The Final For

Use the original Windows power tool to work with lists of files

I’ve been talking about the powerful For tool over the past couple months. For can inject power into any command-line utility, letting it transform a command that normally operates on a single file into one that can work on many files. In the first For column, “The Power of For” (InstantDoc ID 96539), I discussed how For can turn a command loose on an entire folder’s worth of files, and in the second column, “Counting on For” (InstantDoc ID 96704), I showed you how For’s /l option lets you instruct a command to run any number of times.

This month, I show you how For’s /f option lets you tell Windows to apply a single command to a specific list of files.

For for Files
This For functionality came to mind a few weeks ago while I was reviewing the results of a photo shoot. I’d been snapping dozens of close-up photos of a Snowy Egret from a photographer’s blind. I had some good, detailed shots, but most didn’t amount to much. I wanted to burn all the photos to a DVD but keep the most useful ones on my computer’s hard disk.

You’d think separating out a few pictures would be simple—say, by browsing the pictures in one window while dragging the good ones to another folder. But I needed to devote a lot of screen real estate to the image browser and didn’t have enough screen space to hold a couple of Windows Explorer windows on top of that. But I did have enough space for a little Notepad window, in which I could type the names of the files I wanted to keep, leaving me lots of room for the image browser.

I had a folder full of files called C:\newpics, and I had created a text file named keepers.txt that listed the photos I wanted to copy to a folder called C:\goodpics. I wanted to extract each line in keepers.txt and use it as a filename to copy to C:\goodpics. How could I use the Windows command line to accomplish that goal?

I was sure the answer lay in the For command, so I dived into For’s online Help, which reminded me of the tool’s /f option. Here’s the command I came up with:

for /f %i in (C:\newpics\keepers.txt) do copy C:\newpics\%i C:\goodpics

To understand this command, look at the simplified For /f syntax:

for /f <variable> in (<name of file listing the desired objects>) do <an operation involving the variable>

For example, to tell For /f to simply display the files that it will copy, I could type

for /f %i in (C:\newpics\keepers.txt) do echo %i

For /f works its way through keepers.txt by taking one line at a time and putting the contents of that line in a variable, a place in memory that I’ve called “%i.” (Any name works, as long as it’s prefixed with a percent sign.) Then, For /f performs whatever action you’ve typed to the right of do, replacing the two letters “%i” with the actual value that For has most recently extracted from keepers.txt.

Thus, if I type pic1.cr2 on the first line, and pic7.cr2 on the second and final line, For /f would first execute echo pic1.cr2, which would cause Windows to just print pic1. cr2 on the command window, then print pic7.cr2 on that window and stop.

That’s not all For /f can do. Instead of putting a file in the parentheses, you can put a command in there, surrounded by single quotes. For will then execute the command and use each line of the command’s output as a line of text to operate on, just as it operated on the lines in keepers.txt.

For can also accept more than one file as input in the parentheses, as in a variation on the first example, featuring both the file keepers.txt and another named keepers2. txt:

for /f %i in (C:\newpics\keepers.txt C:\newpics\keepers2.txt) do copy C:\newpics\%i C:\goodpics

More to For
There’s more to For, of course. But I think these three visits with “the original Windows power tool” should give you a pretty good starting point toward your own For experimentation. If you learn only one new command-line tool this year, make it For!

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