3.8.2 Redirecting input and output

          Now, let's say that we wanted to send the output of sort to a file, to save our shopping list elsewhere. The shell allows us to redirect standard output to a filename, using the ``>'' symbol. Here's how it works.

/home/larry/papers# sort > shopping-list
bananas
carrots
apples

/home/larry/papers#

As you can see, the result of the sort command isn't displayed, instead it's saved to the file shopping-list. Let's look at this file.

/home/larry/papers# cat shopping-list
apples
bananas
carrots
/home/larry/papers#

Now we can sort our shopping list, and save it, too! But let's suppose that we were storing our unsorted, original shopping list in the file items. One way of sorting the information and saving it to a file would be to give sort the name of the file to read, in lieu of standard input, and redirect standard output as we did above. As so:

/home/larry/papers# sort items > shopping-list
/home/larry/papers# cat shopping-list
apples
bananas
carrots
/home/larry/papers#

      However, there's another way of doing this. Not only can we redirect standard output, but we can redirect standard input as well, using the ``<'' symbol.

/home/larry/papers# sort < items
apples
bananas
carrots
/home/larry/papers#

Technically, sort < items is equivalent to sort items, but the former allows us to demonstrate the point: sort < items behaves as if the data in the file items was typed to standard input. The shell handles the redirection. sort wasn't given the name of the file (items) to read; as far as sort is concerned, it was still reading from standard input as if you had typed the data from your keyboard.

  This introduces the concept of a filter. A filter is a program which reads data from standard input, processes it in some way, and sends the processed data to standard output. Using redirection, standard input and/or standard output can be referenced from files. sort is a simple filter: it sorts the incoming data and sends the result to standard output. cat is even simpler: it doesn't do anything with the incoming data, it simply outputs whatever was given to it.