Classes and Decisions

Table of Contents

Classes are used to apply promises only to particular environments, depending on context. A promise might only apply to Linux systems, or should only be applied on Sundays, or only when a variable has a certain value.

Classes are simply facts that represent the current state or context of a system. The list of set classes classifies the environment at time of execution.

Classes are either set or not set, depending on context. Classes fall into hard classes that are discovered by CFEngine, and soft classes that are user-defined.

Classes have either a namespace or bundle scoped. namespace scoped classes are visible from any bundle. bundle scoped classes can only be checked within that bundle or from a bundle called with inheritance. Hard classes always have a namespace scope.

In CFEngine Enterprise, classes that are defined can be reported to the CFEngine Database Server and can be used there for reporting, grouping of hosts and inventory management. For more information about how this is configured please read the documentation on Enterprise Reporting.

Listing Classes

To see the first order of hard classes and soft classes run cf-promises --show-classes as a privileged user. Alternatively run cf-agent --show-evaluated-classes to get the listing of classes at the end of the agent execution. This will show additional namespace scoped classes that were defined over the course of the agent execution. This output can be convenient for policy testing.


[root@hub masterfiles]# cf-promises --show-classes
Class name                                                   Meta tags
10_0_2_15                                                    inventory,attribute_name=none,source=agent,hardclass
127_0_0_1                                                    inventory,attribute_name=none,source=agent,hardclass
192_168_33_2                                                 inventory,attribute_name=none,source=agent,hardclass
1_cpu                                                        source=agent,derived-from=sys.cpus,hardclass
64_bit                                                       source=agent,hardclass
Afternoon                                                    time_based,source=agent,hardclass
Day22                                                        time_based,source=agent,hardclass

Note that some of the classes are set only if a trusted link can be established with cf-monitord, i.e. if both are running with privilege, and the /var/cfengine/state/env_data file is secure.

The classesmatching() function searches using a regular expression for classes matching a given name and or tag.

See also: The --show-vars option for cf-promises and the --show-evaluated-vars option for cf-agent.


Classes and variables have tags that describe their provenance (who created them) and purpose (why were they created).

While you can provide your own tags for soft classes in policy with the meta attribute, there are some tags applied to hard classes and other special cases. This list may change in future versions of CFEngine.

  • source=agent: this hard class or variable was created by the agent in the C code. This tag is useful when you need to find classes or variables that don't match the other sources below. e.g. linux.
  • source=environment: this hard class or variable was created by the agent in the C code. It reflects something about the environment like a command-line option, e.g. -d sets debug_mode, -v sets verbose_mode, and -I sets inform_mode. Another useful option, -n, sets opt_dry_run.
  • source=bootstrap: this hard class or variable was created by the agent in the C code based on bootstrap parameters. e.g. policy_server is set based on the IP address or host name you provided when you ran cf-agent -B host-or-ip.
  • source=module: this class or variable was created through the module protocol.
  • source=persistent: this persistent class was loaded from storage.
  • source=body: this variable was created by a body with side effects.
  • source=function: this class or variable was created by a function as a side effect, e.g. see the classes that selectservers() sets or the variables that regextract() sets. These classes or variables will also have a function=FUNCTIONNAME tag.
  • source=promise: this soft class was created from policy.
  • inventory: related to the system inventory, e.g. the network interfaces
    • attribute_name=none: has no visual attribute name (ignored by Mission Portal)
    • attribute_name=X: has visual attribute name X (used by Mission Portal)
  • monitoring: related to the monitoring (cf-monitord usually).
  • time_based: based on the system date, e.g. Afternoon
  • derived-from=varname: for a class, this tells you it was derived from a variable name, e.g. if the special variable sys.fqhost is xyz, the resulting class xyz will have the tag derived-from=sys.fqhost.
  • cfe_internal: internal utility classes and variables

Enterprise only:

  • source=ldap: this soft class or variable was created from an LDAP lookup.
  • source=observation: this class or variable came from a measurements system observation and will also have the monitoring tag.

