How to do it...

We can either use Bash's inbuilt debugging tools or write our scripts in such a manner that they become easy to debug; here's how:

1. Add the -x option to enable debug tracing of a shell script.

        $ bash -x script.sh

Running the script with the -x flag will print each source line with the current status.

1. Debug only portions of the script using set -x and set +x. Consider this example:

        #!/bin/bash 
        #Filename: debug.sh 
        for i in {1..6}; 
        do 
            set -x 
            echo $i 
            set +x 
        done 
        echo "Script executed"

In the preceding script, the debug information for echo $i will only be printed, as debugging is restricted to that section using -x and +x.
The script uses the {start..end} construct to iterate from a start to end value, instead of the seq command used in the previous example. This construct is slightly faster than invoking the seq command.

1. The aforementioned debugging methods are provided by Bash built-ins. They produce debugging information in a fixed format. In many cases, we need debugging information in our own format. We can define a _DEBUG environment variable to enable and disable debugging and generate messages in our own debugging style.

Look at the following example code:

        #!/bin/bash 
        function DEBUG() 
        { 
            [ "$_DEBUG" == "on" ] && $@ || : 
        } 
        for i in {1..10} 
        do 
          DEBUG echo "I is $i" 
        done

Run the preceding script with debugging set to "on":

        $ _DEBUG=on ./script.sh

We prefix DEBUG before every statement where debug information is to be printed. If _DEBUG=on is not passed to the script, debug information will not be printed. In Bash, the command : tells the shell to do nothing.