Windows Scripting: Creating a VBScript Shell

How to Create Command Line Interpreter with VBScript

Dec 6, 2008
Mark Alexander Bain

A VBScript Program - Mark Alexander Bain

VBScript does not have a command line interpreter of its own; however a programmer can quickly write a wrapper script that will act as a simple VBScript shell.

Anyone who has used Linux will be aware of the shells (command line interpreters such as Bash, C and Korn) and that's what some languages such as VBScript lack - it's not possible, for example, to open up a DOS window and simply type in a VBScript command.

However, what the programmer can do is to create a VBScript program that will act as wrapper and produce a very simple shell - this will allow the user enter VBScript commands and it will then interpret them.

The VBScript shell will need to:

accept user inputs from the command line
process the input using VBScript
wait for the next user input

Of course the first thing that the script must do is to ensure that the command-based script host (cscript.exe) is running correctly.

Setting up the Command-Based Script Host

VBScript is always run via cscript.exe, and so the first thing that the script must do is to start this running; and to do that the script uses the Windows script host (wscript.exe):

wfullname = wscript.fullname

exe = lcase(mid(wfullname, instrrev(wfullname, "\")+1))

if not exe = "cscript.exe" then

set wshell = createobject("wscript.shell")

wshell.run "cscript.exe """ & wscript.scriptfullname & """"

wscript.quit

end if

With the script hosts in place the script needs to start taking inputs from the user.

Setting up the VBScript Input and Output

The input for the program will be the DOS prompt and the output will be DOS screen:

set input = wscript.stdin

set output = wscript.stdout

Next the user inputs must be handled correctly.

Accepting User Inputs

The process for dealing with the user inputs is quite simple:

display a prompt
read the user input line by line
if the user has asked to exit then do so, otherwise process the VBScript command
go back to displaying the prompt

And the code to do this is:

vbprompt = "$ "

output.write vbprompt

do until input.atendofstream

user_ip = input.readline

if lcase(user_ip) = "exit" then

exit do

else

run_vbscript user_ip

output.write vbprompt

end if

loop

Here the code loops, processing the input, until the word "exit" is entered. In this example another subroutine (called run_vbscript) is required to carry out the VBScript command.

Processing the VBScript Commands

The subroutine for processing the VBScript commands:

accepts the command as text
changes any instances of the word "write" to "output.writeline" (allowing the user to write an output to the screen)

The subroutine uses the executeglobal method to run the VBScript command:

sub run_vbscript (vbcommand)

on error resume next

vbcommand = replace (vbcommand, "write", "output.writeline")

executeglobal vbcommand

end sub

With this small amount of code the programmer is ready to start using the VBScript command line interpreter.

Using the VBScript Command Line Interpreter

The VBScript code should be saved into a file with a .vbs suffix (for example shell.vbs) and then the easiest way to start the shell is to double-click on it in Windows Explorer - this will run the script in a DOS window so that VBScript commands can be written directly at the prompt (remembering to use the write statement to produce an output on the screen). For example:

Microsoft (R) Windows Script Host Version 5.6

$ x = 1

$ write x

$ x = x + 9

$ write x

$ y = array (1,2,3,4)

$ write ubound(y)

The programmer can now test out any VBScript functionality (again remembering to use write to produce an output to the screen).

Using Compound Statements

Compound statements (such as if...then...else and for...next) can be used, but not in the normal way - instead they must be typed in on the same line with a colon separating the statements; for example:

$ for x=1 to 3:write x:next

$ if x<10 then:write "x is less than 10":end if

x is less than 10

And so, the programmer can create a fully functional command line interpreter for VBScript with just a single vbs file.

Summary

In order to use VBScript as a command line interpreter the user must:

call cscript and wscript correctly
accept inputs typed into the DOS window
use the VBScipt executeglobal method to process any commands
use colons to build compound statements

This shell may not be the most advanced in the world, but it will allow the user to easily experiment with VBScript commands.

Mark Alexander Bain - Mark Alexander Bain is a writer, Mo Bro and consultant for all aspects of software development at dsquared. He has also written regularly ...