Batch files are text files with a .BAT extension. You may create batch
files with NotePad or any other other program that edits plain text. Note
that while many word processing programs display plain text on the screen,
the files they save may not be plain text.
Writing Batch files is really high level programming. With the right batch programs you can even write entire applications with batch files.
Batch files may be executed at a command prompt just like a program by
typing the name of the batch file, or you may set up a Windows shortcut with
the name of the batch file on the command line.
In their simplest form, batch files may be used to execute more than one
program in sequence.
Environment Variables
Batch files may access variables in the environment. The environment is
a block of memory that programs and batch files use to store variable
values. The SET command may be used to assign a value to a variable
name, like this:
SET MyVariable=MyValue
After a variable is assigned it may be used (dereferenced) in a batch
by surrounding the variable name with percent signs, like this:
ECHO %MyVariable%
References to variables are case insensitive, so %MyVariable% is
the same as %myvariable%. When an instance of the command interpreter
(command prompt) or any other program is opened, a copy of the global
environment is made for the program. Changes made to this copy of the
environment do not change the global environment. The SETLOCAL and
ENDLOCAL commands may be used within a batch file to localize
environment variables. SETLOCAL may also be used to temporarily
enable Command Extensions (type SETLOCAL /? for details). Variables
assigned after SETLOCAL are local to the batch; ENDLOCAL restores
the environment to what it was before the SETLOCAL command. Windows
assigns a number of variables to the global environment that are
inherited by all processes. These variables include the USERNAME,
USERPROFILE path, and windir (the Windows directory), allowing the
construction of batch files that can run by different users or on
different workstations. Type SET at a command prompt to display all
the environment variables.
The SET command may also be used to check for environment variables
that begin with a sequence of 1 or more letters, calculate a value,
or extract a substring of an environment variable.
(type SET /? for details).
Substring examples:
SET V=1234567890
SET A=%V:~3,2%
SET B=%V:~2%
SET C=%V:~-2%
SET D=%V:~0,-4%
ECHO %A%
ECHO %B%
ECHO %C%
ECHO %D%
45
34567890
90
12
123456
(note: A and B work for Windows NT 4.0 and up, C and D work for Windows 2000
and up).
IF and FOR may be used to execute multiple lines in a batch like this:
FOR %%I IN (*.*) DO (
SET A=%%I
SET B=%A:~0,1%
ECHO %B%
)
The problem with the statement above is that ECHO %B% will only echo the
value of B at the time the entire statement is executed because %B% is
expanded just once when the statement is executed.
On Windows 2000 and up, this may be used:
SETLOCAL ENABLEDELAYEDEXPANSION
FOR %%I IN (*.*) DO (
SET A=%%I
SET B=!A:~0,1!
ECHO !B!
)
SETLOCAL DISABLEDELAYEDEXPANSION
(note the exclamation point is used to immediately dereference variables,
any variable deferenced with % is dereferenced once before the statement
is executed).
On Windows 2000 and up, SET /P may be used to set an environment variable
using the contents of a file:
SET /P V=
(note: only the first line in myfile.txt is used to set the value of the
variable. Piping the output of a command to SET does not work).
Batches may also CALL other batches:
PROG1
CALL BAT2.BAT
PROG2
After PROG1 runs, BAT2 is run. When BAT2 finishes, the calling batch
resumes with PROG2. The called batch shares the same environment with
the calling batch. So, changes to the environment made by the called
batch are seen by the calling batch.
If Command Extensions are enabled, CALL may also be used to run
subroutines within a batch file. GOTO :EOF resumes execution on the
line following the CALL to the subroutine. When used outside the
context of a CALL, GOTO :EOF may be used to exit a batch.
Normally, when a command line program in a batch file is executed,
the batch waits for the program to end before continuing with the
next command in the batch (the program is run synchronously).
However, if the program is a Windows GUI (Graphical User Interface)
program, the batch may not wait for the program to end (the program
is run asynchronously). This behavior may vary with the version of the
operating system and the configuration. The START command may be
used to force whether a program is run synchronously or
asynchronously. To force a batch to wait for a GUI program to close
before continuing, use START with the wait option (START /W).
To force a batch to continue without waiting for the program to
close, use START without the wait option.
