Unix / Linux - File System Basics

A file system is a logical collection of files on a partition or disk. A partition is a container for information and can span an entire hard drive if desired.
Your hard drive can have various partitions which usually contain only one file system, such as one file system housing the /file system or another containing the /home file system.
One file system per partition allows for the logical maintenance and management of differing file systems.
Everything in Unix is considered to be a file, including physical devices such as DVD-ROMs, USB devices, and floppy drives.
Unix uses a hierarchical file system structure, much like an upside-down tree, with root (/) at the base of the file system and all other directories spreading from there.
A Unix filesystem is a collection of files and directories that has the following properties −
It has a root directory (/) that contains other files and directories.
It has a root directory (/) that contains other files and directories.
Each file or directory is uniquely identified by its name, the directory in which it resides, and a unique identifier, typically called an inode.
Each file or directory is uniquely identified by its name, the directory in which it resides, and a unique identifier, typically called an inode.
By convention, the root directory has an inode number of 2 and the lost+found directory has an inode number of 3. Inode numbers 0 and 1 are not used. File inode numbers can be seen by specifying the -i option to ls command.
By convention, the root directory has an inode number of 2 and the lost+found directory has an inode number of 3. Inode numbers 0 and 1 are not used. File inode numbers can be seen by specifying the -i option to ls command.
It is self-contained. There are no dependencies between one filesystem and another.
It is self-contained. There are no dependencies between one filesystem and another.
The directories have specific purposes and generally hold the same types of information for easily locating files. Following are the directories that exist on the major versions of Unix −
/
This is the root directory which should contain only the directories needed at the top level of the file structure
/bin
This is where the executable files are located. These files are available to all users
/dev
These are device drivers
/etc
Supervisor directory commands, configuration files, disk configuration files, valid user lists, groups, ethernet, hosts, where to send critical messages
/lib
Contains shared library files and sometimes other kernel-related files
/boot
Contains files for booting the system
/home
Contains the home directory for users and other accounts
/mnt
Used to mount other temporary file systems, such as cdrom and floppy for the CD-ROM drive and floppy diskette drive, respectively
/proc
Contains all processes marked as a file by process number or other information that is dynamic to the system
/tmp
Holds temporary files used between system boots
/usr
Used for miscellaneous purposes, and can be used by many users. Includes administrative commands, shared files, library files, and others
/var
Typically contains variable-length files such as log and print files and any other type of file that may contain a variable amount of data
/sbin
Contains binary (executable) files, usually for system administration. For example, fdisk and ifconfig utlities
/kernel
Contains kernel files
Now that you understand the basics of the file system, you can begin navigating to the files you need. The following commands are used to navigate the system −
cat filename
Displays a filename
cd dirname
Moves you to the identified directory
cp file1 file2
Copies one file/directory to the specified location
file filename
Identifies the file type (binary, text, etc)
find filename dir
Finds a file/directory
head filename
Shows the beginning of a file
less filename
Browses through a file from the end or the beginning
ls dirname
Shows the contents of the directory specified
mkdir dirname
Creates the specified directory
more filename
Browses through a file from the beginning to the end
mv file1 file2
Moves the location of, or renames a file/directory
pwd
Shows the current directory the user is in
rm filename
Removes a file
rmdir dirname
Removes a directory
tail filename
Shows the end of a file
touch filename
Creates a blank file or modifies an existing file or its attributes
whereis filename
Shows the location of a file
which filename
Shows the location of a file if it is in your PATH
You can use Manpage Help to check complete syntax for each command mentioned here.
The first way to manage your partition space is with the df (disk free) command. The command df -k (disk free) displays the disk space usage in kilobytes, as shown below −
$df -k
Filesystem      1K-blocks      Used   Available Use% Mounted on
/dev/vzfs        10485760   7836644     2649116  75% /
/devices                0         0           0   0% /devices
$

Some of the directories, such as /devices, shows 0 in the kbytes, used, and avail columns as well as 0% for capacity. These are special (or virtual) file systems, and although they reside on the disk under /, by themselves they do not consume disk space.
The df -k output is generally the same on all Unix systems. Here's what it usually includes −
Filesystem
The physical file system name
kbytes
Total kilobytes of space available on the storage medium
used
Total kilobytes of space used (by files)
avail
Total kilobytes available for use
capacity
Percentage of total space used by files
Mounted on
What the file system is mounted on
You can use the -h (human readable) option to display the output in a format that shows the size in easier-to-understand notation.
The du (disk usage) command enables you to specify directories to show disk space usage on a particular directory.
This command is helpful if you want to determine how much space a particular directory is taking. The following command displays number of blocks consumed by each directory. A single block may take either 512 Bytes or 1 Kilo Byte depending on your system.
$du /etc
10     /etc/cron.d
126    /etc/default
6      /etc/dfs
...
$

The -h option makes the output easier to comprehend −
$du -h /etc
5k    /etc/cron.d
63k   /etc/default
3k    /etc/dfs
...
$
A file system must be mounted in order to be usable by the system. To see what is currently mounted (available for use) on your system, use the following 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 directory, by the Unix convention, is where temporary mounts (such as CDROM drives, remote network drives, and floppy drives) are located. If you need to mount a 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 the directory /mnt/cdrom, you can type −
$ mount -t iso9660 /dev/cdrom /mnt/cdrom

This assumes that your CD-ROM device is called /dev/cdrom and that you want to mount it to /mnt/cdrom. Refer to the mount man page for more specific information or type mount -h at the command line for help information.
After mounting, you can use the cd command to navigate the newly available file system through the mount point you just made.
To unmount (remove) the file system from your system, use the umount command by identifying the mount point or device.
For example, to unmount cdrom, use the following command −
$ umount /dev/cdrom

The mount command enables you to access your file systems, but on most modern Unix systems, the automount function makes this process invisible to the user and requires no intervention.
The user and group quotas provide the mechanisms by which the amount of space used by a single user or all users within a specific group can be limited to a value defined by the administrator.
Quotas operate around two limits that allow the user to take some action if the amount of space or number of disk blocks start to exceed the administrator defined limits −
Soft Limit − If the user exceeds the limit defined, there is a grace period that allows the user to free up some space.
Soft Limit − If the user exceeds the limit defined, there is a grace period that allows the user to free up some space.
Hard Limit − When the hard limit is reached, regardless of the grace period, no further files or blocks can be allocated.
Hard Limit − When the hard limit is reached, regardless of the grace period, no further files or blocks can be allocated.
There are a number of commands to administer quotas −
quota
Displays disk usage and limits for a user of group
edquota
This is a quota editor. Users or Groups quota can be edited using this command
quotacheck
Scans a filesystem for disk usage, creates, checks and repairs quota files
setquota
This is a command line quota editor
quotaon
This announces to the system that disk quotas should be enabled on one or more filesystems
quotaoff
This announces to the system that disk quotas should be disabled for one or more filesystems
repquota
This prints a summary of the disc usage and quotas for the specified file systems
You can use Manpage Help to check complete syntax for each command mentioned here.

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