Variables

Table of Contents

Just like classes are defined as promises, variables (or "variable definitions") are also promises. Variables can be defined in any promise bundle. This bundle name can be used as a context when using variables outside of the bundle they are defined in.

CFEngine variables have three high-level types: scalars, lists, and data containers.

  • A scalar is a single value,
  • a list is a collection of scalars.
  • a data container is a lot like a JSON document, it can be a key-value map or an array or anything else allowed by the JSON standard with unlimited nesting.

Scalar Variables

Each scalar may have one of three types: string, int or real. String scalars are sequences of characters, integers are whole numbers, and reals are float pointing numbers.

    vars:
      "my_scalar" string => "String contents...";
      "my_int" int       => "1234";
      "my_real" real     => "567.89";

Integer constants may use suffixes to represent large numbers. The following suffixes can be used to create integer values for common powers of 1000.

  • 'k' = value times 1000
  • 'm' = value times 10002
  • 'g' = value times 10003

Since computing systems such as storage and memory are based on binary values, CFEngine also provide the following uppercase suffixes to create integer values for common powers of 1024.

  • 'K' = value times 1024.
  • 'M' = value times 10242
  • 'G' = value times 10243

However, the values must have an integer numeric part (e.g. 1.5M is not allowed).

In some contexts, % can be used a special suffix to denote percentages.

Lastly, there is a reserved value which can be used to specify a parameter as having no limit at all.

  • 'inf' = a constant representing an unlimited value.

    inf is a special value that in the code corresponds to the magic number of 999999999 (nine nines). Thus any function that accepts a number, can accept inf without a problem. Keep in mind though that you can get a higher number if you set the upper limit manually, but that's almost never a problem.

    For a few functions inf is being treated specially and truly means "there is no limit" instead of "nine nines limit". This is the case for the maxbytes parameter and applies to most read* functions.

CFEngine typing is mostly dynamic, and CFEngine will try to coerce string values into int and real types, and if it cannot it will report an error. However, arguments to built-in functions check the defined argument type for consistency.

Scalar Referencing and Expansion

Scalar variables are referenced by $(my_scalar) (or ${my_scalar}) and expand to the single value they hold at that time. If you refer to a variable by $(unqualified), then it is assumed to belong to the current bundle. To access any other (scalar) variable, you must qualify the name, using the name of the bundle in which it is defined:

$(bundle_name.qualified)

Quoting

When quoting strings cfengine allows the use of ', ", and or `. This allows flexibilty when defining strings that contain quotes. Single or double quotes can be escaped with \ however, please note that backticks (`) can not be escaped.

bundle agent main
{
  vars:
    'single' string => 'single quotes';
    `backtick` string => `backtick quotes`;
    "double" string => "double";
    'single_escape' string => 'You can \'escape\' single quotes';
    "double_escape" string => "You can \"escape\" double quotes";
    `backtick_escape` string => `Note: You can't escape backtick quotes`;

  reports:
    "$(single)";
    `$(backtick)`;
    `$(double)`;
    '$(single_escape)';
    "$(double_escape)";
    `$(backtick_escape)`;
}
R: single quotes
R: backtick quotes
R: double
R: You can 'escape' single quotes
R: You can "escape" double quotes
R: Note: You can't escape backtick quotes

This policy can be found in /var/cfengine/share/doc/examples/quoting.cf and downloaded directly from github.

Scalar Size Limitations

At the moment, up to 4095 bytes can fit into a scalar variable. This limitation may be removed in the future.

If you try to expand strings in a variable or string context that add up to more that 4095 bytes, you will notice this limitation as well. The functions eval() to do math, string_head() and string_tail() to extract a certain number of characters from either end of a string, and string_length() to find a string's length may be helpful.

See readfile() for more detail on reading values from a file.

See data_readstringarray() and data_readstringarrayidx() for a way to read large files' contents into a data container without going through scalar variables or arrays.

Lists

List variables can be of type slist, ilist or rlist to hold lists of strings, integers or reals, respectively.

Every element of a list is subject to the same size limitations as a regular scalar.

