Bash Guide for Beginners
2.3. Debugging Bash scripts
2.3.1. Debugging on the entire script
http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_02_03.html

When things don't go according to plan, you need to determine what exactly causes the script to fail. Bash provides extensive debugging features. The most common is to start up the subshell with the -x option, which will run the entire script in debug mode. Traces of each command plus its arguments are printed to standard output after the commands have been expanded but before they are executed.

There is now a full-fledged debugger for Bash, available at SourceForge. These debugging features are available in most modern versions of Bash, starting from 3.x.

2.3.2. Debugging on part(s) of the script

Using the set Bash built-in you can run in normal mode those portions of the script of which you are sure they are without fault, and display debugging information only for troublesome zones. Say we are not sure what the w command will do in the example commented-script1.sh, then we could enclose it in the script like this:

set -x # activate debugging from here
w

set +x # stop debugging from here

See also:

Advanced Bash-Scripting Guide
An in-depth exploration of the art of shell scripting
Mendel Cooper
http://tldp.org/LDP/abs/html/