Debug Shell Script

Tools for debugging non-working scripts include

1. Inserting echo statements at critical points in the script to trace the variables, and otherwise give a snapshot of what is going on.

2. Using the tee filter to check processes or data flows at critical points.

3. Setting option flags -n -v -x
    sh -n scriptname checks for syntax errors without actually running the script. 
    sh -v scriptname echoes each command before executing it.                                         sh -x scriptname echoes the result each command, but in an abbreviated manner. 

4.  Using an "assert" function to test a variable or condition at critical points in a script. 

5. Using the $LINENO variable and the caller builtin.

6. Trapping at exit.
    The exit command in a script triggers a signal 0, terminating the process, that is, the script itself.