Bash-Beginners-Guide

Chapter 8. Writing interactive scripts
In this chapter we will discuss how to interact with the users of our scripts:
♦ Printing user friendly messages and explanations
♦ Catching user input
♦ Prompting for user input
♦ Using the file descriptors to read from and write to multiple files
8.1. Displaying user messages
8.1.1. Interactive or not?
Some scripts run without any interaction from the user at all. Advantages of non-interactive scripts include:
• The script runs in a predictable way every time.
• The script can run in the background.
Many scripts, however, require input from the user, or give output to the user as the script is running. The
advantages of interactive scripts are, among others:
• More flexible scripts can be built.
• Users can customize the script as it runs or make it behave in different ways.
• The script can report its progress as it runs.
When writing interactive scripts, never hold back on comments. A script that prints appropriate messages is
much more user-friendly and can be more easily debugged. A script might do a perfect job, but you will get a
whole lot of support calls if it does not inform the user about what it is doing. So include messages that tell the
user to wait for output because a calculation is being done. If possible, try to give an indication of how long
the user will have to wait. If the waiting should regularly take a long time when executing a certain task, you
might want to consider integrating some processing indication in the output of your script.
When prompting the user for input, it is also better to give too much than too little information about the kind
of data to be entered. This applies to the checking of arguments and the accompanying usage message as well.
Bash has the echo and printf commands to provide comments for users, and although you should be familiar
with at least the use of echo by now, we will discuss some more examples in the next sections.
8.1.2. Using the echo built-in command
The echo built-in command outputs its arguments, separated by spaces and terminated with a newline
character. The return status is always zero. echo takes a couple of options:
• -e: interprets backslash-escaped characters.
• -n: suppresses the trailing newline.
As an example of adding comments, we will make the feed.sh and penguin.sh from Section 7.2.1.2 a
Chapter 8. Writing interactive scripts 94