SET – Windows CMD Command

Display, set, or remove CMD environment variables. Changes made with SET will remain only for the duration of the current CMD session.

Variable names are not case sensitive but the contents can be.

It is good practice to avoid using any delimiter characters (spaces, commas etc) in the variable name.

Delimiter characters can be used in the value if the complete assignment is surrounded with double quotes to prevent the delimiter being interpreted.

Any extra spaces around either the variable name or the string, will not be ignored, SET is not forgiving of extra spaces like many other scripting languages. So use SET alpha=beta, not SET alpha = beta

The first character of the name must not be numeric. It is a common practice to prefix variable names with either an undescore or a dollar sign _variable or $variable, these prefixes are not required but help to prevent any confusion with the standard built-in Windows Environment variables or any other other command strings.

The CMD shell will fail to read an environment variable if it contains more than 8,191 characters.

Display a variable:

In most contexts, surround the variable name with %’s and the variable’s value will be used
e.g. To display the value of the _department variable with the ECHO command:
ECHO %_department%

If the variable name is not found in the current environment then SET will set %ERRORLEVEL% to 1 .
This can be detected using IF ERRORLEVEL …

Including extra characters can be useful to show any white space:
ECHO [%_department%]
ECHO “%_department%”

Type SET without parameters to display all the current environment variables.

Type SET with a variable name to display that variable
SET _department

The SET command invoked with a string (and no equal sign) will display a wildcard list of all matching variables

Display variables that begin with ‘P’:
SET p
Display variables that begin with an underscore
SET _

Set a variable:

Set a variable that contains a redirection character, note the position of the quotes which are not saved:

SET “_dept=Sales & Marketing”

One variable can be based on another, but this is not dynamic
E.g.

Avoid starting variable names with a number, this will avoid the variable being misinterpreted as a parameter
%123_myvar% < > %1 23_myvar

To display undocumented system variables:

Values with Spaces – using Double Quotes

There is no need to add quotation marks when assigning a value that includes spaces

SET _variable=one two three

For special characters like &, you can surround the entire expression with quotation marks.
The variable contents will not include the surrounding quotes:

SET “_variable=one & two”

If you place quotation marks around the value, then those quotes will be stored:

SET _variable=”one & two”

Variable names with spaces

A variable can contain spaces and also the variable name itself can contain spaces, therefore the following assignment:
SET _var =MyText
will create a variable called “_var ” – note the trailing space

Prompt for user input

The /P switch allows you to set a variable equal to a line of input entered by the user.
The Prompt string is displayed before the user input is read.

The Prompt string can be empty. If the user does not enter anything (just presses return) then the variable will be unchanged and an errorlevel will be set to 1.

The CHOICE command is an alternative to SET /P (but CHOICE accepts only one character/keypress.)

Place the first line of a file into a variable:

Set /P _MyVar=<MyFilename.txt
Echo %_MyVar%

In early versions of CMD, any carriage returns/new lines (CR+LF) before the first line containing text were ignored.

The second and any subsequent lines of text in the file will be discarded.

Delete a variable

Type SET with just the variable name and an equals sign:

SET _department=

Better still, to be sure there is no trailing space after the = place the expression in parentheses or quotes:
(SET _department=)
  or
SET “_department=”

Arithmetic expressions (SET /a)

Placing expressions in “quotes” is optional for simple arithmetic but required for any expression using logical operators.

When refering to a variable in your expression, SET /A allows you to omit the %’s so _myvar instead of %_myvar%

Any SET /A calculation that returns a fractional result will be rounded down to the nearest whole integer.

The expression to be evaluated can include the following operators:

If a variable name is specified as part of the expression, but is not defined in the current environment, then SET /a will use a value of 0.

In a batch script, the Modulus operator (%) must always be doubled up to (%%).

SET /A arithmetic shift operators do not detect overflow which can cause problems for any non-trivial math, e.g. the bitwise invert often incorrectly reverses the + / – sign of the result.

See SET /a examples below and this forum thread for more.
also see SetX, VarSearch and VarSubstring for more on variable manipulation.

Examples:

SET /A will treat any character string in the expression as an environment variable name. This allows you to do arithmetic with environment variables without having to type any % signs to get the values. SET /A _result=5 + _MyVar

Multiple calculations can be performed in one line, by separating each calculation with commas, for example:

_year=1999
Set /a _century=_year/100, _next=_century+1

The numbers must all be within the range of 32 bit signed integer numbers (-2,147,483,648 through 2,147,483,647) to handle larger numbers use PowerShell or VBScript.

Leading Zero will specify Octal

Numeric values are decimal numbers, unless prefixed by
0x for hexadecimal numbers,
for octal numbers.

So 0x10 = 020 = 16 decimal


The octal notation can be confusing – all numeric values that start with zeros are treated as octal but 08 and 09 are not valid octal digits.
For example SET /a _month=07 will return the value 7, but SET /a _month=09 will return an error.

Permanent changes

Changes made using the SET command are NOT permanent, they apply to the current CMD prompt only and remain only until the CMD window is closed.
To permanently change a variable at the command line use SetX
or with the GUI – Control Panel | System | Environment | System/User Variables

Changing a variable permanently with SetX will not affect any CMD prompt that is already open.
Only new CMD prompts will get the new setting.

You can of course use SetX in conjunction with SET to change both at the same time:

Set _Library=T:\Library\
SetX _Library T:\Library\ /m

Change the environment for other sessions

Neither SET nor SetX will affect other CMD sessions that are already running on the machine . This as a good thing, particularly on multi-user machines, your scripts won’t have to contend with a dynamically changing environment while they are running.

It is possible to add permanent environment variables to the registry (HKCU\Environment), but this is an undocumented (and likely unsupported) technique and still it will not take effect until the users next login.

System environment variables can be found in the registry here:
HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Environment

CALL SET

The CALL SET syntax allows a variable substring to be evaluated, the CALL page has more detail on this technique, in most cases, a better approach is to use Setlocal EnableDelayedExpansion

Autoexec.bat

Any SET statement in c:\autoexec.bat will be parsed at boot time
Variables set in this way are not available to 32-bit GUI programs – they won’t appear in the control panel.
They will appear at the CMD prompt.

If autoexec.bat CALLS any secondary batch files, the additional batch files will NOT be parsed at boot.
This behavior can be useful on a dual boot PC.

Errorlevels

When CMD Command Extensions are enabled (the default):

If the variable was successfully changedunchanged, typically this will be 0 but if a previous command set an errorlevel, that will be preserved (this is a bug).
SET No variable found or invalid name.
SET _var=value when _var name starts with “/” and not enclosed in quotes.
SET /P Empty response from user.
1
SET /A Unbalanced parentheses1073750988
SET /A Missing operand1073750989
SET /A Syntax error1073750990
SET /A Invalid number1073750991
SET /A Number larger than 32-bits1073750992
SET /A Division by zero1073750993

SET is an internal command.
If Command Extensions are disabled all SET commands are disabled other than simple assignments like _variable=MyText

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1 Response

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