IF

Performs conditional processing in batch programs. If the 
condition specified by an IF command is true, MS-DOS carries out 
the command that follows the condition. If the condition is 
false, MS-DOS ignores the command. You can use this command only 
in batch programs.

Note:
There are several usages of IF command:

1. IF EXIST format:

IF [NOT] EXIST filename command

filename --- the filename you specify to test. To test whether a 
file exists. If you want to test whether a directory exists, you 
can do it by simply specifying a NUL device of the directory. 
For example, "IF EXIST C:\NUL" tests whether there is a drive C; 
also, "IF EXIST C:\WINDOWS\NUL" tests whether there is a path 
"C:\WINDOWS".

command --- the command you want to execute. It can be a usual 
command, or a batch command, such as GOTO.

NOT --- make the result true to false, false to true.

2. IF string1==string2 format:

IF string1==string2 command

string1, string2 --- two strings to compare. Both shouldn't be 
null, and under Windows 9x or MS-DOS they should be spaceless. 
If one of them is going to include spaces, it should be packed with 
quotation marks. To make both strings not empty, we 
usually add a character before each string. For example:

IF '%no_c%=='yes GOTO a

3. IF ERRORLEVEL format:

IF ERRORLEVEL number command

number --- a decimal integer specifying the errorlevel. 
Errorlevel is a mark returned by a command. See command help 
of the operating system. 

For more information, please refer to MS-DOS Help or Windows NT 
command line command help.