A constant is like a variable, except that its value cannot change throughout the runtime of the program.


In DuckyScript, a constant is initialized using the DEFINE command. One may consider the use of a DEFINE within a payload like a find-and-replace just before the time of compile - within what is called the preprocessor. DEFINE can be used to more easily expose or abstract configuration options used throughout your payload. This means to change a constant value that is described by a DEFINE you only need to change it in one location no matter how many times its used throughout your payload.


  1. DEFINE denotes the start of a constant definition

  2. LABEL is the label or key to be used by the compiler to locate usage within your payload

  3. VALUE is the value to replace matching instances of LABEL throughout your payload. The VALUE is everything past LABEL to the end of the line (minus the first space).

With this in mind its best to keep your LABEL as descriptive as possible. Remember - it will be replaced with the given VALUE - the length of the LABEL will have no affect on the actual length of your compiled payload.

Within PayloadStudio

PayloadStudio takes the guess work out of what will get replaced where by automatically annotating lines that are modified by DEFINE statements throughout your payload.

This also gives you the chance to spot any misconfigurations when compiling your payload as PayloadStudio will list these in the console upon generating your inject.bin


Depending on the format of your LABEL, DEFINE will behave differently in it's find-and-replace method. This is to significantly reduce the likelihood that your DEFINE statement has negative unintended side-affects.

With #

DEFINE #LABEL will replace any instance of #LABEL (except another DEFINE)

DEFINE #myConstant TEST Using this syntax, #myURLConstant will be replaced anywhere within your payload even if it is touching other characters. This is because the LABEL starts with #

DEFINE #myURLConstant
STRING https://www.#myURLConstant

This will result in because #myURLConstant starts with a #

Without #

DEFINE LABEL will replace any instance of LABEL as its own word separated by spaces (except another DEFINE)


Using this syntax, myURLConstant will be replaced anywhere it is not touching other characters (is its own individual word) within your payload. This is because the LABEL does not start with #

DEFINE myURLConstant
STRING my website name is myURLConstant

this will result in my website name is

Best practice is to start your label with # While this method is still supported, it is no longer best practice. Usage of a given LABEL becomes very hard to spot mid-payload making your payload more ambiguous without the help of PayloadStudio.

Consider the following example: DEFINE test 123 STRING This is a test showing the ambiguity Result: This is a 123 showing the ambiguity The instance of test in the above STRING will be replaced but it is not obvious if the DEFINE is not directly above it.


Example as Boolean

REM Example Boolean

DuckyScript developers may find it useful to include defines at the top of their payload which determine whether or not a function will run. This makes it easier for the end-user to customize a shared payload.

Example as Integer

REM Integer

In this example, one may imagine the DELAY_SPEED constant will be used in conjunction with one or more DELAY commands.

Example as STRING

STRING https://

In both cases this will result in "" being typed because the label used starts with a # See above

Example Payload

REM Example constants using DEFINE


DEFINE #MESSAGE2 World! Written with a define!



  • The payload will begin with a 2 second delay, then type "Hello, World! Written with a define!" with a 2 second delay in between #MESSAGE1 and #MESSAGE2.

  • Changing the string values of #MESSAGE1 and #MESSAGE2 will change the outcome of the payload.

  • Changing the integer value of #SPEED will change the delay between the first and second message.

Advanced Example

Considering DEFINE is a effectively an automatic find-and-replace step prior to compile, the VALUE of a DEFINE is not limited to any specific datatypes. Any valid DuckyScript syntax can be the VALUE of a DEFINE




Best Practices

Configurable payload options should be specified in variables or defines at the top of the payload. Define labels should start with # for easy identification throughout your payload.

When writing a payload that calls external resources which may vary depending on the operator, such as a website to open or address to establish a reverse shell with, it is best to use DEFINE.

In addition to comment blocks (like the REM title/author/description lines in the above example), putting your DEFINE commands at the top of your payload makes it easier for someone else to use your payload effectively. Even more so if the constants are commented!

Avoiding Errors

  • Internal variables begin with an underscore, so it is best practice to avoid this style.

  • Spaces cannot be used in naming a constant — however underscore makes for a suitable replacement. For example: DEFINE #REMOTE_HOST

  • Labels should descriptive. For example, #RHST is better than #R, and #REMOTE_HOST is better than #RHOST.

  • Be careful when using the uppercase letter O or lowercase letter l as they may be confused with the numbers 0 and 1.

  • Avoid using the names of commands or internal variables (e.g. ATTACKMODE, STRING, WINDOWS, MAC, $_BUTTON_ENABLED). See the full command and variable reference.

Invalid Usages

DEFINE myURLConstant
STRING https://www.myURLConstant

This will result in https://www.myURLConstant because myURLConstant was not its own word and does not start with #

DEFINEs are excluded from being substituted by other DEFINEs


This will not replace the value of #TEST2 with 123

Last updated