UNIX NOTES1
UNIT1.
How are devices represented in UNIX?
files and other files are named and accessed in the same way. A 'regular file' is just an ordinary data
files and other files are named and accessed in the same way. A 'regular file' is just an ordinary data
2. What
is 'inode'?
All UNIX
files have its description stored in a structure called 'inode'. The inode
contains info about the
Inode
consists of the following fields:
1. File
owner identifier
2. File
type
3. File
access permissions
4. File
access times
5. Number
of links
6. File
size
7.
Location of the file data
3. Brief
about the directory representation in UNIX.
A Unix
directory is a file containing a correspondence between filenames and inodes. A
directory is a special
file that the kernel maintains. Only kernel modifies directories, but processes
can read directories.
The contents of a directory are a list of filename and inode number pairs. When
new directories
are created, kernel makes two entries named '.' (refers to the directory
itself) and '..' (refers
to parent directory). System call for creating directory is mkdir (pathname,
mode).
4. What
are the Unix system calls for I/O?
1.
open(pathname,flag,mode) - open file
2.
creat(pathname,mode) - create file
3.
close(filedes) - close an open file
4.
read(filedes,buffer,bytes) - read data from an open file
5.
write(filedes,buffer,bytes) - write data to an open file
6.
lseek(filedes,offset,from) - position an open file
7.
dup(filedes) - duplicate an existing file descriptor
8.
dup2(oldfd,newfd) - duplicate to a desired file descriptor
9.
fcntl(filedes,cmd,arg) - change properties of an open file
10.
ioctl(filedes,request,arg) - change the behaviour of an open file
11. The
difference between fcntl anf ioctl is that the former is intended for any open
file, while the
latter is
for device-specific operations.
5. How do
you change File Access Permissions?
Every
file has following attributes:
1.
owner's user ID ( 16 bit integer )
2.
owner's group ID ( 16 bit integer )
3. File
access mode word
(r w x) -
(r w x) - (r w x)
(user
permission) - (group permission) - (others permission)
To change
the access mode, we use chmod(filename,mode).
Example
1:
To change
mode of myfile to 'rw-rw-r--' (ie. read, write permission for user - read,write
permission for group -
only read permission for others) we give the args as:
chmod(myfile,0664)
.
Each
operation is represented by discrete values
'r' is 4
'w' is 2
'x' is 1
Therefore,
for 'rw' the value is 6(4+2).
Example
2:
To change
mode of myfile to 'rwxr--r--' we give the args as:
chmod(myfile,0744).
6. What
are links and symbolic links in UNIX file system?
A link is
a second name (not a file) for a file. Links can be used to assign more than
one name to a file, but
cannot be used to assign a directory more than one name or link filenames on
different computers. Symbolic
link 'is' a file that only contains the name of another file.Operation on the
symbolic link is directed
to the file pointed by the it.Both the limitations of links are eliminated in
symbolic links.
Commands
for linking files are:
Link
"ln filename1 filename2"
Symbolic
link "ln -s filename1 filename2"
7. What
is a FIFO?
FIFO are
otherwise called as 'named pipes'. FIFO (first-in-first-out) is a special file
which is said to be data
transient. Once data is read from named pipe, it cannot be read again. Also,
data can be read only in
the order written. It is used in interprocess communication where a process
writes to one end of the
pipe (producer) and the other reads from the other end (consumer).
8. How do
you create special files like named pipes and device files?
The
system call mknod creates special files in the following sequence.
1. kernel
assigns new inode,
2. sets
the file type to indicate that the file is a pipe, directory or special file,
3. If it
is a device file, it makes the other entries like major, minor device numbers.
For
example:
If the
device is a disk, major device number refers to the disk controller and minor
device number is the disk.
9.
Discuss the mount and unmount system calls.
The
privileged mount system call is used to attach a file system to a directory of
another file system; the
unmount system call detaches a file system. When you mount another file system
on to your directory,
you are essentially splicing one directory tree onto a branch in another
directory tree. The first
argument to mount call is the mount point, that is , a directory in the current
file naming system.
The
second argument is the file system to mount to that point. When you insert a
cdrom to your unix system's
drive, the file system in the cdrom automatically mounts to
"/dev/cdrom" in your system.
10. How
does the inode map to data block of a file?
Inode has
13 block addresses. The first 10 are direct block addresses of the first 10
data blocks in the file. The
11th address points to a one-level index block. The 12th address points to a
two-level (double in-direction)
index block. The 13th address points to a three-level(triple in-direction)index
block. This provides
a very large maximum file size with efficient access to large files, but also
small files are accessed
directly in one disk read.
11. What
is a shell?
A shell
is an interactive user interface to an operating system services that allows an
user to enter commands
as character strings or through a graphical user interface. The shell converts
them to system
calls to the OS or forks off a process to execute the command. System call
results and other information
from the OS are presented to the user through an interactive interface.
Commonly used shells
are sh,csh,ks etc.
No comments:
Post a Comment