The working directory when a batch file runs may be critical for the
batch to run correctly. If a batch is run from a shortcut, the working
directory may configured in the shortcut. If running a program or
another batch from a batch file, the CD or CHDIR command may be
used to change the working directory prior to execution. The PUSHD
and POPD commands may be used to save and restore the current
directory when running a process that might change the working
directory. PUSHD saves the current directory and changes to the
directory on the command line. POPD restores the current directory
saved by PUSHD. A period is used to refer to the current working
directory and two periods refers to the parent directory of the
current working directory. So, PUSHD . may be used to save the
current working directory before CALLing a batch that changes to
another directory and POPD executed after the CALLed batch will
restore the working directory. If Command Extensions are enabled,
PUSHD may also be used with UNC paths. The example below
will temporarily map a drive letter to the sharename on the given
servername.
PUSHD \\servername\sharename\path
POPD will restore the current directory and disconnect the temporary
drive map.
Each line in a batch file is a statement which may be a batch command, a
label, an internal DOS command or an external program. DOS
provides a number of command line utility programs.
Lines in a batch which start with REM (remark) are generally ignored,
with the exception of redirection. For example:
REM Here is my remark>redirect.txt
In spite of the fact that the line above is preceeded with REM, the
redirection will create an empty file name redirect.txt.
Normally you would not use redirection in a line that was a comment,
however, another reason to comment a line in a batch file is to
temporariliy disable it. If the line contains redirection, REM will
not do the job. Instead do this:
::DisabledCommand.exe>redirect.txt
The colon character is the prefix for a label, so when the
command processor encounters a line that starts with a colon, it is
ignored, unless the command processor is searching for the label. By
using two colons, the line will not be considered a label. This method
also processes faster than a REM.
These utilities are designed to replace the DOS program with the same name
and have the same base usage and syntax, but offer extended functionality
and/or performance. In order to make sure the Incode Systems version of
the program runs (and not the DOS version), make sure that the Incode
Systems version is in a path that preceeds the DOS command path in your
PATH statement in the environment table. This is the preferred method.
Other methods are deleting or renaming the DOS version of the program
file (a disadvantage is you have to remember to do this each time you
reinstall DOS (or Windows 95), or every time you use these programs,
include a fully qualified path in front of the program name.
To execute a program in a batch, simply enter the program name on the left
end of a line. If the program requires parameters on the command line,
these may be entered to the right of the program name.
Batch files may be passed parameters on the command line which invokes the
batch. Within the batch, these parameters may be referenced by %1 %2 %3
etc., (through 9) where %1 is the first parameter on the command, %2 the
second, and so on. The SHIFT batch command may be used to reference more
than 9 parameters. When SHIFT is executed, the first parameter is
discarded, and the second becomes %1, etc. Also, the %0 parameter is the
name of the batch itself. In Windows 95, command line parameters may be
delimited (separated) by a space, comma, or semicolon. If a single parameter
contains embedded spaces, you may surround it with double quotes. Double
quoted parameters include the double quotes when dereferenced. If the
parameter itself contains a double quote, escape it by doubling the double
quote character. These doubled double quotes will also be contained in the
dereferenced parameters. Prior to Windows 95, parameters were separated by
spaces only. In addition, %* may be used to reference all parameters except
%0 which were passed to the batch. The SHIFT command does not alter the
contents of %*. For example, you could call another batch with all the
parameters passed to the original batch like this:
CALL MyBat.bat %*
SHIFT Example:
:BEGIN
IF "%1"=="" GOTO END
PROG1 %1
SHIFT
GOTO BEGIN
:END
If the example above is in a file named BAT1.bat, the following command
line would execute the command lines shown below it:
Parameters may be dereferenced with special prefix characters to return
modified file names:
%~f1 - expands %1 to a fully qualified path name
%~d1 - expands %1 to a drive letter only
%~p1 - expands %1 to a path only
%~n1 - expands %1 to a file name only
%~x1 - expands %1 to a file extension only
%~s1 - expanded path contains short names only
%~$PATH:1 - searches the directories listed in the PATH
environment variable and expands %i to the
fully qualified name of the first one found.
If the environment variable name is not
defined or the file is not found by the
search, then this modifier expands to the
empty string
In Windows 2000 and newer:
%~1 - expands %1 removing any surrounding double quotes (")
For more information, see usage on the FOR command.
To remove a given number of characters from the beginning of an environment
variable:
SET A=123456789
SET A=%A:~2%
ECHO A=%A%
A=3456789
For more information, see usage on the SET command.
The IF batch command may be used to compare strings or with the
ERRORLEVEL syntax to either branch to another place in a
batch to conditionally execute a command line. Type IF /? for more
information.
ECHO is an internal DOS command which controls whether batch lines are
"echoed" to the screen as they are executed. ECHO ON causes lines to be
echoed, ECHO OFF causes them to NOT be echoed. The "at" character (@)
may also be used at the beginning of a line to cause it to not be echoed.
