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One of the unique things about Unix as an operating system is that regards everything as a file. Files can be divided into three categories; ordinary or plain files, directories, and special or device files. Device files include things such as disks, cdroms, tapes, terminals, serial ports, and sound cards. All files contain data of some kind.
SVR4 Style Directory Tree
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Ordinary or plain files are most likely to contain data that are meaningful to
users. Plain files may contain data that users have entered into the
computer, data that have been generated by programs, and even programs
themselves. Indeed, all programs exist as files, and nearly all
Unix commands result in a program file being retrieved and executed.
This is an important feature and contributes to the flexibility of Unix.
Directories in Unix are properly known as directory files. They contain
a list of file names and inode numbers. That is all! All other information
about files is contained within inodes.
Soft links are files that reference other files. The information in the file
is the name of the file being referenced. When a user
program attempts to read a file that is a soft link, the file system opens
the symbolic link file, reads the reference, and then opens the file that
is referenced. File management programs operate on the
symbolic link file. For example, ls -l reads the symbolic
link file and displays the name of the file being referenced, and
rm deletes the symbolic link file, leaving untouched
the file that is referenced by the link.
Hard links are not really files. They exist when more than one
directory entry references the same inode. Hence, when a hard link
is opened by a user program, the directory entry that is opened
points directly to the inode of the data to be provided. And,
when a hard link is deleted using rm, the directory
entry is removed. Only when one deletes the last remaining directory entry
that points to an inode are the data deleted.
Special files are also known as device files.
In Unix all physical
devices are accessed via device files; they are what programs use to
communicate with hardware. Device files contain information on location,
type, and access mode for a specific device. When a device file is opened,
the kernel uses the information in it to determine which physical device to
access and how to access it.
There are two types of device files;
character and block, as well as two modes of access.
Block device files are used to access block device I/O. Block devices do
buffered I/O, meaning that the the data is collected in a buffer until a full
block can be transfered.
Character device files are associated with character or raw device access.
They are used for unbuffered data transfers to and from a device. Rather than
transferring data in blocks the data is transfered character by character. One
transfer can consist of multiple characters.
Some devices, such as disk partitions, may be accessed in
block or character mode. Because each device file corresponds to a single
access mode, physical devices that have more than one access mode will have
more than one device file. Device files are found in the /dev directory. Each device is assigned
a major and minor device number. The major device number identifies the type of
device, i.e. all SCSI devices would have the same number as would all the
keyboards. The minor device number identifies a specific device, i.e. the
keyboard attached to this workstation. Device files are created using the mknod command. The form for this
command is:
mknod device-name type major minor
The major and minor device numbers are indexed to device switches. There are
two types of device switches; cdevsw for character devices and
bdevsw for block devices. These switches are kernel structures that
hold the names of all the control routines for a device and tell the kernel
which driver module to execute. Device switches are actually tables that
look something like this: Using the ls command in the /dev directory will show entries
that look like: The "b" before the permissions indicates that this is a block device file.
When a user enters /dev/sd1a the kernel sees the file opening,
realizes that it's major device number 1, and calls up the SCSIbus function to
handle it.12.3.3. Unix directory structure
12.3.4. Links
12.3.5. Special
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