ON - Handle Ctrl-Break and errors in batch files

 Purpose:    Execute a command in a batch file when a specific condition 
             occurs. 
             
 Format:     ON BREAK [command ] 
                 or 
             ON ERROR [command ] 
                 or 
             ON ERRORMSG [command ] 
 

 Usage 

 ON can only by used in batch files. 

 ON sets a "watchdog" that remains in effect for the duration of the 
 current batch file.  Whenever a BREAK or ERROR condition occurs after ON 
 has been executed, the corresponding command is automatically executed. 

 ON BREAK will execute the command if the user presses Ctrl-C or 
 Ctrl-Break. 

 ON ERROR and ON ERRORMSG will execute the command after any command 
 processor or operating system error (including critical errors).  That 
 is, they will detect errors such as a disk write error, and CMD.EXE 
 errors such as a COPY command that fails to copy any files, or the use of 
 an unacceptable command option. 

 ON ERROR executes the command immediately after the error occurs, without 
 displaying any command processor error message (operating system errors 
 may still be displayed in some cases).  ON ERRORMSG displays the 
 appropriate error message, then executes the command.  If both are 
 specified, ON ERROR will take precedence, and the ON ERRORMSG setting 
 will be ignored.  The remainder of this section discusses both settings 
 together, using the term "ON ERROR[MSG]". 

 ON BREAK and ON ERROR[MSG] are independent of each other.  You can use 
 either one, or both, in any batch file. 

 Each time ON BREAK or ON ERROR[MSG] is used, it defines a new command to 
 be executed for a break or error, and any old command is discarded.  If 
 you use ON BREAK or ON ERROR[MSG] with no following command, that type of 
 error handling is disabled.  Error handling is also automatically 
 disabled when the batch file exits. 

 ON BREAK and ON ERROR[MSG] only affect the current batch file.  If you 
 CALL another batch file, the first batch file's error handling is 
 suspended, and the CALLed file must define its own error handling.  When 
 control returns to the first batch file, its error handling is 
 reactivated. 

 The command can be any command that can be used on a batch file line by 
 itself.  Frequently, it is a GOTO or GOSUB command.  For example, the 
 following fragment traps any user attempt to end the batch file by 
 pressing Ctrl-C or Ctrl-Break.  It scolds the user for trying to end the 
 batch file and then continues displaying the numbers from 1 to 1000: 

 
         on break gosub gotabreak
         do i = 1 to 1000
            echo %i
         enddo
         quit
         :gotabreak
         echo Hey!  Stop that!!
         return
 

 You can use a command group as the command if you want to execute 
 multiple commands, for example: 

 
         on break (echo Oops, got a break! & quit)
 

 ON BREAK and ON ERROR[MSG] always assume that you want to continue 
 executing the batch file.  After the command is executed, control 
 automatically returns to the next command in the batch file (the command 
 after the one that was interrupted by the break or error).  The only way 
 to avoid continuing the batch file after a break or error is for the 
 command to transfer control to another point with GOTO, end the batch 
 file with QUIT or CANCEL, or start another batch file (without CALLing 
 it). 

 When handling an error condition with ON ERROR[MSG], you may find it 
 useful to use internal variables, particularly %_? and %_SYSERR, to help 
 determine the cause of the error. 

 The ON ERROR[MSG] command will not be invoked if an error occurs while 
 reading or writing a redirected input file, output file, or pipe. 

 Caution:  If a break or error occurs while the command specified in ON 
 BREAK or ON ERROR[MSG] is executing, the command will be restarted.  This 
 means you must use caution to avoid or handle any possible errors in the 
 commands invoked by ON ERROR[MSG], since such errors can cause an 
 infinite loop.