Standard input,output,error

In UNIX systems, there are three special files that are predefined for each process. These special files are

• Standard input is the file that is used for getting data to the command. This file is defined as the keyboard by default. So when you press the keys, system gets the keys and send them to the command using the standard input. If standard input is changed to a regular file, then system gives the contents of that file to the command.
• Standard output is the file that is used for sending the normal output of the command execution. By default this file is defined as the screen you are working at. So you will see the list of the directory on the screen after "ls" command. If standard output is changed to a regular file, then the outputs of command will be written in that file. (you won't see them on the screen any more)
• Standard error is the file that is used for sending abnormal messages while executing a command. This file is also defined as the screen. Usually meaningful error messages are written to this file. If standard error is changed to a regular file, then the error messages are written in that file. (you won't see them on the screen any more)

Note that in UNIX everything can be used as a file. A directory, memory, network I/O and even the keyboard are defined as special types of file. Do not get confused with types of a file. When you see file, it means a regular file. Other special files have their own names.

Nearly every program in UNIX sends its output to the standard output, and errors to standard error. Most commands accept file names as arguments and use them as their input, but if no file name is specified, then standard input is used for input. In the example below, cat command will use its predefined standard input, since no file is specified.

<mendelson:~>cat 
trial and error
trial and error
is it okay????
is it okay????
why is it repeating them two times?
why is it repeating them two times?
^D

As you can see, the command gets each line from the standard input(keyboard) and displays it on standard output (screen). When you press enter, the contents of that line is sent to the command, and it displays them on screen. To tell the command that the standard input is ended (you don't want to enter anything more from keyboard), you should press ^D (Ctrl-d) keys together in a new line. Press the d key for only one instant, don't hold it down. When the command gets this character (^D end of file), it finishes the execution.