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Monitor a specific new files and execute the file, and then re-start the monitoring program itself to continue its monitoring by standby: code:
                   
Output and steps:          
Step 1: The folder          
         
Step 2: Paste a file
         
Step 3: output (had called and run another file)         
         
Step 4: one more layer of the foler            
         
Step 5: paste two files            
         
Step 6: second output together with the first output           
         
Step 7: free the folder again          
         
Step 8: paste the file one more time          
         
Step 9:  Added one more output
         

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Monitor a specific new files and execute the file, and then re-start the monitoring program itself to continue its monitoring by standby (note: this program only has one line difference from the one above, but it acts the same way as the one above): code:
         

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Script ranking: Number 1A. Observer runs in the background and pass the value of obtained events to a/another function call. Note this code has two advantages:
        i) The monitoring program wit watchdog does not stop when the other program crashes because of "exec(open) command".
        ii) The monitoring program executes other programs in series.
          Code:
                
          Output (Failed at running the second function "SimpleCal2" but the the program running in background still runs and still calls functions when new files are uploaded even though there was a failure from calling "SimpleCal2"):
                

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Monitor a specific new files and execute the file, and then re-start the monitoring program itself to continue its monitoring by standby (note: this program only has one line difference from the one above, but it acts the same way as the one above): code:
               

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Monitor specific new files and execute the file: code:
         
         
Output:          
         
Output below once there is a new pdf file in "Downloads" folder:          
         

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Monitor specific new files and execute another file: code 1; and Create a .txt file with calling by another file: code 2:
    Code 1 (the lines marked in red below can avoid secondary excution of the Code 2 file, trigered by the .txt file created by Code 2) -- Warning: this package can be not stable in some cases:
         
     Code 2:
                   
Before excuting Code 1:          
         
After excuting Code 1:          
         
After copying the two files in the red circle into the folder, the .txt file was generated by Code 2 since the file "ImagePool (12).jpg" was included:          
          
         
In contrast, after copying the four files below into the folder, no .txt file was generated by Code 2 since the file "ImagePool (12).jpg" was not included: