hpr-knowledge-base/hpr_transcripts/hpr3740.txt

Episode: 3740
Title: HPR3740: Batch File Variables; Nested Batch Files
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr3740/hpr3740.mp3
Transcribed: 2025-10-25 04:49:23

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This is Hacker Public Radio Episode 3,740 for Friday 2 December 2022.
Today's show is entitled, Batch File Variables Nested Batch Files.
It is part of the series' DOS.
It is hosted by AOKA, and is about 12 minutes long.
It carries a clean flag.
The summary is, more on DOS.
This time, it is using variables in Batch Files and Nesting Batch Files.
Hello, this is AOKA, welcoming you to Hacker Public Radio and another exciting episode
in our DOS series.
We are now coming to the end of our discussion of Batch Files, and we're also coming to
the end of this little series on DOS.
I wrote these 25, 30 years ago, and DOS was used a lot more than, but I hope that still
of some use, the feedback I've been getting is that a lot of people are enjoying these
particular episodes.
But you know, everything has to come to an end at some point.
Well, what I want to talk about today in completing our Batch File look is two things.
First of all, are using variables in Batch Files and Nested Batch Files.
So let's start with the variables.
In DOS Batch Files, you can create and use variables.
The variables are signified by the use of the percentage symbol followed by a number.
For instance, here is a Batch File, which I will call test.bat.
First line is at sign, echo, space, off.
Second line is dir, space, percent, number one.
That's a very simple Batch File, but here's how you can use it.
At the command prompt, type, and now I'm going to assume on all of this you're working
in your DOS temp, temporary directory, and use that as you're working directory.
So at the prompt, type test.bat, space, c, colon, backslash.
So if you've done this correctly, you should see a complete directory listing of the root
directory of c, colon, which is generally your hard drive.
What the Batch File is doing is taking the command, dir, and applying it to whatever directory
you specify.
Now if you neglect to specify a directory, you'll get a listing of the current working directory.
And again, I'm assuming right now that the current working directory is c, colon, backslash,
DOS temp, backslash.
So you would get a listing of that just as if you had issued a dir command there without
specifying the target directory.
If you specify a directory on the command line, that directory is variable number one,
or percent one, as it is signified.
Now suppose you would issued the command at the prompt test.bat, space, c, colon, backslash,
space, c, colon, backslash, windows, backslash.
Now if you try this, you'll see that you will still get the listing of the root directory
of c, but that's all.
After all, your Batch File is only looking for one variable, and besides the dir command
can only take one directory.
Also, you don't need to specify the extension of the Batch File.
I've been doing it as test.bat in all of these examples when you're typing the command.
You don't really need to do that unless you're unlucky enough to have picked a name for
the Batch File that matches one of the DOS external commands or something similar.
When DOS executes commands, it goes in a specified order.
First it looks for DOS key macros, then internal commands, then external commands with the .com
extension.
So we had .com before there was .com.
External commands with the .exe extension, and then finally Batch Files.
So as long as you've never created a DOS key macro called test, you're safe just using
the file name test by itself since there is no DOS command, internal, or external called
test.
Anyway, back to the story here.
Let's rewrite our test.bat file to use a command that takes two arguments so we can
see how this works.
So first we'll create a simple text file called test1.txt using the edit application, just
for practice doing it.
Put a sentence of some kind inside, for instance, now is the time for all good men to come
to the aid of their party, and save that file in your DOS temp working directory.
Then use edit to change your test.bat file.
So now you've got test1.txt that has the sentence in it, now we're going to modify the
Batch file test.bat.
Now it's going to say, at echo space off on the first line, then copy space, percent
one space, percent two, that'll be your second line, and a third line, DIR.
Save it, and then execute the command, test space, test1.txt, space, test2.txt.
If you've done this correctly, you should see a directory listing of your DOS temp working
directory, and you should see three files listed, your test.bat.bat.file, your test1.txt
text file, and a copy of that text file called test2.txt, which was created by your batch
file.
Now let's take a look at nested batch files.
What I mean by nested is that one batch file can be called and run inside of another
batch file.
So let's examine how this works.
We'll begin with a simple pair of batch files, which we'll create in our DOS temp working
directory.
The first file, which we will call nbatch1.bat, will be as follows.
Next line, at sign echo space off, second line, echo space A, space 1, third line, call
space nbatch2.bat, fourth line, echo.
And space A, space 3, exclamation mark space, game space over exclamation mark.
Now the second batch file, and you've already guessed that it's going to be called nbatch2.bat,
because we had just put a call statement in the other one.
Now nbatch2 will have the following contents.
First line, at sign echo space off, second line, echo space, and space A, space 2.
So create both of those using the edit program and save them in your DOS temp working directory.
Now if you then run nbatch1.bat, you'll see something like this.
At the prompt, you'll type nbatch1.bat hit enter, and you will see on the next line, A, space
1, and then line under it, and space A, space 2, and then line under that, and space A, space
3, exclamation mark, space, game space over exclamation mark, and then you're prompt.
So running it is going to print out a 1, and a 2, and a 3 game over.
So what happens here is that the nbatch1.bat file first printed on the screen via the echo command,
the phrase a 1, and then it called the batch file nbatch2.bat, which in turn echoed to
the screen, the phrase, and the 2, and then terminated, and returned control to nbatch1,
which then echoed the final phrase, well phrases really, and a 3 game over, and terminated
in turn, leaving you back at the prompt.
The command call is used in the first batch file to call the second batch file.
Now you might wonder why this is needed, since you never need to use it for any other
command, and batch files are just a collection of commands.
Well, if nbatch2.bat was the last thing listed in the first batch file nbatch1, you could
probably get away without using the call command, but to see what difference it makes, edit
your nbatch1.bat file and remove the call part.
So that your file now reads echo space a space1, next line nbatch2.bat, third line, echo space
and space a space, three exclamation mark space, game space, over exclamation mark.
So save this edited version, and now run nbatch1.
What do you get?
A 1 and a 2.
Aha, the second batch file terminated after it had finished running, but it did not return
to the first batch file to let it finish running.
Using the call command assures that control will always be passed back to the original
batch file.
So you should always make it a practice to use the call command whenever you are nesting
batch files, even if you're sometimes chaining one file after the other.
This comes under the heading of what we might call a best practice.
And so with that, this is a hookah for hacker public radio, winding up our look at DOS,
and encouraging everyone as always to support free software.
Bye bye.
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