

Your hard drive can have different partitions, but usually contains only one file system, such as /file system and the other that /home file system.

A file system partition allows logical maintenance and management of different file systems.

Everything in UNIX is considered a file, including physical devices such as DVD-ROMs, USB devices, floppy drives, and so on.

The directory structure

UNIX uses a file system hierarchy, like an upside-down tree, where the root directory (/) is the bottom of the file system from which all other directories spread.

The UNIX file system is a collection of files and directories with the following properties:

• It has a root (/) that contains other files and directories.
• Use a name to uniquely identify each file or directory, which can be the directory in which it is located, or a unique identifier, often referred to as an inode.
• As is customary, the inode number of the root directory is 2, and the inode number of the lost-found directory is 3. I node numbers 0 and 1 are not used. The file's inode number can be specified by the -i option of the ls command.
• It is self-contained. There is no dependency between one file system and other file systems.

Directories have specific purposes, often storing the same type of information for the purpose of making it easier to locate files. The following are the directories that exist on the major UNIX versions:

Directory	Describe
/	This is the root directory and contains only the directories required for the top-level file structure.
/bin	This is where the executable files are located. They are available to all users.
/dev	These are device drivers.
/etc	Parent directory commands, profiles, disk profiles, valid user lists, groups, Ethernet, hosts, etc. all kinds of places to send important information.
/lib	Contains shared library files, such as other kernel-related files.
/boot	Contains files related to system startup.
/home	Contains the user's home directory and other accounts.
/mnt	Used to mount other temporary file systems, such as CD-ROM drives and floppy drives for discs and floppy disks, respectively.
/proc	Mark as a file that contains all processes that are marked with process numbers or other information. This file is a dynamic system.
/tmp	Contains all temporary files during system startup.
/uer	Used for a variety of purposes and can be used by many users. Includes executive orders, shared files, library files, and more.
/var	Typically contains longer files, such as logs and print files and any other type of file, the amount of data that the file contains can vary.
/sbin	Contains binary (executable) files, typically used for system management. For example, fdisk and ifconfig functions.
/kernel	Contains kernel files.

Browse the file system

Now that you have the basics of the file system, you can start navigating to the files you need. The following are the commands that can be used to navigate to the file system:

Command	Describe
cat filename	The file name is displayed.
cd dirname	Move to a determined directory.
cp file1 file2	Copy a file/directory to the specified location.
file filename	Identify file types (binary, text, etc.).
find filename dir	A file/directory was found.
head filename	Displays the beginning of a file.
less filename	Browse a file from the end or start location.
ls dirname	Displays the contents of the specified directory.
mkdir dirname	Create a specified directory.
more filename	Browse a file from start to finish.
mv file1 file2	Move the location of a file/directory or rename a file/directory.
Pwd	Displays the directory in which the user is currently located.
rm filename	Delete a file.
rmdir dirname	Delete a directory.
tail filename	Displays the end of a file.
touch filename	Create a blank file or modify the properties of an existing file.
whereis filename	The location of a file is displayed.
which filename	If the file is within your path, display its location.

df command

The first way to manage partitioned space is the df (disk idle) command. The command df -k (disk idle) shows disk space usage in thousands of bytes, as follows:

    $df -k
    Filesystem  1K-blocks  Used   Available Use% Mounted on
    /dev/vzfs10485760   7836644 2649116  75% /
    /devices0 0   0   0% /devices
    $

Some directories, such /devices are displayed as 0 in thousands of bytes, with 0% of the available columns and capabilities. These special (or virtual) file systems, although they reside on disk, do not themselves take up disk space.

The output of df -k on all UNIX systems. It generally includes:

Column	Describe
Filesystem	The name of the physical file system.
kbytes	The total byte of free space on the storage media.
used	The total byte of space used by the file.
avail	The total byte of free space.
capacity	The proportion of space and total used by the file.
Mounted on	The file system is being installed.

You -h (readable) option to set the output format for display, easy-to-understand symbols, appropriate size, and so on.

du command

The du (disk usage) command enables you to display disk space usage in a particular directory by specifying a directory.

This command is useful if you want to determine how much space a particular directory is using. T he following command shows the number of blocks consumed by each directory. Depending on the system, a block may require 512 bytes or 1 kbyte.

    $du /etc
    10 /etc/cron.d
    126/etc/default
    6  /etc/dfs
    ...
    $

-h option makes the output easier to understand:

    $du -h /etc
    5k/etc/cron.d
    63k   /etc/default
    3k/etc/dfs
    ...
    $

Mount the file system

The file system must be installed for the normal use of the system. In order to view the file system currently installed (available) on your system, you can use this command:

    $ mount
    /dev/vzfs on / type reiserfs (rw,usrquota,grpquota)
    proc on /proc type proc (rw,nodiratime)
    devpts on /dev/pts type devpts (rw)
    $

The /mnt UNIX contract is where temporary mounts (e.g. CD-ROM drives, remote network drives, floppy disk drives) are installed. If you need to mount the file system, you can use the mount command with the following syntax:

    mount -t file_system_type device_to_mount directory_to_mount_to

For example, if you want to mount a CD-ROM to /mnt/cdrom you can enter:

    $ mount -t iso9660 /dev/cdrom /mnt/cdrom

Suppose your CD-ROM device is /dev/cdrom and you want to mount /mnt/cdrom You can refer to the installation man page for more specific information or types, and enter -h for help information.

After installation, you can use the cd command to browse the available new file systems through mount points.

Uninstall the file system

Uninstall (delete) the file system from your system by identifying mount points or devices. Implement using the umount command.

For example, you can uninstall a disc using the following command:

    $ umount /dev/cdrom

The mount command gives you access to your file system, but in most modern UNIX systems, loading functions make this process invisible to the user and does not require user intervention.

User and group quotas

Mechanisms that provide quotas for users and groups: The amount of space used by individual users or all users in a particular group can be limited by values defined by administrators.

There are two restrictions on quota operations. If the amount of space or disk blocks starts to exceed the administrator-defined limit, allow the user to take action:

• Soft limit: If the user exceeds the definition of the limit, there is a grace limit that allows the user to free up some space.
• Hard Limit: When the hard limit is reached, the grace limit is ignored and no further files or blocks can be allocated.

There are many commands to manage quotas:

Command	Describe
quota	Displays disk usage and restrictions for one user in the group.
edquota	This is a quota editor. You can use this command to edit user or group quotas.
quotacheck	Scan file systems to use, manufacture, inspect, and repair quota files for disk use
setquota	This is also a command line quota editor.
quotaon	The system announces that disk quotas should be enabled on one or more file systems.
quotaoff	The system announces that disk quotas on one or more file systems should be disabled.
repquota	Prints a summary of disk usage and quotas for the specified file system

You can use Manpage help to see the full syntax of each command mentioned here.