Hard Classes

Hard classes are discovered by CFEngine. Each time it wakes up, it discovers and reads properties of the environment or context in which it runs.It turns these properties of the environment into classes. This information is effectively cached and may be used to make decisions about configuration.

You can see all of the hard classes defined on a particular host by running the following command as a privileged user.

    $ cf-promises --show-classes|grep hardclass

These are classes that describe your operating system, the time of day, the week of the year, etc. Time-varying classes (tagged with time_based) will change if you do this a few times over the course of a week.


  • Hard classes can not be undefined. If you try to undefine or cancel a hard class an error will be emitted, for example error: You cannot cancel a reserved hard class 'cfengine' in post-condition classes.

  • CFEngine-specific classes

    • any: this class is always set
    • am_policy_hub, policy_server: set when the file $(workdir)/state/am_policy_hub exists. When a host is bootstrapped, if the agent detects that it is bootstrapping to itself the file is created.
    • bootstrap_mode: set when bootstrapping a host
    • inform_mode, verbose_mode, debug_mode: log verbosity levels in order of noisiness
    • opt_dry_run: set when the --dry-run option is given
    • failsafe_fallback: set when the base policy is invalid and the built-in (see bootstrap.c) is invoked
    • (community, community_edition) and (enterprise, enterprise_edition): the two different CFEngine products, Community and Enterprise, can be distinguished by these mutually exclusive sets of hard classes
    • Component Specific Classes (each component has a class that is always considered defined by that component):
  • Operating System Classes (note that the presence of these classes doesn't imply platform support)

    • Operating System Architecture - arista, big_ip, debian, eos, fedora, Mandrake, Mandriva, oracle, redhat, slackware, smartmachine, smartos, solarisx86, sun4, SuSE, ubuntu, ultrix, the always-favorite unknown_ostype, etc.
    • VM or hypervisor specific: VMware, virt_guest_vz, virt_host_vz, virt_host_vz_vzps, xen, xen_dom0, xen_domu_hv, xen_domu_pv, oraclevmserver, etc.
    • On Solaris-10 systems, the zone name (in the form zone_global, zone_foo, zone_baz).
    • Windows-specific: DomainController, Win2000, WinServer, WinServer2003, WinServer2008, WinVista, WinWorkstation, WinXP
    • have_aptitude, powershell, systemd: based on the detected capabilities of the platform or the compiled-in options
    • See also: sys.arch, sys.class, sys.flavor, sys.os, sys.ostype.
  • Network Classes

    • Unqualified Name of Host. CFEngine truncates it at the first dot. Note: and have the same unqualified name – www
    • The IPv4 address octets of any active interface (in the form ipv4_192_0_0_1, ipv4_192_0_0, ipv4_192_0, ipv4_192)
    • The IPv6 addresses of all active interfaces (with dots replaced by underscores, e.g. ipv6_fe80__a410_6072_21eb_d3fa) added in 3.7.8, 3.10.3, 3.12.0
    • User-defined Group of Hosts
    • mac_unknown: set when the MAC address can't be found
    • See also: sys.domain, sys.hardware_addresses,, sys.interface, sys.interfaces, sys.interface_flags, sys.ipv4, sys.ip_addresses, sys.fqhost, sys.uqhost.
  • Time Classes

