The description below is from the book DOS the
Easy Way by Everett Murdock Ph.D.
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for information about downloading the book.
Type: Internal (2.0 and later)
Syntax:
IF [NOT] EXIST filename (command) [parameters]
IF [NOT] (string1)==(string2) (command) [parameters]
IF [NOT] ERRORLEVEL (number) (command) [parameters]
Purpose: Allows for conditional operations in batch processing.
Discussion
When used in combination with the GOTO command, the IF command provides a
way to transfer control within a batch file dependent on the outcome of a test.
For more information on IF and other batch commands, see Chapter 5,
Using Batch Files, in the downloadable
book DOS the Easy Way .
The IF test is indicated by the condition label in the command format.
It can be one of three types:
Condition 1: IF [NOT] EXIST (filename) (command)
This condition is determined by a test to see if a file exists (or does
not exist) on disk.
When this conditional test is included (it can be on any line of your
batch file), DOS checks to determine if the specified file exists (you must
specify
the path so DOS can find the file). If the condition is true (as
stated), the
specified command is executed. If the condition is found to be false,
the
command is not acted on and processing proceeds to the next line in the
batch file.
When the IF command is used together with the GOTO command, it provides a
way to transfer control to a different line in the batch file dependent on the
outcome of the test.
For example, to display a warning message only if the file DATA.1 does
not exist in the current directory, enter
echo off
if not exist data.1 goto MESSAGE
echo The file DATA.1 exists on the current directory
goto END
MESSAGE
echo The file DATA.1 was not found.
echo The file must be in the current directory
:END
When you enter the name of this batch file, there are two possible
outcomes. If
the file DOES NOT EXIST in the current directory, the program will
display:
echo off
The file DATA.1 was not found.
The file must be on the current directory
If the file DOES EXIST in the current directory, the program will
display:
echo off
echo The file DATA.1 exists on the current directory.
In this example, the IF command is combined with the GOTO command to
determine
the optional branching. The command tells DOS to check the current disk
to
determine if the file DATA.1 exists. Since we are using the NOT EXIST
form of
the command, the condition is true if it DOES NOT find the file; since
the
condition is true, the specified command (in this case, the GOTO command)
is
acted on. The GOTO MESSAGE line passes processing to the :MESSAGE label
and the
messages on the following lines are displayed.
Condition 2: IF [NOT] string1==string2 (command)
This operation checks to determine if the first string you enter is equal
(or is
NOT equal) to the second string you enter. If the condition is true (as
stated), the specified command will be carried out.
DOS judges this condition based on whether or not the two strings are
EXACTLY
identical. DOS WILL detect a difference between uppercase and lowercase
letters.
You can create a very useful version of the IF command that checks to
determine
whether or not a parameter was passed to the batch file from the command
line.
The following batch file acts conditionally depending on a test of
whether or
not any characters were entered on the command line after the batch file
name.
echo off
if not x==%1x goto PRINT
echo You must enter a parameter immediately after batch file name
goto END
:PRINT
echo The passed parameter is [ %1 ]
:END
The first line of this batch file checks to determine whether or not a
parameter
has been passed at the command line. It uses the IF command to determine
if
string one (the character `x`) is equal to string two (the character `x`
plus
the %1 symbol that represents the first passed parameter). If a
parameter WAS
passed to the batch file, the two strings will not match and the
condition,
(stated as NOT equal) is true; the GOTO command will be acted on,
transferring
processing to the label :PRINT.
If the condition is found to be false (the two strings ARE equal), it
means a
parameter WAS NOT entered on the command line. In that case, the GOTO
command
on the same line as the IF command will not be acted on and processing
will be
transferred to the next line of the batch file. The next line includes
the
message You must enter a parameter immediately after the batch file
name. The
following line transfers the processing to the :END label and the program
terminates.
If characters WERE entered after the batch filename, the program will
display
echo off
The passed parameter is [ test ]
If characters WERE NOT entered after the batch filename, the program will
display
echo off
You must enter a parameter immediately after batch file name
Condition 3: IF [NOT] ERRORLEVEL (number) (command)
This operation checks to determine if an ERRORLEVEL number is set (or is
NOT
set) at or greater than the specified value. The ERRORLEVEL number is
set by
programs and the value is retained in the computer`s memory. This
command is
used to check that value. DOS judges this condition to be true if the
ERRORLEVEL code is equal to OR GREATER than the number you specify.
A common use of the IF command is to display a message to indicate that
an
ERRORLEVEL has been set. To display a message whenever any ERRORLEVEL
number
has been set (any value above zero), enter:
echo off
if errorlevel 1 echo The errorlevel is now greater than zero
When this line is acted on (and if the ERRORLEVEL is found to be set to a
value), the ECHO command will be acted on and the program will
display
echo off
The errorlevel is now greater than zero
If DOS does not find a value set for ERRORLEVEL, the program terminates
without displaying the message.
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