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Automate tasks with BAT files on FreeDOS | Opensource.com
Automate tasks with BAT files on FreeDOS
Here's a helpful guide to batch files under FreeDOS.
Even if you haven't used DOS before, you are probably aware of its command-line shell, named simply
COMMAND.COM shell has become synonymous with DOS, and so it's no surprise that FreeDOS also implements a similar shell called "FreeCOM"—but named
COMMAND.COM just as on other DOS systems.
But the FreeCOM shell can do more than just provide a command-line prompt where you run commands. If you need to automate tasks on FreeDOS, you can do that using batch files, also called "BAT files" because these scripts use the
Batch files are much simpler than scripts you might write on Linux. That's because when this feature was originally added to DOS, long ago, it was meant as a way for DOS users to "batch up" certain commands. There's not much flexibility for conditional branching, and batch files do not support more advanced features such as arithmetic expansion, separate redirection for standard output vs error messages, background processes, tests, loops, and other scripting structures that are common in Linux scripts.
Here's a helpful guide to batch files under FreeDOS. Remember to reference environment variables by wrapping the variable name with percent signs (
%) such as
%PATH%. However, note that
FOR loops use a slightly different construct for historical reasons.
Your batch file might need to print messages to the user, to let them know what's going on. Use the
ECHO statement to print messages. For example, a batch file might indicate it is done with a task with this statement:
You don't need quotes in the
ECHO statement. The FreeCOM
ECHO statement will not treat quotes in any special way and will print them just like regular text.
Normally, FreeDOS prints out every line in the batch file as it executes them. This is usually not a problem in a very short batch file that only defines a few environment variables for the user. But for longer batch files that do more work, this constant display of the batch lines can become bothersome. To suppress this output, use the
OFF keyword to the
ECHO statement, as:
To resume displaying the batch lines as FreeDOS runs them, use the
ON keyword instead:
Most batch files include an
ECHO OFF statement on the first line, to suppress messages. But the shell will still print
ECHO OFF to the screen as it executes that statement. To hide that message, batch files often use an at sign (
@) in front. This special character at the start of any line in a batch file suppresses printing that line, even if
ECHO is turned on.
When writing any long batch file, most programmers prefer to use comments to remind themselves about what the batch file is meant to do. To enter a comment in a batch file, use the
REM (for remark) keyword. Anything after
REM gets ignored by the FreeCOM shell.
REM This is a comment
Executing a "secondary" batch file
Normally, FreeCOM only runs one batch file at a time. However, you might need to use another batch file to do certain things, such as set environment variables that are common across several batch files.
If you simply call the second batch file from a "running" batch file, FreeCOM switches entirely to that second batch file and stops processing the first one. To instead run the second batch file "inside" the first batch file, you need to tell the FreeCOM shell to call the second batch file with the
Conditional evaluationBatch files do support a simple conditional evaluation structure with the
IFstatement. This has three basic forms:
- Testing the return status of the previous command
- Testing if a variable is equal to a value
- Testing if a file exists
A common use of the
IF statement is to test if a program returned successfully to the operating system. Most programs will return a zero value if they completed normally, or some other value in case of an error. In DOS, this is called the error level and is a special case to the
To test if a program called
MYPROG exited successfully, you actually want to examine if the program returned a "zero" error level. Use the
ERRORLEVEL keyword to test for a specific value, such as:
IF ERRORLEVEL 0 ECHO Success
Testing the error level with
ERRORLEVEL is a clunky way to examine the exit status of a program. A more useful way to examine different possible return codes for a DOS program is with a special variable FreeDOS defines for you, called
ERRORLEVEL. This stores the error level of the most recently executed program. You can then test for different values using the
You can test if a variable is equal to a value using the
== test with the
IF statement. Like some programming languages, you use
== to directly compare two values. Usually, you will reference an environment variable on one side and a value on the other, but you could also compare the values of two variables to see if they are the same. For example, you could rewrite the above
ERRORLEVEL code with this batch file:
IF %ERRORLEVEL%==0 ECHO Success
And another common use of the
IF statement is to test if a file exists, and take action if so. You can test for a file with the
EXIST keyword. For example, to delete a temporary file called
TEMP.DAT, you might use this line in your batch file:
IF EXIST TEMP.DAT DEL TEMP.DAT
With any of the
IF statements, you can use the
NOT keyword to negate a test. To print a message if a file does not exist, you could write:
IF NOT EXIST TEMP.DAT ECHO No file
One way to leverage the
IF test is to jump to an entirely different part of the batch file, depending on the outcome of a previous test. In the simplest case, you might want to skip to the end of the batch file if a key command fails. Or you might want to execute other statements if certain environment variables are not set up correctly.
You can skip around to different parts of a batch file using the
GOTO instruction. This jumps to a specific line, called a label, in the batch file. Note that this is a strict "go-to" jump; batch file execution picks up at the new label.
Let's say a program needed an existing empty file to store temporary data. If the file did not exist, you would need to create a file before running the program. You might add these lines to a batch file, so your program always has a temporary file to work with:
IF EXIST temp.dat GOTO prog
ECHO Creating temp file...
ECHO Running the program...
Of course, this is a very simple example. For this one case, you might instead rewrite the batch file to create the temporary file as part of the
IF NOT EXIST temp.dat TOUCH temp.dat
ECHO Running the program...
What if you need to perform the same task over a set of files? You can iterate over a set of files with the
FOR loop. This is a one-line loop that runs a single command with a different file each time.
FOR loop uses a special syntax for the iteration variable, which is used differently than other DOS environment variables. To loop through a set of text files so you can edit each one, in turn, use this statement in your batch file:
FOR %F IN (*.TXT) DO EDIT %F
Note that the iteration variable is specified with only one percent sign (
%) if you run this loop at the command line, without a batch file:
C:\> FOR %F IN (*.TXT) DO EDIT %F
FreeDOS provides a simple method to evaluate any command-line options the user might have provided when running batch files. FreeDOS parses the command line, and stores the first nine batch file options in the special variables
%2, .. and so on until
%9. Notice that the eleventh option (and beyond) are not directly accessible in this way. (The special variable
%0 stores the name of the batch file.)
If your batch file needs to process more than nine options, you can use the
SHIFT statement to remove the first option and shift every option down by one value. So the second option becomes
%1, and the tenth option becomes
Most batch files need to shift by one value. But if you need to shift by some other increment, you can provide that parameter to the
SHIFT statement, such as:
Here's a simple batch file that demonstrates shifting by one:
ECHO %1 %2 %3 %4 %5 %6 %7 %8 %9
ECHO Shift by one ..
ECHO %1 %2 %3 %4 %5 %6 %7 %8 %9
Executing this batch file with ten arguments shows how the
SHIFT statement reorders the command line options, so the batch file can now access the tenth argument as
C:\SRC>args 1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9
Shift by one ..
2 3 4 5 6 7 8 9 10