    • note ALL of these have a local and a GMT version. The GMT classes are consistent the world over, in case you need global change coordination.
    • Day of the Week - Monday, Tuesday, Wednesday,...GMT_Monday, GMT_Tuesday, GMT_Wednesday,...
    • Hour of the Day in Current Time Zone - Hr00, Hr01,... Hr23 and Hr0, Hr1,... Hr23
    • Hour of the Day in GMT - GMT_Hr00, GMT_Hr01, ...GMT_Hr23 and GMT_Hr0, GMT_Hr1, ...GMT_Hr23.
    • Minutes of the Hour - Min00, Min17,... Min45,... and GMT_Min00, GMT_Min17,... GMT_Min45,...
    • Five Minute Interval of the Hour - Min00_05, Min05_10,... Min55_00 and GMT_Min00_05, GMT_Min05_10,... GMT_Min55_00. Note the second number indicates up to what minute the interval extends and does not include that minute.
    • Quarter of the Hour - Q1, Q2, Q3, Q4 and GMT_Q1, GMT_Q2, GMT_Q3, GMT_Q4
    • An expression of the current quarter hour - Hr12_Q3 and GMT_Hr12_Q3
    • Day of the Month - Day1, Day2,... Day31 and GMT_Day1, GMT_Day2,... GMT_Day31
    • Month - January, February,... December and GMT_January, GMT_February,... GMT_December
    • Year - Yr1997, Yr2004 and GMT_Yr1997, GMT_Yr2004
    • Period of the Day - Night, Morning, Afternoon, Evening and GMT_Night, GMT_Morning, GMT_Afternoon, GMT_Evening (six hour blocks starting at 00:00 hours).
    • Lifecycle Index - Lcycle_0, Lcycle_1, Lcycle_2 and GMT_Lcycle_0, GMT_Lcycle_1, GMT_Lcycle_2 (the year number modulo 3, used in long term resource memory).
    • See also: sys.cdate,
  • The unqualified name of a particular host (e.g., www). If your system returns a fully qualified domain name for your host (e.g.,, CFEngine will also define a hard class for the fully qualified name, as well as the partially-qualified component names,, and no.

  • An arbitrary user-defined string (as specified in the -D command line option, or defined in a classes promise promise or classes body, restart_class in a processes promise, etc).

  • The IP address octets of any active interface (in the form ipv4_192_0_0_1<!-- /@w -->, ipv4_192_0_0<!-- /@w -->, ipv4_192_0<!-- /@w -->, ipv4_192<!-- /@w -->), provided they are not excluded by a regular expression in the file WORKDIR/inputs/ignore_interfaces.rx.

  • The names of the active interfaces (in the form net_iface_xl0, net_iface_vr0).

  • System status and entropy information reported by cf-monitord.

Soft Classes

Soft classes are user-defined classes which you can use to implement your own classifications.

Soft classes can be set by using the -D or --define options wihtout having to edit the policy. Multiple classes can be defined by separating them with commas (no spaces).

$ cf-agent -Dclass


$ cf-agent --define class1,class2,class3

This can be especially useful when requesting a remote host to run its policy by using cf-runagent to activate policy that is normally dormant.

$ cf-runagent -Demergency_evacuation -H remoteclient

If you're using dynamic inputs this can be useful in combination with cf-promises to ensure that various input combinations syntax is validated correctly. Many people will have this run by pre-commit hooks or as part of a continuous build system like Jenkins or Bamboo.

$ cf-promises -f ./ -D prod
$ cf-promises -f ./ -D dev
./ error: syntax error
   "global1" expression => "any";
./ error: Check previous line, Expected ';', got '"global1"'
   "global1" expression => "any";
./ error: Expected promiser string, got 'expression'
   "global1" expression => "any";
./ error: Expected ';', got '=>'
   "global1" expression => "any";
2014-05-22T13:46:05+0000    error: There are syntax errors in policy files

Note: Classes, once defined, will stay defined either for as long as the bundle is evaluated (for classes with a bundle scope) or until the agent exits (for classes with a namespace scope). See cancel_kept, cancel_repaired, and cancel_notkept in classes body.

This example defines a few soft classes local to the myclasses bundle.

bundle agent myclasses
  "always2" expression => "any";
  "solinux" expression => "linux||solaris";
  "alt_class" or => { "linux", "solaris", fileexists("/etc/fstab") };
  "oth_class" and => { fileexists("/etc/shadow"), fileexists("/etc/passwd") };

    # This will only report "Boo!" on linux, solaris, or any system
    # on which the file /etc/fstab exists
  • The always and always2 soft classes are always defined.

