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Revision as of 13:47, 12 November 2011

CRON - SrvStart utility tutorial

This short tutorial covers the SrvStart utility written by Nick Rozanski. It allows you to run a script as a service!

What is a Service?
A Windows Service is automatically started when your PC starts. Services automatically recover from program crashes and Standby or hibernation modes. Installing software as a Service requires Administrator permissions.


Complexity
Programs (application) designed to run as a service contain code that interface to the Windows service manager. Running a script as a service requires additional code that allows it to access the Windows service manager. This code registers your script allowing it to be run as a service in addition it must provide responses to command requests from the service manager and have the ability to provide your scripts status.

Not a trivial exercise however this complexity is hidden when using SrvStart.


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Download SrvStart

Download SRVStart.exe from http://www.rozanski.org.uk/software

  • Navigate to section SRVSTART.EXE
  • Click on the image named "Executable"
  • This starts the download.
  • Save file srvstart_run.v110.zip to any folder.
  • Extract contents of this file to where you saved it.

Install SRVStart

The folder srvstart_run.v110 contains the following files:

  • logger.dll (28KB)
  • srvstart.dll (140KB)
  • srvstart.exe (size 36KB)
  • svc.exe

We are interest only in files srvstart.exe, srvstart.dll and logger.dll
For this tutorial create a new folder c:\service_test and copy the above three files into it.


Note 1:
The documentation states copy files srvstart.exe, srvstart.dll, logger.dll and msvcrt.dll to C:\WINDOWS\system32 (system path). For XP onwards msvcrt.dll is already installed on a system path. Other three files need not be installed on a system path see below. We are going to run srvstart.exe from its installation folder. This allows it to be portable by not installing anything on a host PC (assumes running from memory stick).

Note 2:
When installing a service it does write to the registry hence remember to stop and uninstall the service you create especially if running from a memory stick.


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Script to run

For this tutorial we require a script to run. Create a new text document named my_script.vbs with the following content:

Set oWS = WScript.CreateObject("WScript.Shell")
oWS.Run "%comspec% /c echo " & Chr(07), 0, True 

Set oWS = WScript.CreateObject("WScript.Shell")
oWS.Run "%comspec% /c echo " & Chr(07), 0, True 

This is an annoying VBScript it produces beeps when run.


Double click on my_script.vbs and confirm it produces two beeps.

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Limitation

An application to be run as a service must have one of the following file extensions .exe, .com or .bat The script we want to run has an extension .vbs hence cannot be run directly.

At first this looks very restrictive however a batch file will run any application. To run our script as a service create a batch file named my_script.bat with the following content:

start my_script.vbs

start Runs our vb script and closes. It does not wait for our vb script to finish
Double click on my_script.bat and confirm it produces two beeps.

Note 1: Running my_script.bat a command window opens for a short time. When run as a service this is hidden.

Note 2: Removing the "start" keyword our vb script runs and the command window remains open until vb script finishes. Again when run as a service this is hidden.


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SrvStart configuration file

To create our application (script) service SrvStart requires a configuration file.

  • Each service to be run is contained in a separate block.
  • Every block starts with a service name enclosed in square brackets.
  • Each block must contain a startup command, which specifies the program (application/script) to launch.

Create a file named srvstart.ini with the following content:

[z_test]
startup=C:\service_test\my_script.bat
startup_dir=C:\service_test

Service name is z_test
startup Full path to script
startup_dir Folder to start looking in

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Control_batch_files

We are now ready to install and run our service this can be done using a command prompt and issuing the appropriate commands. I prefer to put these commands into separate batch files as follows:

z_install.bat

Create a file named z_install.bat with the following content:

srvstart.exe install z_test -c C:\service_test\srvstart.ini
pause
  • srvstart.exe - Run srvstart program
  • install - Command to run is install
  • z_test - Name of service to install
  • -c Configuration file to use

z_uninstall_service.bat

Create a file named z_uninstall_service.bat with the following content:

srvstart.exe remove z_test
pause
  • srvstart.exe - Run srvstart program
  • remove - Command to run is install
  • z_test - Name of service to uninstall

z_service_start.bat

Create a file named z_service_start.bat with the following content:

net start z_test
pause
  • net - Command is used to manage services
  • start - Start the named service
  • z_test - Name of service to start

z_service_stop.bat

Create a file named z_service_stop.bat with the following content:

net stop z_test
pause
  • net - Command is used to manage services
  • stop - Stops the named service
  • z_test - Name of service to start

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Test - Perceived problems

Run z_install.bat

You will see "Successfully created non-desktop service 'z_test'"

Run z_service_start.bat

Confirm two bleeps are produced. Note the following messages

  • The z_test service is starting.
  • The z_test service could not be started.
  • The service did not report an error.
  • More help is available by typing NET HELPMSG 3534.

