error − this even is raised whenever a command which is sent to the operating system results in an error.
failure − this even is raised whenever a command which is sent to the operating system results in a failure.
halt − this is normally raised whenever an operation is dependent on another operation. an example is if an i/o operation is being halted for any reason.
novalue − this event is raised when a value has not been assigned to a variable.
notready − this is raised by any i/o device which is not ready to accept any operation.
syntax − this event is raised if there is any syntax error in the code.
lostdigits − this event is raised when an arithmetic operation results in a loss of digits during the operation.

trapping errors

errors are trapped with the help of the signal command. let’s take a look at the syntax and an example of this.

syntax

signal on [errorcondition]

where,

errorcondition − this is the error condition which is given above.

example

let’s take a look at an example on this.

/* main program */ 
signal on error 
signal on failure 
signal on syntax 
signal on novalue beep(1) 
signal off error 
signal off failure
signal off syntax 
signal off novalue 
exit 0 
error: failure: syntax: novalue: 
say 'an error has occured'

in the above example, we first turn the error signals on. we then add a statement which will result in an error. we then have the error trap label to display a custom error message.

the output of the above program will be as shown below.

an error has occurred.

an example of error codes is shown in the following program.

/* main program */ 
signal on error 
signal on failure 
signal on syntax 
signal on novalue beep(1) 
exit 0 
error: failure: syntax: novalue: 

say 'an error has occured' 
say rc 
say signal

the output of the above program will be as shown below.

an error has occured 
40 
6

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