UNIX- Directory Management & File System Basics

A directory is a file the solo job of which is to store the file names and the related information. All the files, whether ordinary, special, or directory, are contained in directories.

Unix uses a hierarchical structure for organizing files and directories. This structure is often referred to as a directory tree. The tree has a single root node, the slash character (/), and all other directories are contained below it.

Home Directory

The directory in which you find yourself when you first login is called your home directory.

You will be doing much of your work in your home directory and subdirectories that you'll be creating to organize your files.

You can go in your home directory anytime using the following command −

$cd ~
$

Here ~ indicates the home directory. Suppose you have to go in any other user's home directory, use the following command −

$cd ~username
$

To go in your last directory, you can use the following command −

$cd -
$

Absolute/Relative Pathnames

Directories are arranged in a hierarchy with root (/) at the top. The position of any file within the hierarchy is described by its pathname.

Elements of a pathname are separated by a /. A pathname is absolute, if it is described in relation to root, thus absolute pathnames always begin with a /.

Following are some examples of absolute filenames.

/etc/passwd
/users/sjones/chem/notes
/dev/rdsk/Os3

A pathname can also be relative to your current working directory. Relative pathnames never begin with /. Relative to user amrood's home directory, some pathnames might look like this −

chem/notes
personal/res

To determine where you are within the filesystem hierarchy at any time, enter the command pwd to print the current working directory −

$pwd
/user0/home/amrood

$

Listing Directories

To list the files in a directory, you can use the following syntax −

$ls dirname

Following is the example to list all the files contained in /usr/local directory −

$ls /usr/local

X11       bin          gimp       jikes       sbin
ace       doc          include    lib         share
atalk     etc          info       man         ami

Creating Directories

We will now understand how to create directories. Directories are created by the following command −

$mkdir dirname

Here, directory is the absolute or relative pathname of the directory you want to create. For example, the command −

$mkdir mydir
$

Creates the directory mydir in the current directory. Here is another example −

$mkdir /tmp/test-dir
$

This command creates the directory test-dir in the /tmp directory. The mkdir command produces no output if it successfully creates the requested directory.

If you give more than one directory on the command line, mkdir creates each of the directories. For example, −

$mkdir docs pub
$

Creates the directories docs and pub under the current directory.

Creating Parent Directories

We will now understand how to create parent directories. Sometimes when you want to create a directory, its parent directory or directories might not exist. In this case, mkdir issues an error message as follows −

$mkdir /tmp/amrood/test
mkdir: Failed to make directory "/tmp/amrood/test"; 
No such file or directory
$

In such cases, you can specify the -p option to the mkdir command. It creates all the necessary directories for you. For example −

$mkdir -p /tmp/amrood/test
$

The above command creates all the required parent directories.

Removing Directories

Directories can be deleted using the rmdir command as follows −

$rmdir dirname
$

Note − To remove a directory, make sure it is empty which means there should not be any file or sub-directory inside this directory.

You can remove multiple directories at a time as follows −

$rmdir dirname1 dirname2 dirname3
$

The above command removes the directories dirname1, dirname2, and dirname3, if they are empty. The rmdir command produces no output if it is successful.

Changing Directories

You can use the cd command to do more than just change to a home directory. You can use it to change to any directory by specifying a valid absolute or relative path. The syntax is as given below −

$cd dirname
$

Here, dirname is the name of the directory that you want to change to. For example, the command −

$cd /usr/local/bin
$

Changes to the directory /usr/local/bin. From this directory, you can cd to the directory /usr/home/amrood using the following relative path −

$cd ../../home/amrood
$

Renaming Directories

The mv (move) command can also be used to rename a directory. The syntax is as follows −

$mv olddir newdir
$

You can rename a directory mydir to yourdir as follows −

$mv mydir yourdir
$

The directories . (dot) and .. (dot dot)

The filename . (dot) represents the current working directory; and the filename .. (dot dot) represents the directory one level above the current working directory, often referred to as the parent directory.

If we enter the command to show a listing of the current working directories/files and use the -a option to list all the files and the -l option to provide the long listing, we will receive the following result.

$ls -la
drwxrwxr-x    4    teacher   class   2048  Jul 16 17.56 .
drwxr-xr-x    60   root              1536  Jul 13 14:18 ..
----------    1    teacher   class   4210  May 1 08:27 .profile
-rwxr-xr-x    1    teacher   class   1948  May 12 13:42 memo
$

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.

Directory Structure

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.
• 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.
• 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 −

S.No.	Directory & Description
1	/ This is the root directory which should contain only the directories needed at the top level of the file structure
2	/bin This is where the executable files are located. These files are available to all users
3	/dev These are device drivers
4	/etc Supervisor directory commands, configuration files, disk configuration files, valid user lists, groups, ethernet, hosts, where to send critical messages
5	/lib Contains shared library files and sometimes other kernel-related files
6	/boot Contains files for booting the system
7	/home Contains the home directory for users and other accounts
8	/mnt Used to mount other temporary file systems, such as cdrom and floppy for the CD-ROM drive and floppy diskette drive, respectively
9	/proc Contains all processes marked as a file by process number or other information that is dynamic to the system
10	/tmp Holds temporary files used between system boots
11	/usr Used for miscellaneous purposes, and can be used by many users. Includes administrative commands, shared files, library files, and others
12	/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
13	/sbin Contains binary (executable) files, usually for system administration. For example, fdisk and ifconfig utlities
14	/kernel Contains kernel files

Navigating the File System

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 −

S.No.	Command & Description
1	cat filename Displays a filename
2	cd dirname Moves you to the identified directory
3	cp file1 file2 Copies one file/directory to the specified location
4	file filename Identifies the file type (binary, text, etc)
5	find filename dir Finds a file/directory
6	head filename Shows the beginning of a file
7	less filename Browses through a file from the end or the beginning
8	ls dirname Shows the contents of the directory specified
9	mkdir dirname Creates the specified directory
10	more filename Browses through a file from the beginning to the end
11	mv file1 file2 Moves the location of, or renames a file/directory
12	pwd Shows the current directory the user is in
13	rm filename Removes a file
14	rmdir dirname Removes a directory
15	tail filename Shows the end of a file
16	touch filename Creates a blank file or modifies an existing file or its attributes
17	whereis filename Shows the location of a file
18	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 df Command

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 −

S.No.	Column & Description
1	Filesystem The physical file system name
2	kbytes Total kilobytes of space available on the storage medium
3	used Total kilobytes of space used (by files)
4	avail Total kilobytes available for use
5	capacity Percentage of total space used by files
6	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 Command

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
...
$

Mounting the File System

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.

Unmounting the File System

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.

User and Group Quotas

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.
• 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 −

S.No.	Command & Description
1	quota Displays disk usage and limits for a user of group
2	edquota This is a quota editor. Users or Groups quota can be edited using this command
3	quotacheck Scans a filesystem for disk usage, creates, checks and repairs quota files
4	setquota This is a command line quota editor
5	quotaon This announces to the system that disk quotas should be enabled on one or more filesystems
6	quotaoff This announces to the system that disk quotas should be disabled for one or more filesystems
7	repquota This prints a summary of the disc usage and quotas for the specified file systems

Anurag