on error

Communicating error levels to a calling application from with in VBScript

We’ve discussed error handling previously (via the On Error Goto 0 command) in a entry titled, Using error control in a VBScript to recover from odd errors. However, occasionally you need to communicate a condition back to an application or batch file that calls a script as part of its processing. One reason you may need to do this is if you need to call a separate script or application depending on the outcome of the script. You can do this with what is called an error level. Keep in mind this may not have anything to do with an error in the script but can, instead, be used as a method to control how another applicatioin responds to a particular condition.

For example, say you already have a executable that processes a file into a database– but you don’t want to run the executable on a old file because it would cause all sorts of duplicated entries in the database. you might not be able to go back to the person who wrote the executable and have them change it or the executable might not have even been written by the company where you work! You could, however write a script to check the date of a file and if it was created during a time frame return a error level back to a batch file that calls your script and preforms the logic to decide if the executable needed to be ran. Take this batch file for example:

Echo off
cscript Yourscript.vbs
If %errorlevel% == 3 goto runexe
Echo “File to old”
goto EndBatch
:runexe
processingprogram.exe
:EndBatch

This would only run the executable IF your script returned a error level of 3! So you can then preform work and checks never intended by the executable when it was written only if the conditions are correct for it to run.

The line of code in VBScript you would add to your  script to communicate back to the batch file would be:

WScript.quit(3)

This will return a error level of 3 back to the batch file and cause the batch file to THEN run your executable.

So, there you have it! A quick and simple method to extend the life of those aging processes that work fine– just need a minor tweak because your environment has changed.

VBScript Statements: Explanation of the On Error Statement

The on error statement allows you to somewhat control how the scripting engine handles errors in the scripts you write. It allows you to turn error handling on and off for the scripting engine so that you can handle them in your script.

 The Statement essentially has two ways it is used. First to turn off the script engine error checking:

On Error Resume Next

The second is to turn it on, thereby allowing the script engine to stop execution of a script when an error is encountered. Like so:

On Error Goto 0

One word of caution, It is very tempting to simply place “On error resume next” in a complex script block and not worry about annoying hard to find errors. However, It is generally better to try to find the source of the errors since ignoring the errors can cause unintended consequences– or at the very least difficult to find issues in another piece of code.

In general, if you use On Error Resume Next you should always check for errors and handle them in your code somehow.