KerboScript Syntax Specification

This describes what is and is not a syntax error in the KerboScript programming language. It does not describe what function calls exist or which commands and built-in variables are present. Those are contained in other documents.

General Rules

Whitespace consisting of consecutive spaces, tabs, and line breaks are all considered identical to each other. Because of this, indentation is up to you. You may indent however you like.


Statements are ended with a period character (”.”).

The following are reserved command keywords and special operator symbols:

Arithmetic Operators:

+  -  *  /  ^  e  (  )

Logic Operators:

not  and  or  true  false  <>  >=  <=  =  >  <

Instructions and keywords:

add all at batch break clearscreen compile copy declare delete
deploy do do edit else file for from from function global if
in list local lock log off on once parameter preserve print reboot
remove rename run set shutdown stage step switch then to toggle
unlock unset until volume wait when

Other symbols:

{  }  [  ]  ,  :  //

Comments consist of everything from a “//” symbol to the end of the line:

set x to 1. // this is a comment.

Identifiers: Identifiers consist of: a string of (letter, digit, or underscore). The first character must be a letter or an underscore. The rest may be letters, digits or underscores.

Identifiers are case-insensitive. The following are identical identifiers:



New in version 1.1.0: Kerboscript accepts Unicode source code, encoded using the UTF-8 encoding method. Because of this, the definition of a “letter” character for an identifier includes letters from many languages’ alphabets, including accented Latin alphabet characters, Cyrllic characters, etc. Not all languages have been tested but in principle they should work as long as they have a Unicode standard accepted definition of what counts as a “letter”. We defer to the .NET libraries’ definition of what constitutes the “same” letter in uppercase and lowercase forms, and we hope this is right for most alphabets.


The same case-insensitivity applies throughout the entire language, with all keywords and when comparing literal strings. The values inside the strings are also case-insensitive, for example, the following will print “equal”:

if "hello" = "HELLO" {
    print "equal".
} else {
    print "unequal".


New in version 1.1.0: Again, depending on the alphabet being used, the concept of “uppercase” and “lowercase” might not make sense in some languages. kOS defers to .NET’s interpretation of what letters in Unicode are paired together as the “upper” and “lower” versions of the same letter. For obvious reasons, the kOS developers cannot test every language and verify if this is correct or not.


Some variable types are structures that contain sub-portions. The separator between the main variable and the item inside it is a colon character (:). When this symbol is used, the part on the right-hand side of the colon is called the “suffix”:

list parts in mylist.
print mylist:length. // length is a suffix of mylist

Suffixes can be chained together, as in this example:

print ship:velocity:orbit:x.

In the above example you’d say “velocity is a suffix of ship”, and “orbit is a suffix of ship:velocity”, and “x is a suffix of ship:velocity:orbit”.

Numbers (scalars)

Numbers in kerboscript are referred to as “scalars”, to distinguish them from the many cases where a values will be represnted as a vectors. You are allowed to use integers, decimal fractional numbers (numbers with a decimal point and a fractional part), and scientific notation numbers.

The following are valid scalar syntax:

12_345_678 (The underscores are ignored as just visual spacers)

Kerobscript does not support imaginary numbers or irrational numbers or rational numbers that cannot be represented as a finite decimal (i.e. sqrt(-1) returns a Not-a-Number error. Pi will have to be an approximation. “One third”, ends up being something like 0.333333333).)

Under the hood, these numbers are stored as either 32-bit integers or as 64-bit double floats, depending on the need, but kerboscript attempts to hide this detail from the programmer as much as possible.

Braces (statement blocks)

Anywhere you feel like, you may insert braces around a list of statements to get the language to treat them all as a single statement block.

For example: the IF statement expects one statement as its body, like so:

if x = 1
  print "it's 1".

But you can put multiple statements there as its body by surrounding them with braces, like so:

if x = 1 { print "it's 1".  print "yippieee.".  }

(Although this is usually preferred to be indented as follows):

if x = 1 {
  print "it's 1".
  print "yippieee.".


if x = 1
  print "it's 1".
  print "yippieee.".

Kerboscript does not require proper indentation of the brace sections, but it is a good idea to make things clear.

You are allowed to just insert braces anywhere you feel like even when the language does not require it, as shown below:

declare x to 3.
print "x here is " + x.
  declare x to 5.
  print "x here is " + x.
    declare x to 7.
    print "x here is " + x.

The usual reason for doing this is to create a local scope section for yourself. In the above example, there are actually 3 different variables called ‘x’ - each with a different scope.

Functions (built-in)

There exist a number of built-in functions you can call using their names. When you do so, you can do it like so:

functionName( *arguments with commas between them* ).

For example, the ROUND function takes 2 arguments:

print ROUND(1230.12312, 2).

The SIN function takes 1 argument:

print SIN(45).

When a function requires zero arguments, it is legal to call it using the parentheses or not using them. You can pick either way:

// These both work:

Suffixes as Functions (Methods)

Some suffixes are actually functions you can call. When that is the case, these suffixes are called “method suffixes”. Here are some examples:

set x to ship:partsnamed("rtg").
print x:length().

Suffixes as Lexicon keys

The special type called a Lexicon can be used with this suffix syntax as an alternate way to get the value for a key, as in the example below:

// Given this setup...
set MyLex to Lexicon().
MyLex:ADD( "key1", "value1").
// ...these two lines have the same effect:
print MyLex["key1"]. // key used in the usual way as an "index".
print MyLex:key1.    // key used in an alternate way as a "suffix".

There are some limits to using this syntax, as described in more detail in the documentation for the Lexion type.

User Functions

Help for the new user - What is a Function?

In programming terminology, there is a commonly used feature of many programming languages that works as follows:

    1. Create a chunk of program instructions that you don’t intend to execute YET.
    1. Later, when executing other parts of the program, do the following:
      1. Remember the current location in the program.
      1. Jump to the previously created chunk of code from (1) above.
      1. Run the instructions there.
      1. Return to where you remembered from (2.A) and continue from there.

This feature goes by many different names, with slightly different precise meanings: Subroutines, Procedures, Functions, etc. For the purposes of kerboscript, we will refer to all uses of this feature with the term Function, whether it technically fits the mathematical definition of a “function” or not.

In kerboscript, you can make your own user functions using the DECLARE FUNCTION command, which is structured as follows:

declare function identifier { statements } optional dot (.)

Functions are a long enough topic as to require a separate documentation page, here.

Built-In Special Variable Names

Some variable names have special meaning and will not work as identifiers. Understanding this list is crucial to using kOS effectively, as these special variables are the usual way to query flight state information. The full list of reserved variable names is on its own page.

What does not exist (yet?)

Concepts that many other languages have, that are missing from KerboScript, are listed below. Many of these are things that could be supported some day, but at the moment with the limited amount of developer time available they haven’t become essential enough to spend the time on supporting them.

user-made structures or classes
Several of the built-in variables of kOS are essentially “classes” with methods and fields, however there’s currently no way for user code to create its own classes or structures. Supporting this would open up a large can of worms, as it would then make the kOS system more complex.