Redirection is one of Unix's strongest points.
Whenever you run a program you get some output at the shell prompt. In case you don't want that output to appear in the shell window, you can redirect it elsewhere. you can make the output go into a file...or maybe go directly to the printer.
This is known as Redirection. Not only can the output of programs be redirected, you can also redirect the input for programs.
the program has 3 important files to work with. They are standard input, standard output, and standard error. These are 3 files that are always open when a program runs.
File Descriptor Descriptor Points to -
0 Standard Input (Generally Keyboard)
1 Standard output (Generally Display/Screen)
2 Standard Error Ouput (Generally Display/Screen)
Output Redirection
The most common use of Redirection is to redirect the output (that normally goes to the terminal) from a command to a file instead. This is known as Output Redirection. This is generally used when you get a lot of output when you execute your program. Often you see that screens scroll past very rapidly. You could get all the output in a file and then even transfer that file elsewhere or mail it to someone.
The way to redirect the output is by using the ' > ' operator in shell command you enter. This is shown below. The ' > ' symbol is known as the output redirection operator. Any command that outputs its results to the screen can have its output sent to a file.
$ ls > listing
The ' ls ' command would normally give you a directory listing. Since you have the ' > ' operator after the ' ls ' command, redirection would take place. What follows the ' > ' tells Unix where to redirect the output. In our case it would create a file named ' listing ' and write the directory listing in that file. You could view this file using any text editor or by using the cat command.
Note: If the file mentioned already exists, it is overwritten. So care should be taken to enter a proper name. In case you want to append to an existing file, then instead of the ' > ' operator you should use the ' >> ' operator. This would append to the file if it already exists, else it would create a new file by that name and then add the output to that newly created file.
Input Redirection
Input Redirection is not as popular as Output Redirection. Since most of the times you would expect the input to be typed at the keyboard. But when it is used effectively, Input Redirection can be of great use. The general use of Input Redirection is when you have some kind of file, which you have ready and now you would like to use some command on that file.
You can use Input Redirection by typing the ' < ' operator. An excellent example of Input Redirection has been shown below.
$ mail cousin < my_typed_letter
The above command would start the mail program with contents of the file named ' my_typed_letter ' as the input since the Input Redirection operator was used.
Note: You can't have Input Redirection with any program/command. Only those commands that accept input from keyboard could be redirected to use some kind of text files as their input. Similarly Output Redirection is also useful only when the program sends its output to the terminal. In case you are redirecting the output of a program that runs under X, it would be of no use to you.
Error Redirection
This is a very popular feature that many Unix users are happy to learn. In case you have worked with Unix for some time, you must have realised that for a lot of commands you type you get a lot of error messages. And you are not really bothered about those error messages. For example whenever I perform a search for a file, I always get a lot of permission denied error messages. There may be ways to fix those things. But the simplest way is to redirect the error messages elsewhere so that it doesn't bother me. In my case I know that errors I get while searching for files would be of no use to me.
Here is a way to redirect the error messages
$ myprogram 2>errorsfile
This above command would execute a program named ' myprogram ' and whatever errors are generated while executing that program would all be added to a file named ' errorsfile ' rather than be displayed on the screen. Remember that 2 is the error output file descriptor. Thus ' 2> ' means redirect the error output.
$ myprogram 2>>all_errors_till_now
The above command would be useful in case you have been saving all the error messages for some later use. This time the error messages would append to the file rather than create a new file.
You might realize that in the above case since I wasn't interested in the error messages generated by the program I redirected the output to a file. But since those error messages don't interest me I would have to go and delete that file created every time I run that command. Else I would have several such files created all over whenever I redirect my unwanted error output. An excellent way around is shown below
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