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MS DOS Command: SET

Reference from Microsoft WinXP cmd help

Description

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

Syntax

SET [variable=[string]]

Parameters

variable	Specifies the environment-variable name.
string	Specifies a series of characters to assign to the variables.

Remarks

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.

Two new switches have been added to the SET command:

SET /A expression

SET /P variable=[promptString]

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:

()	grouping
! ~ _:-	unary operators
* / %	arithmetic operators
+ -	arithmetic operators
<< >>	logical shift
&	bitwise and
^	bitwise exclusive or
\|	bitwise or
= *= /= %= += -= &= ^= \|= <<= >>=	assignment
,	expression separator

If you use any of the logical or modulus operators, you 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 t 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, and 0 for octal numbers. So 0x12 is the same as 18 isthe same as 022. Please note that the octal notation can be confusing:08 and 09 are not valid numbers 8 and 9 are not valid octal digits.

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.

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 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.

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 reminder of thevariable value. 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.

Finally, support for delayed environment variable expansion has beed added. This support is alway disabled by default, but may be enabled/disabled via the /V command line switch to CMD.EXE.

Delayed environment variable expansion is useful for gettign 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 command statement. So the IF inside the compound statement is really comparing the "before" with "after" wihich will never be equal.

Similarly, the following example will not work as expected:

set LIST=

for %i in (*) do set LIST=%LST% %i

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 %i in (*) do set LIST= %i

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

)