I just stumbled upon a nice way to enhance the bash debug trace output format (See here for the source).

Just declare

declare -r PS4='|${LINENO}> \011${FUNCNAME[0]:+${FUNCNAME[0]}(): }'

and you will get following output

framp@obelix:~/scripts$ bash -x ./dbg.sh 3
+ declare -r 'PS4=|${LINENO}> \011${FUNCNAME[0]:+${FUNCNAME[0]}(): }'
|16> main(): fib 3
|6> fib(): (( 3 <= 0 ))
|8> fib(): (( 3 == 1 ))
||11> fib(): fib 1
||6> fib(): (( 1 <= 0 ))
||8> fib(): (( 1 == 1 ))
||9> fib(): echo 1
||11> fib(): fib 2
||6> fib(): (( 2 <= 0 ))
||8> fib(): (( 2 == 1 ))
|||11> fib(): fib 0
|||6> fib(): (( 0 <= 0 ))
|||7> fib(): echo 0
|||11> fib(): fib 1
|||6> fib(): (( 1 <= 0 ))
|||8> fib(): (( 1 == 1 ))
|||9> fib(): echo 1
||11> fib(): echo 1
|11> fib(): echo 2
2

when you execute following code to calculate the Fibonacci numbers:

1: #!/bin/bash
  
3: declare -r PS4='|${LINENO}> \011${FUNCNAME[0]:+${FUNCNAME[0]}(): }'
 
5: function fib(){
6: if (( $1 <= 0 )); then
7: echo 0
8: elif (( $1 == 1 )); then
9: echo 1
10: else
11: echo $[ $(fib $(($1-2)) ) + $(fib $(($1 - 1)) ) ]
12: fi
 
14: }
 
16: fib $1