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<strong>Bash</strong> <strong>Guide</strong> for <strong>Beginners</strong><br />
Once you found the buggy part of your script, you can add echo statements before each command of which<br />
you are unsure, so that you will see exactly where and why things don't work. In the example<br />
commented-script1.sh script, it could be done like this, still assuming that the displaying of users gives<br />
us problems:<br />
echo "debug message: now attempting to start w command"; w<br />
In more advanced scripts, the echo can be inserted to display the content of variables at different stages in the<br />
script, so that flaws can be detected:<br />
echo "Variable VARNAME is now set to $VARNAME."<br />
2.4. Summary<br />
A shell script is a reusable series of commands put in an executable text file. Any text editor can be used to<br />
write scripts.<br />
Scripts start with #! followed by the path to the shell executing the commands from the script. Comments are<br />
added to a script for your own future reference, and also to make it understandable for other users. It is better<br />
to have too many explanations than not enough.<br />
Debugging a script can be done using shell options. Shell options can be used for partial debugging or for<br />
analyzing the entire script. Inserting echo commands at strategic locations is also a common troubleshooting<br />
technique.<br />
2.5. Exercises<br />
This exercise will help you to create your first script.<br />
1. Write a script using your favorite editor. The script should display the path to your homedirectory and<br />
the terminal type that you are using. Additionally it shows all the services started up in runlevel 3 on<br />
your system. (hint: use HOME, TERM and ls /etc/rc3.d/S*)<br />
2. Add comments in your script.<br />
3. Add information for the users of your script.<br />
4. Change permissions on your script so that you can run it.<br />
5. Run the script in normal mode and in debug mode. It should run without errors.<br />
6. Make errors in your script: see what happens if you misspell commands, if you leave out the first line<br />
or put something unintelligible there, or if you misspell shell variable names or write them in lower<br />
case characters after they have been declared in capitals. Check what the debug comments say about<br />
this.<br />
Chapter 2. Writing and debugging scripts 28
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