SET

Windows NT 4..Windows 7 Syntax

Displays, sets, or removes cmd.exe environment variables.

SET [variable=[string]]

Type SET without parameters to display the current environment variables.

If Command Extensions are enabled SET changes as follows:

SET command invoked with just a variable name, no equal sign or value will display the value of all variables whose prefix matches the name given to the SET command. For example:

SET P

would display all variables that begin with the letter 'P'

SET command will set the ERRORLEVEL to 1 if the variable name is not found in the current environment.

SET command will not allow an equal sign (=) to be part of the name of a variable (with the exception of some dynamic environment variables).
However, SET command will allow an equal sign in the value of an environment variable in any position other than the first character.

Integer Math

The /A switch was introduced in Windows NT 4:

SET /A expression

The /A switch specifies that the string to the right of the equal sign is a numerical expression that is evaluated. The expression evaluator is pretty simple and supports the following operations, in decreasing order of precedence:

If you use any of the logical or modulus operators, you will need to enclose the expression string in quotes.
Any non-numeric strings in the expression are treated as environment variable names whose values are converted to numbers before using them.
If an environment variable name is specified but is not defined in the current environment, then a value of zero is used. This allows you to do arithmetic with environment variable values without having to type all those % signs to get their values.
If SET /A is executed from the command line outside of a command script, then it displays the final value of the expression.
The assignment operator requires an environment variable name to the left of the assignment operator.
Numeric values are decimal numbers, unless prefixed by 0x for hexadecimal numbers, 0b for binary numbers and 0 for octals numbers. So 0x12 is the same as 0b10010 is the same as 022 is the same as 18.
Please note that the octal notation can be confusing: 08 and 09 are not valid numbers because 8 and 9 are not valid octal digits.

More on integer math, and its limitations, can be found on my Batch math and PHP based batch files pages.

Prompt For Input

The /P switch was introduced in Windows 2000:

SET /P variable=[promptString]

The /P switch allows you to set the value of a variable to a line of input entered by the user. Displays the specified promptString before reading the line of input. The promptString can be empty.

String Substitution

Environment variable substitution has been enhanced as follows:

%PATH:str1=str2%

would expand the PATH environment variable, substituting each occurrence of "str1" in the expanded result with "str2". "str2" can be the empty string to effectively delete all occurrences of "str1" from the expanded output. "str1" can begin with an asterisk, in which case it will match everything from the beginning of the expanded output to the first occurrence of the remaining portion of str1.

Substrings

May also specify substrings for an expansion.

%PATH:~10,5%

would expand the PATH environment variable, and then use only the 5 characters that begin at the 11th (offset 10) character of the expanded result.
If the length is not specified, then it defaults to the remainder of the variable value.
As of Windows 2000, if either number (offset or length) is negative, then the number used is the length of the environment variable value added to the offset or length specified.

%PATH:~-10%

would extract the last 10 characters of the PATH variable.

%PATH:~0,-2%

would extract all but the last 2 characters of the PATH variable.

Delayed Variable Expansion

Finally, support for delayed environment variable expansion has been introduced in Windows 2000. This support is always disabled by default, but may be enabled/disabled via the /V command line switch to CMD.EXE. See CMD /?

Delayed Variable Expansion

Delayed environment variable expansion is useful for getting around the limitations of the current expansion which happens when a line of text is read, not when it is executed.
The following example demonstrates the problem with immediate variable expansion:

SET VAR=before
IF "%VAR%" == "before" (
	SET VAR=after;
	IF "%VAR%" == "after" @ECHO If you see this, it worked
)

would never display the message, since the %VAR% in BOTH IF statements is substituted when the first IF statement is read, since it logically includes the body of the IF, which is a compound statement.
So the IF inside the compound statement is really comparing "before" with "after" which will never be equal.
Similarly, the following example will not work as expected:

SET LIST=
FOR %A IN (*) DO SET LIST=%LIST% %A
ECHO %LIST%

in that it will NOT build up a list of files in the current directory, but instead will just set the LIST variable to the last file found.
Again, this is because the %LIST% is expanded just once when the FOR statement is read, and at that time the LIST variable is empty.
So the actual FOR loop we are executing is:

FOR %A IN (*) DO SET LIST= %A

which just keeps setting LIST to the last file found.

Delayed environment variable expansion allows you to use a different character (the exclamation mark) to expand environment variables at execution time.
If delayed variable expansion is enabled, the above examples could be written as follows to work as intended:

SET VAR=before
IF "%VAR%" == "before" (
	SET VAR=after
	IF "!VAR!" == "after" @ECHO If you see this, it worked
)

SET LIST=
FOR %A IN (*) DO SET LIST=!LIST! %A
ECHO %LIST%

Dynamic Environment Variables

As of Windows 2000, if Command Extensions are enabled, then there are several dynamic environment variables that can be expanded but which don't show up in the list of variables displayed by SET.
These variable values are computed dynamically each time the value of the variable is expanded.
If the user explicitly defines a variable with one of these names, then that definition will override the dynamic one described below:

ErrorLevels

Andrew Pixley reported that in Windows XP, "unsetting" a variable (SET variable=) will set the ErrorLevel to 1, whereas setting it again (SET variable=value) will not reset the ErrorLevel to 0.
In Windows 7, SET only sets the ErrorLevel to 1 with a SET variable command (no equal sign) if variable is not set.