For example, the program wc (for word count) reads text from its input stream and produces as its output stream three numbers indicating the number of lines, words, and characters that it saw. You could invoke this directly:

   wc
   Hello.
   How are you?
   ^D

in which case, you would see as output:

     2     4    20

For this to be very useful, however, we need to make it accept a file as input. This is done by using the < operator in the shell. Think of the < as an arrow indicating data flowing towards the command from a filename: If hello.c is this file:

#include <stdio.h> 
int main () 
{
  printf ("Hello from C!\n"); 
  return 0;
}

then the command

wc < hello.c

produces the output

     6    13    80

On the output end, the shell operator > redirects the standard output into a file (again, think of this as an arrow indicating data flowing into a filename from the command):

wc < hello.c > hello.wc

produces no output on the screen, but creates a file called hello.wc. That file will contain the output

     6    13    80

of the wc command.

The output redirection operator has a couple of important variants. First, the shell generally does not allow you to redirect into an existing file. If you give the command

   wc < hello.c > hello.wc

a second time, the shell will refuse to perform the command. You can force the shell to delete an existing file and create a new one for redirection by changing the > to >!.

cd ~/playing
echo Hello > greeting.txt
more greeting.txt
echo Goodbye > greeting.txt
more greeting.txt
echo Farewell >! greeting.txt
more greeting.txt

Second, sometimes we would like to add output to the end of an existing file instead of replacing that file. This is done with the operator >>. So the code sequence

   wc < hello.c >! hello.wc
   wc < hello.c >> hello.wc

would result in a file hello.wc with contents

     6    13    80
     6    13    80

regardless of whether hello.wc had existed previously.

cd ~/playing
echo Aloha >> greeting.txt
more greeting.txt

To pipe the output of one command into the input of another, use the shell operator |. A common example of a pipe is to take a command that may have a large amount of output and to pipe it through more to facilitate viewing.

   ls /bin | more

As you gain facility with a greater variety of Unix text manipulation commands, you will find that redirection and pipes can be a powerful combination. For example, suppose that you have written program myprog that emits a great deal of output, among which might be some error messages starting with the phrase “ ERROR: ”. If you wanted to read only the error messages, you could, of course, just view all the output, watching for the occasional error message:

   myprog | more

But if the program produces a lot of output, this will quickly become tedious. However, we previously encountered the program grep, which scans its input stream, printing only those lines matching a given regular expression. By piping the myprog output through grep, we can limit the output to the part we really want to see:

   myprog | grep "ERROR:" | more