when a command is enterted how does it display the output on screen..... what is the internal working???
In UNIX land most commands are written to operate as a filter. It accepts input from a thing called Standard Input, and if it has any out put it writes it to a thing called Standard Output. Normally the Standard Input accepts input from the keyboard, and Standard Output is written to the screen. There is also another "standard" output channel, its called (if I remember correctly) 'Standard Error" which is almost always mapped to the screen. All of these can be redirected from their normal mapping to other sources and sinks. Many UNIX commands work with a special remapping symbol "|" called the "pipe". The pipe takes the standard output from one command and feeds it into the standard input of the following command. An example is "ls | more". The "ls" reads the current directory and lists the files. The "more" command accepts input and displays it on the screen, if the input is such that it would more than fill the screen then it paginates the output so that the output does not run off the top of the screen. So "ls | more" will read the current directory and display the list one page at a time.
i appreciate ur answer, but i wanna know when i typed
$ echo hello
hello is the output, how does this output displayed in screen, i mean what is the internal working behind it........... is it the interection with shell and then kernel........ plz clear this out in detail.
Well, there is certainly no simple 25 word answer for this question, but it is a good one.
The shell takes input from the terminal. The appropriate kernel keyboard driver gets notified when the user types a character on the keyboard. It takes the key typed and passes that to the shell via the shell's standard input file handle (stdin). The shell sends a copy to its standard output file handle (stdout). When you type one of a number of special keys, such as Enter or Ctrl-C, Tab, Esc, etc. then the shell will act accordingly. If it is the Enter key, it will take everything input up to that point and process that. Assuming that you weren't in the middle of a quoted string, braces, parentheses, etc, then it will break the line into tokens, using the first token as a command. Assuming that the command does not have any path characters (a slash) in it, It takes the PATH environment variable and prepends the first path + slash to the command and looks to see if that file exists and is executable. If not, it does the same thing with the next directory found in the PATH environment. If it doesn't find any suitable executable files, it gives the "<command>: command not found" error. If it does find it, then executes the command, passing the other tokens as an array of arguments to the command, and also passes control of the terminal to the command being executed. When the command is done, control is passed back to the shell.
Output works like input, but in reverse. The program passes each output character to the terminal output driver, which buffers that output until it receives a command from the application (or shell) to flush it (cooked mode), or immediately (uncooked/raw mode). So, the echo command looks at all its arguments and sends them to stdout, each separated with a space. The driver associated with the device attached to stdout will then do whatever it needs to do. For example, if stdout is piped to /dev/null, it gets dropped into the bit bucket. If stdout is attached to a GUI command window, the GUI driver deals with it. If the output is a printer, the printer driver deals with it.
Anyway, I'm sure there are good resources on the Internet to help you better understand how this works.