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GUIDE TO (mostly) HARMLESS HACKING
Microsoft-only version Number 2-2
Hacking with Win95/NT: Batch File Programming
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by Nezah
We've learned what a batch file is and how to write them.
Now it's time to get all that batch files can bring us.
Lets start.
1.- The IF command
It's easy to find out what it is for. The if command evaluates an
a condition and, in case of true result, it executes a command.
There are three ways to use IF. The different sintax are these:
IF [NOT] EXIST file command
IF [NOT] string1==string2 command
IF [NOT] ERRORLEVEL number command
Where command is the order (only one) you want to execute.
The NOT word is optional, and it makes the condition inverese.
Lets see one by one.
**IF [NOT] EXIST file command.
What it does is to find out if there is or not a file. If the file
exist (or not, if the NOT word is typed) then the command is executed.
Otherwise, the command is ignored.
=========================================================
TIP: Maybe you don't want to verify a file, but a drive or a directory.
In this case, you have to look for the "file" NULL, that is present in
any directory. For example, if you want to verify the c:\nezah
directory, type this:
IF EXIST C:\NEZAH\NULL command
To test if a disquette is in, type this:
IF EXIST A:\NULL command
=========================================================
**IF [NOT] string1==string2 command
Compares the two strings string1 and string2. In case that every
character is equal in both (case sensitive and blank spaces ignored), the
command is executed.
It is useful to play with the parameters (read GTmHH Micro$oft 2).
Note now that, when a parameter call is found (for example, %1), DOS replaces
it for the text of the parameter, no matter where it is. For example, if %1
is Happy, when DOS finds:
00%100
replaces it for:
00Happy00
The same with "%1" ---> "Happy"
" %1 " ---> " Happy "
============================================================================
NOTE: If a parameter does not exist, DOS replaces it with a blank space or
just with nothing (in W95 OSR2 is nothing). To avoid lose control of
the program, is useful to put every string into quotes. So, for example,
if you want to see if a parameter is present, type:
IF "%1"==" " command ---> DOS 6.x
IF "%1"=="" command ---> In W95 is empty string ""
In any other cases, quotes are not needed and strings are compared normally.
=======================================================
**IF [NOT] ERRORLEVEL number command
This evaluates the last errorlevel number present. Errorlevels are
generated by programs to inform about the way they finished their execution.
For example, format returns errorlevel 3 when is Ctrl-C is pressed, and
errorlevel 0 when it finish normally. Not all the programs return errorlevels,
and errorlevels are lost when another program is runned.
This is the most useful feature of IF command. We'll explain it later,
with the CHOICE command.
2.- Labels and GOTO command.
Labels are used to identify a line of the batch file. The way to
put a label inside a .bat progam is simply to put : before the label name.
For example, to create the "example" label, do this:
:example
And that's all.
Labels get useful when you use the order GOTO. GOTO simply goes to
the labeled line you want. For example, to go to the :example label just type:
GOTO :example
That makes the execution of the .bat file continue above that line,
no matter if it was far below the current line or before it.
======================================================
NOTE: The label does not defines a function (like in programming languajes).
It makes the execution continue below the :label line.
When, in normal execution, a :label line is found, it is ignored. The
only use of lables is with the order GOTO.
======================================================
======================================================
TIP: You can create a :end label at the end of the file, so that, whenever
you want to finish the program, you just have to type "GOTO :end".
======================================================
======================================================
TIP: Remember that, in the IF sentence, only was allowed one command.
This sucks, I know. But now, you can create a label and make this
only one command be GOTO :label. Then, in this :label you can have
as many commands as you want, and finish the execution or return to
the program point you wanted with :end or :new_label
======================================================
3.- CHOICE
Now we're ready to get the maximum power from the choice command.
We saw before that errorlevels are some kind'a number that programs returned
and so on. That was not very useful, I know, but for the choice program.
