Answer Posted / ravi

* ordinary files
* directories
* symbolic links
* block and character device files

You determine a file’s type by issuing the ls -l command and
reading the first character of each row of the output.
The typical output of the ls command is as follows:

$ ls –l
total 8
-rw-r–r– 1 root root 22 Oct 6 15:33 anormalfile
brw-rw—- 2 root disk 41, 0 May 5 1998 blockdev
crw-rw-rw- 2 root root 5, 0 May 5 1998
characterdev
drwxr-xr-x 2 root root 4096 Oct 6 15:33 subdir
lrwxrwxrwx 1 root root 11 Oct 6 15:35
symbolic -> anormalfile

Ordinary files begin with a dash (-), directories begin with
d, symbolic links begin with the character l, block devices
are prefaced with the character b, and character devices
begin with the letter c.

Ordinary files
An ordinary file can consist of any kind of data, including
executable programs. Most of the files in the Linux file
system are of this type.

Directories
A directory is a file that contains other files and
directories, and provides pointers to them.
It performs a similar function to a folder in a filing
cabinet, in that it enables you to group related files in an
organized fashion. However, whereas folders can normally
contain files only, directories can contain additional
directories, often referred to as subdirectories.

Symbolic links
A symbolic – or soft – link points to the name and location
of a completely separate file. So when you open, copy, move
or otherwise refer to the link, the operation is in fact
performed on the referenced file. This distinction is
usually invisible to the user. If the referenced file is
removed or renamed, the link is broken and an error occurs
if you try to open it.

You can also create hard links. A hard link points to the
actual data in a file in exactly the same way as an ordinary
file does. Therefore, other than the name, there is no
difference between the original file and a hard link that
points to the same data, and both can be regarded as
ordinary files. You can distinguish a hard link from any
other ordinary file only by the number of links that each
one has. The number of links is displayed in the second
field of an ls -l listing. If this number is greater than
one, then you know there are additional hard links to the data.

Device files
All the physical devices that Linux uses are represented by
device files.
Device files can be classified as character special or block
special. Character-special files represent devices that
interact with Linux on a character-by-character, or serial,
basis. Printers and terminals are examples of this type of
device. Block-special files represent devices such as hard
or floppy disks and CD-ROMs, which interact with Linux using
blocks of data.

All the device files are contained in the /dev directory –
for example, the file associated with the system’s first
floppy drive is /dev/fd0.

Device files are extremely powerful because they enable
users to access hardware devices such as disk drives,
modems, and printers as though they were data files.
Therefore, you can move, copy, and transfer data between
such devices easily, often without having to use special
commands or syntax.

Filenames and pathnames

Every file is assigned a filename, which can be up to 256
characters long. This name can consist of a mixture of
uppercase and lowercase letters, numbers, and certain
punctuation marks such as the period, dash, or underscore.

Certain characters cannot be used in filenames. For example,
you cannot use characters that represent a field separator –
such as a comma – or other special characters that have
particular meaning to the shell. The special characters that
you cannot use are

! @ # $ % ^ & * ( ) [ ] { } ‘ ” \ / | ; < > ‘

Pathnames
You can navigate between directories on the command line
using pathnames. To use pathnames, you must understand the
directory structure of the Linux file system. The
highest-level directory in the Linux file system is the root
directory, which is represented by a forward slash (/).
Located under the root directory are the top-level
directories, followed by one or more subdirectory levels.

File structure of a Linux file system
You can move between directories using relative or absolute
pathnames.

A relative pathname starts with your current directory. For
example, if you want to change to the expenses directory
from within your home directory, you enter

cd expenses

Relative pathnames can begin with the name of a file or
directory, or with symbolic references to the current
directory (.) or its parent directory (..), but never with a
forward slash.

A simple example of a Linux file system
An absolute pathname shows the full pathname from the root
directory (/). For example, the following command allows you
to move from your current directory directly to the applic
subdirectory in the usr directory that’s located under the
root directory:

cd /usr/applic

Inodes, blocks, and special files

Inodes
Every file is assigned a unique inode number. An inode is a
structure that defines the file’s location and attributes.
You can check for a file’s inode number using the -i option
with the ls command. You can view the information that a
file’s inode contains using the stat filename syntax. This
command output (the stat results) displays the information
related to the “results” file’s inode.

$ stat results
File: ”results”
Size: 8
Filetype: Regular file
Mode: (0644/-rw-r–r–)
Uid: ( 0/ root)
Gid: ( 0/ root)
Device: 3,7 Inode: 123256 Links: 1
Access: Tue Jul 25 16:45:00 2000 (00072.18:31:07)
Modify: Thu Jul 20 12:35:20 2000 (00077.22:40:47)
Change: Thu Jul 20 12:35:20 2000 (00077.22:40:47)

In this example, some of the attributes that are displayed
include the file type, file size, the owner’s User ID (UID),
the number of hard links associated with it, and the file’s
creation, access, and modification times.

An inode does not store a file’s name. Filenames are stored
in directories with their associated inode numbers. In the
example of the stat command, the name of the file is
obtained from the filename parameter that you entered.

Blocks
In the Linux file system, files are stored in blocks, which
are identically sized segments of disk space. Generally, the
size of a block varies from 512 bytes to 32 KB, depending on
the Linux installation. The maximum size of a file depends
on the block size used in the file system. For example, the
maximum file size for an ext2 file system is 2 GB if it uses
512-byte blocks.

Disk systems retrieve data in block-sized chunks, so the
larger the block size the more efficient the access. The
problem with creating large blocks is that it can waste disk
space. For example, if the block size is 4 KB and most files
contain only a few bytes of data, most of the 4096 bytes of
disk space is wasted for each file. But if you make the
block sizes small, disk access will be relatively less
efficient.

Some commands, such as df, output disk information in 1 KB
blocks, even if the installation stores files in a different
block size. A simple way to check your system’s block size
is to use the du command to display the disk usage. In this
example, the disk usage for all files beginning with “m” is
displayed.

$ du -h m*
36k mail
4.0k mail.rc
12k mailcap
12k mailcap.vga
4.0k man.config
4.0k mc.global
148k midi
40k mime-magic
104k mime-magic.dat
8.0k mime.types
4.0k minicom.users
0 motd
4.0k mtab

In this example, the du command output displays the file
sizes on the disk in increments of the block size, which is
4.0 KB in this case. One exception to this rule is when zero
is displayed, which indicates that the file is completely empty.

Special files
In addition to other file types, Linux makes use of special
files, which are system-defined files that perform unique
functions when accessed.

Special files and their functions

linux file types

For example, if you need to get rid of unwanted output from
a command, you can redirect the output to the /dev/null
file. In this example, any errors generated by the find
command are redirected from stderr to /dev/null.

$ find / -n myfile 2> /dev/null

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