I've never done it this way, just by overriding those methods in the class definition containing the property, but I do know in that case Set has to take a value as an argument. Maybe it's not recognizing your Set method because it doesn't match the signature of the usual Set method. When you override it in a class, it looks like this:

Method NewProperty1Set(Arg As %String) As %Status [ ServerOnly = 1 ]

In that case, Arg is the value that the process is trying to set the value of the property to. So it might have to be something like:

Method Set(Arg as %String){ S %val = "asd" Q 1}

Or if you wanted to some something with the input value, you could use Arg to do that?

Yes, you do have to tread lightly and program very carefully if you mess with those. Errors in them could make it impossible to start or log into IRIS. This is straight from the documentation about those routines:

Make sure that the routines are well-behaved under all possible conditions. They should be written defensively. That is, they should check to make sure that all the resources needed to complete their task are at hand and, if possible, reserved to them before computation starts. Errors which occur are reported as failures of that system function so it is important to think about the design from the viewpoint of error containment and handling. Failure to properly account for recovery in the face of missing resources or the presence of errors has varied consequences: InterSystems IRIS may fail to start; major functions such as Studio may act strangely; or more subtle and insidious consequences may occur which are not immediately detected. It is strongly recommended that these routines be carefully written and debugged under simulated conditions, and then tested under simulated environment conditions before being put into production systems.

It doesn't sound like you're trying to do anything too crazy, but do make sure you trap or catch any potential errors anyway.

When using $EXTRACT, you use a * to signify an offset from the end of a string. So if you did $EXTRACT(Str,1,*-1) you would have the string with the last character removed.

Also note that the arguments for the $EXTRACT are the string, the starting character, and the ending character, so in the examples you gave, you're actually telling it to extract from Str starting at Length(Str)-1. You need to have a 1 in there as the second argument to go from the beginning to that character.

You can also use the $ROLES special variable to do that. It contains both the user's assigned roles and any roles you've added during the process. You can't change the user roles, but if you set it that will add a role. So you could do:

set $ROLES = "%All"

Or whatever role you need to add, then do the stuff that requires that roles, then do:

set $ROLES = ""

That will take away the %All role you added, reverting the process to just the user's normal roles.

Just to be clear, are you trying to run the OS-level command on the server, or on the client?

If you're trying to run it on the server, check the documentation for the $ZF -100 command.

If you're trying to run it on the client, you're going to run into all kinds of security issues, and for very good reasons.

Edit: just saw that you're on Cache 2013. You might have to use $ZF -1 instead of -100.