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Processing Multiple Items with the For Command

You'll often want to write batch files that process "all" of a certain type of file. Command-line programs can deal with filename wildcards: For example, you can type delete *.dat to delete all files whose name ends with .dat. In batch files, you can accomplish this sort of thing with the for loop.

The basic version of the for command scans through a set or list of names and runs a command once for each. The format for batch files is

for %%x in (set of names) do command 

where set of names is a list of words separated by spaces. The for command executes command once for each item it finds in the set. At each iteration, variable x contains the current name, and any occurrences of %%x in the command are replaced by the current value of x. You can choose any alphabetic letter for the variable name. Also, upper- and lowercase matters, meaning a and A are different to the for command.

For example, the command

for %%x in (a b c d) do echo %%x 

prints four lines: a, b, c, and d. What makes for especially useful is that if any item in the set contains the wildcard characters ? or *, for will assume that the item is a filename and will replace the item with any matching filenames. The command

for %%x in (*.tmp *.out *.dbg) do delete %%x 

will delete any occurrences of files ending with .tmp, .out, or .dbg in the current directory. If no such files exist, the command will turn into

for %%x in () do delete %%x 

which is fine...it does nothing. To get the same "silent" result when specifying the wildcards directly in the delete command, you'd have to enter

if exist *.tmp delete *tmp 
if exist *.out delete *.out
if exist *.dbg delete *.dbg

because delete complains if it can't find any files to erase.

As another example, the command

for %%F in ("%ALLUSERSPROFILE%\Documents\My Faxes\Received Faxes\*.tif") do echo %%~nF: received %%~tF 

prints a list of all faxes received from the Windows Fax service and the time they were received.

Several other forms of the for command are covered in Chapter 11, and you should make sure you are familiar with them. The extended for command lets you scan for directories, recurse into subdirectories, and several other useful things that you can't do any other way.

Using Multiple Commands in a For Loop

CMD lets you use multiple command lines after a for loop. This makes the Windows XP for command much more powerful than the old DOS version. In cases where you would have had to call a batch file subroutine in the past, you can now use parentheses to perform complex operations.

For example, this batch file examines a directory full of Windows bitmap (BMP) files and makes sure that there is a corresponding GIF file in another directory; if the GIF file doesn't exist, it uses an image-conversion utility to create one:

@echo off 
setlocal
echo Searching for new .BMP files...

for %%F in (c:\incoming\*.bmp) do (
    rem output file is input file name with extension .GIF
    set outfile=c:\outgoing\%%~nF.gif
    if not exist %outfile% (
         echo ...Creating %outfile%
         imgcnv -gif %%F %outfile%
    )
)

Therefore, every time you run this batch file, it makes sure there is a converted GIF file in the \outgoing folder for every BMP file in the \incoming folder. This sample script uses several of the techniques we've discussed in this chapter:

  • A setlocal statement keeps environment variable changes in the batch file from persisting after the batch is finished.
  • The for loop and if command use parentheses to group several statements.
  • The environment variable outfile is used as a "working" variable.
  • The batch file uses echo statements to let you know what it's doing as it works.

A batch file like this can make short work of maintaining large sets of files. You might accidentally overlook a new file if you were trying to manage something like this manually, but the batch file won't.

As a final example, the following handy batch file tells you what file is actually used when you type a command by name. I call this program which.bat, and when I want to know what program is run by, say, the ping command, I type the following:

which ping 

The batch file searches the current folder, then every folder in the PATH list. In each folder, it looks for a specific file, if you typed a specific extension with the command name, or it tries all the extensions in the PATHEXT list, which contains EXE, COM, BAT, and the other usual suspects:

@rem                                               Example file which.bat 
@echo off

if "%1" == "" (
    echo Usage: which command
    echo Locates the file run when you type 'command'.
    exit /b
)

for %%d in (. %path%) do (
    if "%~x1" == "" (
        rem the user didn't type an extension so use the PATHEXT list
        for %%e in (%pathext%) do (
            if exist %%d\%1%%e (
                echo %%d\%1%%e
                exit /b
            )
        )
    ) else (
        rem the user typed a specific extension, so look only for that
        if exist %%d\%1 (
            echo %%d\%1
            exit /b
        )
    )
)
echo No file for %1 was found

As you can see, the for command lets you write powerful, useful programs that can save you time and prevent errors, and the syntax is cryptic enough to please even a Perl programmer.

Delayed Expansion

Environment variables and command-line arguments marked with % are replaced with their corresponding values when CMD reads each command line. However, when you're writing for loops and compound if statements, this can cause some unexpected results.

For example, you might want to run a secondary batch file repeatedly with several files, with the first file handled differently, like this:

call anotherbatch firstfile.txt FIRST 
call anotherbatch secondfile.txt MORE
call anotherbatch xfiles.txt MORE

You might want to do this so that the first call will create a new output file, while each subsequent call will add on to the existing file.

You might be tempted to automate this process with the for command, using commands like this:

set isfirst=FIRST 
for %%f in (*.txt) do (
    call anotherbatch %%f %isfirst%
    set isfirst=MORE
)

The idea here is that the second argument to anotherbatch will be FIRST for the first file and MORE for all subsequent files. However, this will not work. CMD will replace %isfirst% with its definition MORE when it first encounters the for statement. When CMD has finished processing % signs, the command will look like this:

set isfirst=FIRST 
for %%f in (*.txt) do (
    call anotherbatch %%f FIRST
    set isfirst=MORE
)

Because FIRST is substituted before the for loop starts running, anotherbatch will not see the value of isfirst change, and the result will be

call anotherbatch firstfile.txt FIRST 
call anotherbatch secondfile.txt FIRST
call anotherbatch xfiles.txt FIRST

which is not at all what you wanted.

There is a way to fix this: Delayed expansion lets you specify environment variables with exclamation points rather than percent signs, as an indication that they are to be expanded only when CMD actually intends to really execute the command. Because ! has not traditionally been a special character, this feature is disabled by default. To enable delayed expansion, specify /V:ON on the CMD command line or use SETLOCAL to enable this feature inside the batch file. The statements

setlocal enabledelayedexpansion 
set isfirst=FIRST
for %%f in (*.txt) do (
    call anotherbatch %%f !isfirst!
    set isfirst=MORE
)

will work correctly.

Although delayed expansion is disabled by default in Windows XP, you can change the default setting through the Registry key HKLM\Software\Microsoft\Command Processor\EnableExtensions. If this DWORD value is present and set to 0, command extensions are disabled by default. Any nonzero value for EnableExtensions enables them.

Another place delayed expansion is useful is to collect information into an environment variable in a for list. The following batch file adds c:\mystuff and every folder under it to an environment variable named dirs:

setlocal ENABLEDELAYEDEXPANSION 
set dirs=
for /R c:\mystuff %%d in (.) do set dirs=!dirs!;%%d

The for statement recursively visits every folder, starting in c:\mystuff, and %%d takes on the name of each folder in turn. The set statement adds each directory name to the end of the dirs variable.

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