The choice program takes a letter from the keyboard and returns an
errorlevel in consecuence of the key pressed. The sintax is:
CHOICE [text] [/C[:]keys] [/S] [/N] [/T[:]key,secs]
Text is no more than the text to show when choice is runned.
/C:keys Defines the possibles keys to be pressed. If no option is
present, Y/N are the keys. For example:
CHOICE /C:ABCD
Defines A,B,C,D as possible keys.
If you press a not defined key, you'll hear a beep and will
continue as if nothing was pressed.
/S Makes CHOICE case sensitive. By default, Z is equal to z for
choice. With /S flag present, Z and z are different.
/N Choice shows the possible keys into brackets when is called.
With the /N flag present, it does not, so that only the text
you typed (if so) is shown.
/T:key,secs defines a key that is taken as default when secs seconds
are passed. For example:
CHOICE Chose drive /C:AC /T:C,5
Shows the message "Chose drive [A,C]" (without quotes) and,
if no key pressed, passed 5 seconds, choses C.
**Way it works
Now we know how to make the CHOICE sentence. Lets see what happens
when CHOICE runs. It returns an errorlevel number corresponding to the key
position in the /C flag. What???. Well, lets se an example:
CHOICE /C:ABCD
|||L____> D gives errorlevel 5
||L_____> C gives errorlevel 4
|L______> B gives errorlevel 3
L_______> A gives errorlevel 1
Now you see that the errorlevel number depends on nothing but the
position of the key that you gave to CHOICE. That is, when you type
CHOICE /C:2567
Pressing 2, CHOICE will give us errorlevel 1, 5 will give
errorlevel 2, 6 errorlevel 3 and 7 errorlevel 4.
Lets see now what to do with errorlevel. If you remember the IF
section, there was an:
IF [NOT] ERRORLEVEL number command
This evaluates the current errorlevel number. If condition is true
the command is executed.
===================================================
IMPORTANT NOTE: The evaluation of an errorlevel is true when the current
errorlevel is equal OR HIGHER than the number compared. That means
that, in:
IF ERRORLEVEL 3 GOTO :label
The condition is true for errorlevel 3, 4, 5... and every errorlevel
equal or greater than 3.
===================================================
To clarify this, read the next example.
@ECHO OFF
ECHO.
ECHO 1.- Runs Windoze 3.11
ECHO 2.- Runs Dosshell
ECHO 3.- Runs Quake
ECHO X.- Exit program
ECHO.
CHOICE "Choose your option " /C:123x /N
IF ERRORLEVEL 4 GOTO end
IF ERRORLEVEL 3 c:\quake\quake -listen 16
IF ERRORLEVEL 2 dosshell
IF ERRORLEVEL 1 win
:end
This is a complete batch program. Lets analyze it.
First line turns ECHO off: the command lines inside the program will
not be shown.
Second line prints an empty line (like pressing RETURN).
Lines 3 to 7 prints the messages for the program.
Line 8 is choice. This will show you only the text (because of the
/N flag) and will only allow you to press 1,2,3,4 or x key. It is case
unsensitive (no /S flag given). Note that the text is into quotes. This is
to make CHOICE respect the blank space at the end. Quotes will not be shown,
and are not required. If you unuse quotes, CHOICE will not print the blank
spaces of the begining or the end of the text.
Lines 9 to 12 evaluates the errorlevel. Note that:
Only numbers are evaluated. No X letter is writen.
It is in decreasing order. That is because the evaluation is
true if the current errorlevel is equal or higher. So
if I start with errorlevel 1, and errorlevel 4 (an X) is pressed,
I will execute the command anyway.
The execution of the batch file continues after the program
you called is runned. So, if the program returns errorlevels, you may get an error.
Commands CLS,CD and DIR doesn't reset the current errorlevel, and
other DOS commands returns their own errorlevels. So, when you call a DOS order,
is better to make a GOTO anyway, to avoid stupid errors.
In the first IF ERRORLEVEL... the goto order does not have
: before the label name. This is because they are not required in the goto call.
Line 13 is the label end. When you call it, the programs finish.