  • The solinux soft class is defined as a combination of the linux or the solaris hard classes. This class will be set if the operating system family is either of these values.

  • The alt_class soft class is defined as a combination of linux, solaris, or the presence of a file named /etc/fstab. If one of the two hard classes evaluate to true, or if there is a file named /etc/fstab, the alt_class class will also be set.

  • The oth_class soft class is defined as the combination of two fileexists functions - /etc/shadow and /etc/passwd. If both of these files are present the oth_class class will also be set.

Negative Knowledge

If a class is set, then it is certain that the corresponding fact is true. However, that a class is not set could mean that something is not the case, or that something is simply not known. This is only a problem with soft classes, where the state of a class can change during the execution of a policy, depending on the order in which bundles and promises are evaluated.

Making Decisions based on classes

Class guards are the most common way to restrict a promise to a specific context. Once stated the restriction applies until a new context is specified. A new promise type automatically resets to an unrestricted context (the unrestricted context is typically referred to as any).

This example illustrates how a class guard applies (to multiple promises) until a new context is specified.

bundle agent __main__
      "This promise is not restricted.";

      "Neither is this promise restricted, 'any' is always defined.";

      "This promise is restricted to hosts that have the class 'linux' defined.";

      "This promise is also restricted to hosts that have the class 'linux' defined.";

      "This promise is restricted to hosts that have both the 'linux' class AND the 'cfengine_4' class.";

      "This promise will never be actuated.";

      "Message" string => "Hello World!";

      "And this promise is again unrestricted";
R: This promise is not restricted.
R: Neither is this promise restricted, 'any' is always defined.
R: This promise is restricted to hosts that have the class 'linux' defined.
R: This promise is also restricted to hosts that have the class 'linux' defined.
R: And this promise is again unrestricted

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

Another Example:

bundle agent greetings
      "Good morning!";

      "Good evening!";

      "! any"::
      "This report won't ever be seen.";

      # whitespace allowed only in 3.8 and later
      Friday . Evening::
      "It's Friday evening, TGIF!";

      "Monday . Evening"::
      "It's Monday evening.";

In this example, the report "Good morning!" is only printed if the class Morning is set, while the report "Good evening!" is only printed when the class Evening is set.

The "! any" context will never be evaluated. Note that since CFEngine 3.8 context expressions can contain spaces for legibility.

The "Monday . Evening" context will only be true on Monday evenings. The Friday . Evening context will only be true on Friday evenings. See below for more on context operators.

Variable class expressions

Sometimes it's convenient to put class names in variables. Variables can be used in quoted class expressions as well as the if and unless promise attributes.

Variables used in quoted class expressions apply to all promises following the quoted class expression until the end of the promise type or until a new class expression is stated.

Note: The double colon (::) at the end of the class expression should remain outside of the quoted string.

This example shows two ways to execute code conditionally based on such variables:

bundle agent greetings
    "myclassname" string => "Evening";

       "Good evening!";
       "What a wonderful sunset";

       "Good evening too!" if => "$(myclassname)";

As you saw above, the class predicate if (unless is also available) can be used if variable class expressions are required. It is ANDed with the normal class expression, and is evaluated together with the promise. Both may contain variables as long as the resulting expansion is a legal class expression.

    bundle agent example
              "french_cities"  slist => { "toulouse", "paris" };
              "german_cities"  slist => { "berlin" };
              "italian_cities" slist => { "milan" };
              "usa_cities"     slist => { "lawrence" };

              "all_cities" slist => { @(french_cities), @(german_cities), @(italian_cities), @(usa_cities) };

          "italy"   or => { @(italian_cities) };
          "germany" or => { @(german_cities) };
          "france"  or => { @(french_cities) };

        "It's $( here";

          "Good morning from Italy",
            if => "$(all_cities)";

          "Good afternoon from Germany",
            if => "$(all_cities)";

          "Hello from France",
            if => "$(all_cities)";

            unless => "france";

          "Hello from $(all_cities)";

