Guy's Scripting Ezine 14 - Do...Loop Until

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Contents of Guy's Scripting Ezine No 14 - Do...Loop Until

• This week's secret
• Do... Loop - Introducing the Family
• Example  - Simple Do...Loop Until
• Example  - To add a 'Description' to a users property
• LDIFDE Example - A different method to change 'Description'
• Summary

1) This week's secret

Many VBScripts need an instruction that repeats the same sequence for multiple objects.  For example, creating hundreds of computer accounts.  The Do...Loop is my secret for telling the script to keep cycling through the same lines, until it reaches the end of the list.  Perhaps you have already used 'For...Next' loops in other programming languages?  In which case you will already have a good idea of what Do...Loop can achieve.

2) Do...Loop - Introducing the Family

I would like to introduce you to the family of Do ...Loop commands

        Do....Loop Until and its 'brother', Do....Until Loop

        Do....Loop While and its 'sister',  Do....While Loop

The job of this family is to cycle through repetitive tasks like adding users from rows in spreadsheets or reading through hundreds of objects in Active Directory.

This Do...Loop family operates by going through line after line of commands, then circling back to the beginning and repeating commands for the next item. 

The secret of avoiding endless loops is having an effective command to stop.  To halt the loop, you have a choice of two prepositions: Until or While.  Both 'Loop Until' and 'Loop While' achieve the same goal - breaking the cycle.  It is just that their attitude is different;  'Do...Loop Until', is adventurous, it keeps on going until stopped.  On the other hand, 'Do...Loop While' is cautious, it only goes through the loop if the condition is met.

Do...Loop Examples

Another trade secret.  I do not have the perfect Do...Loop example for Windows 2003.  Instead I have two VBScripts, the first is ridiculously easy, but doesn't do anything useful.  The second example is powerful and interesting, but it is technically demanding.

3) Example - Simple Do...Loop Until

Copy the VBScript below, then paste into notepad and save as a .vbs.  Double click your .vbs file and see a message with the answer to the question: What is the sum of 1 to 7?

Learning Points

Note 1:  inti is the counter; while intx is keeping the score.

Note 2:  With each loop, the script adds another number to the sequence 1+2+3+4+5+6+7

Note 3:  Beware, as with all Loop scripts, they must have a break statement.  Loop until inti = 7 is our command to break the loop.

Note 4:  In the WScript.Echo statement I have added the two variables inti and intx.  My reasoning is this, if you do change Line 13: Loop Until inti = 7 to Loop Until inti = 10, then you still get a meaningful message.

4) Example  - To add a 'Description' to a users property

Instructions. 

1. Warning this script will alter the users description.
2. As usual, copy and paste into notepad and save as a .vbs.  You will need a domain to get this script to work. 
3. You must change line 21 for the VBScript to work in your domain.  Find then alter:  ou=Worcester, dc=cp,dc=com.  I also suggest that you create an OU and test user as necessary.
4. Go to Active Directory Users and Computers, User, Property, then you will see what changes your script has made.

Learning Points

Note 1:  Sorry for repeating the warning, but this script will alter your users properties.  However, I have chosen a relatively harmless property = description.

Note 2:  By introducing the variable strChange it makes it a little easier for you to edit the actual comment; it also makes it a lot easier to display what the script did in the message box.

Note 3:  Here is my challenge to you; carefully select another User Property, then change the strProperty.  Example strProperty =  "Office"

5)Use LDIFDE to achieve the same goal - change a user's 'Description' property

Method use:  changetype: modify

Even if you cannot get the 'unicodePwd' to work, you can at least master the changetype: modify.  Another bonus of doing this exercise is that you will learn the strange syntax of the - (dash).

1. Assumption: You have a user called Guy Thomas in the Worcester OU.

2. Modification: Change DC=cp to reflect yourdomain.

3. Note well :  Syntax is crucial.  Observe carefully the single -  (dash) on line 5

4. Amendment:  Change the description below, then use notepad and save to a file: for example guydes.ldp
    

5. dn: CN=Guy Thomas, OU=Worcester, DC=cp, DC=com
   changetype: modify
   replace: description
   description: Supremo
   -

    

   Table 1

6. At the command prompt type
   LDIFDE -i -f  guydes.ldf -k

7. Check the properties of Guy Thomas in Active Directory Users and Computers

Troubleshooting LDIFDE

Note if there is no - (Dash) on the last line, this is the error you get.

There is a syntax error in the input file
Failed on line 4. The last token starts with 'P'.
The change-modify entry is missing the terminator '-'.
An error has occurred in the program
 

The disadvantage of LDIFDE is that it does not 'feed off' spreadsheets, and I find the format a bit clunky. See more about LDIFDE here.

Summary

All good scripter need a 'Looping' command in their repertoire.  Here we investigate a simple loop to cut our teeth, then a more useful example to change attributes in Active Directory. 

Guy Thomas recommends Computer Training Software. 

Their topics and material are ideal for getting you started with VBScript.  The videos are easy to follow and you can control the pace.  Try their free demo material and then see if you want to buy the full package. See more about VB Script Training CD.

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