Linux® Command Line and Shell Scripting Bible

Chapter 12. Presenting Data

IN THIS CHAPTER

• Revisiting redirection

• Standard input and output

• Reporting errors

• Throwing away data

• Creating log files

So far the scripts shown in this book display information either by echoing data to the monitor or by redirecting data to a file. Chapter 8 demonstrated how to redirect the output of a command to a file. This chapter expands on that topic by showing you how you can redirect the output of your script to different locations on your Linux system.

Understanding Input and Output

So far, you've seen two methods for displaying the output from your scripts:

• Display output on the monitor screen

• Redirect output to a file

Both methods produced an all-or-nothing approach to data output. There are times though when it would be nice to display some data on the monitor and other data in a file. For these instances, it comes in handy to know how Linux handles input and output so that you can get your script output to the right place.

The following sections describe how to use the standard Linux input and output system to your advantage, to help direct script output to specific locations.

Standard file descriptors

The Linux system handles every object as a file. This includes the input and output process. Linux identifies each file object using a file descriptor. The file descriptor is a non-negative integer, which uniquely identifies open files in a session. Each process is allowed to have up to nine open file descriptors at a time. The bash shell reserves the first ...