Wednesday, November 29, 2006
UNIX Command SummaryThere are MANY commands available for you in a UNIX shell account. A list of them follows (in no particular order). Remember, you can always read the manpage on each command for more information.
ls ................. show directory, in alphabetical order
logout ............. logs off system
mkdir .............. make a directory
rmdir .............. remove directory (rm -r to delete folders with files)
rm ................. remove files
cd ................. change current directory
man (command) ...... shows help on a specific command
talk (user) ........ pages user for chat - (user) is a email address
write (user) ....... write a user on the local system (control-c to end)
pico (filename) .... easy to use text editor to edit files
pine ............... easy to use mailer
more (file) ........ views a file, pausing every screenful
sz ................. send a file (to you) using zmodem
rz ................. recieve a file (to the unix system) using zmodem
telnet (host) ...... connect to another Internet site
ftp (host) ......... connects to a FTP site
archie (filename) .. search the Archie database for a file on a FTP site
irc ................ connect to Internet Relay Chat
lynx ............... a textual World Wide Web browser
gopher ............. a Gopher database browser
tin, trn ........... read Usenet newsgroups
passwd ............. change your password
chfn ............... change your "Real Name" as seen on finger
chsh ............... change the shell you log into
grep ............... search for a string in a file
tail ............... show the last few lines of a file
who ................ shows who is logged into the local system
w .................. shows who is logged on and what they're doing
finger (emailaddr).. shows more information about a user
df ................. shows disk space available on the system
du ................. shows how much disk space is being used up by folders
chmod .............. changes permissions on a file
bc ................. a simple calculator
make ............... compiles source code
gcc (file.c) ....... compiles C source into a file named 'a.out'
gzip ............... best compression for UNIX files
zip ................ zip for IBM files
tar ................ combines multiple files into one or vice-versa
lharc, lzh, lha .... un-arc'ers, may not be on your system
dos2unix (file) (new) - strips CR's out of dos text files
unix2dos (file) (new) - adds CR's to unix text files
posted @ 8:37 PM
Sunday, November 26, 2006
1.) operating systemLast modified: Friday, January 04, 2002
The most important program that runs on a computer. Every general-purpose computer must have an operating system to run other programs. Operating systems perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and directories on the disk, and controlling peripheral devices such as disk drives and printers.
For large systems, the operating system has even greater responsibilities and powers. It is like a traffic cop -- it makes sure that different programs and users running at the same time do not interfere with each other. The operating system is also responsible for security, ensuring that unauthorized users do not access the system.
Operating systems can be classified as follows:
· multi-user : Allows two or more users to run programs at the same time. Some operating systems permit hundreds or even thousands of concurrent users.
· multiprocessing : Supports running a program on more than one CPU.
· multitasking : Allows more than one program to run concurrently.
· multithreading : Allows different parts of a single program to run concurrently.
· real time: Responds to input instantly. General-purpose operating systems, such as DOS and UNIX, are not real-time.
Operating systems provide a software platform on top of which other programs, called application programs, can run. The application programs must be written to run on top of a particular operating system. Your choice of operating system, therefore, determines to a great extent the applications you can run. For PCs, the most popular operating systems are DOS, OS/2, and Windows, but others are available, such as Linux.
As a user, you normally interact with the operating system through a set of commands. For example, the DOS operating system contains commands such as COPY and RENAME for copying files and changing the names of files, respectively. The commands are accepted and executed by a part of the operating system called the command processor or command line interpreter. Graphical user interfaces allow you to enter commands by pointing and clicking at objects that appear on the screen.
2) What are the major components of OS?
An operating system has two functions. It acts as a resource manager on the one side and a virtual machine on the other side. The resource manager of the operating system manages allocation of memory, processing time and disk space. The virtual machine should provide a nice user interface, other than the bare hardware would provide.
UNIX for instance has a resource manager, which allocates memory and processing time in a very efficient way. However the virtual machine leaves a lot to be desired on most UNIX systems. The Apple Macintosh in contrast provides a very nice virtual machine with its graphical user interface, but lacks features like protecting programs from each other and fair distribution of processor time. MS-DOS unfortunately provides neither a powerful resource manager nor a friendly virtual machine.
An operating system provides two main services: Filemanagement and Processmanagement.
Files
Processes
Shells
Files
Files are used to store data and directories are a way of organising files in a hierarchical structure. What UNIX does differently from DOS and other systems is the way it organises different physical storage devices.
In DOS there are individual drive names (actually letters from A to Z) denoting the different drives. In UNIX there is essentially just one virtual drive. All physical drives are sub-directories of this virtual drive or other drives.
Example: In DOS the first floppy disk drive is always A: and the first hard disk is always C:. When copying a file from the floppy disk directory flop_dir to the hard disk directory hard_dir, we issue the following instruction:
copy A:\flop_dir\file C:\hard_dir
Processes
A process is basically a program in execution. It consists of a program, its data, its ownership information (UID), the current position at which the computer is in the programs execution and all the other information needed to run the program. The UNIX operating system will occasionally decide to halt a process running and run another process. This gives every process its fair share of processing time. A process is also halted if it is waiting for some I/O operation to finish, like accessing a printer.
When a process is halted the operating system has to store all the information about the processes current state, so that the process can later be restarted exactly where it left off.
Processes can start other processes. These child processes have exactly the same ownership and permissions as the parent processes. Parents can control their children and send them signals. The operating system can also send signals to processes. Their are a wide variety of signals, and most programs react to signals by dying. There are a number of programs though, which are mostly operating system related and take more intelligent action towards signals sent to them.
Shells
The strict definition of an operating system limits it to only the basic program which runs the computer and provides systems calls to the users' programs. Usually the command interpreter or shell is not considered part of the operating system. But without the shell, which executes users' commands and programs, and is the primary interface between the computer and the user, a computer would be virtually impossible to use.
The basic function of a shell is to allow users to execute programs. Many advanced shells though have features which allow writing of scripts, command-line completion and command-line histories.
posted @ 8:13 PM