ECHO is ON by default. ECHO Message displays Message, even if ECHO
is OFF
To ECHO a blank line, use ECHO.
For example:
IF "%1"=="HOWDY" GOTO LABEL2
ECHO HOWDY is not parameter 1
GOTO MOREBAT
:LABEL2
ECHO HOWDY is parameter 1
:MOREBAT
The statement above checks to see if the first parameter on the batch file
command line is HOWDY, if it is, the batch jumps to LABEL2, if not it
continues with the next line.
Using double-quote characters (ASCII 34) to delimit a parameter in an
IF statement can cause a problem when the batch is passed a parameter
which is double-quoted. If a parameter passed to a batch file contains
embedded spaces, it must be surrounded by double-quote characters. The
parameter value dereferenced within the batch file contains the
double-quote characters, so they are effectively double-quoted a second
time when IF "%1" is used. The batch file will exit with a message containing
was unexpected at this time. To avoid this problem, you can use a
different character to delimit the parameter marker. For example:
IF `%1`==`HOWDY` GOTO LABEL2
The character used to surround %1 is a leading quote character (ASCII 96).
There is nothing special about ASCII 96, I just selected it because it looks
like a quote character and it is not commonly used.
IF %1. == HOWDY. GOTO LABEL2
Ending each string with a period is another way to avoid this problem.
ELSE may also be used. Here is a one line IF with ELSE followed by a
multiline IF ELSE:
IF %1. == HOWDY. (ECHO Yes) ELSE ECHO No
IF %1. == HOWDY. (
ECHO This and
ECHO That
) ELSE (
ECHO Other and
ECHO Another
)
Under Windows 2000 and newer, this will work:
IF "%~1"=="HOWDY" (ECHO Yes) ELSE ECHO No
PAUSE is an internal DOS Command which displays:
Press any key to continue . . .
and waits for a keystroke before continuing. If you want to branch based on
which key is pressed, use GETKEY or CONFIRM
FOR may be used to run a program multiple times.
FOR examples:
FOR %%F IN (*.TXT) DO PROG1 %%F
FOR %%F IN (First,Second,Third) DO PROG1 %%F
FOR %%F IN (D:\MyPath\*.txt) DO PROG1 %%F
The first FOR statement above runs the program named PROG1 once for each
instance of a file with a TXT extension. Note that %%F contains two
percent signs. That is because a percent sign used in a batch must be
doubled unless it refers to a parameter (%1 - %9). The same statement
may be run from a command prompt with a single percent sign. If there are
three files matching *.txt in the current directory: 1.txt, 2.txt, and 3.txt,
the following command lines would be executed:
PROG1 1.TXT
PROG1 2.TXT
PROG1 3.TXT
The second FOR statement runs the program named PROG1 three times
with the parameters listed. Note that parameters listed this way need not be
file names. The following command lines would be executed:
PROG1 First
PROG1 Second
PROG1 Third
The third FOR statement would run the following command lines if there were
three TXT files named 1.TXT, 2.TXT, and 3.TXT:
Now, suppose you want to rename them so the number in the file name appears
first. The following line in a BAT file will perform the commands that
follow:
FOR %%v IN (*_*.txt) DO FOR /F "tokens=1-3 delims=_." %%i IN ("%%v") DO REN "%%v" "%%j_%%i.%%k"
The FOR command may be used to run another program:
FOR /F "delims==" %i IN ('set') DO @echo %i
Batch file programming tips
How to detect if a drive letter is unavailable:
VOL F: >NUL 2>NUL
IF ERRORLEVEL 1 GOTO NO_F
The internal VOL command will set ERRORLEVEL to 1 if the drive is not mapped
or the drive is not available, like a DVD drive with no disk in the drive.
Redirection is used to hide the output of the VOL command as
well as any error messages.
DIR \\server\>NUL 2>NUL
IF ERRORLEVEL 1 GOTO NO_SERVER
How to detect if a path exists:
IF EXIST may be used to check for the existence of a file, like this:
IF EXIST FileName.ext GOTO Found
To check for the existence of a directory, check for NUL in the path, like this:
IF EXIST MyPath\NUL GOTO Found
How to change the working directory to the BAT file directory:
When you write a batch file to process files which are dropped on the batch
file in Windows Explorer (or a shortcut to your batch file), the working
directory is likely to be the Windows, not the directory where the batch
file exists or the file that was dropped. If your batch contains other files
needed to process the file that was dropped, you will need to change the
working directory to the directory where the batch file exists.
Incode Systems, Inc.