        "Hello from $(all_cities), if edition",
          if => "$(all_cities)";

Example Output:

    cf-agent -Kf -D lawrence -b example
    R: It's Tue May 28 16:47:33 2013 here
    R: Hello from lawrence
    R: Hello from lawrence, if edition

    cf-agent -Kf -D paris -b example
    R: It's Tue May 28 16:48:18 2013 here
    R: Hello from France
    R: Hello from paris
    R: Hello from paris, if edition

    cf-agent -Kf -D milan -b example
    R: It's Tue May 28 16:48:40 2013 here
    R: Hello from milan
    R: Hello from milan, if edition

    cf-agent -Kf -D germany -b example
    R: It's Tue May 28 16:49:01 2013 here

    cf-agent -Kf -D berlin -b example
    R: It's Tue May 28 16:51:53 2013 here
    R: Good afternoon from Germany
    R: Hello from berlin
    R: Hello from berlin, if edition

In this example, lists of cities are defined in the vars section and these lists are combined into a list of all cities. These variable lists are used to qualify the greetings and to make the policy more concise. In the classes section a country class is defined if a class described on the right hand side evaluates to true. In the reports section the current time is always reported but only agents found to have the Morning and italy classes defined will report "Good morning from Italy", this is further qualified by ensuring that the report is only generated if one of the known cities also has a class defined.

Automatic canonification on class definition

Classes are automatically canonified when they are defined. Classes are not automatically canonified when they are checked.

This example shows how classes are automatically canonified when they are defined and that you must explicitly canonify when verifying classes.

bundle agent main



      # We search to see what class was defined:
      "$(with)" with => join( " ", classesmatching( "my.illegal.class" ) );

      # We see that the illegal class is explicitly not defined.
      "my-illegal-class is NOT defined (as expected, its invalid)"
        unless => "my-illegal-class";

      # We see the canonified form of the illegal class is defined.
      "my_illegal_class is defined"
        if => canonify("my-illegal-class");

      # Note, if takes expressisons, you couldn't do that if it were
      # automatically canonified. Here I canonify the string using with, and use
      # it as part of the expression which contains an invalid classcharacter, but
      # its desireable for constructing expressions.

      "Slice and dice using `with`"
        with => canonify( "my-illegal-class" ),
        if => "linux|$(with)";


First we promise to define my-illegal-class. When the promise is actuated it is automatically canonified and defined. This automatic canonification is logged in verbose logs (verbose: Class identifier 'my-illegal-class' contains illegal characters - canonifying). Next several reports prove which form of the class was defined. The last report shows how if takes a class expression, and if you are checking a class that contains invalid characters you must canonify it.

R: my_illegal_class
R: my-illegal-class is NOT defined (as expected, its invalid)
R: my_illegal_class is defined
R: Slice and dice using `with`

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

Operators and Precedence

Classes promises define new classes based on combinations of old ones. This is how to make complex decisions in CFEngine, with readable results. It is like defining aliases for class combinations. Such class 'aliases' may be specified in any kind of bundle.

Since CFEngine 3.8, whitespace is allowed between operators. It was not allowed up to 3.7.

For example a . b is equivalent to a.b and perhaps more readable.

Classes may be combined with the operators listed here in order from highest to lowest precedence:

  • ‘()':: ~ The parenthesis group operator.

  • ‘!’:: ~ The NOT operator.

  • ‘.’:: ~ The AND operator.

  • ‘&’:: ~ The AND operator (alternative).

  • ‘|’:: ~ The OR operator.

  • ‘||’:: ~ The OR operator (alternative).

