The Power of For

It’s the ultimate Windows power tool

Over the past couple months, I've been discussing Chml, a tool I wrote to manipulate a new Windows Vista feature called integrity levels. When I created the tool, unfortunately, I never got around to building in support for wildcards. So, I was a bit frustrated when I recently needed to raise the integrity level of all files with names starting with "s" to the High integrity level. Ideally, I could just type

chml s* -i:h 

to get the job done. Then, I realized I didn't need to build in wildcard support, because a very old and powerful Windows utility—the For command—could provide that functionality it for me.

What For Is For
For is the ultimate Windows power tool. Essentially, For's job is to automatically select a set of files or folders based on a criterion that you specify, then to execute a given command repeatedly—once for each file. For's syntax looks like

for %<variablename> in (<filenamefilter>) do <command>

where filenamefilter tells For which files to select, and command tells For which command to run. In my example, I want to specify a filename filter of (s*), which specifies all files (and folders) whose names start with the letter "s." So, I would type

for %a in (s*) do chml %a -i:h 

In this command, For does the wildcard processing for me by looking in the current folder, seeking out the files whose names start with "s," invoking Chml once for each of those files, then returning to the folder to search for any more matching files. Running this For statement is the exact equivalent of an administrator first figuring out which files have names starting with "s," then typing a Chml statement for that file—except, of course, that it's a lot easier to let For do the work.

The command I originally wanted to run looked like

chml <fill in the filename> -i:h. 

The variablename variable accomplishes the fill in the filename part of that command. As For works its way through the sequence of files that match the filename filter, it needs a place to hold the file. That's what %a is doing in my original example—%a is what Windows refers to a replaceable parameter or variable. It's a place in memory where the For command, after it finds a matching file, can insert that value into the command, replacing %a with the filename.

Thus, if my current directory contains three files—sit .txt, hi.exe, and salt.dat—For would first find the sit.txt file and place it into the %a variable. For would then progress to the command

chml %a -i:h 

and substitute sit.txt for %a, resulting in a command of

chml sit.txt -i:h

which is the exact text of the command that For would then execute. After executing that command, For would find a match in salt.dat (remember that hi.exe wouldn't match the "s*" pattern) and again build a Chml command, this time executing

chml salt.dat -i:h 

For would then find no more matches and would stop.

This most basic of For's formulations will cause For to find file matches in the current folder. You can extend that behavior in two ways. First, adding the /r switch after the For command causes For to search not just the current folder but also any subfolders (and sub-subfolders, and so on) in that folder. For example,

for /r %a in (s*) do chml %a -i:h

Watch for More For
For is one of those little unsung Windows heroes, and even some long-time Windows power users might not be aware of it. I've only scratched the surface of its power, so join me next month for more For.

TAGS: Windows 8
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