Environment variables contain information about the environment for the system and for the user who’s currently logged on. Knowing how to use these variables can save you time and keystrokes in batch files and shortcuts and at the command line. In addition, some programs use environment variables to determine where to store user data and temporary files. To use an environment variable in a command or a dialog box, you surround the variable name with percent signs; this tells Windows to look up the current value for that variable in the registry and use that value in the command or path where you entered it.

Two environment variables that I use regularly are %appdata% and %userprofile%. By typing either of these strings in the Run dialog box and pressing Enter, I can jump straight to the Application Data folder or the root of the User Profile folder for the currently logged-on user. I don’t need to know what drive Windows is installed on, and I don’t need to know the name of the current user. Similarly, typing cd %allusersprofile% at a command prompt switches to the All Users profile folder, where shared documents and settings are stored. In a default installation, the full path name is nearly twice as long: C:\Documents and Settings\All Users. You can also use environment variables to create shared shortcuts that work differently for each user. A shortcut that points to the %UserProfile%\My Documents folder, for instance, will always open the My Documents folder of the user who’s currently logged on.

Windows XP creates a group of default system variables, including several that define the location of Windows system files. Only an administrator can change a system variable. However, any user can add, edit, or remove user variables, which are exclusive to the user who created them and can be used in scripts and batch files.

To view and edit all variables, open the System option in Control Panel, and click the Environment Variables button on the Advanced tab. Click the New button to add a variable name and value, click the Edit button to change an existing variable, or click the Delete button to remove a variable.

You can also see a list of all currently defined environment variables by opening a Command Prompt window and entering the set command.

Yet another Web site posted yet another “tip” today recommending that you clean out your Prefetch folder to improve performance of Windows. Arrrggghhh! I’ve written about this repeatedly (here and here and here, for instance), but the message doesn’t seem to be spreading very fast. Maybe this quote from “Misinformation and the Prefetch Flag” by Ryan Myers, a developer on Microsoft’s Windows Client Performance Team, will help:

XP systems have a Prefetch directory underneath the windows root directory, full of .pf files — these are lists of pages to load. The file names are generated from hashing the EXE to load — whenever you load the EXE, we hash, see if there’s a matching (exename)-(hash).pf file in the prefetch directory, and if so we load those pages. (If it doesn’t exist, we track what pages it loads, create that file, and pick a handful of them to save to it.) So, first off, it is a bad idea to periodically clean out that folder as some tech sites suggest. For one thing, XP will just re-create that data anyways; secondly, it trims the files anyways if there’s ever more than 128 of them so that it doesn’t needlessly consume space. So not only is deleting the directory totally unnecessary, but you’re also putting a temporary dent in your PC’s performance. [emphasis in original]

Bottom line: You will not improve Windows performance by cleaning out the Prefetch folder. You will, in fact, degrade Windows performance by cleaning out the Prefetch folder. I’ve done performance testing that establishes this definitively. In all the many sites that offer this bogus tip, I have yet to see a single piece of actual performance testing.

Oh, and for anyone who cites this TechRepublic article as a source, let me just say that it contains more serious factual errors than I can count. For instance:

As you boot your workstation or access programs on your workstation, XP’s prefetcher copies portions of those files to the Prefetch area of your hard drive.

That’s completely wrong. The files in the Prefetch folder contain lists of pages that that should be loaded when a program starts. Each file is essentially an index. Windows XP doesn’t copy portions of any files to the Prefetch folder.

When your workstation boots, XP prefetches portions of the files you use most frequently and has any application you’ve recently run waiting and ready to go.

This is equally absurd. If this were true, it would mean that Windows was actually loading into memory every program you’ve ever used, every time you start Windows. That’s not the way it works at all. When your PC starts up, Windows looks in the Prefetch folder to determine how best to load Windows. It doesn’t do a thing with the .pf files for applications (unless, of course, you’ve configured one of those apps to start up with Windows).

If you’re frequently using the same few applications over and over again, prefetching can greatly increase the apparent speed of a system. Rather than waiting for you to click an icon to start a program, and then loading all of the associated files, libraries, and pointers necessary to run the program, XP has all the components of your programs preloaded. When you click an icon to start the program, most of the hard work is already done.

