Input/Output Redirection and Filter Commands

So far, we might have learned how to use some commands in their standard form, e.g. to display the content of a text file, to view a directory listing or to view the tree structure of directories. We can view the output directly on the screen.

Using MS-DOS command in standard form, we type in our instruction at the command prompt, through the keyboard. It’s a standard input.

Then, MS-DOS will process your input and display the result on the screen. It’s a standard output. Up to this point, the job is simply done.

Now, if we want to record a directory listing for further reference or get a hard copy immediately, we need to instruct MS-DOS not to display the result on the screen but to save it to a file or print it out directly. It’s called output redirection.

On the other hand, if we want to process a job by using data on an existing file, we need to instruct MS-DOS to use data from a given file. It’s called input redirection.

In some cases, moreover, we may need to process a job by executing more than one command. By the way, the output from the first command will not be displayed on the screen but to be redirected into the input of the next command. It involves both output and input redirection. The logical connection betwwen these two commands is called piping.

Piping is associated with filter commands:

1. find,

2. sort and

3. more.

They are called filter commands because they work much like a filter in a water system, which takes incoming water, changes it in some way, and sends it along the system (a good explanation from Van Wolverton’s Running MS-DOS).

The application of these filter commands are listed as follow. They can be used simply in an input redirection or through a logical piping connection.

• Find searches input data for a text string.

• Sort arranges input data in order.

• More displays output one screen at a time.

The following special characters are needed in Input/Output redirection and Piping:

Examples