These operators can be combined to form complex expressions. For example, the following expression would be only true on Mondays or Wednesdays from 2:00pm to 2:59pm on Windows XP systems:


Operands that are functions

If an operand is another function and the return value of the function is undefined, the result of the logical operation will also be undefined. For this reason, when using functions as operators, it is safer to collapse the functions down to scalar values and to test if the values are either true or false before using them as operands in a logical expression.


            expression => fileexists("/etc/aliases.db");

    or => { isnewerthan("/etc/aliases", "/etc/aliases.db"),
    "!variable_1" };

The function, isnewerthan can return "undefined" if one or other of the files does not exist. In that case, result would also be undefined. By checking the validity of the return value before using it as an operand in a logical expression, unpredictable results are avoided. i.e negative knowledge does not necessarily imply that something is not the case, it could simply be unknown. Checking if each file exists before calling isnewerthan would avoid this problem.

Operands that are JSON booleans

If an operand is true it will succeed, even though there doesn't have to be a class named true. If an operand is false it will fail, even though there may be a class named false. This allows JSON booleans from data containers to be used in context expressions:

bundle agent main
      "checks" data => '[true, false]';
      # find all classes named
      "classes_named_true" slist => classesmatching('true');

      # always defined
      "first_check" expression => "$(checks[0])";
      # never defined
      "second_check" expression => "$(checks[1])";

      # prints nothing, there are no classes named 'true'
      "Classes named 'true': $(classes_named_true)";

      "The class was defined from '$(checks[0])'";
      "The class was NOT defined from '$(checks[0])'";
      "The class was defined from '$(checks[1])'";
      "The class was NOT defined from '$(checks[1])'";


R: The class was defined from 'true'
R: The class was NOT defined from 'false'

Class scope

Classes defined by classes type promises in bundles of type common are namespace scoped by default (globally available, can be seen from any bundle), whereas classes defined in all other bundle types are local aka bundle scoped (they can not be seen from other bundles). Classes are evaluated when the bundle is evaluated (and the bundles are evaluated in the order specified in the bundlesequence).

Note that any class promise must have one - and only one - value constraint. That is, you may not leave expression in the example above and add and or xor constraints to the single promise.

Additionally classes can be defined or undefined as the result of a promise by using a classes body. To set a class if a promise is repaired, one might write:

        classes => if_repaired("signal_class");

These classes are namespace scoped by default. The scope attribute can be used to make them local to the bundle.

It is recommended to use bundle scoped classes whenever possible. This example will define signal_class prefixed classes with a suffix matching the promise outcome (_kept, _repaired, _notkept).

        classes => results("bundle", "signal_class");


        "Some aspect of the promise was repaired.";
        "The agent made a change to take us closer to the desired state";

        "Some aspect of the promise was kept";

        "Some aspect of the promsie was unable to be repaired";

        "All promise aspects were as desired";

Classes defined by the module protocol are namespace scoped.

Finally, restart_class classes in processes are global.

Class Scopes: A More Complex Example

    body common control
        bundlesequence => { "global","local_one", "local_two" };


    bundle common global
            # The soft class "zero" is always satisfied,
            # and is global in scope
            "zero" expression => "any";


    bundle agent local_one
            # The soft class "one" is always satisfied,
            # and is local in scope to local_one
            "one" expression => "any";


    bundle agent local_two
            # The soft class "two" is always satisfied,
            # and is local in scope to ls_2
            "two" expression => "any";

                # This report will be generated

In this example, there are three bundles. One common bundle named global with a global scope. Two agent bundles define classes one and two which are local to those bundles.

The local_two bundle promises a report "Success" which applies only if zero.!one.two evaluates to true. Within the local_two scope this evaluates to true because the one class is not set.


By default classes are re-computed on each agent execution. Once a class has been defined, it persists until the end of that agent execution. Persistent classes are always global and can not be set to local by scope directive. Classes can persist for a period of time. This can be useful to avoid the expense of re-evaluation, communicate states across multiple agent runs on the same host.

The standard library in the Masterfiles Policy Framework contains the feature bundle which implements a useful model for defining classes for a period of time as well as canceling them on demand.

See also: persistance classes attribute, persist_time in classes body, lib/ in the MPF, lib/ in the MPF

Canceling classes

You can cancel a class with a classes body. See the cancel_kept, cancel_notkept, and cancel_repaired attributes.