The author just made this up. The .pf files don’t get used at all until you run a program. What actually happens when you click an icon is that Windows uses the information in the Prefetch folder to decide which program segments to load and in what order to load those pages. There’s plenty of documentation for this, including Ryan Myers’ article and this definitive article by Mark Russinovitch and David Solomon, Windows XP Kernel Improvements Create a More Robust, Powerful, and Scalable OS.

The drawback to prefetching is that XP will prefetch a program even if you use it only once or twice. XP will retain a copy of a portion of it in the Prefetch folder. From there, it will prefetch the program, taking resources from your workstation even though you may have no intention of ever using the program again.

Again, the author just pulled this out of who-knows-where. When you run a program, Windows creates a .pf file for it in the Prefetch folder. When you run the program again, Windows looks for this .pf file and uses it to determine how to load the program. The hash doesn’t contain any portion of the original program code. If you never run the program again, that .pf file never gets used, and in fact it gets deleted eventually.

I used to write for TechRepublic. I’ve tried to contact someone there to get them to correct this silly article but have yet to receive a response. It would be really, really great if some of the other sites that have propagated this urban legend would also correct it.

In yesterday’s tip, I explained how to move the My Documents folder. To move the default folders that Windows XP uses for music, pictures, or video, you use a slightly different technique: Select the folder you want to move (My Music, My Pictures, My Video), hold down the right mouse button, and drag it to its new location. When you release the mouse button, choose Move Here. Windows XP automatically updates all references to the folder, including those in the Start menu.

This tip is especially useful if you have a large music or video collection. By moving the My Music or My Videos folder (or both) out of My Documents, you greatly reduce the size of your documents folder and make the task of backing it up faster and easier. A separate drive is the ideal location for bulky music and video files.

The My Documents folder is the default location for your data files. If disk space is at a premium, you might choose to relocate this folder. For instance, if you add a second hard drive to your system, you might decide to use it exclusively for document storage. Here’s how to relocate all your data files to the new drive.

In Windows XP, click the Start button and then right-click the shortcut for My Documents. Click Properties. On the Target tab, click Move and choose the new location for the folder. (If necessary, you can create a new folder using the Browse dialog box.)

When you click OK, your document files are moved to the new location. Clicking the My Documents icon on the Start menu or in a common dialog box now takes you to this folder instead of the original location.

One caution: Be aware that some programs (notably Microsoft Outlook and Outlook Express) continue to store data in the hidden Application Data folders within your user profile. When you move your My Documents folder, these files remain in their original location. Make sure your backup strategy includes all your data files, not just those in the My Documents folder.

Marc Orchant passes along a tip from PC Magazine that explains how to edit the registry to add “Send” and “Copy” commands to Windows Explorer and concludes with this comment:

After performing this tweak, you’ll have two new context menu commands: “Move to Folder” and “Copy to Folder”. Apparently this stuff is built into Windows but hidden for some mysterious reason.

Mysterious perhaps, but the reason is explained by no less an authority than Raymond Chen, the author of the original power toy for Windows, Tweak UI, in his February 2004 blog post, “What goes wrong when you add “Copy To” to the context menu”:

The “Copy to Folder” and “Move to Folder” options weren’t designed to be on the context menu. They were only meant to be placed in Explorer’s toolbar. (Right-click a blank space on your toolbar, select Customize, and pick “Move To” or “Copy To” from the list of available buttons.) If you add them to the context menu, you may notice that the “Copy To” and “Move To” dialogs start showing up when you really aren’t expecting them, for example, whenever you double-click an attachment in Outlook.

If you do the customization that Raymond suggests, you end up with toolbar buttons that look like this:

There are other tweaks widely publicized on various tip sites that have unpleasant side-effects as well. In this case, the effects are only annoying. If you want to add the context menus to Explorer, nothing bad will happen, but be prepared for an occasional annoyance!

(Oh, and Marc, how come I’m not in your blogroll?!)