4.- The command FOR
Now we'll se how to make a batch file smaller. The for order is not
very useful, but sometimes is exactly what you need. So lets see it.
The FOR command makes a "variable" change it's value between the
posibilities you gave it, and executes a command every time it changes.
The sintax is that:
FOR %%A IN (list of values) DO command
Here %%A is the name of the variable. (list of values) is just
the list of values (easy, uh?) between which will change the variable %%A.
The different values are separed by a blank space, and are only considered
as strings.
=======================================================
Programes NOTE: The variables are not true variables. They are only valid
in the FOR command, and will lose any meaning after FOR is finished.
=======================================================
Lets see an example to explain it:
FOR %%B IN (Hello Happy Hacker) DO ECHO %%B
This will make the variable %%B change it's value. First time will
be Hello. Second time Happy, and third Hacker. Every time the %%B value is
changed, the command after the DO word (ECHO %%B in our example) is executed.
So this line in a batch file will print in our screns:
Hello Happy Hacker
There is something more to say about FOR. I have never tried it, but
I read that: when you give FOR a wildcard, it will take care to substitute it
for every existing filename that fits the wildcard. That is that, if you have
FOR %%A in (AUTO*.BAT) DO copy %%A c:\backup
the copy command will recive every time complete file names and no wildcards.
This will execute, for example:
copy AUTOEXEC.BAT c:\backup
copy AUTODOS.BAT c:\backup
copy AUTODOS7.BAT c:\backup
And not something like:
copy AUTO*.BAT c:\backup.
This may be useful with programs with no wildcards alowed, like the
expand command, that expand a compressed file.
5.- Other orders.
**REM and the commets.
To comment a complete line, you should use REM at the begining of it.
You can also use : before tha line. But this will make the line a label, I
conseder this quite rough. It's up to you.
========================================================
Newbie note: A comment is something ignored in the execution of the program.
Every programming language has at least one way to make comments.
This is extremely useful to make your program easier to read to other
people and to yourself in future.
========================================================
**CALL
When you call an .exe of a .com or a DOS command inside a batch file,
after the execution of this program, the .bat file will continue at the point
it was before. But this is not this way when you call a .bat file. I you run
another .bat file inside another, the second one will take control and won't
return it after is finished. To run a .bat file inside another and after
continue the program that called the second, we have to use the order CALL.
That's what CALL does: execute a .bat file and return to ours after it is
finished.
**PAUSE
PAUSE is used to stop the execution till a key is pressed. It gives
a message ("Pulse una tecla para continuar" in Spain), and there is no way
to change it. I you want to insert you own message, the best way is to
redirect the text of pause to NULL (NULL is some kind'a black hole). This
will make pause with no message, and you can now insert yours:
ECHO This is my message. Press a key.
PAUSE > NULL
Another way to do this is with the CHOICE order, but this limits
the valid keys to be pressed.
**SHIFT
If you notice, you can only use 9 parameters: from %1 to %9. In most
of cases this it more than enough. But, if you need to use more than 9
parameters in your program, you can use the order SHIFT. SHIFT makes the
parameters lose their current value and will give them the value of the next
numbered parameters. Quite confusing, I know. Lets see it.
If you give the order SHIFT, %1 lose it's value and, from now, has
second parameter as content. The same with %2, that now refers to the third
parameter... and so on till the %9 parameter. Now, the %9 values the 10th
parameter, that was unaccesible before.
You can execute SHIFT as many times as you want. But note that the
maximun DOS order width is 255 characters, so you don't make a program that
needs 50 parameters because you won't be able to type all of them.
Well, that's all I know about batch files. I hope you've enjoyed it
and excuse my spelling, my grammar... my English in general.
Nezah
nezah@zoom.es
http://www.zoom.es/~nezah/
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GUIDE TO (mostly) HARMLESS HACKING on your Web site as long as you leave
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___________________________________________________________
Carolyn Meinel
M/B Research -- The Technology Brokers
http://techbroker.com
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