If

The IF command is used to provide conditional processing in batch programs.

Syntax:

IF condition command

IF NOT condition command

condition	IF will test for any of three conditions: string1==string2 EXIST filename ERRORLEVEL x IF tests condition and determines whether it is True or False. For: IF condition command    command is executed should condition be TRUE and skipped if FALSE. For: IF NOT condition command    command is executed should condition be FALSE and skipped if TRUE.
command	Any Dos command or executable program along with parameters and/or switches as required. Only one command can be specified. To have more than one command dependent on the result of condition, it is necessary to branch to a label or second batch file and have the series there .

Although IF works in the same way for each of the above conditions, they are used in such different circumstances that they could almost be considered three separate commands:

1. Comparing Strings

Syntax:

IF [NOT] string1==string2 command

Notes:

1. When comparing strings, note the use of the double equal signs!
   (Why it was considered that a single equal sign was insufficient is beyond me).
    
   
2. String1 and/or string2 can be literal strings, batch variables (eg. %1), or environmental variables (eg. %windir%).
    
   
3. The comparison is case sensitive
   ie.

   IF %windir% == C:\WINDOWS IF %windir% == c:\windows

   will be seen as True will be seen as False

   
    
   
4. An error occurs if either string doesn't have at least one character. This can happen if one of the strings is a variable, and very likely will happen should the IF statement be contained within a loop in which the variable is changed in each cycle.
   To avoid this error arising it is typical to add dummy character(s) to both sides as:

   IF [NOT] "string1"=="string2" command
   or
   IF [NOT] string1x==string2x command

   Any character can be used as the dummy but it makes for simplicity to use the same character consistently. Indeed, the dummy can often be used as one of the trademarks of some batch file authors.
    
   

5. Neither literal nor variable strings may contain = ; | < >.
    
   
6. The parsing mechanism in IF seems to be able to handle embedded spaces in either string (literal or variable) but does not recognize quotes as special characters holding such strings together. Should the test result require command to be carried out, and string2 contains embedded spaces IF takes the second word as the command - typically resulting in a "Bad command" error message (see Examples below).
   I'm sure there is a good reason for IF's aversion to spaces - but it's a right nuisance!

Examples

1. to determine whether all command line parameters have been processed in a loop containing a SHIFT command: .....
     :Step1
     IF %1"==" GOTO Step2
     SHIFT
     COPY %1 a:
     GOTO Step1
     :Step2
     .....
   
    
   
2. Say the environmental variable "Name" could contain information in the form Fred Smith and "Name1" is Fred Smith (ie. with embedded spaces), then:

   Command	If condition is True	If condition is False
   IF %Name%"==" GOTO CreateName	OK - branch to label CreateName	OK - command not executed
   IF NOT %Name%"==" GOTO Step2	OK - command not executed	OK - branch to label Part2
   IF %Name%"==%Name1%" GOTO Part2	Error - "Bad command"	OK - command not executed
   IF Fred Smith"==%Name%" GOTO Step2	Error - "Bad command"	OK - command not executed

2. Checking for the existence of a file (or directory)

Syntax:

IF [NOT] EXIST [path]filename command

filename

A file or Dos device

Notes:

1. path must be included unless filename is in the current drive and directory. Entries in any Dos PATH statement are not recognized.
    
   
2. [path]filename can use either short or long filename form (or a mixture) - but long names must be enclosed in quotes if they include spaces.
    
   
3. Directories cannot be detected directly. However, the Dos devices (NUL, AUX, CON, COM1-4, LPT1-3, and PRN) are accessible from every directory and it follows that if directoryname\device exists, then so does the directory. Conversely, if directoryname\device doesn't exist, then nor does the directory.

   Although, theoretically, any of the above devices can be used for the test, every reference I have seen uses the NUL device. A quick test on my machine indicated that all devices were recognized except for LPT1/PRN which resulted in an error message:  General failure reading device
     Abort, Retry, Fail?
   For reliability, NUL is probably the best choice.
    