They are declared as follows:

     vars:
         "my_slist" slist => { "list", "of", "strings" };
         "my_ilist" ilist => { "1234", "5678" };
         "my_rlist" rlist => { "567.89" };

List Substitution and Expansion

An entire list is referenced with the symbol ‘@’ and can be passed in their entirety in any context where a list is expected as @(list). For example, the following variable definition references a list named "shortlist":

    vars:
        "shortlist" slist => { "you", "me" };
        "longlist" slist => { @(shortlist), "plus", "plus" };

The declaration order does not matter – CFEngine will understand the dependency, and execute the promise to assign the variable @(shortlist) before the promise to assign the variable @(longlist).

Using the @ symbol in a string scalar will not result in list substitution. For example, the string value "My list is @(mylist)" will not expand this reference.

Using the scalar reference to a local list variable, will cause CFEngine to iterate over the values in the list. E.g. suppose we have local list variable @(list), then the scalar $(list) implies an iteration over every value of the list.

In some function calls, listname instead of @(listname) is expected. See the specific function's documentation to be sure.

Data Container Variables

The data containers can contain several levels of data structures, e.g. list of lists of key-value arrays. They are used to store structured data, such as data read from JSON or YAML files. The variable type is data.

Data containers are obtained from functions that return data types, such as readjson() or parsejson(), readyaml() or parseyaml(), or from merging existing containers.

They can NOT be modified, once created, but they can be re-defined.

Data containers do not have the size limitations of regular scalar variables.

bundle agent example_reference_values_inside_data
{
  vars:

    "data" data => '{
  "Key1": "Value1",
  "Key2": "Value2",
  "Key3": [
    "Value3",
    "Value4"
  ]
}';

  reports:
    "Key1 contains '$(data[Key1])'";
    "Key2 contains '$(data[Key2])'";
    "Key3 iterates and contains '$(data[Key3])'";
}
bundle agent __main__
{
   methods: "example_reference_values_inside_data";
}
R: Key1 contains 'Value1'
R: Key2 contains 'Value2'
R: Key3 iterates and contains 'Value3'
R: Key3 iterates and contains 'Value4'

This policy can be found in /var/cfengine/share/doc/examples/reference_values_inside_data.cf and downloaded directly from github.

Associative Arrays

Associative arrays in CFEngine are fundamentally a collection of individual variables that together represent a data structure with key value pairs. They can be built up, dynamically one key at a time and individual keys can be re-defined.

While in many cases associative arrays can be used interchangeably with data variables (e.g. as input to a function) if there is not explicit need to use an associative array for it's ability to be built up dynamically or for managing the size of individual variables use of a data variable is recommended.

Every value in an associative array is subject to the same size limitations as a regular scalar.

Associative array variables are written with [ and ] brackets that enclose an arbitrary key.

Arrays are associative and may be of type scalar or list. Enumerated arrays are simply treated as a special case of associative arrays, since there are no numerical loops in CFEngine. Special functions exist to extract lists of keys from array variables for iteration purposes.

Example:

bundle common g
{
  vars:

      "array[key1]" string => "one";
      "array[key2]" string => "two";
}

bundle agent __main__
{
  vars:

      "thing[1][color]" string => "red";
      "thing[1][name]" string => "one";
      "thing[2][color]" string => "blue";
      "thing[2][name]" string => "two";

      "_thing_idx" slist => sort( getindices( thing ), lex );

  reports:

      "Keys in default:g.array = $(with)"
        with => join( ", ", getindices( "default:g.array" ));

      "Keys of default:main.thing[1] = $(with)"
        with => join( ", ", getindices( "default:main.thing[1]" ));

      "Thing $(thing[$(_thing_idx)][name]) is $(thing[$(_thing_idx)][color])";
}
R: Keys in default:g.array = key1, key2
R: Keys of default:main.thing[1] = color, name
R: Thing one is red
R: Thing two is blue

This policy can be found in /var/cfengine/share/doc/examples/arrays.cf and downloaded directly from github.

See also: getindices(), getvalues()