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Recursion

Microsoft® Windows® 2000 Scripting Guide

Recursion is a programming technique in which a subroutine or a function calls itself. You have probably seen photographs of a person staring into a mirror. In turn, his or her reflection is staring into a mirror located in the background, which reflects the person in the mirror staring into the mirror, and so on. This is the basic idea behind recursion: Subroutine A calls subroutine A, which calls subroutine A, which calls subroutine A, and so on.

Although the concept of recursion might seem a bit bizarre, it actually has important uses in system administration scripting. For example, suppose you want to enumerate all the files in a shared folder. You can easily connect to the shared root folder and list all the files, but what happens if the shared root folder has subfolders? And what happens if those subfolders contain other subfolders? Recursion allows you to enumerate all items and subitems, even if you have no advance knowledge of the folder structure.

For example, the folder structure shown in Figure 2.28 shows a root folder, Scripts, and main subfolders, Subfolder 1 and Subfolder 2. Each of those subfolders contains other subfolders.

Figure 2.28 Sample Folder Structure

sas_fil_03c

To enumerate all the folders shown in this figure, you will have to start with the Scripts folder, return a list of subfolders, bind to each subfolder, return a list of subfolders within those folders, and so on.

To perform this task, you can use a script similar to the following. This script uses a function named ShowSubFolders that is called over and over until every folder and subfolder has been enumerated.



Set FSO = CreateObject("Scripting.FileSystemObject")
ShowSubfolders FSO.GetFolder("C:\Scripts")
Sub ShowSubFolders(Folder)
    For Each Subfolder in Folder.SubFolders
        Wscript.Echo Subfolder.Path
        ShowSubFolders Subfolder
    Next
End Sub

The function ShowSubFolders does the following:

  1. Retrieves a collection consisting of all the subfolders of the root folder, C:\Scripts. This collection has two items: Subfolder1 and Subfolder 2.

  2. Takes the first item in the collection, Subfolder 1, and echoes the folder path. It then uses the name of that folder as a parameter passed to itself. In other words, it now runs the function ShowSubFolders using Subfolder 1 as the parameter.

  3. Retrieves a collection consisting of all the subfolders of Subfolder 1. This collection has two items: Subfolder1A and Subfolder 1B.

  4. Takes the first item in the collection, Subfolder 1A, and echoes the folder path. It then uses the name of that folder as a parameter passed to itself. In other words, it now runs the function ShowSubFolders using Subfolder 1A as the parameter.

  5. Because Subfolder 1A has no subfolders, control passes to the next item in the collection, Subfolder 1B. The function calls itself using Subfolder 1B as the parameter.

  6. Because Subfolder 1B has no subfolders, the function has finishes recursing through Subfolder 1. It thus returns to the second item in the original collection (subfolders of C:\Scripts), and repeats the entire process.

When the script runs, the following output appears:



C:\scripts\Subfolder 1
C:\scripts\Subfolder 1\Subfolder 1A
C:\scripts\Subfolder 1\Subfolder 1B
C:\scripts\Subfolder 2
C:\scripts\Subfolder 2\Subfolder 2A
C:\scripts\Subfolder 2\Subfolder 2B
C:\scripts\Subfolder 2\Subfolder 2C

Recursion is an extremely powerful technique for exploring data stored in a tree structure, including Active Directory as well as the file system.

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