With the confirmation of two beeps our script was successfully run. Reason for could not be started message; the batch file closes immediately when run. Srvstart regularly checks running status of an application it started, it immediately sees the script no longer running and reports back to the service control it could not be started. The service did not report an error because it’s not an error.

Run z_service_stop.bat

Note the following messages

  • C:\service_test>net stop z_test
  • The z_test service is not started.
  • More help is available by typing NET HELPMSG 3521.

The batch file is not running hence the service as reported is not running see above. Above is not a problem its the way we want to run our scrip.

Run z_uninstall_service.bat 

Note the following message

  • Deletion of service 'z_test' succeeded

Confirms correct operation

Note: The above results may be a little confusing, remember our VBScript does not run continuously its a run once program. Its purpose is to familiarise yourself with the Srvstart program.

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Service Status

With the service created, it will now appear in the Windows Services list and can be configured like any other Windows Service. The service which is installed has the following characteristics.

  • Its short name and display name are both z_test.
  • It is set to Manual startup.
  • It starts using LocalSystem and has no dependencies.
  • If cannot interact with the desktop.

Manual startup

Service start type specifies the services behavior after reboot:

  • Auto - Automatically start after a re-boot
  • Manual - Need to manually start service once PC started
  • Disabled - Deactivated

We want our script to start when the PC is powered up. Either change the Manual state to Auto using service manger. Alternatively use a script to set the service state. Create a new file named start_automatic.vbs with the following content:

Set objWMIService = GetObject("winmgmts:\\.\root\cimv2") 

Set colRunningServices = objWMIService.ExecQuery ("Select * from Win32_Service")
 For Each objService in colRunningServices
   If objService.DisplayName = "z_test" Then
        errReturn = objService.Change( , , , , "Automatic")
   End If
 Next

The above script will change the service state to Automatic. After you have installed your service it only requires running once.


For testing this script starts our service and changes it to Automatic.

Set objWMIService = GetObject("winmgmts:\\.\root\cimv2") 

Set colRunningServices = objWMIService.ExecQuery ("Select * from Win32_Service")
 For Each objService in colRunningServices
   If objService.DisplayName = "z_test" Then
     If objService.State = "Running" Then
        MsgBox "Is Running"
     End If
     If objService.State= "Stopped" Then
        errReturn = objService.StartService()
        errReturn = objService.Change( , , , , "Automatic")
     End If
   End If
 Next

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Simulating service program crash

Our batch file is ideal for simulating a service program crash and demonstrating recovery. The batch file only runs once (simulating a program crash), srvstat can be set to automatically restart the program using directive auto_restart=y, setting restart_interval=seconds will pause before starting.

This can be put to good use; it allows a one time run script to be periodically called. Modify the configuration file as shown below:

[z_test]
startup=C:\service_test\my_script.bat
startup_dir=C:\service_test
auto_restart=y
restart_interval=10

Our test script will now be run every 10 seconds

Note 1: auto_restart will restart the service program if it exits for any reason. If restart_interval is defined, then before restarting, SRVSTART will wait seconds seconds.


Note 2: auto_restart does not; restart the service program if it is stopped by request (eg NET STOP or Control Panel|Services).


Note 3: auto_restart does not restart the service program if it thinks that Windows NT is shutting down. If you are irritated by services restarting during Windows NT shutdown, then increase the value of restart_interval to, say, a minute.

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Conclusion

The above has shown how to run a simple VBScipt as a service. When Uniform Server is installed as a permanent installation concepts outlined above can be used for kicking portable cron into life. Portable cron is intended to be run from a memory stick and still requires manually starting.

Where to next

Related to the above service is Uniform Server Cron both portable cron and running as a service have been implemented for details see Cron.


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