is one of Unix's strongest points. Ramnick explains this
concept in this article. He talks about Input, Output Redirection.
He cites many simple and useful ways in which we can put redirection
to good use.
those of you'll who have no idea what Redirection means, let
me explain it in a few words. 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.. or you could make it disappear
is known as Redirection. Not only can the output of programs
be redirected, you can also redirect the input for programs.
I shall be explaining all this in detail in this article. Lets
important thing you have to know to understand Redirection is
file descriptors. In Unix every file has a no. associated
with it called the file descriptor. And in Unix everything is
a file. Right from your devices connected to your machine to
the normal text files storing some information - all of these
are looked at, as files by the Operating System.
Similarly even your screen on which your programs display their
output are files for Unix. These have file descriptors associated
with it. So when a program actually executes it sends its output
to this file descriptor and since this particular file descriptor
happens to be pointing to the screen, the output gets displayed
on the screen. Had it been the file descriptor of the printer,
the output would have been printed by the printer. (There are
ofcourse other factors which come into play, but I guess you
got the idea of how everything is a file and you send whatver
you want to particular files descriptors)
any program is executed (i.e. when the user types a command)
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. You could kind of
consider them to be inherently present for all programs (For
the techies.. basically when a child process is forked from
a parent process, these 3 files are made available to the child
process). For the rest, just remember that you always have these
3 files with you whenever you type any command at the prompt.
As explained before a file descriptor, is associated with each
of these files -
Points to -
Input (Generally Keyboard)
output (Generally Display/Screen)
Error Ouput (Generally Display/Screen)
could redirect any of these files to other files. In short if
you redirect 1 (standard output) to the printer, your programs
output would start getting printed instead of being displayed
on the screen.
is the standard input? That would be your keyboard. Most of
the times since you enter commands with your keyboard, you could
consider 0 to be your keyboard. Since you get the output
of your command on the screen, 1 would be the screen (display)
and the errors as well are shown on the screen to you, so 2
would also be the screen.
those of you'll who like to think ahead of what is being discussed...
you'll must have already understood that you can now avoid all
those irritating, irrelevant error messages you often get while
executing some programs. You could just redirect the standard
error (2) to some file and avoid seeing the error messages on
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.
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
' 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.
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.
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.
can use Input Redirection by typing the ' < ' operator.
An excellent example of Input Redirection has been shown below.
mail cousin < my_typed_letter
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.
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.
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.
is a way to redirect the error messages
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.
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.
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
find / -name s*.jpg 2>/dev/null
/dev/null ????? That something like a black hole. Whatever is
sent to the ' /dev/null ' never returns. Neither does one know
where it goes. It simple disappears. Isn't that fantastic !!
So remember.. whenever you want to remove something.. something
that you don't want ...you could just send it to /dev/null
Unix wonderful !!!
Different ways to use Redirection Operators
you want to create a text file quickly
cat > filename
This is some text that I want in this file
it!! Once you type the ' cat ' command, use the Redirection
operator and add a name for a file. Then start typing your line.
And finally press Ctrl+D. You will have a file named ' filename
' in the same directory.
you want to add a single line to an existing file.
echo "this is a new line" >> exsisting_file
would add the new line to the file named ' existing_file ' .
Remember to use ' >> ' instead of ' >
' else you would overwrite the file.
you wanted to join 2 files
cat file2 >> file1
That a much neater way then to open a text editor and copy paste.
The contents of ' file2 ' would be added to ' file1 ' .
you want to join a couple of files
cat file1 file2 > file3
would add the contents of ' file1 ' and ' file2 ' and then write
these contents into a new file named ' file3 ' .
Redirection works with many commands besides normal ones such
as ' cat ' or ' ls ' . One example I could give
you is in
case you are programming using any language you could redirect
the output messages of the compilation of your code
so that you can view them later on. There are lots of commands
where you can use Redirection. The more you use Unix the more
you will come to know.
the Author - Ramnick G currently works for Realtech Systems
based in Brazil. He has been passionate about Linux since
early 90s and has been developing on Linux machines for
the last couple of years. When he finds some free time,
he prefers to spend it listening to Yanni.
Copyright 2002 by Ramnick G. All rights reserved. Contact
the author for permissions.