   

4. As well as the essential Dos devices (listed above) being accessible from any existing directory, any installed Dos devices (as recorded in an MEM /D display) are also so available and are recognized by IF [NOT] EXIST. Note that the names by which these devices are recognized is not the same as that of the device driver loaded in Config.sys and seem to be generally of the form:

   Device Driver filename	Installed Device Name
   HIMEM.SYS	XMSXXXX0
   EMM386.EXE	EMMXXXX0
   Display.sys	CON
   Dblbuff.sys	DblBuff$
   Ifshlp.sys	IFS$HLP$

Examples

1. To copy a font to c:\windows\fonts from a CD-ROM (drive x:) only if it isn't already installed:   IF NOT EXIST c:\windows\fonts\maxi.ttf COPY x:\maxi.ttf c:\windows\fonts
    
   
2. To create the directory "tests" if it doesn't already exist:   IF NOT EXIST c:\tests\NUL MD c:\tests
    
   
3. To check that Himem.sys is currently installed:   IF EXIST XMSXXXX0 echo Himem installed

Oddities

1. the file is created;
2. Should the condition be True, when executed in a batch file, the entire next line (including prompt and [Enter]) is copied to the file. When working from the command line, the prompt along with any text entered from the keyboard up to and including [Enter] is put into the file. (Text entered is not displayed).
3. Should the condition be False, a zero byte file is created.

3. Checking the exit code of a previous command

Syntax:

IF [NOT] ERRORLEVEL x command

Notes:

1. When external Dos commands terminate, an exit code is returned to MS-Dos. For example, a value of 0 is typically used to indicate that a program was successfully executed. IF [NOT] ERRORLEVEL x tests the exit code returned by the last command and returns True when the exit code is greater than or equal to x and False when the exit code is less than x.

   Should, say, the exit code from the last command be 2 then:

   IF ERRORLEVEL 0 command0 IF ERRORLEVEL 1 command1 IF ERRORLEVEL 2 command2 IF ERRORLEVEL 3 command3	(True) (True) (True) (False)	command0 is executed command1 is executed command2 is executed command3 is skipped
   IF NOT ERRORLEVEL 0 command0 IF NOT ERRORLEVEL 1 command1 IF NOT ERRORLEVEL 2 command2 IF NOT ERRORLEVEL 3 command3	(True) (True) (True) (False)	command0 is skipped command1 is skipped command2 is skipped command3 is executed

1. To associate a particular command with an exit code, it is necessary to use one of two constructions:
2. Arrange the IF commands to test for exit codes in descending order.
   However, note that, unless commandx dictates otherwise, each IF line will be acted on in sequence. There are three ways to prevent valid, but irrelevant, commands being processed:
   1. Have the batch file branch to a label (using GOTO label);
   2. Irreversibly branch to a new batch file (by simply naming the file);
   3. Stopping the batch file with EXIT.
   
    
   
3. Use a nested construction: IF ERRORLEVEL x IF NOT ERRORLEVEL x+1 commandx (where x is the exit code). The logic here is that commandx is only executed when x is True and any number larger than x is False. This structure is useful when each exit code is to result in a single command and a series of GOTOs would be cumbersome.

Examples

1. A series of IF commands is to follow a CHOICE command to display the key that was pressed in response to the prompt:    CHOICE /C:ABC Press A, B, or C:
      IF ERRORLEVEL 1 IF NOT ERRORLEVEL 2 ECHO You pressed "A"
      IF ERRORLEVEL 3 IF NOT ERRORLEVEL 4 ECHO You pressed "C"
      IF ERRORLEVEL 2 IF NOT ERRORLEVEL 3 ECHO You pressed "B"
   Note: that the order of IF commands is not important.
    
   
2. XCOPY is used in a batch file to back up any new or modified spreadsheet files. A series of IF commands are used to display various status message at the conclusion of the copying process.    XCOPY d:\spreadsheets\*.* f:\spreadsheets\ /s/m
      IF ERRORLEVEL 3 GOTO Problems
      IF ERRORLEVEL 2 GOTO Cancel
      IF ERRORLEVEL 1 GOTO NoFiles
      IF ERRORLEVEL 0 GOTO OK
   
      :Problems
      ECHO You have problems!
      ECHO Modified/New files have not been backed up.
      GOTO end
   
      :Cancel
      ECHO Copying process terminated.
      GOTO end
   
      :NoFiles
      ECHO No files found to copy.
      GOTO continue
   
      :OK
      ECHO All new/modified files backed up.
      GOTO continue
   
      :continue
      ........
   
      :end
   Note: that the order of IF commands is most important!
    
   
3. Oh yes, the (apparent) exception. To set an environmental variable according to a keypress (say), either of the above approaches will work just fine. However, simply to use    CHOICE /C:ABC Press A, B, or C:
      IF ERRORLEVEL 3 SET key=C
      IF ERRORLEVEL 2 SET key=B
      IF ERRORLEVEL 1 SET key=A
   is tempting - but doesn't work! Instead the order should be reversed as:
      CHOICE /C:ABC Press A, B, or C:
      IF ERRORLEVEL 1 SET key=A
      IF ERRORLEVEL 2 SET key=B
      IF ERRORLEVEL 3 SET key=C
   

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